Change search
Refine search result
12 1 - 50 of 91
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 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. 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.

  • 4. 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.

  • 5. 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.

  • 6. 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.

  • 7. 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.

  • 8. 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.
    Bergholtz, Emil J.
    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.

  • 9.
    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.

  • 10.
    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.

  • 11.
    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.

  • 12.
    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.

  • 13.
    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.

  • 14.
    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.

  • 15. 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.

  • 16. 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).

  • 17. 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.

  • 18.
    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.

  • 19.
    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-T-c cuprate superconductors YBa2Cu3O7-x and the colossal magnetoresistive (CMR) manganite ferromagnets La2/3X1/3MnO3+delta (X=Ca or Sr). 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.

  • 20.
    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.

  • 21. 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.

  • 22. 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.

  • 23.
    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.

  • 24.
    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.

  • 25.
    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.

  • 26.
    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.

  • 27. 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.

  • 28. Fransson, J.
    et al.
    Black-Schaffer, A. M.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Institute for Materials Science, Los Alamos, USA.
    Magnon Dirac materials2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 7, article id 075401Article in journal (Refereed)
    Abstract [en]

    We demonstrate how a Dirac-like magnon spectrum is generated for localized magnetic moments forming a two-dimensional honeycomb lattice. The Dirac crossing point is proven to be robust against magnon-magnon interactions, as these only shift the spectrum. Local defects induce impurity resonances near the Dirac point, as well as magnon Friedel oscillations. The energy of the Dirac point is controlled by the exchange coupling, and thus a two-dimensional array of magnetic dots is an experimentally feasible realization of Dirac magnons with tunable dispersion.

  • 29. Gani, Yohanes S.
    et al.
    Abergel, David S. L.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Rossi, Enrico
    Electronic structure of graphene nanoribbons on hexagonal boron nitride2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 20, article id 205415Article in journal (Refereed)
    Abstract [en]

    Hexagonal boron nitride is an ideal dielectric to form two-dimensional heterostructures due to the fact that it can be exfoliated to be just a few atoms thick and its very low density of defects. By placing graphene nanoribbons on high quality hexagonal boron nitride it is possible to create ideal quasi-one-dimensional systems with very high mobility. The availability of high quality one-dimensional electronic systems is of great interest also given that when in proximity to a superconductor they can be effectively engineered to realize Majorana bound states. In this work we study how a boron nitride substrate affects the electronic properties of graphene nanoribbons. We consider both armchair and zigzag nanoribbons. Our results show that for some stacking configurations the boron nitride can significantly affect the electronic structure of the ribbons. In particular, for zigzag nanoribbons, due to the lock between spin and sublattice degree of freedom at the edges, the hexagonal boron nitride can induce a very strong spin splitting of the spin-polarized, edge states. We find that such spin splitting can be as high as 40 meV.

  • 30.
    Geilhufe, R. Matthias
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; University of Connecticut, USA.
    Symmetry analysis of odd- and even-frequency superconducting gap symmetries for time-reversal symmetric interactions2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 2, article id 024507Article in journal (Refereed)
    Abstract [en]

    Odd-frequency superconductivity describes a class of superconducting states where the superconducting gap is an odd function in relative time and Matsubara frequency. We present a group theoretical analysis based on the linearized gap equation in terms of Shubnikov groups of the second kind. By discussing systems with spin-orbit coupling and an interaction kernel which is symmetric under the reversal of relative time, we show that both even-and odd-frequency gaps are allowed to occur. Specific examples are discussed for the square lattice, the octahedral lattice, and the tetragonal lattice. For irreducible representations that are even under the reversal of relative time the common combinations of s- and d-wave spin singlet and p-wave spin triplet gaps are revealed, irreducible representations that are odd under reversal of relative time give rise to s- and d-wave spin triplet and p-wave spin singlet gaps. Furthermore, we discuss the construction of a generalized Ginzburg-Landau theory in terms of the associated irreducible representations. The result complements the established classification of superconducting states of matter.

