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  • 1.
    Abouelkomsan, Ahmed
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Geometry, Topology and Emergence in Moiré Systems2022Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The experimental discovery of correlated insulators and superconductivity in highly tunable Van der Waals heterostructures, such as twisted bilayer graphene, has highlighted the role of moiré patterns, resulting from tiny relative twists or lattice constant mismatches, in realizing strongly correlated physics. A key ingredient is the existence of very narrow flat bands where interaction effects are dominant.

    In this thesis and the accompanying papers, we theoretically study a number of experimentally relevant moiré systems. We generally show that strong interactions combined with the geometry and the topology of the underlying flat bands can result in a plethora of distinct quantum many-body phases ranging from topological order to multiferroicity. Of particular importance are lattice analogues of the fractional quantum Hall effect known as fractional Chern insulators. They harbour peculiar phenomena such as fractional charge and statistics and provide a route towards realizing topologically ordered states at high temperature. A ubiquitous feature of the many-body physics is the emergence of unique particle-hole dualities driven by the geometry of band-projected interactions.

    Fulltekst (pdf)
    Geometry, Topology and Emergence in Moiré Systems
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  • 2.
    Abouelkomsan, Ahmed
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Strongly Correlated Moiré Materials2021Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Recent advances in materials science have established Moiré materials as a new highly tunable and versatile form of quantum matter. When two dimensional atomic layers are brought into proximity, a tiny relative twist or a slight lattice mismatch produces Moiré patterns manifested in a superlattice structure with a lattice constant that is much larger than the lattice constants of the constituent layers. The new length scale has dramatic consequences for the underlying properties. A particular distinctive feature of Moiré materials is the emergence of nearly flat bands upon tuning external parameters such as the twist angle or the applied gate voltage. In a flat band, the kinetic energy is quenched, and interactions are enhanced bringing us to the realm of strongly correlated systems. A prime example of Moiré materials is twisted bilayer graphene, formed by taking two graphene layers and twisting them relative to each other.

    On the other hand, a famous class of interaction-induced phases of matter are fractional quantum Hall states and their lattice analogues known as fractional Chern insulators. These topologically ordered phases represent a departure from the conventional Landau symmetry breaking classification of matter, seen in the absence of local order parameters and the presence of global topological properties insensitive to local perturbations. Identifying and manufacturing materials that could host fractional Chern insulator states has a great potential for technological use.

    In this thesis, we provide the necessary background required for understanding the results of the accompanying papers [Phys. Rev. Lett. 124, 106803 & Phys. Rev. Lett. 126, 026801]. The theory of fractional Chern insulators is discussed followed by an introduction to the Moiré models used. In the two accompanying papers, we theoretically study a number of flat band Moiré materials aiming at identifying the possible phases that occur at fractional band fillings using a combination of analytical and numerical techniques. By reformulating the problem in terms of holes instead of electrons, it's possible to identify a variety of emergent weakly interacting Fermi liquids from an initial strongly interacting problem. In addition, our findings also include several high temperature fractional Chern insulator states at different fillings without external magnetic field.

  • 3.
    Abouelkomsan, Ahmed
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Johansson Bergholtz, Emil
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Chatterjee, Shubhayu
    Department of Physics, University of California, Berkeley. .
    Multiferroicity and Topology in Twisted Transition Metal DichalcogenidesManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    Van der Waals heterostructures have recently emerged as an exciting platform for investigating the effects of strong electronic correlations, including various forms of magnetic or electrical orders. Here, we perform an unbiased exact diagonalization study of the effects of interactions on topological flat bands of twisted transition metal dichalcogenides (TMDs) at odd integer fillings. We find that Chern insulator phases, expected from interaction-induced spin and valley polarization of the bare band structure, are quite fragile, and give way to spontaneous multiferroic order -- coexisting ferroelectricity and ferromagnetism, in presence of long-range Coulomb repulsion. We provide a simple real-space picture to understand the phase diagram as a function of interaction range and strength. Our findings establish twisted TMDs as a novel and highly tunable platform for multiferroicity, with potential applications to electrical control of magnetism. 

  • 4.
    Abouelkomsan, Ahmed
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Yang, Kang
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Johansson Bergholtz, Emil
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Quantum metric induced phases in Moiré materials2023Inngår i: Physical Review Research, E-ISSN 2643-1564, Vol. 5, nr 1, artikkel-id L012015Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We show that, quite generally, quantum geometry plays a major role in determining the low-energy physics in strongly correlated lattice models at fractional band fillings. We identify limits in which the Fubini-Study metric dictates the ground states and show that this is highly relevant for Moiré materials leading to symmetry breaking and interaction driven Fermi liquids. This phenomenology stems from a remarkable interplay between the quantum geometry and interaction which is absent in continuum Landau levels but generically present in lattice models where these terms tend to destabilize, e.g., fractional Chern insulators. We explain this as a consequence of the fundamental asymmetry between electrons and holes for band projected normal ordered interactions, as well as from the perspective of a self-consistent Hartree-Fock calculation. These basic insights about the role of the quantum metric, when dominant, turn an extremely strongly coupled problem into an effectively weakly coupled one, and may also serve as a guiding principle for designing material setups. We argue that this is a key ingredient for understanding symmetry-breaking phenomena recently observed in Moiré materials.

  • 5. Adak, Vivekananda
    et al.
    Roychowdhury, Krishanu
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Das, Sourin
    Spin-polarized voltage probes for helical edge state: A model study2022Inngår i: Physica. E, Low-Dimensional systems and nanostructures, ISSN 1386-9477, E-ISSN 1873-1759, Vol. 139, artikkel-id 115125Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Theoretical models of a spin-polarized voltage probe (SPVP) tunnel-coupled to the helical edge states (HES) of a quantum spin Hall system (QSHS) are studied. Our first model of the SPVP comprises Np spin-polarized modes (subprobes), each of which is locally tunnel-coupled to the HES, while the SPVP, as a whole, is subjected to a self-consistency condition ensuring zero average current on the probe. We carry out a numerical analysis which shows that the optimal situation for reading off spin-resolved voltage from the HES depends on the interplay of the probe-edge tunnel-coupling and the number of modes in the probe (Np). We further investigate the stability of our findings by introducing Gaussian fluctuations in (i) the tunnel-coupling between the subprobes and the HES about a chosen average value and (ii) spin-polarization of the subprobes about a chosen direction of the net polarization of SPVP. We also perform a numerical analysis corresponding to the situation where four such SPVPs are implemented in a self-consistent fashion across a ferromagnetic barrier on the HES and demonstrate that this model facilitates the measurements of spin-resolved four-probe voltage drops across the ferromagnetic barrier. As a second model, we employ the edge state of a quantum anomalous Hall state (QAHS) as the SPVP which is tunnel-coupled over an extended region with the HES. A two-dimensional lattice simulation for the quantum transport of the proposed device setup comprising a junction of QSHS and QAHS is considered and a feasibility study of using the edge of the QAHS as an efficient spin-polarized voltage probe is carried out including disorder.

  • 6.
    Adranno, Brando
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Renier, Olivier
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Bousrez, Guillaume
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Paterlini, Veronica
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Baryshnikov, Glib V.
    Smetana, Volodymyr
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Tang, Shi
    Ågren, Hans
    Metlen, Andreas
    Edman, Ludvig
    Mudring, Anja-Verena
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK). Aarhus University, Denmark.
    Rogers, Robin D.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK). The Queen's University of Belfast, Northern Ireland; The University of Alabama, USA.
    The 8-Hydroxyquinolinium Cation as a Lead Structure for Efficient Color-Tunable Ionic Small Molecule Emitting Materials2023Inngår i: Advanced Photonics Research, E-ISSN 2699-9293, Vol. 4, nr 3, artikkel-id 2200279Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Albeit tris(8-hydroxyquinolinato) aluminum (Alq3) and its derivatives are prominent emitter materials for organic lighting devices, and the optical transitions occur among ligand-centered states, the use of metal-free 8-hydroxyquinoline is impractical as it suffers from strong nonradiative quenching, mainly through fast proton transfer. Herein, it is shown that the problem of rapid proton exchange and vibration quenching of light emission can be overcome not only by complexation, but also by organization of the 8-hydroxyquinolinium cations into a solid rigid network with appropriate counter-anions (here bis(trifluoromethanesulfonyl)imide). The resulting structure is stiffened by secondary bonding interactions such as π-stacking and hydrogen bonds, which efficiently block rapid proton transfer quenching and reduce vibrational deactivation. Additionally, the optical properties are tuned through methyl substitution from deep blue (455 nm) to blue-green (488 nm). Time-dependent density functional theory (TDFT) calculations reveal the emission to occur from which an unexpectedly long-lived S1 level, unusual for organic fluorophores. All compounds show comparable, even superior photoluminescence compared to Alq3 and related materials, both as solids and thin films with quantum yields (QYs) up to 40–50%. In addition, all compounds show appreciable thermal stability with decomposition temperatures above 310 °C.

