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  • 1. A. Madsen, Kevin
    et al.
    J. Bergholtz, Emil
    Stockholm University, Faculty of Science, Department of Physics.
    Brouwer, Piet W.
    Josephson effect in a Weyl SNS junction2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 6, article id 064511Article in journal (Refereed)
    Abstract [en]

    We calculate the Josephson current density j (phi) for a Weyl superconductor-normal-metal-superconductor junction for which the outer terminals are superconducting Weylmetals and the normal layer is a Weyl (semi) metal. We describe the Weyl (semi) metal using a simple model with two Weyl points. The model has broken time-reversal symmetry, but inversion symmetry is present. We calculate the Josephson current for both zero and finite temperature for the two pairing mechanisms inside the superconductors that have been proposed in the literature, zero-momentum BCS-like pairing and finite-momentum FFLO-like pairing, and assuming the short-junction limit. For both pairing types we find that the current is proportional to the normal-state junction conductivity, with a proportionality coefficient that shows quantitative differences between the two pairing mechanisms. The current for the BCS-like pairing is found to be independent of the chemical potential, whereas the current for the FFLO-like pairing is not.

  • 2.
    Abergel, David S. L.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Mucha-Kruczynski, Marcin
    Infrared absorption of closely aligned heterostructures of monolayer and bilayer graphene with hexagonal boron nitride2015In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 92, no 11, article id 115430Article in journal (Refereed)
    Abstract [en]

    We model optical absorption of monolayer and bilayer graphene on hexagonal boron nitride for the case of closely aligned crystal lattices. We show that perturbations with different spatial symmetry can lead to similar absorption spectra. We suggest that a study of the absorption spectra as a function of the doping for an almost completely full first miniband is necessary to extract meaningful information about the moire characteristics from optical absorption measurements and to distinguish between various theoretical proposals for the physically realistic interaction. Also, for bilayer graphene, the ability to compare spectra for the opposite signs of electric-field-induced interlayer asymmetry might provide additional information about the moire parameters.

  • 3. Adak, Vivekananda
    et al.
    Roychowdhury, Krishanu
    Stockholm University, Faculty of Science, Department of Physics. Cornell University, USA.
    Das, Sourin
    Spin Berry phase in a helical edge state: S-z nonconservation and transport signatures2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 3, article id 035423Article in journal (Refereed)
    Abstract [en]

    Topological protection of edge state in quantum spin Hall systems relies only on time-reversal symmetry. Hence, S-z conservation on the edge can be relaxed which can have an interferometric manifestation in terms of spin Berry phase. Primarily it could lead to the generation of spin Berry phase arising from a closed loop dynamics of electrons. Our work provides a minimal framework to generate and detect these effects by employing both spin-unpolarized and spin-polarized leads. We show that spin-polarized leads could lead to resonances or antiresonances in the two-terminal conductance of the interferometer. We further show that the positions of these antiresonances (as a function of energy of the incident electron) get shifted owing to the presence of spin Berry phase. Finally, we present simulations of a device setup using KWANT package which put our theoretical predictions on a firm footing.

  • 4. Ahmed, Aamna
    et al.
    Ramachandran, Ajith
    Khaymovich, Ivan
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Sharma, Auditya
    Flat band based multifractality in the all-band-flat diamond chain2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, no 20, article id 205119Article in journal (Refereed)
    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.

  • 5.
    Amundsen, Morten
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    V. Bobkova, Irina
    Kamra, Akashdeep
    Magnonic spin Joule heating and rectification effects2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, no 14, article id 144411Article in journal (Refereed)
    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.

  • 6. Andrade, Tomas
    et al.
    Krikun, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Thermoelectric transport properties of gapless pinned charge density waves2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, no 4, article id L041118Article in journal (Refereed)
    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.

  • 7.
    B. Brant Carvalho, Paulo H.
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mace, Amber
    Andersson, Ove
    Tulk, Chris A.
    Molaison, Jamie
    Lyubartsev, Alexander P.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Nangoi, Inna M.
    Leitão, Alexandre A.
    Häussermann, Ulrich
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Pressure-induced amorphization of noble gas clathrate hydrates2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 103, no 6, article id 064205Article in journal (Refereed)
    Abstract [en]

    The high-pressure structural behavior of the noble gas (Ng) clathrate hydrates Ar center dot 6.5 H2O and Xe center dot 7.2 H2O featuring cubic structures II and I, respectively, was investigated by neutron powder diffraction (using the deuterated analogues) at 95 K. Both hydrates undergo pressure-induced amorphization (PIA), indicated by the disappearance of Bragg diffraction peaks, but at rather different pressures, at 1.4 and above 4.0 GPa, respectively. Amorphous Ar hydrate can be recovered to ambient pressure when annealed at >1.5 GPa and 170 K and is thermally stable up to 120 K. In contrast, it was impossible to retain amorphous Xe hydrate at pressures below 3 GPa. Molecular dynamics (MD) simulations were used to obtain general insight into PIA of Ng hydrates, from Ne to Xe. Without a guest species, both cubic clathrate structures amorphize at 1.2 GPa, which is very similar to hexagonal ice. Filling of large-sized H cages does not provide stability toward amorphization for structure II, whereas filled small-sized dodecahedral D cages shift PIA successively to higher pressures with increasing size of the Ng guest. For structure I, filling of both kinds of cages, large-sized T and small-sized D, acts to stabilize in a cooperative fashion. Xe hydrate represents a special case. In MD, disordering of the guest hydration structure is already seen at around 2.5 GPa. However, the different coordination numbers of the two types of guests in the crystalline cage structure are preserved, and the state is shown to produce a Bragg diffraction pattern. The experimentally observed diffraction up to 4 GPa is attributed to this semicrystalline state.

  • 8.
    Balabanov, Oleksandr
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ortega-Taberner, Carlos
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Hermanns, Maria
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Quantization of topological indices in critical chains at low temperatures2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, no 4, article id 045116Article in journal (Refereed)
    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.

  • 9. Balafendiev, Rustam
    et al.
    Simovski, Constantin
    Millar, Alexander J.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Belov, Pavel
    Wire metamaterial filled metallic resonators2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, no 7, article id 075106Article in journal (Refereed)
    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.