  • 31.
    Geilhufe, R. Matthias
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden.
    Bouhon, Adrien
    Borysov, Stanislav S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden.
    Three-dimensional organic Dirac-line materials due to nonsymmorphic symmetry: A data mining approach2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 4, article id 041103Article in journal (Refereed)
    Abstract [en]

    A datamining study of electronic Kohn-Sham band structures was performed to identify Dirac materials within the Organic Materials Database. Out of that, the three-dimensional organic crystal 5,6-bis(trifluoromethyl)-2-methoxy-1H-1,3-diazepine was found to host different Dirac-line nodes within the band structure. From a group theoretical analysis, it is possible to distinguish between Dirac-line nodes occurring due to twofold degenerate energy levels protected by the monoclinic crystalline symmetry and twofold degenerate accidental crossings protected by the topology of the electronic band structure. The obtained results can be generalized to all materials having the space group P2(1)/c (No. 14, C-2h(5)) by introducing three distinct topological classes.

  • 32.
    Geilhufe, R. Matthias
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Guinea, Francisco
    Juričić, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Hund nodal line semimetals: The case of a twisted magnetic phase in the double-exchange model2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 2, article id 020404Article in journal (Refereed)
    Abstract [en]

    We propose a class of topological metals, which we dub Hund nodal line semimetals, arising from the strong Coulomb interaction encoded in the Hund's coupling between itinerant electrons and localized spins. We here consider a particular twisted spin configuration, which is realized in the double-exchange model which describes the manganite oxides. The resulting effective tetragonal lattice of electrons with hoppings tied to the local spin features an antiunitary nonsymmorphic symmetry that, in turn, together with another nonsymmorphic but unitary glide-mirror symmetry, protects crossings of a double pair of bands along a high-symmetry line on the Brillouin zone boundary. We also discuss the stability of Hund nodal line semimetals with respect to symmetry breaking arising from various perturbations of the twisted phase. Our results motivate further studies of other realizations of this state of matter, for instance, in different spin backgrounds, properties of its drumhead surface states, as well as its stability to disorder and interactions among the itinerant electrons.

  • 33. Gerhardt, Stefan
    et al.
    Deppisch, Michael
    Betzold, Simon
    Harder, Tristan H.
    Liew, Timothy C. H.
    Predojevic, Ana
    Stockholm University, Faculty of Science, Department of Physics.
    Hoefling, Sven
    Schneider, Christian
    Polarization-dependent light-matter coupling and highly indistinguishable resonant fluorescence photons from quantum dot-micropillar cavities with elliptical cross section2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 11, article id 115305Article in journal (Refereed)
    Abstract [en]

    We study the optical properties of coupled quantum dot-microcavity systems with an elliptical cross section. First, we develop an analytic model that describes the spectrum of the cavity modes that are split due to the reduced symmetry of the resonator. By coupling the quantum dot (QD) emission to the polarized fundamental cavity modes, we observe the vectorial nature of the Purcell enhancement, which depends on the intrinsic polarization of the quantum dot and its relative alignment with respect to the cavity axis. The variable interaction strength of the QD with the polarized cavity modes leads to the observation of strong and weak coupling. Finally, we demonstrate the capability of elliptical micropillars to emit single and highly indistinguishable photons (visibility of 87%).

  • 34.
    Hellsvik, Johan
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). KTH Royal Institute of Technology, Sweden.
    Thonig, Danny
    Modin, Klas
    Iusan, Diana
    Bergman, Anders
    Eriksson, Olle
    Bergqvist, Lars
    Delin, Anna
    General method for atomistic spin-lattice dynamics with first-principles accuracy2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 10, article id 104302Article in journal (Refereed)
    Abstract [en]