  • 7. Ahmed, Aamna
    et al.
    Ramachandran, Ajith
    Khaymovich, Ivan
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Sharma, Auditya
    Flat band based multifractality in the all-band-flat diamond chain2022Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, nr 20, artikkel-id 205119Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the effect of quasiperiodic Aubry-Andre disorder on the energy spectrum and eigenstates of a one-dimensional all-band-flat (ABF) diamond chain. The ABF diamond chain possesses three dispersionless flat bands with all the eigenstates compactly localized on two unit cells in the zero disorder limit. The fate of the compact localized states in the presence of the disorder depends on the symmetry of the applied potential. We consider two cases here: a symmetric one, where the same disorder is applied to the top and bottom sites of a unit cell and an antisymmetric one, where the disorder applied to the top and bottom sites are of equal magnitude but with opposite signs. Remarkably, the symmetrically perturbed lattice preserves compact localization, although the degeneracy is lifted. When the lattice is perturbed antisymmetrically, not only is the degeneracy is lifted but compact localization is also destroyed. Fascinatingly, all eigenstates exhibit a multifractal nature below a critical strength of the applied potential. A central band of eigenstates continue to display an extended yet nonergodic behavior for arbitrarily large strengths of the potential. All other eigenstates exhibit the familiar Anderson localization above the critical potential strength. We show how the antisymmetric disordered model can be mapped to a π/4 rotated square lattice with the nearest and selective next-nearest-neighbor hopping and a staggered magnetic field-such models have been shown to exhibit multifractality. Surprisingly, the antisymmetric disorder (with an even number of unit cells) preserves chiral symmetry-we show this by explicitly writing down the chiral operator.

  • 8.
    Amundsen, Morten
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Juričić, Vladimir
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Universidad Técnica Federico Santa María, Chile.
    Controlling Majorana modes by p-wave pairing in two-dimensional p + id topological superconductors2022Inngår i: Physical Review Research, E-ISSN 2643-1564, Vol. 4, nr 1, artikkel-id 013088Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We show that corner Majorana zero modes in a two-dimensional p + id topological superconductor can be controlled by the manipulation of the parent p-wave superconducting order. Assuming that the p-wave superconducting order is in either a chiral or helical phase, we find that when a dx2y2 wave superconducting order is induced, the system exhibits quite different behavior depending on the nature of the parent p-wave phase. In particular, we find that while in the helical phase, a localized Majorana mode appears at each of the four corners, in the chiral phase, it is localized along only two of the four edges. We furthermore demonstrate that the Majoranas can be directly controlled by the form of the edges, as we explicitly show in the case of circular edges. We argue that the application of strain may provide additional means of fine-tuning the Majorana zero modes in the system; in particular, it can partially gap them out. Our findings may be relevant for probing the topology in two-dimensional mixed-pairing superconductors.

  • 9.
    Amundsen, Morten
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Juričić, Vladimir
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Universidad Técnica Federico Santa María, Chile.
    Grain-boundary topological superconductor2023Inngår i: Communications Physics, E-ISSN 2399-3650, Vol. 6, nr 1, artikkel-id 232Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Majorana zero modes (MZMs) are of central importance for modern condensed matter physics and quantum information due to their non-Abelian nature, which thereby offers the possibility of realizing topological quantum bits. We here show that a grain boundary (GB) defect can host a topological superconductor (SC), with a pair of cohabitating MZMs at its end when immersed in a parent two-dimensional gapped topological SC with the Fermi surface enclosing a nonzero momentum. The essence of our proposal lies in the magnetic-field driven hybridization of the localized MZMs at the elementary blocks of the GB defect, the single lattice dislocations, due to the MZM spin being locked to the Burgers vector. Indeed, as we show through numerical and analytical calculations, the GB topological SC with two localized MZMs emerges in a finite range of both the angle and magnitude of the external magnetic field. Our work demonstrates the possibility of defect-based platforms for quantum information technology and opens up a route for their systematic search in future.

  • 10.
    Amundsen, Morten
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Norwegian University of Science and Technology, Norway.
    Linder, Jacob
    Robinson, Jason W. A.
    Žutić, Igor
    Banerjee, Niladri
    Colloquium: Spin-orbit effects in superconducting hybrid structures2024Inngår i: Reviews of Modern Physics, ISSN 0034-6861, E-ISSN 1539-0756, Vol. 96, nr 2, artikkel-id 021003Artikkel, forskningsoversikt (Fagfellevurdert)
    Abstract [en]

    Spin-orbit coupling (SOC) relates to the interaction between an electron’s motion and its spin and is ubiquitous in solid-state systems. Although the effect of SOC in normal-state phenomena has been extensively studied, its role in superconducting hybrid structures and devices elicits many unexplored questions. In conjunction with broken symmetries and material inhomogeneities within superconducting hybrid structures, SOC may have contributions beyond its effects in homogeneous materials. Notably, even with well-established magnetic or nonmagnetic materials and conventional 𝑠-wave spin-singlet superconductors, SOC leads to emergent phenomena including equal-spin-triplet pairing and topological superconductivity (hosting Majorana states), a modified current-phase relationship in Josephson junctions, and nonreciprocal transport, including superconducting diode effects. SOC is also responsible for transforming quasiparticles in superconducting structures, which enhances the spin Hall effect and changes the spin dynamics. Taken together, SOC in superconducting hybrid structures and the potential for electric tuning of the SOC strength create interesting possibilities to advance superconducting spintronic devices for energy-efficient computing and enable topological fault-tolerant quantum computing. By providing a description of experimental techniques and theoretical methods to study SOC, this Colloquium describes the current understanding of resulting phenomena in superconducting structures and offers a framework to select and design a growing class of materials systems where SOC plays an important role.

  • 11.
    Amundsen, Morten
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    V. Bobkova, Irina
    Kamra, Akashdeep
    Magnonic spin Joule heating and rectification effects2022Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, nr 14, artikkel-id 144411Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Nonlinear devices, such as transistors, enable contemporary computing technologies. We theoretically investigate nonlinear effects, bearing a high fundamental scientific and technical relevance, in magnonics with emphasis on superconductor-ferromagnet hybrids. Accounting for a finite magnon chemical potential, we theoretically demonstrate magnonic spin Joule heating, the spin analog of conventional electronic Joule heating. Besides suggesting a key contribution to magnonic heat transport in a broad range of devices, it provides insights into the thermal physics of nonconserved bosonic excitations. Considering a spin-split superconductor self-consistently, we demonstrate its interface with a ferromagnetic insulator to harbor large tunability of spin and thermal conductances. We further demonstrate hysteretic rectification I-V characteristics in this hybrid, where the hysteresis results from the superconducting state bistability.

  • 12. Andrade, Tomas
    et al.
    Krikun, Alexander
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Thermoelectric transport properties of gapless pinned charge density waves2022Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, nr 4, artikkel-id L041118Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Quantum strongly correlated matter exhibits properties which are not easily explainable in the conventional framework of Fermi liquids. Universal effective field theory tools are applicable in these cases regardless of the microscopic details of the quantum system, since they are based on symmetries. It is necessary, however, to construct these effective tools in full generality, avoiding restrictions coming from particular microscopic descriptions which may inadequately constrain the coefficients that enter in the effective theory. In this work we demonstrate with explicit examples how the hydrodynamic coefficients, which have been recently reinstated in the effective theory of pinned charge density waves (CDWs), can affect the phenomenology of the thermoelectric transport in strongly correlated quantum matter. Our examples, based on two classes of holographic models with pinned CDW, have microscopics which are conceptually different from Fermi liquids. Therefore, the above transport coefficients are nonzero, contrary to the conventional approach. We show how these coefficients allow one to take into account the change of sign of the Seebeck coefficient and the low resistivity of the CDW phase of the cuprate high temperature superconductors, without referring to the effects of Fermi surface reconstruction.