  • 10. Banerjee, R.
    et al.
    Liew, T. C. H.
    Kyriienko, Oleksandr
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Realization of Hofstadter's butterfly and a one-way edge mode in a polaritonic system2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 7, article id 075412Article in journal (Refereed)
    Abstract [en]

    We present a scheme to generate an artificial gauge field for the system of neutral bosons, represented by polaritons in micropillars arranged into a square lattice. The splitting between the two polarizations of the micropillars breaks the time-reversal symmetry (TRS) and results in the effective phase-dependent hopping between cavities. This can allow for engineering a nonzero flux on the plaquette, corresponding to an artificial magnetic field. Changing the phase, we observe a characteristic Hofstadter's butterfly pattern and the appearance of chiral edge states for a finite-size structure. For long-lived polaritons, we show that the propagation of wave packets at the edge is robust against disorder. Moreover, given the inherent driven-dissipative nature of polariton lattices, we find that the system can exhibit topological lasing, recently discovered for active ring cavity arrays. The results point to a static way to realize artificial magnetic field in neutral spinful systems, avoiding the periodic modulation of the parameters or strong spin-orbit interaction. Ultimately, the described system can allow for high-power topological single-mode lasing which is robust to imperfections.

  • 11. Bastien, G.
    et al.
    Roslova, Maria
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Technische Universität Dresden, Germany.
    Haghighi, M. H.
    Mehlawat, K.
    Hunger, J.
    Isaeva, A.
    Doert, T.
    Vojta, M.
    Büchner, B.
    Wolter, A. U. B.
    Spin-glass state and reversed magnetic anisotropy induced by Cr doping in the Kitaev magnet alpha-RuCl32019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 21, article id 214410Article in journal (Refereed)
    Abstract [en]

    Magnetic properties of the substitution series Ru1-xCrxCl3 were investigated to determine the evolution from the anisotropic Kitaev magnet alpha-RuCl3 with J(eff) = 1/2 magnetic Ru(3+ )ions to the isotropic Heisenberg magnet CrCl3 with S = 3/2 magnetic Cr3+ ions. Magnetization measurements on single crystals revealed a reversal of the magnetic anisotropy under doping, which we argue to arise from the competition between anisotropic Kitaev and off-diagonal interactions on the Ru-Ru links and approximately isotropic Cr-Ru and isotropic Cr-Cr interactions. In addition, combined magnetization, ac susceptibility, and specific-heat measurements clearly show the destabilization of the long-range magnetic order of alpha-RuCl3 in favor of a spin-glass state of Ru1-xCrxCl3 for a low doping of x similar or equal to 0.1. The corresponding freezing temperature as a function of Cr content shows a broad maximum around x similar or equal to 0.45.

  • 12. Behrends, Jan
    et al.
    Kunst, Flore K.
    Stockholm University, Faculty of Science, Department of Physics.
    Sbierski, Björn
    Transversal magnetotransport in Weyl semimetals: Exact numerical approach2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 6, article id 064203Article in journal (Refereed)
    Abstract [en]

    Magnetotransport experiments on Weyl semimetals are essential for investigating the intriguing topological and low-energy properties of Weyl nodes. If the transport direction is perpendicular to the applied magnetic field, experiments have shown a large positive magnetoresistance. In this work we present a theoretical scattering matrix approach to transversal magnetotransport in a Weyl node. Our numerical method confirms and goes beyond the existing perturbative analytical approach by treating disorder exactly. It is formulated in real space and is applicable to mesoscopic samples as well as in the bulk limit. In particular, we study the case of clean and strongly disordered samples.

  • 13. Belonoshko, Anatoly B.
    et al.
    Simak, Sergei I.
    Olovsson, Weine
    Vekilova, Olga Yu.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Elastic properties of body-centered cubic iron in Earth's inner core2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 18, article id L180102Article in journal (Refereed)
    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.

  • 14.
    Bhalla, Pankaj
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Rostami, Habib
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Second harmonic helicity and Faraday rotation in gated single-layer 1T ' -WTe22022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 23, article id 235132Article in journal (Refereed)
    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.

  • 15.
    Bhalla, Pankaj
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Vignale, Giovanni
    Rostami, Habib
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Pseudogauge field driven acoustoelectric current in two-dimensional hexagonal Dirac materials2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 12, article id 125407Article in journal (Refereed)
    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.

  • 16. Björnson, Kristofer
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; ETH Institute for Theoretical Studies, Switzerland.
    Black-Schaffer, Annica M.
    Superconducting order parameter pi-phase shift in magnetic impurity wires2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 10, article id 104521Article in journal (Refereed)
    Abstract [en]

    It has previously been found that amagnetic impurity in a conventional s-wave superconductor can give rise to a local pi-phase shift of the superconducting order parameter. By studying a finite wire of ferromagnetic impurities, we are able to trace the origin of the pi-phase shift to a resonance condition for the Bogoliubov-de Gennes quasiparticle states. When nonresonating states localized at the impurity sites are pulled into the condensate for increasing magnetic strength, the superconducting order parameter is reduced in discrete steps, eventually resulting in a pi-phase shift. We also show that for a finite spin-orbit coupling, the pi-phase shift is preserved and occurs in a large portion of the topologically nontrivial phase.

  • 17.
    Bouhon, Adrien
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Uppsala University, Sweden.
    Bzdušek, Tomáš
    Slager, Robert-Jan
    Geometric approach to fragile topology beyond symmetry indicators2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 11, article id 115135Article in journal (Refereed)
    Abstract [en]

    We present a framework to systematically address topological phases when finer partitionings of bands are taken into account, rather than only considering the two subspaces spanned by valence and conduction bands. Focusing on C2T-symmetric systems that have gained recent attention, for example, in the context of layered van-der-Waals graphene heterostructures, we relate these insights to homotopy groups of Grassmannians and flag varieties, which in turn correspond to cohomology classes and Wilson-flow approaches. We furthermore make use of a geometric construction, the so-called Plucker embedding, to induce windings in the band structure necessary to facilitate nontrivial topology. Specifically, this directly relates to the parametrization of the Grassmannian, which describes partitioning of an arbitrary band structure and is embedded in a better manageable exterior product space. From a physical perspective, our construction encapsulates and elucidates the concepts of fragile topological phases beyond symmetry indicators as well as non-Abelian reciprocal braiding of band nodes that arises when the multiple gaps are taken into account. The adopted geometric viewpoint most importantly culminates in a direct and easily implementable method to construct model Hamiltonians to study such phases, constituting a versatile theoretical tool.