    We present a computationally efficient and general first-principles based method for spin-lattice simulations for solids and clusters. The method is based on a coupling of atomistic spin dynamics and molecular dynamics simulations, expressed through a spin-lattice Hamiltonian, where the bilinear magnetic term is expanded up to second order in displacement. The effect of first-order spin-lattice coupling on the magnon and phonon dispersion in bcc Fe is reported as an example, and we observe good agreement with previous simulations. We also illustrate the coupled spin-lattice dynamics method on a more conceptual level, by exploring dissipation-free spin and lattice motion of small magnetic clusters (a dimer, trimer, and tetramer). The method discussed here opens the door for a quantitative description and understanding of the microscopic origin of many fundamental phenomena of contemporary interest, such as ultrafast demagnetization, magnetocalorics, and spincaloritronics.

  • 35. Houchins, Gregory
    et al.
    Crook, Charles B.
    Zhu, Jian-Xin
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Haraldsen, Jason T.
    Voltage-dependent spin flip in magnetically substituted graphene nanoribbons: Towards the realization of graphene-based spintronic devices2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 15, article id 155450Article in journal (Refereed)
    Abstract [en]

    We examine the possibility of using graphene nanoribbons (GNRs) with directly substituted chromium atoms as a spintronic device. Using density functional theory, we simulate a voltage bias across a constructed GNR in a device setup where a magnetic dimer has been substituted into the lattice. Through this first-principles approach, we calculate the electronic and magnetic properties as a function of Hubbard U, voltage, and magnetic configurations. By calculating the total energy of each magnetic configuration, we determine that the initial antiferromagnetic ground state flips to a ferromagnetic state with applied bias. Mapping this transition point to the calculated conductance for the system reveals that there is a distinct change in conductance through the GNR, which indicates the possibility of a spin valve. We also show that this corresponds to a distinct shift in the induced magnetization within graphene.

  • 36.
    Iovan, Adrian
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Signatures of the spin-triplet current in a Josephson spin valve: A micromagnetic analysis2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 1, article id 014511Article in journal (Refereed)
    Abstract [en]

    A Josephson spin valve is a ferromagnetic spin valve sandwiched between two superconducting electrodes. It has been predicted theoretically that such a device may exhibit a long-range proximity effect due to generation of unconventional odd-frequency spin-triplet and long-range spin-singlet components of the supercurrent. In this work we present a comprehensive numerical analysis of Josephson spin-valve characteristics. Our analysis is based on micromagnetic simulations for Ni-based spin valves. The supercurrent through the spin valve depends on shapes and sizes of components, the magnetic domain structure, and the flux quantization. For very small monodomain spin valves, the triplet current is manifested by a dissimilar double maximum in the magnetic field dependence of the critical current Ic (H). However, this feature is washed away in larger devices due to appearance of domains and flux quantization. The only remaining signature of the triplet current in this case are beatings in Ic (H) with a half-flux quantum periodicity. The complexity of the device can make it difficult to identify the spin-triplet supercurrent without a detailed knowledge of the spin-valve state. However, we argue that unambiguous conclusions can be made from a systematic analysis of size, thickness, and shape dependencies of the Josephson spin-valve characteristics.

  • 37. Islam, M. F.
    et al.
    Canali, C. M.
    Pertsova, Anna
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Mahatha, S. K.
    Carbone, C.
    Barla, A.
    Kokh, K. A.
    Tereshchenko, O. E.
    Jimenez, E.
    Brookes, N. B.
    Gargiani, P.
    Valvidares, M.
    Schatz, S.
    Peixoto, T. R. F.
    Bentmann, H.
    Reinert, F.
    Jung, J.
    Bathon, T.
    Fauth, K.
    Bode, M.
    Sessi, P.
    Systematics of electronic and magnetic properties in the transition metal doped Sb2Te3 quantum anomalous Hall platform2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 15, article id 155429Article in journal (Refereed)
    Abstract [en]