  • 13. Araya Day, Isidora
    et al.
    Akhmerov, Anton R.
    Varjas, Dániel
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Topological defects in a double-mirror quadrupole insulator displace diverging charge2022Inngår i: Scipost Physics Core, ISSN 2666-9366, Vol. 5, nr 4, artikkel-id 053Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We show that topological defects in quadrupole insulators do not host quantized fractional charges, contrary to what their Wannier representation indicates. In particular, we test the charge quantization hypothesis based on the Wannier representation of a disclination and a parametric defect. Since disclinations necessarily strain the lattice and parametric defects require closed curves in parameter space, both defects break four-fold rotation symmetry, even away from their origin. The Wannier representation of the defects is thus determined by local reflection symmetries. Contrary to the hypothesis, we find that the local charge density decays as ∼1/r2 with distance, leading to a diverging defect charge. Because topological defects are incompatible with four-fold rotation symmetry, we conclude that defect charge quantization is protected by sublattice symmetry, and not higher order topology.

  • 14. Ashuiev, Anton
    et al.
    Allouche, Florian
    Islam, Md. Ashraful
    Carvalho, José P.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Sanders, Kevin J.
    Conley, Matthew P.
    Klose, Daniel
    Lapadula, Giuseppe
    Wörle, Michael
    Baabe, Dirk
    Walter, Marc D.
    Pell, Andrew J.
    Copéret, Christophe
    Jeschke, Gunnar
    Pintacuda, Guido
    Andersen, Richard A.
    Geometry and electronic structure of Yb(iii)[CH(SiMe3)2]3 from EPR and solid-state NMR augmented by computations2024Inngår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 26, nr 11, s. 8734-8747Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge, and – still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f-shell. Here, we develop a methodology based on the combined use of state-of-the-art magnetic resonance spectroscopies (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the electronic structure and geometry of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, a prototypical example, which contains notable structural features according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex. Taken together, both EPR and NMR, augmented by quantum chemical calculations, provide a detailed and complementary understanding of such paramagnetic compounds. In particular, the EPR and NMR signatures point to the presence of three-centre–two-electron Yb-γ-Me-β-Si secondary metal–ligand interactions in this otherwise tri-coordinate metal complex, similarly to its diamagnetic Lu analogues. The electronic structure of Yb(III) can be described as a single 4f13 configuration, while an unusually large crystal-field splitting results in a thermally isolated ground Kramers doublet. Furthermore, the computational data indicate that the Yb–carbon bond contains some π-character, reminiscent of the so-called α-H agostic interaction.

  • 15.
    B. Arnardottir, Kristin
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. University of Iceland, Iceland.
    Kyriienko, O.
    Shelykh, I. A.
    Hall effect for indirect excitons in an inhomogeneous magnetic field2012Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, nr 24, artikkel-id 245311Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the effect of an inhomogeneous out-of-plane magnetic field on the behavior of two-dimensional (2D) spatially indirect excitons. Due to the difference of the magnetic field acting on electrons and holes, the total Lorentz force affecting the center of mass motion of an indirect exciton appears. Consequently, an indirect exciton acquires an effective charge proportional to the gradient of the magnetic field. The appearance of the Lorentz force causes the Hall effect for neutral bosons, which can be detected by measurement of the spatially inhomogeneous blueshift of the photoluminescence using a counterflow experiment.

  • 16.
    B. Brant Carvalho, Paulo H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Pressure-Induced Amorphization and Distinct Amorphous States of Clathrate Hydrates2023Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis summarizes a study on the pressure-induced amorphization (PIA) and structures of amorphous states of clathrate hydrates (CHs).

    PIA involves the transition of a crystalline material into an amorphous solid in response of mechanical compression at temperatures well below the melting point. The first material observed to undergo PIA was hexagonal ice. More recently it was shown that compounds of water undergo the same phenomenon without decomposition, despite the presence of solutes. CHs, which are crystalline inclusion compounds consisting of water molecules encaging small guest species, undergo PIA at ca. 1–4 GPa below 145 K. The obtained amorphous CH phase can be further densified on isobaric heating at high pressure. This annealing step enables to retain an amorphous material on pressure release. There has been a significant amount of studies into the understanding of the nature of PIA and transformations between amorphous phases of pure ice. The aim of this thesis has been the understanding of the PIA in CHs and its relation to pure ice. New information on the nature of PIA and subsequent amorphous-amorphous transitions in CH systems were gained from structural studies and in situ neutron diffraction played pivotal role due to the sensitivity of neutrons to the light element hydrogen. Here a generalized understanding of the PIA in CHs and a clear image of amorphous CH structures are presented.

    Fulltekst (pdf)
    Pressure-Induced Amorphization and Distinct Amorphous States of Clathrate Hydrates
    Download (jpg)
    presentationsbild
  • 17.
    B. Brant Carvalho, Paulo H.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Ivanov, Mikhail
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Andersson, Ove
    Loerting, Thomas
    Bauer, Marion
    Tulk, Chris A.
    Haberl, Bianca
    Daemen, Luke L.
    Molaison, Jamie J.
    Amann-Winkel, Katrin
    Lyubartsev, Alexander P.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Bull, Craig L.
    Funnell, Nicholas P.
    Häussermann, Ulrich
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Neutron scattering study of polyamorphic THF·17(H2O) – toward a generalized picture of amorphous states and structures derived from clathrate hydrates2023Inngår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, nr 21Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    From crystalline tetrahydrofuran clathrate hydrate, THF–CH (THF·17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF–CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77–140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, VHDA, upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third form, recovered amorphous (RA). Results from neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF–CH and liquid THF·17H2O solution (∼2.5 M). Although fully amorphous, HDA is heterogeneous with two length scales for water–water correlations (less dense local water structure) and guest–water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest–host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ∼23H2O. The local water structure in HDA is reminiscent of pure HDA-ice featuring 5-coordinated H2O. In VHDA, the hydration structure of HDA is maintained but the local water structure is densified and resembles pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ∼18 H2O molecules and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous.

  • 18.
    Balabanov, Oleksandr
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ortega-Taberner, Carlos
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Hermanns, Maria
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Quantization of topological indices in critical chains at low temperatures2022Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, nr 4, artikkel-id 045116Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Various types of topological phenomena at criticality are currently under active research. In this paper we suggest to generalize the known topological quantities to finite temperatures, allowing us to consider gapped and critical (gapless) systems on the same footing. It is then discussed that the quantization of the topological indices, also at critically, is retrieved by taking the low-temperature limit. This idea is explicitly illustrated on a simple case study of chiral critical chains where the quantization is shown analytically and verified numerically. The formalism is also applied for studying robustness of the topological indices to various types of disordering perturbations.

  • 19. Balafendiev, Rustam
    et al.
    Simovski, Constantin
    Millar, Alexander J.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC). Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Belov, Pavel
    Wire metamaterial filled metallic resonators2022Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, nr 7, artikkel-id 075106Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this work we study electromagnetic properties of a resonator recently suggested for the search of axions—a hypothetical candidate to explain dark matter. A wire medium loaded resonator (called a plasma haloscope when used to search for dark matter) consists of a box filled with a dense array of parallel wires electrically connected to top and bottom walls. We show that the homogenization model of a wire medium works for this resonator without mesoscopic corrections, and that the resonator quality factor Q at the frequency of our interest drops versus the growth of the resonator volume V until it is dominated by resistive losses in the wires. We find that even at room temperature metals like copper can give quality factors in the thousands—an order of magnitude higher than originally assumed. Our theoretical results for both loaded and unloaded resonator quality factors were confirmed by building an experimental prototype. We discuss ways to further improve wire medium loaded resonators.

  • 20. Balducci, Federico
    et al.
    Beau, Mathieu
    Yang, Jing
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Luxembourg, Grand Duchy of Luxembourg.
    Gambassi, Andrea
    del Campo, Adolfo
    Large Deviations beyond the Kibble-Zurek Mechanism2023Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 131, nr 23, artikkel-id 230401Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Kibble-Zurek mechanism (KZM) predicts that the average number of topological defects generated upon crossing a continuous or quantum phase transition obeys a universal scaling law with the quench time. Fluctuations in the defect number near equilibrium are approximately of Gaussian form, in agreement with the central limit theorem. Using large deviations theory, we characterize the universality of fluctuations beyond the KZM and report the exact form of the rate function in the transverse-field quantum Ising model. In addition, we characterize the scaling of large deviations in an arbitrary continuous phase transition, building on recent evidence establishing the universality of the defect number distribution.