  • 18. Boyko, D.
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Haraldsen, J. T.
    Evolution of magnetic Dirac bosons in a honeycomb lattice2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 1, article id 014433Article in journal (Refereed)
    Abstract [en]

    We examine the presence and evolution of magnetic Dirac nodes in the Heisenberg honeycomb lattice. Using linear spin theory, we evaluate the collinear phase diagram as well as the change in the spin dynamics with various exchange interactions. We show that the ferromagnetic structure produces bosonic Dirac and Weyl points due to the competition between the interactions. Furthermore, it is shown that the criteria for magnetic Dirac nodes are coupled to the magnetic structure and not the overall crystal symmetry, where the breaking of inversion symmetry greatly affects the antiferromagnetic configurations. The tunability of the nodal points through variation of the exchange parameters leads to the possibility of controlling Dirac symmetries through an external manipulation of the orbital interactions.

  • 19. Budich, Jan Carl
    et al.
    Carlström, Johan
    Stockholm University, Faculty of Science, Department of Physics.
    Kunst, Flore K.
    Stockholm University, Faculty of Science, Department of Physics.
    J. Bergholtz, Emil
    Stockholm University, Faculty of Science, Department of Physics.
    Symmetry-protected nodal phases in non-Hermitian systems2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 4, article id 041406Article in journal (Refereed)
    Abstract [en]

    Non-Hermitian (NH) Hamiltonians have become an important asset for the effective description of various physical systems that are subject to dissipation. Motivated by recent experimental progress on realizing the NH counterparts of gapless phases such as Weyl semimetals, here we investigate how NH symmetries affect the occurrence of exceptional points (EPs), that generalize the notion of nodal points in the spectrum beyond the Hermitian realm. Remarkably, we find that the dimension of the manifold of EPs is generically increased by one as compared to the case without symmetry. This leads to nodal surfaces formed by EPs that are stable as long as a protecting symmetry is preserved, and that are connected by open Fermi volumes. We illustrate our findings with analytically solvable two-band lattice models in one and two spatial dimensions, and show how they are readily generalized to generic NH crystalline systems.

  • 20.
    Budich, Jan Carl
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Trauzettel, Björn
    Sangiovanni, Giorgio
    Fluctuation-driven topological Hund insulators2013In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 87, no 23, article id 235104Article in journal (Refereed)
    Abstract [en]

    We investigate the role of electron-electron interaction in a two-band Hubbard model based on the Bernevig-Hughes-Zhang Hamiltonian exhibiting the quantum spin Hall (QSH) effect. By means of dynamical mean-field theory, we find that a system with topologically trivial noninteracting parameters can be driven into a QSH phase at finite interaction strength by virtue of local dynamical fluctuations. For very strong interaction, the system reenters a trivial insulating phase by going through a Mott transition. We obtain the phase diagram of our model by direct calculation of the bulk topological invariant of the interacting system in terms of its single-particle Green's function.

  • 21. Bugaiko, O.
    et al.
    Gorbar, E.
    Sukhachov, Pavlo O.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Surface plasmon polaritons in strained Weyl semimetals2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 8, article id 085426Article in journal (Refereed)
    Abstract [en]

    Surface plasmon polaritons in a strained slab of a Weyl semimetal with broken time-reversal symmetry are investigated. It is found that the strain-induced axial gauge field reduces frequencies of these collective modes for intermediate values of the wave vector. Depending on the relative orientation of the separation of Weyl nodes in momentum space, the surface normal, and the direction of propagation, the dispersion relation of surface plasmon polaritons could be nonreciprocal even in a thin slab. In addition, strain-induced axial gauge fields can significantly affect the localization properties of the collective modes. These effects allow for an in situ control of the propagation of surface plasmon polaritons in Weyl semimetals and might be useful for creating nonreciprocal devices.

  • 22.
    Campanini, Donato
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Diao, Zhu
    Stockholm University, Faculty of Science, Department of Physics. Halmstad University, Sweden.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Raising the superconducting T-c of gallium: In situ characterization of the transformation of alpha-Ga into beta-Ga2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 18, article id 184517Article in journal (Refereed)
    Abstract [en]

    Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable beta-Ga with a critical temperature T-c = 6.04(5) K, while stable alpha-Ga has a much lower T-c < 1.2 K. Here we use a membrane-based nanocalorimeter to initiate the transition from alpha-Ga to beta-Ga on demand, as well as study the specific heat of the two phases on one and the same sample. The in situ transformation is initiated by bringing the temperature to about 10 K above the melting temperature of alpha-Ga. After such treatment, the liquid supercools down to 232 K, where beta-Ga solidifies. We find that beta-Ga is a strong-coupling type-I superconductor with Delta(0)/k(B)T(c) = 2.00(5) and a Sommerfeld coefficient gamma(n) = 1.53(4) mJ/molK(2), 2.55 times higher than that in the alpha phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases.

  • 23. Cappelluti, Emmanuele
    et al.
    Silva-Guillén, Jose Angel
    Rostami, Habib
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Bath, United Kingdom.
    Guinea, Francisco
    Flat-band optical phonons in twisted bilayer graphene2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 108, no 12, article id 125401Article in journal (Refereed)
    Abstract [en]

    Twisting bilayer sheets of graphene have been proven to be an efficient way to manipulate the electronic Dirac-like properties, resulting in flat bands at magic angles. Inspired by the electronic model, we develop a continuum model for the lattice dynamics of twisted bilayer graphene and we show that a remarkable band flattening applies to almost all the high-frequency in-plane lattice vibration modes, including the valley Dirac phonon, valley optical phonon, and zone-center optical phonon bands. Utilizing an approximate approach, we estimate small but finite magic angles at which a vanishing phonon bandwidth is expected. In contrast to the electronic case, the existence of a restoring potential prohibits the emergence of a magic angle in a more accurate modeling. The predicted phonon band flattening is highly tunable by the twist angle and this strong dependence is directly accessible by spectroscopic tools.