    The quantum anomalous Hall effect (QAHE) has recently been reported to emerge in magnetically doped topological insulators. Although its general phenomenology is well established, the microscopic origin is far from being properly understood and controlled. Here, we report on a detailed and systematic investigation of transition metal (TM) doped Sb2Te3. By combining density functional theory calculations with complementary experimental techniques, i.e., scanning tunneling microscopy, resonant photoemission, and x-raymagnetic circular dichroism, we provide a complete spectroscopic characterization of both electronic and magnetic properties. Our results reveal that the TM dopants not only affect the magnetic state of the host material, but also significantly alter the electronic structure by generating impurity-derived energy bands. Our findings demonstrate the existence of a delicate interplay between electronic and magnetic properties in TM doped topological insulators. In particular, we find that the fate of the topological surface states critically depends on the specific character of the TM impurity: while V-and Fe-doped Sb2Te3 display resonant impurity states in the vicinity of the Dirac point, Cr and Mn impurities leave the energy gap unaffected. The single-ion magnetic anisotropy energy and easy axis, which control the magnetic gap opening and its stability, are also found to be strongly TM impurity dependent and can vary from in plane to out of plane depending on the impurity and its distance from the surface. Overall, our results provide general guidelines for the realization of a robust QAHE in TM doped Sb2Te3 in the ferromagnetic state.

  • 38. Islam, M. F.
    et al.
    Pertsova, Anna
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Canali, C. M.
    Impurity potential induced gap at the Dirac point of topological insulators with in-plane magnetization2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 15, article id 155401Article in journal (Refereed)
    Abstract [en]

    The quantum anomalous Hall effect (QAHE), characterized by dissipationless quantized edge transport, relies crucially on a nontrivial topology of the electronic bulk band structure and a robust ferromagnetic order that breaks time-reversal symmetry. Magnetically doped topological insulators (TIs) satisfy both these criteria, and are the most promising quantum materials for realizing the QAHE. Because the spin of the surface electrons aligns along the direction of the magnetic-impurity exchange field, only magnetic TIs with an out-of-plane magnetization are thought to open a gap at the Dirac point (DP) of the surface states, resulting in the QAHE. Using a continuum model supported by atomistic tight-binding and first-principles calculations of transition-metal doped Bi2Se3, we show that a surface-impurity potential generates an additional effective magnetic field which spin polarizes the surface electrons along the direction perpendicular to the surface. The predicted gap-opening mechanism results from the interplay of this additional field and the in-plane magnetization that shifts the position of the DP away from the Gamma point. This effect is similar to the one originating from the hexagonal warping correction of the band structure but is one order of magnitude stronger. Our calculations show that in a doped TI with in-plane magnetization the impurity-potential-induced gap at the DP is comparable to the one opened by an out-of-plane magnetization.

  • 39.
    Jacobs, Thorsten
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Katterwe, Sven-Olof
    Stockholm University, Faculty of Science, Department of Physics.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Superconducting correlations above T-c in the pseudogap state of Bi2Sr2CaCu2O8+delta cuprates revealed by angular-dependent magnetotunneling2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 22, article id 220501Article in journal (Refereed)
    Abstract [en]

    We present an angular-dependent magnetotunneling technique, which facilitates unambiguous separation of superconducting (supporting circulating screening currents) and nonsuperconducting (not supporting screening currents) contributions to the pseudogap phenomenon in layered Bi2Sr2CaCu2O8+delta cuprates. Our data indicate persistence of superconducting correlations at temperatures up to 1.5T(c) in a form of both phase and amplitude fluctuations of the superconducting order parameter. However, despite a profound fluctuations region, only a small fraction of the pseudogap spectrum is caused by superconducting correlations, while the dominating part comes from a competing nonsuperconducting order, which does not support circulating orbital currents.