  • 21.
    Barros Brant Carvalho, Paulo Henrique
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Ivanov, Mikhail
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Andersson, Ove
    Department of Physics, Umeå University.
    Loerting, Thomas
    Institute of Physical Chemistry, University of Innsbruck.
    Bauer, Marion
    Institute of Physical Chemistry, University of Innsbruck.
    Tulk, Chris A.
    Neutron Scattering Division, Oak Ridge National Laboratory.
    Haberl, Bianca
    Neutron Scattering Division, Oak Ridge National Laboratory.
    Daemen, Luke L.
    Neutron Scattering Division, Oak Ridge National Laboratory.
    Molaison, Jamie J.
    Neutron Scattering Division, Oak Ridge National Laboratory.
    Amann-Winkel, Katrin
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Lyubartsev, Alexander P.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK), Avdelningen för fysikalisk kemi. Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Science for Life Laboratory (SciLifeLab).
    Bull, Craig L.
    ISIS Neutron and Muon Source, Rutherford Appleton Laboratory.
    Funnell, Nicholas P.
    ISIS Neutron and Muon Source, Rutherford Appleton Laboratory.
    Häussermann, Ulrich
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK), Avdelningen för oorganisk kemi och strukturkemi.
    Neutron scattering study of polyamorphic THF ∙ (H2O)17 – toward a generalized picture of amorphous states and structures derived from clathrate hydratesManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    From crystalline tetrahydrofuran clathrate hydrate, THF-CH (THF ∙ 17H2O, cubic structure II), three distinct polyamorphs can be derived. First, THF-CH undergoes pressure-induced amorphization when pressurized to 1.3 GPa in the temperature range 77–140 K to a form which, in analogy to pure ice, may be called high-density amorphous (HDA). Second, HDA can be converted to a densified form, very-HDA (VHDA), upon heat-cycling at 1.8 GPa to 180 K. Decompression of VHDA to atmospheric pressure below 130 K produces the third, recovered amorphous (RA) form. Results from a compilation of neutron scattering experiments and molecular dynamics simulations provide a generalized picture of the structure of amorphous THF hydrates with respect to crystalline THF-CH and liquid THF ∙ 17H2O solution (~2.5 M). The calculated density of (only in situ observable) HDA and VHDA at 2 GPa and 130 K is 1.287 and 1.328 g/cm3, respectively, whereas that of RA (at 1 atm) is 1.081 g/cm3. Although fully amorphous, HDA is heterogeneous with two length scales for water-water correlations (less dense local water structure) and guest-water correlations (denser THF hydration structure). The hydration structure of THF is influenced by guest-host hydrogen bonding. THF molecules maintain a quasiregular array, reminiscent of the crystalline state, and their hydration structure (out to 5 Å) constitutes ~23 H2O. The local water structure in HDA is reminiscent of pure HDA-ice, featuring 5-coordinated H2O. In VHDA, this structure is maintained but the local water structure is densified to resemble pure VHDA-ice with 6-coordinated H2O. The hydration structure of THF in RA constitutes ~18 H2O and the water structure corresponds to a strictly 4-coordinated network, as in the liquid. Both VHDA and RA can be considered as homogeneous, solid solutions of THF and water. The local water structure of water-rich (1:17) amorphous CHs resembles most that of the corresponding amorphous water ices when compared to guest-rich CHs, e.g., Ar ∙ ~6H2O. The proposed significance of different contributions of water local environments presents a simple view to justify neutron structure factor features.

  • 22.
    Barth, A
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för biokemi och biofysik.
    Hauser, K
    Andersson, J
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för biokemi och biofysik.
    Karjalainen, E-L
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för biokemi och biofysik.
    Proton countertransport by the Ca2+ pump: Pathways and protonation sites studied by infrared spectroscopy and computation2007Konferansepaper (Annet (populærvitenskap, debatt, mm))
  • 23.
    Barth, Andreas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för biokemi och biofysik.
    Infrared spectroscopy of proteins2007Inngår i: Biochim Biophys Acta: Bioenergetics, ISSN 0006-3002, Vol. 1767, nr 9, s. 1073-101Artikkel, forskningsoversikt (Annet (populærvitenskap, debatt, mm))
    Abstract [en]

    This review discusses the application of infrared spectroscopy to the study of proteins. The focus is on the mid-infrared spectral region and the study of protein reactions by reaction-induced infrared difference spectroscopy.

  • 24.
    Barth, Andreas
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för biokemi och biofysik.
    Structural dynamics of the Ca2+-ATPase studied by time-resolved infrared spectroscopy2008Inngår i: Spectroscopy, Vol. 22, s. 63-82Artikkel, forskningsoversikt (Annet (populærvitenskap, debatt, mm))
    Abstract [en]

    Protonation of acidic residues in the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA 1a) was studied by multiconformation continuum electrostatic calculations in the Ca(2+)-bound state Ca(2)E1, in the Ca(2+)-free state E2(TG) with bound thapsigargin, and in the E2P (ADP-insensitive phosphoenzyme) analog state with MgF(4)(2-) E2(TG+MgF(4)(2-)). Around physiological pH, all acidic Ca(2+) ligands (Glu(309), Glu(771), Asp(800), and Glu(908)) were unprotonated in Ca(2)E1; in E2(TG) and E2(TG+MgF(4)(2-)) Glu(771), Asp(800), and Glu(908) were protonated. Glu(771) and Glu(908) had calculated pK(a) values larger than 14 in E2(TG) and E2(TG+MgF(4)(2-)), whereas Asp(800) titrated with calculated pK(a) values near 7.5. Glu(309) had very different pK(a) values in the Ca(2+)-free states: 8.4 in E2(TG+MgF(4)(2-)) and 4.7 in E2(TG) because of a different local backbone conformation. This indicates that Glu(309) can switch between a high and a low pK(a) mode, depending on the local backbone conformation. Protonated Glu(309) occupied predominantly two main, very differently orientated side-chain conformations in E2(TG+MgF(4)(2-)): one oriented inward toward the other Ca(2+) ligands and one oriented outward toward a protein channel that seems to be in contact with the cytoplasm. Upon deprotonation, Glu(309) adopted completely the outwardly orientated side-chain conformation. The contact of Glu(309) with the cytoplasm in E2(TG+MgF(4)(2-)) makes this residue unlikely to bind lumenal protons. Instead it might serve as a proton shuttle between Ca(2+)-binding site I and the cytoplasm. Glu(771), Asp(800), and Glu(908) are proposed to take part in proton countertransport.

  • 25.
    Basini, Martina
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pancaldi, M.
    Wehinger, B.
    Udina, M.
    Unikandanunni, Vivek
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Tadano, T.
    Hoffmann, M. C.
    Balatsky, A. V.
    Bonetti, Stefano
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Ca’ Foscari University of Venice, Italy; Rara Foundation – Sustainable Materials and Technologies, Italy.
    Terahertz electric-field-driven dynamical multiferroicity in SrTiO32024Inngår i: Nature, ISSN 0028-0836, E-ISSN 1476-4687Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The emergence of collective order in matter is among the most fundamental and intriguing phenomena in physics. In recent years, the dynamical control and creation of novel ordered states of matter not accessible in thermodynamic equilibrium is receiving much attention1,2,3,4,5,6. The theoretical concept of dynamical multiferroicity has been introduced to describe the emergence of magnetization due to time-dependent electric polarization in non-ferromagnetic materials7,8. In simple terms, the coherent rotating motion of the ions in a crystal induces a magnetic moment along the axis of rotation. Here we provide experimental evidence of room-temperature magnetization in the archetypal paraelectric perovskite SrTiO3 due to this mechanism. We resonantly drive the infrared-active soft phonon mode with an intense circularly polarized terahertz electric field and detect the time-resolved magneto-optical Kerr effect. A simple model, which includes two coupled nonlinear oscillators whose forces and couplings are derived with ab initio calculations using self-consistent phonon theory at a finite temperature9, reproduces qualitatively our experimental observations. A quantitatively correct magnitude was obtained for the effect by also considering the phonon analogue of the reciprocal of the Einstein–de Haas effect, which is also called the Barnett effect, in which the total angular momentum from the phonon order is transferred to the electronic one. Our findings show a new path for the control of magnetism, for example, for ultrafast magnetic switches, by coherently controlling the lattice vibrations with light.