  • 24.
    Carlström, Johan
    Stockholm University, Faculty of Science, Department of Physics.
    Diagrammatic Monte Carlo procedure for the spin-charge transformed Hubbard model2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 7, article id 075119Article in journal (Refereed)
    Abstract [en]

    Using a dual representation of lattice fermion models that is based on spin-charge transformation and fermionization of the original description, I derive an algorithm for diagrammatic Monte Carlo simulation of strongly correlated systems. This scheme allows eliminating large expansion parameters, as well as large corrections to the density matrix that generally prevent diagrammatic methods from being efficient in this regime. As an example, I compute the filling factor for the Hubbard model at infinite on-site repulsion and compare the results to controllable data obtained from numerical linked-cluster expansion. I find excellent agreement between the two methods, as well as rapid convergence of the diagrammatic series. I also report results for the momentum distribution and kinetic energy of the electrons.

  • 25.
    Carlström, Johan
    Stockholm University, Faculty of Science, Department of Physics.
    Strong-coupling diagrammatic Monte Carlo technique for correlated fermions and frustrated spins2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 103, no 19, article id 195147Article in journal (Refereed)
    Abstract [en]

    We describe a controllable and unbiased strong-coupling diagrammatic Monte Carlo technique that is applicable to a wide range of fermionic systems and spin models. Unlike previous strong coupling methods that generally rely on the Grassmannian Hubbard-Stratonovich transformation, our construction is based on Wick's theorem and a recursive procedure to group contractions into effective connected vertices that are nonperturbative in all local physics and can be calculated exactly. The resulting expansion is described by simple diagrammatic rules that make it suitable for systematic treatment via stochastic sampling. Benchmarks against numerical linked cluster expansion display excellent agreement.

  • 26.
    Carlström, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bergholt, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Strongly interacting Weyl semimetals: Stability of the semimetallic phase and emergence of almost free fermions2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 24, article id 241102Article in journal (Refereed)
    Abstract [en]

    Using a combination of analytical arguments and state-of-the-art diagrammatic Monte Carlo simulations, we show that the corrections to the dispersion in interacting Weyl semimetals are determined by the ultraviolet cutoff and the inverse screening length. If both of these are finite, then the diagrammatic series is convergent even in the low-temperature limit, which implies that the semimetallic phase remains stable. Meanwhile, the absence of a UV cutoff or screening results in logarithmic divergences at zero temperature. These results highlight the crucial impact of Coulomb interactions and screening, mediated, e.g., through the presence of parasitic bands, which are ubiquitous effects in real-world materials. Also, despite sizable corrections from Coulomb forces, the contribution from the frequency-dependent part of the self-energy remains extremely small, thus giving rise to a system of effectively almost free fermions with a strongly renormalized dispersion.

  • 27.
    Carlström, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Symmetry-enforced stability of interacting Weyl and Dirac semimetals2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 16, article id 161102Article in journal (Refereed)
    Abstract [en]

    The nodal and effectively relativistic dispersion featuring in a range of novel materials including two-dimensional graphene and three-dimensional Dirac and Weyl semimetals has attracted enormous interest during the past decade. Here, by studying the structure and symmetry of the diagrammatic expansion, we show that these nodal touching points are in fact perturbatively stable to all orders with respect to generic two-body interactions. For effective low-energy theories relevant for single and multilayer graphene, type-I and type-II Weyl and Dirac semimetals, as well as Weyl points with higher topological charge, this stability is shown to be a direct consequence of a spatial symmetry that anticommutes with the effective Hamiltonian while leaving the interaction invariant. A more refined argument is applied to the honeycomb lattice model of graphene showing that its Dirac points are also perturbatively stable to all orders. We also give examples of nodal Hamiltonians that acquire a gap from interactions as a consequence of symmetries different from those of Weyl and Dirac materials.

  • 28.
    Carlström, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Stålhammar, Marcus
    Stockholm University, Faculty of Science, Department of Physics.
    Budich, Jan Carl
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Knotted non-Hermitian metals2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 16, article id 161115Article in journal (Refereed)
    Abstract [en]

    We report on the occurrence of knotted metallic band structures as stable topological phases in non-Hermitian (NH) systems. These knotted NH metals are characterized by open Fermi surfaces, known in mathematics as Seifert surfaces, that are bounded by knotted lines of exceptional points. Quite remarkably, and in contrast to the situation in Hermitian systems, no fine tuning or symmetries are required in order to stabilize these exotic phases of matter. By explicit construction, we derive microscopic tight-binding models hosting knotted NH metals with strictly short-ranged hopping, and investigate the stability of their topological properties against perturbations. Building up on recently developed experimental techniques for the realization of NH band structures, we discuss how the proposed models may be experimentally implemented in photonic systems.

  • 29. Chan, AtMa P. O.
    et al.
    Kvorning, Thomas
    Stockholm University, Faculty of Science, Department of Physics.
    Ryu, Shinsei
    Fradkin, Eduardo
    Effective hydrodynamic field theory and condensation picture of topological insulators2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, no 15, article id 155122Article in journal (Refereed)
    Abstract [en]

    While many features of topological band insulators are commonly discussed at the level of single-particle electron wave functions, such as the gapless Dirac boundary spectrum, it remains elusive to develop a hydrodynamic or collective description of fermionic topological band insulators in 3+1 dimensions. As the Chern-Simons theory for the 2+1-dimensional quantum Hall effect, such a hydrodynamic effective field theory provides a universal description of topological band insulators, even in the presence of interactions, and that of putative fractional topological insulators. In this paper, we undertake this task by using the functional bosonization. The effective field theory in the functional bosonization is written in terms of a two-form gauge field, which couples to a U(1) gauge field that arises by gauging the continuous symmetry of the target system [the U(1) particle number conservation]. Integrating over the U(1) gauge field by using the electromagnetic duality, the resulting theory describes topological band insulators as a condensation phase of the U(1) gauge theory (or as a monopole condensation phase of the dual gauge field). The hydrodynamic description of the surface of topological insulators and the implication of its duality are also discussed. We also touch upon the hydrodynamic theory of fractional topological insulators by using the parton construction.