  • 40.
    Jiang, Qing-Dong
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Wilczek, Frank
    Stockholm University, Faculty of Science, Department of Physics. Massachusetts Institute of Technology, USA; Shanghai Jiao Tong University, China; Arizona State University, USA.
    Axial Casimir force2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 16, article id 165402Article in journal (Refereed)
    Abstract [en]

    Quantum fluctuations in vacuum can exert a dissipative force on moving objects, which is known as Casimir friction. Especially, a rotating particle in the vacuum will eventually slow down due to the dissipative Casimir friction. Here, we identify a dissipationless force by examining a rotating particle near a bi-isotropic media that generally breaks parity symmetry or/and time-reversal symmetry. The direction of the dissipationless vacuum force is always parallel with the rotating axis of the particle. We therefore call this dissipationless vacuum force the axial Casimir force.

  • 41.
    Jiang, Qing-Dong
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Wilczek, Frank
    Stockholm University, Faculty of Science, Department of Physics. Massachusetts Institute of Technology, USA; Shanghai Jiao Tong University, China; Arizona State University, USA.
    Chiral Casimir forces: Repulsive, enhanced, tunable2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 12, article id 125403Article in journal (Refereed)
    Abstract [en]

    Both theoretical interest and practical significance attach to the sign and strength of Casimir forces. A famous, discouraging no-go theorem states that the Casimir force between two bodies with reflection symmetry is always attractive. Here, we identify a promising way to avoid the assumptions of the no-go theorem, and propose a universal way to realize repulsive Casimir forces. We show that the sign and strength of Casimir forces can be adjusted by inserting optically active or gyrotropic media between bodies, and modulated by external fields.

  • 42.
    Jiang, Qing-Dong
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Wilczek, Frank
    Stockholm University, Faculty of Science, Department of Physics. Massachusetts Institute of Technology, USA; Shanghai Jiao Tong University, China; Arizona State University, USA.
    Quantum atmospherics for materials diagnosis2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 20, article id 201104Article in journal (Refereed)
    Abstract [en]

    Symmetry-breaking states of matter can transmit symmetry breaking to nearby atoms or molecular complexes, perturbing their spectra. We calculate one such effect, involving the axion electrodynamics relevant to topological insulators, quantitatively, and identify a signature for T violating superconductivity. We provide an operator framework whereby effects of this kind can be analyzed systematically.

  • 43. Jukimenko, O.
    et al.
    Modestov, Mikhail
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Princeton University, USA.
    Dion, C. M.
    Marklund, M.
    Bychkov, V.
    Multilevel model for magnetic deflagration in nanomagnet crystals2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 17, article id 174403Article in journal (Refereed)
    Abstract [en]

    We extend the existing theoretical model for determining the characteristic features of magnetic deflagration in nanomagnet crystals. For the first time, all energy levels are accounted for calculation of the the Zeeman energy, the deflagration velocity, and other parameters. It reduces the final temperature and significantly changes the propagation velocity of the spin-flipping front. We also consider the effect of a strong transverse magnetic field, and show that the latter significantly modifies the spin-state structure, leading to an uncertainty concerning the activation energy of the spin flipping. Our front velocity prediction for a crystal of Mn-12 acetate in a longitudinal magnetic field is in much better agreement with experimental data than the previous reduced-model results.

  • 44.
    Juricic, Vladimir
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Abergel, David S. L.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; ETH Institute for Theoretical Studies, Switzerland.
    First-order quantum phase transition in three-dimensional topological band insulators2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 16, article id 161403Article in journal (Refereed)
    Abstract [en]

    Topological states of matter are characterized by global topological invariants which change their value across a topological quantum phase transition. It is commonly assumed that the transition between topologically distinct noninteracting gapped phases of fermions is necessarily accompanied by the closing of the band gap as long as the symmetries of the system are maintained. We show that such a quantum phase transition is possible without closing the gap in the case of a three-dimensional topological band insulator. We demonstrate this by calculating the free energy of the minimal model for a topological insulator, the Bernevig-Hughes-Zhang model, and show that as the band curvature continuously varies, a jump between the band-gap minima corresponding to the topologically trivial and nontrivial insulators occurs. Therefore, this first-order phase transition is a generic feature of three-dimensional topological band insulators. For a certain parameter range we predict a reentrant topological phase transition. We discuss our findings in connection with the recent experimental observation of a discontinuous topological phase transition in a family of topological crystalline insulators.