  • 26.
    Basini, Martina
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Udina, M.
    Pancaldi, M.
    Unikandanunni, Vivek
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Bonetti, Stefano
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Ca’ Foscari University of Venice, Italy.
    Benfatto, L.
    Terahertz ionic Kerr effect: Two-phonon contribution to the nonlinear optical response in insulators2024Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 109, nr 2, artikkel-id 024309Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The THz Kerr effect measures the birefringence induced in an otherwise isotropic material by a strong THz pulse driving the Raman-active excitations of the systems. Here we provide experimental evidence of a sizable Kerr response in insulating SrTiO3 due to infrared-active lattice vibrations. Such a signal, named the ionic Kerr effect, is associated with the simultaneous excitation of multiple phonons. Thanks to a theoretical modeling of the time, polarization, and temperature dependence of the birefrengence, we can disentangle the ionic Kerr effect from the off-resonant electronic excitations, providing an alternative tunable mechanism to modulate the refractive index on ultrashort timescales via infrared active phonons.

  • 27.
    Baym, Gordon
    et al.
    Univ Illinois, Dept Phys, Urbana, IL, USA.
    Beck, D. H.
    Niels Bohr Inst, Niels Bohr Int Acad, Copenhagen O, Denmark.
    Pethick, Christopher
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Niels Bohr Inst, Niels Bohr Int Acad, Copenhagen , Denmark.
    Transport in very dilute solutions of He-3 in superfluid He-42013Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, nr 1, s. 014512-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Motivated by a proposed experimental search for the electric dipole moment of the neutron (nEDM) utilizing neutron-He-3 capture in a dilute solution of He-3 in superfluid He-4, we derive the transport properties of dilute solutions in the regime where the He-3 are classically distributed and rapid He-3-He-3 scatterings keep the He-3 in equilibrium. Our microscopic framework takes into account phonon-phonon, phonon-He-3, and He-3-He-3 scatterings. We then apply these calculations to measurements by Rosenbaum et al. [J. Low Temp. Phys. 16, 131 (1974)] and by Lamoreaux et al. [Europhys. Lett. 58, 718 (2002)] of dilute solutions in the presence of a heat flow. We find satisfactory agreement of theory with the data, serving to confirm our understanding of the microscopics of the helium in the future nEDM experiment.

    Fulltekst (pdf)
    Fulltext
  • 28. Belonoshko, Anatoly B.
    et al.
    Simak, Sergei I.
    Olovsson, Weine
    Vekilova, Olga Yu.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Elastic properties of body-centered cubic iron in Earth's inner core2022Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, nr 18, artikkel-id L180102Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The solid Earth's inner core (IC) is a sphere with a radius of about 1300 km in the center of the Earth. The information about the IC comes mainly from seismic studies. The composition of the IC is obtained by matching the seismic data and properties of candidate phases subjected to high pressure (P) and temperature (T). The close match between the density of the IC and iron suggests that the main constituent of the IC is iron. However, the stable phase of iron is still a subject of debate. One such iron phase, the body-centered cubic phase (bcc), is dynamically unstable at pressures of the IC (330–364 GPa) and low T but gets stabilized at high T characteristic of the IC (5000–7000 K). So far, ab initio molecular dynamics (AIMD) studies attempted to compute the bcc elastic properties for a small (order of 102) number of atoms. The mechanism of the bcc stabilization cannot be enabled in such cells and that has led to erroneous results. Here we apply AIMD to compute elastic moduli and sound velocities of the Fe bcc phase for a 2000 Fe atom computational cell, which is a cell of unprecedented size for ab initio calculations of iron. Unlike in previous ab initio calculations, both the longitudinal and the shear sound velocities of the Fe bcc phase closely match the properties of the IC material at P = 360 GPa and T = 6600 K, likely the PT conditions in the IC. The calculated density of the bcc iron at these PT conditions is just 3% higher than the density of the IC material according to the Preliminary Earth Model. This suggests that the widely assumed amount of light elements in the IC may need a reconsideration. The anisotropy of the bcc phase is an exact match to the most recent seismic studies.

  • 29.
    Benetis, Nikolas
    Stockholms universitet.
    Nuclear spin relaxation in paramagnetic metal complexes and the slow motion problem for the electron spin1984Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    A general theory is derived for the relaxation of ligand nuclei in a solution of slowly tumbling paramagnetic complexes with a static zero-field splitting interaction. The Solomon-Bloembergen-Morgan theory, which is the approach traditionally taken in studies of this kind, is shown to be the limit of the new theory when the electron Zeeman interaction dominates over the zerofield splitting interaction and when the exchange rate is significant.

    The spin-lattice and spin-spin relaxation processes for the nuclear spin are treated, for both the dipole-dipole and the scalar interaction between the nuclear and the electron spin. In addition, within the framework of the new theory, the effect of the interference of the dipole-dipole interaction, with the scalar interaction is also shown to be of significance.

    In the final section of this study, the new theory is used to reproduce the variable temperature and field 15N-relaxation data for a paramagnetic complex of Ni2+ with aniline.

  • 30.
    Bergholtz, E.J.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Karlhede, A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    A simple view on the quantum Hall system2007Inngår i: Bulletin of the American Physical Society, 2007Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    The physics of the quantum Hall system becomes very simple on a thin torus. Remarkably, however, the rich structure of the system still exists in this limit. On the very thin torus the many body problem reduces to a one-dimensional classical electrostatic problem and both the abelian and the non-abelian quantum Hall states are manifested as gapped one-dimensional crystals, Tao-Thouless states, with fractionally charged excitations that appear as domain walls between degenerate ground states. These states represent, but are extreme forms of, the observed states in the bulk and their qualitative properties (such as quasiparticle degeneracies, quantum numbers, relative size of the gaps etc.) remain the same. For the gapless states, there is a phase transition at finite thickness to phases different from the gapped crystals. At half-filling in the lowest Landau level, this new phase is a Luttinger liquid of neutral dipoles which is adiabatically connected to the gapless state observed in the bulk. The existence of the gapless phase on the thin (but finite) torus provides an explicit microscopic example of how weakly interacting quasiparticles moving in a reduced (zero) magnetic field emerge as the low energy sector of strongly interacting electrons in a strong magnetic field.

  • 31.
    Bergholtz, E.J.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Karlhede, A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    An Exact Solution for the Half-filled Lowest Landau Level2006Inngår i: Bulletin of the American Physical Society, 2006Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    We present an exact solution for the interacting electron gas in the half-filled lowest Landau level on a thin torus. The low energy sector consists of non-interacting, one-dimensional, neutral fermions (dipoles). The ground state, which is homogeneous, is the Fermi sea obtained by filling the negative energy states and the excited states are the gapless neutral excitations out of this one-dimensional sea. We identify this ground state as a version of the Rezayi-Read state, and find that it develops continuously, as the circumference grows, into the Rezayi-Read state that is believed to describe the observed metallic phase in the two-dimensional system. This suggests a Luttinger liquid description of the half-filled Landau level.

  • 32.
    Bergholtz, Emil J
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Karlhede, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Half-filled lowest Landau level on a thin torus.2005Inngår i: Phys Rev Lett, ISSN 0031-9007, Vol. 94, nr 2, s. 026802-Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    We solve a model that describes an interacting electron gas in the half-filled lowest Landau level on a thin torus, with radius of the order of the magnetic length. The low-energy sector consists of noninteracting, one-dimensional, neutral fermions. The ground state, which is homogeneous, is the Fermi sea obtained by filling the negative energy states, and the excited states are gapless neutral excitations out of this one-dimensional sea. Although the limit considered is extreme, the solution has a striking resemblance to the composite fermion description of the bulk \nu = 1/2 state—the ground state is homogeneous and the excitations are neutral and gapless. This suggests a one-dimensional Luttinger liquid description, with possible observable effects in transport experiments, of the bulk state where it develops continuously from the state on a thin torus as the radius increases.

  • 33.
    Bergholtz, Emil J
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Karlhede, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    'One-dimensional' theory of the quantum Hall system2006Inngår i: Journal of Statistical Mechanics: Theory and Experiment, nr L04001Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We consider the lowest Landau level on a torus as a function of its circumference L_1. When L_1 → 0, the ground state at general rational filling fraction is a crystal with a gap—a Tao–Thouless state. For filling fractions ν = p/(2pm + 1), these states are the limits of Laughlin's or Jain's wavefunctions describing the gapped quantum Hall states when L_1 → ∞. For the half-filled Landau level, there is a transition to a Fermi sea of non-interacting neutral fermions (dipoles), or rather to a Luttinger liquid modification thereof, at L_1 ~ 5 magnetic lengths. Using exact diagonalization we identify this state as a version of the Rezayi–Read state, and find that it develops continuously into the state that is believed to describe the observed metallic phase as L_1 → ∞. Furthermore, the effective Landau level structure that emerges within the lowest Landau level is found to be a consequence of the magnetic symmetries.