  • 30. Chapai, R.
    et al.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Smylie, M. P.
    Chung, D. Y.
    Zheng, H.
    Koshelev, A. E.
    Pearson, J. E.
    Kwok, W.-K.
    Mitchell, J. F.
    Welp, U.
    Superconducting properties of the spin Hall candidate Ta3Sb with eightfold degeneracy2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 18, article id 184510Article in journal (Refereed)
    Abstract [en]

    We report the synthesis and characterization of phase pure Ta3Sb, a material predicted to be topological with eightfold degenerate fermionic states [Bradlyn et al., Science 353, aaf5037 (2016)] and to exhibit a large spin Hall effect [Derunova et al., Sci. Adv. 5, eaav8575 (2019)]. We observe superconductivity in Ta3Sb with Tc∼0.67 K in both electrical resistivity ρ(T) and specific heat c(T) measurements. Field-dependent measurements yield the superconducting phase diagram with an upper critical field of Hc2(0)∼ 0.95 T, corresponding to a superconducting coherence length of ξ≈18.6nm. The gap ratio deduced from specific heat anomaly 2Δ0/kBTc is 3.46, a value close to the Bardeen-Cooper-Schrieffer value of 3.53. From a detailed analysis of both the transport and thermodynamic data within the Ginsburg-Landau (GL) framework, a GL parameter of κ≈90 is obtained, identifying Ta3Sb as an extreme type-II superconductor. The observation of superconductivity in an eightfold degenerate fermionic compound with topological surface states and predicted large spin Hall conductance positions Ta3Sb as an appealing platform to further explore exotic quantum states in multifold degenerate systems.

  • 31. Chen, Siyu
    et al.
    Bouhon, Adrien
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Slager, Robert-Jan
    Monserrat, Bartomeu
    Non-Abelian braiding of Weyl nodes via symmetry-constrained phase transitions2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 8, article id L081117Article in journal (Refereed)
    Abstract [en]

    Weyl semimetals are arguably the most paradigmatic form of a gapless topological phase. While the stability of Weyl nodes, as quantified by their topological charge, has been extensively investigated, recent interest has shifted to the manipulation of the location of these Weyl nodes for non-Abelian braiding. To accomplish this braiding it is necessary to drive significant Weyl node motion using realistic experimental parameter changes. We show that a family of phase transitions characterized by certain symmetry constraints impose that the Weyl nodes have to reorganize by a large amount, shifting from one high-symmetry plane to another. Additionally, for a subset of pairs of nodes with nontrivial Euler class topology, this reorganization can only occur through a braiding process with adjacent nodes. As a result, the Weyl nodes are forced to move a large distance across the Brillouin zone and to braid, all driven by small temperature changes, a process we illustrate with Cd2Re2O7.

  • 32. Colmenarez, Luis
    et al.
    Luitz, David J.
    Khaymovich, Ivan M.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Max Planck Institute for the Physics of Complex Systems, Germany; Russian Academy of Sciences, Russia.
    De Tomasi, Giuseppe
    Subdiffusive Thouless time scaling in the Anderson model on random regular graphs2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 105, no 17, article id 174207Article in journal (Refereed)
    Abstract [en]

    The scaling of the Thouless time with system size is of fundamental importance to characterize dynamical properties in quantum systems. In this work, we study the scaling of the Thouless time in the Anderson model on random regular graphs with on-site disorder. We determine the Thouless time from two main quantities: the spectral form factor and the power spectrum. Both quantities probe the long-range spectral correlations in the system and allow us to determine the Thouless time as the timescale after which the system is well described by random matrix theory. We find that the scaling of the Thouless time is consistent with the existence of a subdiffusive regime anticipating the localized phase. Furthermore, to reduce finite-size effects, we break energy conservation by introducing a Floquet version of the model and show that it hosts a similar subdiffusive regime.

  • 33. Commeau, Benjamin
    et al.
    Geilhufe, R. Matthias
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Fernando, Gayanath W.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Structural and electronic properties of alpha-(BEDT-TTF)(2)I-3, ss-(BEDT-TTF)(2)I-3, and kappa-(BEDT-TTF)(2)X-3 (X = I, F, Br, Cl) organic charge transfer salts2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 12, article id 125135Article in journal (Refereed)
    Abstract [en]

    (BEDT-TFF)(2)I-3 charge transfer salts are reported to show superconductivity and pressure-induced quasi-twodimensional Dirac cones at the Fermi level. By performing state of the art ab initio calculations in the framework of density functional theory, we investigate the structural and electronic properties of the three structural phases alpha, beta, and kappa(.) We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and the origin of band crossings. Additionally, we discuss the chemically induced strain in kappa-(BEDT-TTF)(2)I-3 achieved by replacing the iodine layer with other halogens: fluorine, bromine, and chlorine. In the case of kappa-(BEDT-TTF)(2)F-3, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the nonsymmorphic nature of the crystal. The calculated electronic structures presented here are added to the organic materials database (OMDB).

  • 34. Curcio, Davide
    et al.
    Pakdel, Sahar
    Volckaert, Klara
    Miwa, Jill A.
    Ulstrup, Søren
    Lanatà, Nicola
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Aarhus University, Denmark.
    Bianchi, Marco
    Kutnyakhov, Dmytro
    Pressacco, Federico
    Brenner, Günter
    Dziarzhytski, Siarhei
    Redlin, Harald
    Agustsson, Steinn Ymir
    Medjanik, Katerina
    Vasilyev, Dmitry
    Elmers, Hans-Joachim
    Schönhense, Gerd
    Tusche, Christian
    Chen, Ying-Jiun
    Speck, Florian
    Seyller, Thomas
    Bühlmann, Kevin
    Gort, Rafael
    Diekmann, Florian
    Rossnagel, Kai
    Acremann, Yves
    Demsar, Jure
    Wurth, Wilfried
    Lizzit, Daniel
    Bignardi, Luca
    Lacovig, Paolo
    Lizzit, Silvano
    Sanders, Charlotte E.
    Hofmann, Philip
    Ultrafast electronic linewidth broadening in the C 1s core level of graphene2021In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 104, no 16, article id L161104Article in journal (Refereed)
    Abstract [en]

    We show that the presence of a transiently excited hot electron gas in graphene leads to a substantial broadening of the C 1s line probed by time-resolved x-ray photoemission spectroscopy. The broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron gas, rather than by vibrational excitations. This interpretation is supported by a quantitative line-shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in good agreement with electronic temperature values obtained from valence band data. Furthermore, we show how the momentum change of the outgoing core electrons leads to a detectable but very small change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the presence of a narrow σ band in the C 1s state.

  • 35. Călugăru, Dumitru
    et al.
    Juričić, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Roy, Bitan
    Higher-order topological phases: A general principle of construction2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 4, article id 041301Article in journal (Refereed)
    Abstract [en]

    We propose a general principle for constructing higher-order topological (HOT) phases. We argue that if a D-dimensional first-order or regular topological phase involves m Hermitian matrices that anticommute with additional p - 1 mutually anticommuting matrices, it is conceivable to realize an nth-order HOT phase, where n = 1, ..., p, with appropriate combinations of discrete symmetry-breaking Wilsonian masses. An nth-order HOT phase accommodates zero modes on a surface with codimension n. We exemplify these scenarios for prototypical three-dimensional gapless systems, such as a nodal-loop semimetal possessing SU(2) spin-rotational symmetry, and Dirac semimetals, transforming under (pseudo)spin-1/2 or 1 representations. The former system permits an unprecedented realization of a fourth-order phase, without any surface zero modes. Our construction can be generalized to HOT insulators and superconductors in any dimension and symmetry class.

  • 36. D'Ambrosio, Federico
    et al.
    Juričić, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Barkema, Gerard T.
    Discontinuous evolution of the structure of stretching polycrystalline graphene2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 16, article id 161402Article in journal (Refereed)
    Abstract [en]

    Polycrystalline graphene has an inherent tendency to buckle, i.e., develop out-of-plane, three-dimensional structure. A force applied to stretch a piece of polycrystalline graphene influences the out-of-plane structure. Even if the graphene is well relaxed, this happens in nonlinear fashion: Occasionally, a tiny increase in stretching force induces a significant displacement, in close analogy to avalanches, which in turn can create vibrations in the surrounding medium. We establish this effect in computer simulations: By continuously changing the strain, we follow the displacements of the carbon atoms that turn out to exhibit a discontinuous evolution. Furthermore, the displacements exhibit a hysteretic behavior upon the change from low to high stress and back. These behaviors open up another direction in studying dynamical elasticity of polycrystalline quasi-two-dimensional systems, and in particular the implications on their mechanical and thermal properties.

  • 37.
    de Andrés Prada, Roberto
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Fribourg, Switzerland.
    Gaina, R.
    Biškup, N.
    Varela, M.
    Stahn, J.
    Bernhard, C.
    Controlling the strength of ferromagnetic order in YBa2Cu3O7/La2/3Ca1/3MnO3 multilayers2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 11, article id 115129Article in journal (Refereed)
    Abstract [en]

    With dc magnetization and polarized neutron reflectometry we studied the ferromagnetic response of YBa2Cu3O7/La2/3Ca1/3MnO3 (YBCO/LCMO) multilayers that are grown with pulsed laser deposition. We found that whereas for certain growth conditions (denoted as A type) the ferromagnetic moment of the LCMO layer is strongly dependent on the structural details of the YBCO layer on which it is deposited, for others (B type) the ferromagnetism of LCMO is much more robust. Both kinds of multilayers are of similar structural quality, but electron energy-loss spectroscopy studies with a scanning transmission electron microscope reveal an enhanced average Mn oxidation state of +3.5 for the A-type as opposed to the B-type samples, for which it is close to the nominal value of +3.33. The related, additional hole doping of the A-type LCMO layers, which likely originates from La and/or Mn vacancies, can explain their fragile ferromagnetic order, since it places them close to the boundary of the ferromagnetic order at which even weak perturbations can induce an antiferromagnetic or glassy state. On the other hand, we show that the B-type samples allow one to obtain YBCO/LCMO heterostructures with very thick YBCO layers and, yet, strongly ferromagnetic LCMO layers.

  • 38.
    de Andrés Prada, Roberto
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Fribourg, Switzerland.
    Golod, Taras
    Stockholm University, Faculty of Science, Department of Physics.
    Kapran, Olena M.
    Stockholm University, Faculty of Science, Department of Physics.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Bernhard, Ch.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics. Moscow Institute of Physics and Technology, Russia.
    Memory-functionality superconductor/ferromagnet/superconductor junctions based on the high-Tc cuprate superconductors YBa2Cu3O7−x and the colossal magnetoresistive manganite ferromagnets La2/3X1/3MnO3+δ(X=Ca,Sr)2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 21, article id 214510Article in journal (Refereed)
    Abstract [en]

    Complex oxides exhibit a variety of unusual physical properties, which can be used for designing novel electronic devices. Here we fabricate and study experimentally nanoscale superconductor/ferromagnet/ superconductor junctions with the high-Tc cuprate superconductors YBa2Cu3O7−x and the colossal magnetoresistive (CMR) manganite ferromagnets La2/3X1/3MnO3+δ(X=CaorSr). We demonstrate that in a broad temperature range the magnetization of a manganite nanoparticle, forming the junction interface, switches abruptly in a monodomain manner. The CMR phenomenon translates the magnetization loop into a hysteretic magnetoresistance loop. The latter facilitates a memory functionality of such a junction with just a single CMR ferromagnetic layer. The orientation of the magnetization (stored information) can be read out by simply measuring the junction resistance in a finite magnetic field. The CMR facilitates a large readout signal in a small applied field. We argue that such a simple single-layer CMR junction can operate as a memory cell both in the superconducting state at cryogenic temperatures and in the normal state up to room temperature.

  • 39. De Tomasi, Giuseppe
    et al.
    Khaymovich, Ivan
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Non-Hermiticity induces localization: Good and bad resonances in power-law random banded matrices2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 108, no 18, article id L180202Article in journal (Refereed)
    Abstract [en]

    The power-law random banded matrix (PLRBM) is a paradigmatic ensemble to study the Anderson localization transition (AT). In d dimensions, the PLRBMs are random matrices with algebraic decaying off-diagonal elements H-nm similar to 1/|n-m|(alpha), having AT at alpha=d. In this work, we investigate the fate of the PLRBM to non-Hermiticity (nH). We consider the case where the random on-site diagonal potential takes complex values, mimicking an open system, subject to random gain-loss terms. We understand the model analytically by generalizing the Anderson-Levitov resonance counting technique to the nH case. We identify two competing mechanisms due to nH: favoring localization and delocalization. The competition between the two gives rise to AT at d/2 <=alpha <= d. The value of the critical alpha depends on the strength of the on-site potential, like in Hermitian disordered short-range models in d>2. Within the localized phase, the wave functions are algebraically localized with an exponent alpha even for alpha<d. This result provides an example of non-Hermiticity-induced localization and finds immediate application in phase transitions driven by weak measurements.

  • 40. De Tomasi, Giuseppe
    et al.
    Khaymovich, Ivan M.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Max Planck Institute for the Physics of Complex Systems, Germany; Russian Academy of Sciences, Russia.
    Non-Hermitian Rosenzweig-Porter random-matrix ensemble: Obstruction to the fractal phase2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, no 9, article id 094204Article in journal (Refereed)
    Abstract [en]

    We study the stability of nonergodic but extended (NEE) phases in non-Hermitian systems. For this purpose, we generalize the so-called Rosenzweig-Porter random-matrix ensemble, known to carry a NEE phase along with the Anderson localized and ergodic ones, to the non-Hermitian case. We analyze, both analytically and numerically, the spectral and multifractal properties of the non-Hermitian case. We show that the ergodic and localized phases are stable against the non-Hermitian nature of matrix entries. However, the stability of the fractal phase depends on the choice of the diagonal elements. For purely real or imaginary diagonal potential, the fractal phase is intact, while for a generic complex diagonal potential the fractal phase disappears, giving way to a localized one.

  • 41.
    de Wijn, Astrid S.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Norwegian University of Science and Technology, Norway.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    How square ice helps lubrication2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 16, article id 165433Article in journal (Refereed)
    Abstract [en]

    In the context of friction we use atomistic molecular-dynamics simulations to investigate water confined between graphene sheets over a wide range of pressures. We find that thermal equilibration of the confined water is hindered at high pressures. We demonstrate that, under the right conditions, square ice can form in an asperity, and that it is similar to cubic ice VII and ice X. We simulate sliding of atomically flat graphite on the square ice and find extremely low friction due to structural superlubricity. The conditions needed for square ice to form correspond to low sliding speeds, and we suggest that the ice observed in experiments of friction on wet graphite is of this type.

  • 42. Delacrétaz, Luca
    et al.
    Goutéraux, Blaise
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Université Paris-Saclay, France.
    Hartno, Sean A.
    Karlsson, Anna
    Theory of collective magnetophonon resonance and melting of a field-induced Wigner solid2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 8, article id 085140Article in journal (Refereed)
    Abstract [en]

    Electron solid phases of matter are revealed by characteristic vibrational resonances. Sufficiently large magnetic fields can overcome the effects of disorder, leading to a weakly pinned collective mode called the magnetophonon. Consequently, in this regime it is possible to develop a tightly constrained hydrodynamic theory of pinned magnetophonons. The behavior of the magnetophonon resonance across thermal and quantum melting transitions has been experimentally characterized in two-dimensional electron systems. Applying our theory to these transitions we explain several key features of the data. Firstly, violation of the Fukuyama-Lee sum rule as the transition is approached is shown to be a consequence of the non-Lorentzian form taken by the resonance. Secondly, this non-Lorentzian shape is shown to be caused by dissipative channels that become especially important close to melting: proliferating dislocations and uncondensed charge carriers.

  • 43. Delacrétaz, Luca V.
    et al.
    Goutéraux, Blaise
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Hartnoll, Sean A.
    Karlsson, Anna
    Theory of hydrodynamic transport in fluctuating electronic charge density wave states2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 19, article id 195128Article in journal (Refereed)
    Abstract [en]

    We describe the collective hydrodynamic motion of an incommensurate charge density wave state in a clean electronic system. Our description simultaneously incorporates the effects of both pinning due to weak disorder and also phase relaxation due to proliferating dislocations. We show that the interplay between these two phenomena has important consequences for charge and momentum transport. For instance, it can lead to metal-insulator transitions. We furthermore identify signatures of fluctuating density waves in frequency and spatially resolved conductivities. Phase disordering is well known to lead to a large viscosity. We derive a precise formula for the phase relaxation rate in terms of the viscosity in the dislocation cores. We thereby determine the viscosity of the superconducting state of BSCCO from the observed melting dynamics of Abrikosov lattices and show that the result is consistent with dissipation into Bogoliubov quasiparticles.

  • 44.
    Dunnett, Kirsty
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Ferrier, A.
    Zamora, A.
    Dagvadorj, G.
    Szymanska, M. H.
    Properties of the signal mode in the polariton optical parametric oscillator regime2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 16, article id 165307Article in journal (Refereed)
    Abstract [en]

    Theoretical analyses of the polariton optical parametric oscillator (OPO) regime often rely on a mean-field approach based on the complex Gross-Pitaevskii equations in a three-mode approximation, where only three momentum states, the signal, pump, and idler, are assumed to be significantly occupied. This approximation, however, lacks a constraint to uniquely determine the signal and idler momenta. In contrast, multimode numerical simulations and experiments show a unique momentum structure for the OPO states. In this work we show that an estimate for the signal momentum chosen by the system can be found from a simple analysis of the pump-only configuration. We use this estimate to investigate how the chosen signal momentum depends on the properties of the drive.

  • 45.
    Dunnett, Kirsty
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Narayan, Awadhesh
    Spaldin, N. A.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; University of Connecticut, USA.
    Strain and ferroelectric soft-mode induced superconductivity in strontium titanate2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 14, article id 144506Article in journal (Refereed)
    Abstract [en]

    We investigate the effects of strain on superconductivity with particular reference to SrTiO3. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviors under uniaxial and biaxial strain can be understood.

  • 46.
    Edvardsson, Elisabet
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ardonne, Eddy
    Stockholm University, Faculty of Science, Department of Physics.
    Sensitivity of non-Hermitian systems2022In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 106, no 11, article id 115107Article in journal (Refereed)
    Abstract [en]

    Understanding the extreme sensitivity of the eigenvalues of non-Hermitian Hamiltonians to the boundary conditions is of great importance when analyzing non-Hermitian systems, as it appears generically and is intimately connected to the skin effect and the breakdown of the conventional bulk boundary correspondence. Here we describe a method to find the eigenvalues of one-dimensional one-band models with arbitrary boundary conditions. We use this method on several systems to find analytical expressions for the eigenvalues, which give us conditions on the parameter values in the system for when we can expect the spectrum to be insensitive to a change in boundary conditions. By stacking one-dimensional chains, we use the derived results to find corresponding conditions for insensitivity for some two-dimensional systems with periodic boundary conditions in one direction. This would be hard by using other methods to detect skin effect, such as the winding of the determinant of the Bloch Hamiltonian. Finally, we use these results to make predictions about the (dis)appearance of the skin effect in purely two-dimensional systems with open boundary conditions in both directions.

  • 47.
    Edvardsson, Elisabet
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kunst, Flore K.
    Stockholm University, Faculty of Science, Department of Physics.
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Non-Hermitian extensions of higher-order topological phases and their biorthogonal bulk-boundary correspondence2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 8, article id 081302Article in journal (Refereed)
    Abstract [en]

    Non-Hermitian Hamiltonians, which describe a wide range of dissipative systems, and higher-order topological phases, which exhibit novel boundary states on corners and hinges, comprise two areas of intense current research. Here we investigate systems where these frontiers merge and formulate a generalized biorthogonal bulk-boundary correspondence, which dictates the appearance of boundary modes at parameter values that are, in general, radically different from those that mark phase transitions in periodic systems. By analyzing the interplay between corner/hinge, edge/surface and bulk degrees of freedom we establish that the non-Hermitian extensions of higher-order topological phases exhibit an even richer phenomenology than their Hermitian counterparts and that this can be understood in a unifying way within our biorthogonal framework. Saliently this works in the presence of the non-Hermitian skin effect, and also naturally encompasses genuinely non-Hermitian phenomena in the absence thereof.

  • 48.
    Ertan, Emelie
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kimberg, Victor
    Gel'mukhanov, Faris
    Hennies, Franz
    Rubensson, Jan-Erik
    Schmitt, Thorsten
    Strocov, Vladimir N.
    Zhou, Kejin
    Iannuzzi, Marcella
    Foehlisch, Alexander
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Pietzsch, Annette
    Theoretical simulations of oxygen K-edge resonant inelastic x-ray scattering of kaolinite2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 14, article id 144301Article in journal (Refereed)
    Abstract [en]

    Near-edge x-ray absorption fine structure (NEXAFS) and resonant inelastic x-ray scattering (RIXS) measurements at the oxygen K edge were combined with theoretical spectrum simulations, based on periodic density functional theory and nuclear quantum dynamics, to investigate the electronic structure and chemical bonding in kaolinite Al2Si2O5(OH)(4). We simulated NEXAFS spectra of all crystallographically inequivalent oxygen atoms in the crystal and RIXS spectra of the hydroxyl groups. Detailed insight into the ground-state potential energy surface of the electronic states involved in the RIXS process were accessed by analyzing the vibrational excitations, induced by the core excitation, in quasielastic scattering back to the electronic ground state. In particular, we find that the NEXAFS pre-edge is dominated by features related to OH groups within the silica and alumina sheets, and that the vibrational progression in RIXS can be used to selectively probe vibrational modes of this subclass of OH groups. The signal is dominated by the OH stretching mode, but also other lower vibrational degrees of freedom, mainly hindered rotational modes, contribute to the RIXS signal.

  • 49. Eschmann, Tim
    et al.
    Mishchenko, Petr A.
    O'Brien, Kevin
    Bojesen, Troels A.
    Kato, Yasuyuki
    Hermanns, Maria
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Motome, Yukitoshi
    Trebst, Simon
    Thermodynamic classification of three-dimensional Kitaev spin liquids2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 102, no 7, article id 075125Article in journal (Refereed)
    Abstract [en]

    In the field of frustrated magnetism, Kitaev models provide a unique framework to study the phenomena of spin fractionalization and emergent lattice gauge theories in two and three spatial dimensions. Their ground states are quantum spin liquids, which can typically be described in terms of a Majorana band structure and an ordering of the underlying Z(2) gauge structure. Here we provide a comprehensive classification of the gauge physics of a family of elementary three-dimensional Kitaev models, discussing how their thermodynamics and ground state order depends on the underlying lattice geometry. Using large-scale, sign-free quantum Monte Carlo simulations we show that the ground-state gauge order can generally be understood in terms of the length of elementary plaquettes-a result which extends the applicability of Lieb's theorem to lattice geometries beyond its original scope. At finite temperatures, the proliferation of (gapped) vison excitations destroys the gauge order at a critical temperature scale, which we show to correlate with the size of vison gap for the family of three-dimensional Kitaev models. We also discuss two notable exceptions where the lattice structure gives rise to gauge frustration or intertwines the gauge ordering with time-reversal symmetry breaking In a more general context, the thermodynamic gauge transitions in such 3D Kitaev models are one of the most natural settings for phase transitions beyond the standard Landau-Ginzburg-Wilson paradigm.

  • 50. Evans, Michael J.
    et al.
    Wu, Yang
    Kranak, Verina F.
    Arizona State University, USA.
    Newman, N.
    Reller, Armin
    Garcia-Garcia, F. Javier
    Häussermann, Ulrich
    Arizona State University, USA.
    Structural properties and superconductivity in the ternary intermetallic compoundsMAB (M=Ca, Sr, Ba; A=Al, Ga, In; B=Si, Ge, Sn)2009In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 80, article id 064514Article in journal (Refereed)
    Abstract [en]

    The ternary intermetallic compounds MAB=CaAlSi, SrAlSi, BaAlSi, CaGaSi, SrGaSi, BaGaSi, SrAlGe, BaAlGe, CaGaGe, SrGaGe, BaGaGe, BaInGe, BaAlSn, CaGaSn, SrGaSn, and BaGaSn have been prepared by arc-melting stoichiometric elemental mixtures and structurally characterized by a combination of x-ray powder and electron diffraction. They crystallize as variants of the simple hexagonal AlB2 structure type where trivalent and tetravalent A- and B-type atoms, respectively, form commonly a planar hexagon layer, and structural variations arise from A/B ordering and/or puckering of hexagon layers. The silicides (B=Si) were previously investigated for their superconducting properties. By dc magnetization measurements it is demonstrated that also the germanides SrAlGe, BaAlGe, SrGaGe, and BaGaGe and the stannide BaAlSn are superconductors above 2 K.

12345 1 - 50 of 218
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