  • 45.
    Kalenyuk, Aleksey A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Institute of Metal Physics of National Academy of Sciences of Ukraine, Ukraine.
    Pagliero, Alessandro
    Stockholm University, Faculty of Science, Department of Physics.
    Borodianskyi, Evgenii A.
    Stockholm University, Faculty of Science, Department of Physics.
    Aswartham, S.
    Wurmehl, S.
    Büchner, B.
    Chareev, D. A.
    Kordyuk, A. A.
    Krasnov, Vladimir M.
    Stockholm University, Faculty of Science, Department of Physics.
    Unusual two-dimensional behavior of iron-based superconductors with low anisotropy2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 13, article id 134512Article in journal (Refereed)
    Abstract [en]

    We study angular-dependent magnetoresistance in iron-based superconductors Ba1-xNaxFe2As2 and FeTe1-xSex. Both superconductors have relatively small anisotropies gamma similar to 2 and exhibit a three-dimensional (3D) behavior at low temperatures. However, we observe that they start to exhibit a profound two-dimensional behavior at elevated temperatures and in applied magnetic field parallel to the surface. We conclude that the unexpected two-dimensional (2D) behavior of the studied low-anisotropic superconductors is not related to layeredness of the materials, but is caused by appearance of surface superconductivity when magnetic field exceeds the upper critical field H-c2(T) for destruction of bulk superconductivity. We argue that the corresponding 3D-2D bulk-to-surface dimensional transition can be used for accurate determination of the upper critical field.

  • 46.
    Kedem, Yaron
    Stockholm University, Faculty of Science, Department of Physics.
    Novel pairing mechanism for superconductivity at a vanishing level of doping driven by critical ferroelectric modes2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 22, article id 220505Article in journal (Refereed)
    Abstract [en]

    Superconductivity occurring at low densities of mobile electrons is still a mystery since the standard theories do not apply in this regime. We address this problem by using a microscopic model for ferroelectric (FE) modes, which mediate an effective attraction between electrons. When the dispersion of modes, around zero momentum, is steep, forward scattering is the main pairing process and the self-consistent equation for the gap function can be solved analytically. The solutions exhibit unique features: Different momentum components of the gap function are decoupled, and at the critical regime of the FE modes, different frequency components are also decoupled. This leads to effects that can be observed experimentally: The gap function can be nonmonotonic in temperature and the critical temperature can be independent of the chemical potential. The model is applicable to lightly doped polar semiconductors, in particular, strontium titanate.

  • 47.
    Kjäll, Jonas A.
    Stockholm University, Faculty of Science, Department of Physics.
    Many-body localization and level repulsion2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 3, article id 035163Article in journal (Refereed)
    Abstract [en]

    Insertion of disorder in thermal interacting quantum systems decreases the amount of level repulsion and can lead to many-body localization. In this paper we use the many-body picture to perturbatively study the effect of level repulsion in the localized phase. We find that most eigenstates can be described accurately in an approximate way, including many with rare resonances. A classification of the rare resonances shows that most types are exponentially rare and requires exponential fine tuning in an approximate description. The classification confirms that no rare thermal eigenstates exist in a fully localized phase and we argue that all types of resonances need to become common if a continuous transition into a thermal phase should occur.