  • 34.
    Berkowicz, Sharon
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    The Role of Molecular Heterogeneity in the Structural Dynamics of Aqueous Solutions2022Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Aqueous solutions constitute the basis of life, yet their complex and anomalous nature is far from well-understood. Transient molecular ordering gives rise to microscopic spatial liquid heterogeneities and fluctuations which are believed to play a key role in biochemical processes as well as in pure water, the latter of which could fundamentally alter our view on water as life's solvent. In this thesis, we experimentally investigate the structural dynamics in aqueous solutions with the aim to understand the role of molecular heterogeneity in the complex solution dynamics. To extract dynamic information, we utilize coherent light- and X-ray scattering techniques, such as dynamic light scattering (DLS) and X-ray photon correlation spectroscopy (XPCS), which can resolve structural dynamics on a broad range of length and time scales. We explore the influence of hypothesized water fluctuations in the dynamics of simple model solutes, finding that their diffusive dynamic behaviour is effectively similar and independent of solute size down to molecular scale, whilst significantly different from that of single water molecules. Secondly, in a first proof-of-concept experiment, we successfully combine the spatial resolution of nanofocused coherent X-ray beams with dynamic measurements by XPCS, the results of which indicate that `nano-XPCS' could prove a valuable tool in the quest to resolve nanoscale fluctuations. Finally, an outlook is given where the next steps in these investigations are discussed, such as the use of aqueous-organic mixtures as model systems for spatially heterogeneous dynamics in biological solutions.

  • 35.
    Berkowicz, Sharon
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    The Role of Molecular Heterogeneity in the Structural Dynamics of Aqueous Solutions2023Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The liquid-liquid critical point hypothesis suggests that liquid water exists in two liquid states with different local structures, so-called high- and low-density liquid (HDL, LDL). At ambient pressure water locally fluctuates between these two states, with the fluctuations becoming more pronounced as the liquid is supercooled. In this thesis, we explore the role of molecular heterogeneity in the structural dynamics of aqueous solutions, specifically investigating the interplay of different solutes in water with the hypothesized HDL-LDL fluctuations. In our experimental approach, we utilize coherent light and X-ray scattering techniques, including small- and wide-angle X-ray scattering (SAXS, WAXS), as well as correlation methods, such as dynamic light scattering (DLS) and X-ray photon correlation spectroscopy (XPCS), that enable us to probe structural dynamics at a broad range of length and time scales. 

    Using DLS, we measure the diffusive dynamic behaviour of differently sized nanomolecular probes in supercooled water, finding that it is effectively similar and independent of probe size down to molecular scales of ≈1 nm. In contrast to single water molecules, these probes experience a similar dynamic environment, which coincides with the bulk viscosity. These results could suggest that anomalous influence from the hypothesized water fluctuations becomes apparent first on sub-nm length scales. Furthermore, we explore how the presence of small polar-organic solutes modulates the water phase diagram, utilizing glycerol-water solutions as a model system. By outrunning freezing with the rapid evaporative cooling technique, combined with ultrafast X-ray scattering at X-ray free-electron lasers (XFELs), we are able to probe the liquid structure in deeply supercooled dilute glycerol-water solutions. Our findings indicate the existence of HDL- and LDL-like fluctuations upon supercooling, with a Widom line shifted to slightly lower temperatures compared to pure water. Further experiments on deeply supercooled glycerol-water solutions at intermediate glycerol concentrations, combining WAXS and SAXS/XPCS, provide additional insights. These results reveal a first-order-like liquid-liquid transition involving discontinuous changes in the inter-atomic liquid structure and nanoscale liquid dynamics, which precedes ice crystallization. 

    Lastly, with the aim of developing powerful tools for resolving dynamics within spatially heterogeneous systems, including aqueous solutions, we combine the spatial resolution of nanofocused coherent X-ray beams with dynamic measurements by XPCS. Here, we successfully demonstrate a first proof-of-concept experiment of so-called nanofocused XPCS at MAX IV synchrotron radiation facility. In future experiments, we plan to go beyond standard XPCS at synchrotrons, towards accessing ultrafast atomic-scale liquid dynamics by X-ray speckle visibility spectroscopy (XSVS) at XFELs.

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  • 36.
    Berkowicz, Sharon
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Andronis, Iasonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Filianina, Mariia
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Bin, Maddalena
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Girelli, Anita
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nam, Kyeongmin
    Shin, Myeongsik
    Kowalewski, Markus
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Katayama, Tetsuo
    Kim, Kyung Hwan
    Perakis, Fivos
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Unveiling the Structure and Thermodynamics of Deeply Supercooled Glycerol-Water Microdroplets with Ultrafast X-ray ScatteringManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    The liquid-liquid critical point (LLCP) hypothesis of water suggests that water exists in two structurally distinct liquid states, high- and low-density liquid (HDL, LDL), with an LLCP hidden in the supercooled regime at elevated pressures. However, its consequences for solvation and structural dynamics in aqueous solutions remain to be explored. Here, we probe the structure and thermodynamics of deeply supercooled microdroplets of prototypical aqueous solutions of glycerol. The combination of rapid evaporative cooling with ultrafast small- and wide-angle X-ray scatter-ing (SAXS, WAXS) allows us to outrun crystallization and gain access to the largely unexplored deeply supercooled dilute regime (3.2 mol% glycerol) down to T ≈ 229 K, which is not accessible by conventional cooling methods. The experimental results, and complementary molecular dynamics(MD) simulations, indicate an increase in the tetrahedral coordination and enhancement of HDL-and LDL-like density fluctuations upon supercooling. In addition, the extended temperature range of the MD simulations reveals a maximum in the isothermal compressibility at T ≈ 220 K, indicating the location of a Widom line shifted to slightly lower temperatures compared to that of pure water. We conclude that the apparent effect of the presence of glycerol molecules on the water hydrogen-bond structure resembles that of pressure. This opens the possibility to search for the existence of an LLCP in these aqueous solutions simply by varying the solute concentration.

  • 37.
    Berkowicz, Sharon
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Filianina, Mariia
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Bin, Maddalena
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Tyburski, Robin
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Bauer, Robert
    Chushkin, Yuriy
    Zontone, Federico
    Westermeier, Fabian
    Sprung, Michael
    Roseker, Wojciech
    Lehmkühler, Felix
    Perakis, Fivos
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Resolving nanoscale dynamics during a liquid-liquid transition in supercooled glycerol-water solutionsManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    It is proposed that a liquid-liquid transition (LLT), related to the hypothesized transition between high- and low-density liquids (HDL, LDL) in pure water, also exists in supercooled aqueous mixtures. However, experimental observations of the LLT in the supercooled solution is often complicated by the overlap with freezing. Here, we conducted an experiment probing the hypothesized LLT in deeply supercooled 16.5 mol% glycerol-water solution, combining X-ray photon correlation spectroscopy (XPCS), ultra small-angle X-ray scattering (USAXS) and wide-angle X-ray scattering (WAXS). This approach allows us to capture simultaneous, discontinuous structural and dynamic changes within the supercooled liquid following quenching to cryogenic temperatures (172-182 K). We observe changes in the inter-atomic liquid structure (from WAXS) as well as in the nanoscale structure and dynamics (from USAXS/XPCS), resembling a first-order LLT between HDL-like to LDL-like liquid. Importantly, we find that the LLT precedes the onset of ice crystalliization, which we can distinguish based on the advent of ice bragg peaks in WAXS. In addition, analysis of the two-time correlation (TTC) function from XPCS enables us to follow the dynamics during the LLT, which indicates super-diffusive ballistic-like motion and a gradual slowdown towards an arrested state upon freezing, consistent with an LLT via spinodal decomposition. We conclude that these results indicate the existence of a first-order LLT in supercooled glycerol-water solutions at intermediate glycerol concentrations, similar to that hypothesized for pure water at elevated pressures.

  • 38. Bhalla, Pankaj
    et al.
    Rostami, Habib
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Bath, United Kingdom.
    Light-induced nonlinear spin Hall current in single-layer WTe22024Inngår i: New Journal of Physics, E-ISSN 1367-2630, Vol. 26, nr 2, artikkel-id 023042Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this theoretical investigation, we analyze light-induced nonlinear spin Hall currents in a gated single-layer 1T′-WTe2, flowing transversely to the incident laser polarization direction. Our study encompasses the exploration of the second and third-order rectified spin Hall currents using an effective low-energy Hamiltonian and employing the Kubo's formalism. We extend our analysis to a wide frequency range spanning both transparent and absorbing regimes, investigating the influence of light frequency below and above the optical band gap. Additionally, we investigate the influence of an out-of-plane gate potential on the system, disrupting inversion symmetry and effectively manipulating both the strength and sign of nonlinear spin Hall responses. We predict a pronounced third-order spin Hall current relative to its second-order counterpart. The predicted nonlinear spin currents show strong anisotropic dependence on the laser polarization angle. The outcomes of our study contribute to a generalized framework for nonlinear response theory within the spin channel will impact the development of emerging field of opto-spintronic.

  • 39.
    Bhalla, Pankaj
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Rostami, Habib
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Second harmonic helicity and Faraday rotation in gated single-layer 1T ' -WTe22022Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, nr 23, artikkel-id 235132Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A single layer of the 1T' phase of WTe2 provides a rich platform for exotic physical properties such as the nonlinear Hall effect and high-temperature quantum spin Hall transport. Utilizing a continuum model and the diagrammatic method, we calculate the second harmonic conductivity of monolayer 1T'-WTe2 modulated by an external vertical electric field and electron doping. We obtain a finite helicity and Faraday rotation for the second harmonic signal in response to linearly polarized incident light in the presence of time-reversal symmetry. The second harmonic signal's helicity is highly controllable by altering the bias potential and serves as an optical indicator of the nonlinear Hall current. Our study motivates future experimental investigation of the helicity spectroscopy of two-dimensional materials.

  • 40.
    Bhalla, Pankaj
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Vignale, Giovanni
    Rostami, Habib
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Pseudogauge field driven acoustoelectric current in two-dimensional hexagonal Dirac materials2022Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, nr 12, artikkel-id 125407Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using a diagrammatic scheme, we study the acoustoelectric effects in two-dimensional (2D) hexagonal Dirac materials due to the sound-induced pseudogauge field. We analyze both uniform and spatially dispersive currents in response to copropagating and counterpropagating sound waves, respectively. In addition to the longitudinal acoustoelectric current, we obtain an exotic transverse charge current flowing perpendicular to the sound propagation direction owing to the interplay of transverse and longitudinal gauge field components jTALAT. In contrast to the almost isotropic directional profile of the longitudinal uniform current, a highly anisotropic transverse component jT∼sin(6θ) is achieved that stems from the inherited threefold symmetry of the hexagonal lattice. However, both longitudinal and transverse parts of the dispersive current are predicted to be strongly anisotropic ∼sin2(3θ) or cos2(3θ). We quantitatively estimate the pseudogauge field contribution to the acoustoelectric current that can be probed in future experiments in graphene and other 2D hexagonal Dirac materials.

  • 41.
    Bonetti, Stefano
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Ca Foscari University of Venice, Italy.
    Terahertz metamaterials: accessing nonlinear spin and phonon dynamics2019Inngår i: META 2019 Lisbon - Portugal: The 10th International Conference on Metamaterials, Photonic Crystals and Plasmonics / [ed] Said Zouhdi; Antonio Topa, META Conference , 2019, s. 585-586Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this talk, I will describe a few metamaterials design that can be used to drive large amplitude, nonlinear dynamics in condensed matter systems, focusing in particular to the spin and lattice degrees of freedom. For this scope, I will discuss different designs aimed at enhancing either the electric or the magnetic field component of terahertz radiation. The metamaterial structures have been investigated thoroughly using three-dimensional finite element simulations in both the frequency and the time domain. Preliminary results of the observation of the large amplitude dynamics induced by such metamaterials will be shown, detected using timeresolved pump-probe microscopy.

  • 42.
    Bonetti, Stefano
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    X-ray imaging of spin currents and magnetisation dynamics at the nanoscale2017Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, nr 13, artikkel-id 133004Artikkel, forskningsoversikt (Fagfellevurdert)
    Abstract [en]

    Understanding how spins move in time and space is the aim of both fundamental and applied research in modern magnetism. Over the past three decades, research in this field has led to technological advances that have had a major impact on our society, while improving the understanding of the fundamentals of spin physics. However, important questions still remain unanswered, because it is experimentally challenging to directly observe spins and their motion with a combined high spatial and temporal resolution. In this article, we present an overview of the recent advances in x-ray microscopy that allow researchers to directly watch spins move in time and space at the microscopically relevant scales. We discuss scanning x-ray transmission microscopy (STXM) at resonant soft x-ray edges, which is available at most modern synchrotron light sources. This technique measures magnetic contrast through the x-ray magnetic circular dichroism (XMCD) effect at the resonant absorption edges, while focusing the x-ray radiation at the nanometre scale, and using the intrinsic pulsed structure of synchrotron-generated x-rays to create time-resolved images of magnetism at the nanoscale. In particular, we discuss how the presence of spin currents can be detected by imaging spin accumulation, and how the magnetisation dynamics in thin ferromagnetic films can be directly imaged. We discuss how a direct look at the phenomena allows for a deeper understanding of the the physics at play, that is not accessible to other, more indirect techniques. Finally, we present an overview of the exciting opportunities that lie ahead to further understand the fundamentals of novel spin physics, opportunities offered by the appearance of diffraction limited storage rings and free electron lasers.

  • 43.
    Bonfils, Anthony
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    The growth of waves by wind as a problem in nonequilibrium statistical mechanics2020Licentiatavhandling, monografi (Annet vitenskapelig)
    Abstract [en]

    In 1948, Casimir predicted a net attractive force between two perfectly conducting parallel plates due to electromagnetic vacuum fluctuations. By analogy, the interaction of two ships on a wavy sea has been named Maritime Casimir effect. This is an example of force generation in non-equilibrium systems. Lee, Vella and Wettlaufer showed it to be oscillatory as it is induced by the sharply peaked energy spectrum measured in the sixties by Pierson and Moskowitz for a fully developed sea; a sea whose state is independent of the distance over which the wind blows and the time for which it has been blowing. The aim of this project is to construct a theory for that spectrum and understand how the Maritime Casimir effect emerges from wind-wave interaction. Waves in the absence of wind, so-called water waves, are mainly characterized by dispersion and weak non-linearity. The coupling of both results in the instability of a wave packet to side-band perturbations in deep water. The growth rate can be calculated thanks to a non-linear Schrödinger equation, which is a universal model for weakly non-linear waves in a dispersive medium. Furthermore, this instability can be understood in the even more general framework of resonant wave-wave interaction. The evolution of deep water gravity waves is actually a sum of four-wave interaction processes and triadic interactions should be added for capillary waves. That evolution is strongly affected by the presence of turbulent wind because it transfers energy to the waves. The growth rate of wind waves was calculated by Miles in 1957 on the basis of the weak air-water coupling. His formula involves the solution of the hydrodynamic Rayleigh equation at the critical level, which is the height at which the phase speed of the wave is equal to the wind speed. We develop an efficient numerical scheme to compute it and then compare the theory with the observational data compiled by Plant. We eventually propose asymptotic solutions of the Rayleigh equation for a generic wind profile, which will be useful to get a better understanding of the experimental results.

  • 44. Borisov, Vladislav
    et al.
    Xu, Qichen
    Ntallis, Nikolaos
    Clulow, Rebecca
    Shtender, Vitalii
    Cedervall, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för material- och miljökemi (MMK).
    Sahlberg, Martin
    Wikfeldt, Kjartan Thor
    Thonig, Danny
    Pereiro, Manuel
    Bergman, Anders
    Delin, Anna
    Eriksson, Olle
    Tuning skyrmions in B20 compounds by 4d and 5d doping2022Inngår i: Physical Review Materials, E-ISSN 2475-9953, Vol. 6, nr 8, artikkel-id 084401Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the 3d sublattice of the B20 structure by substitution of heavier 4d and 5d elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for 5d-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal the spin-spiral nature of the magnetic ground state and field-induced skyrmions for all these systems. Especially small skyrmions ∼50nm are predicted for Co0.75Os0.25Si, compared to ∼148nm for Fe0.75Co0.25Si. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.

  • 45.
    Borodianskyi, Evgenii A.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Krasnov, Vladimir M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Josephson emission with frequency span 1–11 THz from small Bi2Sr2CaCu2O8+δ mesa structures2017Inngår i: Nature Communications, E-ISSN 2041-1723, Vol. 8, artikkel-id 1742Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. They can be used for generation of high-frequency electromagnetic waves. Here we analyze Josephson emission from small-but-high mesas (with a small area, but containing many stacked junctions). We have found strong evidence for tunable terahertz emission with a good efficacy in a record high-frequency span 1–11 THz, approaching the theoretical upper limit for this superconductor. Emission maxima correspond to in-phase cavity modes in the mesas, indicating coherent superradiant nature of the emission. We conclude that terahertz emission requires a threshold number of junctions N ~ 100. The threshold behavior is not present in the classical description of stacked Josephson junctions and suggests importance of laser-like cascade amplification of the photon number in the cavity. 

  • 46.
    Borysov, Stanislav S.
    et al.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Platz, Daniel
    de Wijn, Astrid S.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Forchheimer, Daniel
    Tolen, Eric A.
    Balatsky, Alexander V.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Los Alamos Natl Lab, Div Theoret, Los Alamos, NM 87545 USA.
    Haviland, David B.
    Reconstruction of tip-surface interactions with multimodal intermodulation atomic force microscopy2013Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, nr 11, s. 115405-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We propose a theoretical framework for reconstructing tip-surface interactions using the intermodulation technique when more than one eigenmode is required to describe the cantilever motion. Two particular cases of bimodal motion are studied numerically: one bending and one torsional mode, and two bending modes. We demonstrate the possibility of accurate reconstruction of a two-dimensional conservative force field for the former case, while dissipative forces are studied for the latter.

  • 47. Bossini, D.
    et al.
    Juraschek, D. M.
    Geilhufe, R. M.
    Nagaosa, N.
    Balatsky, Alexander V.
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Connecticut, United States of America.
    Milanović, M.
    Srdić, V. V.
    Šenjug, P.
    Topić, E.
    Barišić, D.
    Rubčić, M.
    Pajić, D.
    Arima, T.
    Savoini, M.
    Johnson, S. L.
    Davies, C. S.
    Kirilyuk, A.
    Magnetoelectrics and multiferroics: theory, synthesis, characterisation, preliminary results and perspectives for all-optical manipulations2023Inngår i: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 56, nr 27, artikkel-id 273001Artikkel, forskningsoversikt (Fagfellevurdert)
    Abstract [en]

    Solid state compounds exhibiting multiple and coupled macroscopic orders, named multiferroics, represent a challenge for both theoretical and experimental modern condensed-matter physics. Spins and the electric polarisation in conventional magnetic and ferroelectric materials can be manipulated on their fundamental timescales, by means of femtosecond laser pulses. In view of the resounding success and popularity of the all-optical approach, it is only natural to wonder about the application of this scheme to study the intrinsic coupling between spins and charges in multiferroics. Deeply fundamental questions arise: can ultrashort laser pulses deterministically activate, enhance or suppress the magnetoelectric coupling on the femtosecond timescale? Can these processes be triggered in a fully coherent fashion, thus being unrestrained by any thermal load? Which mechanism of spin-charge coupling is most favourable to overcome these overarching and daunting challenges? This problem is interdisciplinary in nature, requiring contributions from materials science and condensed matter physics from both theoretical and experimental perspectives. High-quality materials suitable for optical investigations have to be identified, synthetized and characterised. General and valid models offer then a guide to the plethora of possible light-induced processes, resulting in the desired ultrafast multiferroic manipulations. Finally, healthy experimental schemes, able to unambiguously track the ultrafast dynamics of either the ferroelectric or the magnetic order parameter have to be developed and implemented. Our motivation to write this review is to lay a broad and multidisciplinary foundation, which may be employed as a starting point for non-equilibrium approaches to the manipulation of the multiferroicity on the femtosecond timescale. This was also one of the main goals of the COST Action MAGNETOFON, whose network constitutes the core of the authors of this review. The present work thus represents a part of the scientific legacy of MAGNETOFON itself.

  • 48. Brero, Francesca
    et al.
    Arosio, Paolo
    Albino, Martin
    Cicolari, Davide
    Porru, Margherita
    Basini, Martina
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Mariani, Manuel
    Innocenti, Claudia
    Sangregorio, Claudio
    Orsini, Francesco
    Lascialfari, Alessandro
    1H-NMR Relaxation of Ferrite Core-Shell Nanoparticles: Evaluation of the Coating Effect: Evaluation of the Coating Effect2023Inngår i: Nanomaterials, E-ISSN 2079-4991, Vol. 13, nr 5, artikkel-id 804Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigated the effect of different organic coatings on the 1H-NMR relaxation properties of ultra-small iron-oxide-based magnetic nanoparticles. The first set of nanoparticles, with a magnetic core diameter ds1 = 4.4 ± 0.7 nm, was coated with polyacrylic acid (PAA) and dimercaptosuccinic acid (DMSA), while the second set, ds2 = 8.9 ± 0.9 nm, was coated with aminopropylphosphonic acid (APPA) and DMSA. At fixed core diameters but different coatings, magnetization measurements revealed a similar behavior as a function of temperature and field. On the other hand, the 1H-NMR longitudinal r1 nuclear relaxivity in the frequency range ν = 10 kHz ÷ 300 MHz displayed, for the smallest particles (diameter ds1), an intensity and a frequency behavior dependent on the kind of coating, thus indicating different electronic spin dynamics. Conversely, no differences were found in the r1 relaxivity of the biggest particles (ds2) when the coating was changed. It is concluded that, when the surface to volume ratio, i.e., the surface to bulk spins ratio, increases (smallest nanoparticles), the spin dynamics change significantly, possibly due to the contribution of surface spin dynamics/topology.

  • 49. Brondin, Carlo Alberto
    et al.
    Ghosh, Sukanya
    Debnath, Saikat
    Genuzio, Francesca
    Genoni, Pietro
    Jugovac, Matteo
    Bonetti, Stefano
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Binggeli, Nadia
    Stojic, Natasa
    Locatelli, Andrea
    Mentes, Tevfik Onur
    Tailoring Magnetic Anisotropy in Ultrathin Cobalt by Surface Carbon Chemistry2024Inngår i: Advanced Electronic Materials, E-ISSN 2199-160X, Vol. 10, nr 4, artikkel-id 2300579Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The ability to manipulate magnetic anisotropy is essential for magnetic sensing and storage tools. Surface carbon species offer cost-effective alternatives to metal-oxide and noble metal capping layers, inducing perpendicular magnetic anisotropy in ultrathin ferromagnetic films. Here, the different mechanisms by which the magnetism in a few-layer-thick Co thin film is modified upon adsorption of carbon monoxide (CO), dispersed carbon, and graphene are elucidated. Using X-ray microscopy with chemical and magnetic sensitivity, the in-plane to out-of-plane spin reorientation transition in cobalt is monitored during the accumulation of surface carbon up to the formation of graphene. Complementary magneto-optical measurements show weak perpendicular magnetic anisotropy (PMA) at room temperature for dispersed carbon on Co, while graphene-covered cobalt exhibits a significant out-of-plane coercive field. Density-functional theory (DFT) calculations show that going from CO/Co to C/Co and to graphene/Co, the magnetocrystalline and magnetostatic anisotropies combined promote out-of-plane magnetization. Anisotropy energies weakly depend on carbidic species coverage. Instead, the evolution of the carbon chemical state from carbidic to graphitic is accompanied by an exponential increase in the characteristic domain size, controlled by the magnetic anisotropy energy. Beyond providing a basic understanding of the carbon-ferromagnet interfaces, this study presents a sustainable approach to tailor magnetic anisotropy in ultrathin ferromagnetic films. Magnetic properties of Co ultrathin films are shown to undergo dramatic changes upon surface carbon accumulation. Chemical transformation from molecular carbon monoxide to surface carbide and to a graphene layer progressively enhances the perpendicular magnetic anisotropy of Co. Calculations reveal that magnetocrystalline and magnetostatic contributions play distinctly different roles for the different carbon species.image

  • 50.
    Budich, Jan Carl
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Charge conservation protected topological phases2013Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, nr 16, s. 161103-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We discuss the relation between particle number conservation and topological phases. In four spatial dimensions, we find that systems belonging to different topological phases in the presence of a U(1) charge conservation can be connected adiabatically, i.e., without closing the gap, upon intermediately breaking this local symmetry by a superconducting term. The time reversal preserving topological insulator states in two and three dimensions which can be obtained from the four-dimensional parent state by dimensional reduction inherit this protection by charge conservation. Hence, all topological insulators can be adiabatically connected to a trivial insulating state without breaking time reversal symmetry, provided an intermediate superconducting term is allowed during the adiabatic deformation. Conversely, in one spatial dimension, non-symmetry-protected topological phases occur only in systems that break U(1) charge conservation. These results can intuitively be understood by considering a natural embedding of the classifying spaces of charge conserving Hamiltonians into the corresponding Bogoliubov-de Gennes classes.

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