  • 48.
    Kunst, Flore K.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Dwivedi, Vatsal
    Non-Hermitian systems and topology: A transfer-matrix perspective2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 24, article id 245116Article in journal (Refereed)
    Abstract [en]

    Topological phases of Hermitian systems are known to exhibit intriguing properties such as the presence of robust boundary states and the famed bulk-boundary correspondence. These features can change drastically for their non-Hermitian generalizations, as exemplified by a general breakdown of bulk-boundary correspondence and a localization of all states at the boundary, termed the non-Hermitian skin effect. In this paper, we present a completely analytical unifying framework for studying these systems using generalized transfer matrices, a real-space approach suitable for systems with periodic as well as open boundary conditions. We show that various qualitative properties of these systems can be easily deduced from the transfer matrix. For instance, the connection between the breakdown of the conventional bulk-boundary correspondence and the existence of a non-Hermitian skin effect, previously observed numerically, is traced back to the transfer matrix having a determinant not equal to unity. The vanishing of this determinant signals real-space exceptional points, whose order scales with the system size. We also derive previously proposed topological invariants such as the biorthogonal polarization and the Chern number computed on a complexified Brillouin zone. Finally, we define an invariant for and thereby clarify the meaning of topologically protected boundary modes for non-Hermitian systems.

  • 49.
    Kunst, Flore K.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Freie Universität Berlin, Germany.
    Trescher, Maximilian
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics. Freie Universität Berlin, Germany.
    Anatomy of topological surface states: Exact solutions from destructive interference on frustrated lattices2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 8, article id 085443Article in journal (Refereed)
    Abstract [en]

    The hallmark of topological phases is their robust boundary signature whose intriguing properties-such as the one-way transport on the chiral edge of a Chern insulator and the sudden disappearance of surface states forming open Fermi arcs on the surfaces of Weyl semimetals-are impossible to realize on the surface alone. Yet, despite the glaring simplicity of noninteracting topological bulk Hamiltonians and their concomitant energy spectrum, the detailed study of the corresponding surface states has essentially been restricted to numerical simulation. In this work, however, we show that exact analytical solutions of both topological and trivial surface states can be obtained for generic tight-binding models on a large class of geometrically frustrated lattices in any dimension without the need for fine-tuning of hopping amplitudes. Our solutions derive from local constraints tantamount to destructive interference between neighboring layer lattices perpendicular to the surface and provide microscopic insights into the structure of the surface states that enable analytical calculation of many desired properties including correlation functions, surface dispersion, Berry curvature, and the system size dependent gap closing, which necessarily occurs when the spatial localization switches surface. This further provides a deepened understanding of the bulkboundary correspondence. We illustrate our general findings on a large number of examples in two and three spatial dimensions. Notably, we derive exact chiral Chern insulator edge states on the spin-orbit-coupled kagome lattice, and Fermi arcs relevant for recently synthesized slabs of pyrochlore-based Eu2Ir2O7 and Nd2Ir2O7, which realize an all-in-all-out spin configuration, as well as for spin-ice-like two-in-two-out and one-in-three-out configurations, which are both relevant for Pr2Ir2O7. Remarkably, each of the pyrochlore examples exhibit clearly resolved Fermi arcs although only the one-in-three-out configuration features bulk Weyl nodes in realistic parameter regimes. Our approach generalizes to symmetry protected phases, e.g., quantum spin Hall systems and Dirac semimetals with time-reversal symmetry, and can furthermore signal the absence of topological surface states, which we illustrate for a class of models akin to the trivial surface of Hourglass materials KHgX where the exact solutions apply but, independently of Hamiltonian details, yield eigenstates delocalized over the entire sample.

  • 50.
    Kunst, Flore K.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    van Miert, Guido
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Boundaries of boundaries: A systematic approach to lattice models with solvable boundary states of arbitrary codimension2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 8, article id 085426Article in journal (Refereed)
    Abstract [en]

    We present a generic and systematic approach for constructing D−dimensional lattice models with exactly solvable d−dimensional boundary states localized to corners, edges, hinges, and surfaces. These solvable models represent a class of “sweet spots” in the space of possible tight-binding models—the exact solutions remain valid for any tight-binding parameters as long as they obey simple locality conditions that are manifest in the underlying lattice structure. Consequently, our models capture the physics of both (higher order) topological and nontopological phases as well as the transitions between them in a particularly illuminating and transparent manner.

12 1 - 50 of 91
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf