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  • 51.
    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.

  • 52.
    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.

  • 53.
    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.

  • 54.
    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.
    Extended Bloch theorem for topological lattice models with open boundaries2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 8, article id 085427Article in journal (Refereed)
    Abstract [en]

    While the Bloch spectrum of translationally invariant noninteracting lattice models is trivially obtained by a Fourier transformation, diagonalizing the same problem in the presence of open boundary conditions is typically only possible numerically or in idealized limits. Here we present exact analytic solutions for the boundary states in a number of lattice models of current interest, including nodal-line semimetals on a hyperhoneycomb lattice, spin-orbit coupled graphene, and three-dimensional topological insulators on a diamond lattice, for which no previous exact finite-size solutions are available in the literature. Furthermore, we identify spectral mirror symmetry as the key criterium for analytically obtaining the entire (bulk and boundary) spectrum as well as the concomitant eigenstates, and exemplify this for Chern and Z2 insulators with open boundaries of codimension one. In the case of the two-dimensional Lieb lattice, we extend this further and show how to analytically obtain the entire spectrum in the presence of open boundaries in both directions, where it has a clear interpretation in terms of bulk, edge, and corner states.

  • 55.
    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.
    Lattice models with exactly solvable topological hinge and corner states2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 24, article id 241405Article in journal (Refereed)
    Abstract [en]

    We devise a generic recipe for constructing D-dimensional lattice models whose d-dimensional boundary states, located on surfaces, hinges, corners, and so forth, can be obtained exactly. The solvability is rooted in the underlying lattice structure and as such does not depend on fine tuning, allowing us to track their evolution throughout various phases and across phase transitions. Most saliently, our models provide boundary solvable examples of the recently introduced higher-order topological phases. We apply our general approach to breathing and anisotropic kagome and pyrochlore lattices for which we obtain exact corner eigenstates, and to periodically driven two-dimensional models as well as to three-dimensional lattices where we present exact solutions corresponding to one-dimensional chiral states at the hinges of the lattice. We relate the higher-order topological nature of these models to reflection symmetries in combination with their provenance from lower-dimensional conventional topological phases.

  • 56.
    Kyriienko, Oleksandr
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). ITMO University, Russia.
    Kibis, O.
    Shelykh, I. A.
    Optically induced topological states on the surface of mercury telluride2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 11, article id 115411Article in journal (Refereed)
    Abstract [en]

    We developed the theory which describes the Floquet engineering of surface electronic modes in bulk mercury telluride (HgTe) by a circularly polarized electromagnetic field. The analysis shows that the field results in the appearance of the surface states which arise from the mixing of conduction and valence bands of HgTe. Their branches lie near the center of the Brillouin zone and have the Dirac dispersion characteristic for topological states. Besides them, the irradiation induces the gap between the conduction and valence bands of HgTe. Thus, the irradiation can turn mercury telluride into a topological insulator from a gapless semiconductor. It is demonstrated that the optically induced states differ substantially from the nontopological surface states existing in HgTe without irradiation. The structure of the found states is studied both analytically and numerically in the broad range of their parameters.

  • 57.
    Kyriienko, Oleksandr
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). ITMO University, Russia.
    Sigurdsson, H.
    Liew, T. C. H.
    Probabilistic solving of NP-hard problems with bistable nonlinear optical networks2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 19, article id 195301Article in journal (Refereed)
    Abstract [en]

    We study theoretically a lattice of locally bistable driven-dissipative nonlinear cavities. The system is found to resemble the classical Ising model and enables its effective simulation. First, we benchmark the performance of driven-dissipative nonlinear cavities for spin-glass problems, and study the scaling of the ground-state-energy deviation and success probability as a function of system size. Next, we show how an effective bias field can be included in an optical model and use it for probabilistic solving of optimization problems. As particular examples we consider NP-hard problems embedded in the Ising model, namely graph partitioning and the knapsack problem. Finally, we confirm that locally bistable polariton networks act as classical optimizers and can potentially provide an improvement within the exponential complexity class.

  • 58. Liu, Zhao
    et al.
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Fractional quantum Hall states with gapped boundaries in an extreme lattice limit2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 19, article id 195122Article in journal (Refereed)
    Abstract [en]

    We present a detailed microscopic investigation of fractional quantum Hall states with gapped boundaries in a coupled bilayer lattice model featuring holes whose counterpropagating chiral edge states are hybridized and gapped out. We focus on an lattice limit for cold-atom experiments, in which each hole just consists of a single removed site. Although the holes distort the original band structure and lead to in-gap remnants of the continuum edge modes, we find that the lowest nearly flat band representing a higher-genus system may naturally form by controlling the local hopping terms that gap out the boundaries. Remarkably, local interactions in this new flat band lead to various Abelian and non-Abelian fractional quantum Hall states with gapped boundaries residing on emergent higher-genus surfaces, which we identify by extracting the nontrivial topological ground-state degeneracies and the fractional statistics of quasiparticles. These results demonstrate the feasibility of realizing novel fractional quantum Hall states with gapped boundaries even in the extreme lattice limit, thus enabling a possible new route towards universal topological quantum computation.

  • 59. Liu, Zhao
    et al.
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Romito, Alessandro
    Meidan, Dganit
    Interacting Majorana chain: Transport properties and signatures of an emergent two-dimensional weak topological phase2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 20, article id 205442Article in journal (Refereed)
    Abstract [en]

    We study a one-dimensional chain of 2N Majorana bound states, which interact through a local quartic interaction. This model describes for example the edge physics of a quasi-one-dimensional (1D) stack of 2N Kitaev chains with modified time-reversal symmetry T gamma iT-1 = gamma(i), which precludes the presence of quadratic coupling. The ground state of our 1D Majorana chain displays a fourfold periodicity in N, corresponding to the four distinct topological classes of the stacked Kitaev chains. We analyze the transport properties of the 1D Majorana chain, when probed by local conductors located at its ends. We find that for finite but large N, the scattering matrix partially reflects the fourfold periodicity, and the chain exhibits strikingly different transport properties for different chain lengths. In the thermodynamic limit, the 1D Majorana chain hosts a robust many-body zero mode, which indicates that the corresponding stacked two-dimensional bulk system realizes a weak topological phase.

  • 60. Lledó, C.
    et al.
    Mavrogordatos, Themistoklis
    Stockholm University, Faculty of Science, Department of Physics.
    Szymańska, M. H.
    Driven Bose-Hubbard dimer under nonlocal dissipation: A bistable time crystal2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 5, article id 054303Article in journal (Refereed)
    Abstract [en]

    We investigate the critical behavior of the open coherently-driven Bose-Hubbard dimer under nonlocal dissipation. A conserved quantity arises from the nonlocal nature of the dissipation, rendering the dimer multistable. In the weak-coupling semiclassical limit, the displayed criticality takes the form of amplitude bistability and breaking of spatial and temporal symmetries. A period-bistable time crystal is formed, consisting of Josephson-like oscillations. Mean-field dynamics and quantum trajectories complement the spectral analysis of the Liouvillian in the approach to the semiclassical limit.

  • 61.
    Mahyaeh, Iman
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ardonne, Eddy
    Stockholm University, Faculty of Science, Department of Physics.
    Exact results for a Z(3)-clock-type model and some close relatives2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 24, article id 245104Article in journal (Refereed)
    Abstract [en]

    In this paper, we generalized the Peschel-Emery line of the interacting transverse field Ising model to a model based on three-state clock variables. Along this line, the model has exactly degenerate ground states, which can be written as product states. In addition, we present operators that transform these ground states into each other. Such operators are also presented for the Peschel-Emery case. We numerically show that the generalized model is gapped. Furthermore, we study the spin-S generalization of interacting Ising model and show that along a Peschel-Emery line they also have degenerate ground states. We discuss some examples of excited states that can be obtained exactly for all these models.

  • 62. Milovanovic, M. V.
    et al.
    Ciric, M. Dimitrijevic
    Juricic, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Pairing instabilities of Dirac composite fermions2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 11, article id 115304Article in journal (Refereed)
    Abstract [en]

    Recently, a Dirac (particle-hole symmetric) description of composite fermions in the half-filled Landau level (LL) was proposed [D. T. Son, Phys. Rev. X 5, 031027 (2015)], and we study its possible consequences on BCS (Cooper) pairing of composite fermions (CFs). One of the main consequences is the existence of anisotropic states in single-layer and bilayer systems, which was previously suggested in Jeong and Park [J. S. Jeong and K. Park, Phys. Rev. B 91, 195119 (2015)]. We argue that in the half-filled LL in the single-layer case the gapped states may sustain anisotropy, because isotropic pairings may coexist with anisotropic ones. Furthermore, anisotropic pairings with the addition of a particle-hole symmetry-breaking mass term may evolve into rotationally symmetric states, i.e., Pfaffian states of Halperin-Lee-Read (HLR) ordinary CFs. On the basis of the Dirac formalism, we argue that in the quantum Hall bilayer at total filling factor 1, with decreasing distance between the layers, weak pairing of p-wave paired CFs is gradually transformed from Dirac to ordinary, HLR-like, with a concomitant decrease in the CF number. Global characterization of low-energy spectra based on the Dirac CFs agrees well with previous calculations performed by exact diagonalization on a torus. Finally, we discuss features of the Dirac formalism when applied in this context.

  • 63. Mishchenko, Petr A.
    et al.
    Kato, Yasuyuki
    O'Brien, Kevin
    Bojesen, Troels A.
    Eschmann, Tim
    Hermanns, Maria
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Trebst, Simon
    Motome, Yukitoshi
    Chiral spin liquids with crystalline Z(2) gauge order in a three-dimensional Kitaev model2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 4, article id 045118Article in journal (Refereed)
    Abstract [en]

    Chiral spin liquids (CSLs) are time-reversal-symmetry-breaking ground states of frustrated quantum magnets that show no long-range magnetic ordering but instead exhibit topological order and fractional excitations. Their realization in simple and tractable microscopic models has, however, remained an open challenge for almost two decades until it was realized that Kitaev models on lattices with odd-length loops are natural hosts for such states, even in the absence of a time-reversal-symmetry-breaking magnetic field. Here we report on the formation of CSLs in a three-dimensional Kitaev model on a hypernonagon lattice composed of nine-site loops, which differ from their widely studied two-dimensional counterparts; namely, they exhibit a crystalline ordering of the Z(2) gauge fluxes and thereby break some of the underlying lattice symmetries. We study the formation of these unconventional CSLs via extensive quantum Monte Carlo simulations and demonstrate that they are separated from the featureless paramagnet at high temperatures by a single first-order phase transition at which both time-reversal and lattice symmetries are simultaneously broken. Using variational approaches for the ground state, we explore the effect of varying the Kitaev couplings and find at least five distinct CSL phases, all of which possess crystalline ordering of the Z(2) gauge fluxes. For some of these phases, the complementary itinerant Majorana fermions exhibit gapless band structures with topological features such as Weyl nodes or nodal lines in the bulk and Fermi arc or drumhead surface states.

  • 64. Nietner, A.
    et al.
    Krumnow, C.
    Johansson Bergholtz, Emil
    Stockholm University, Faculty of Science, Department of Physics. Freie Universität Berlin, Germany.
    Eisert, J.
    Composite symmetry-protected topological order and effective models2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 23, article id 235138Article in journal (Refereed)
    Abstract [en]

    Strongly correlated quantum many-body systems at low dimension exhibit a wealth of phenomena, ranging from features of geometric frustration to signatures of symmetry-protected topological order. In suitable descriptions of such systems, it can be helpful to resort to effective models, which focus on the essential degrees of freedom of the given model. In this work, we analyze how to determine the validity of an effective model by demanding it to be in the same phase as the original model. We focus our study on one-dimensional spin-1/2 systems and explain how nontrivial symmetry-protected topologically ordered (SPT) phases of an effective spin-1 model can arise depending on the couplings in the original Hamiltonian. In this analysis, tensor network methods feature in two ways: on the one hand, we make use of recent techniques for the classification of SPT phases using matrix product states in order to identify the phases in the effective model with those in the underlying physical system, employing Kunneth's theorem for cohomology. As an intuitive paradigmatic model we exemplify the developed methodology by investigating the bilayered Delta chain. For strong ferromagnetic interlayer couplings, we find the system to transit into exactly the same phase as an effective spin-1 model. However, for weak but finite coupling strength, we identify a symmetry broken phase differing from this effective spin-1 description. On the other hand, we underpin our argument with a numerical analysis making use of matrix product states.

  • 65.
    Olsthoorn, Bart
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Physics.
    Mass fluctuations and absorption rates in dark-matter sensors based on Dirac materials2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 4, article id 045120Article in journal (Refereed)
    Abstract [en]

    We study the mass fluctuations in gapped Dirac materials by treating the mass term as both a continuous and discrete random variable. Gapped Dirac materials were proposed to be used as materials for dark-matter sensors. One thus would need to estimate the role of disorder and fluctuations on the interband absorption of dark matter. We find that both continuous and discrete fluctuations across the sample introduce tails (e.g., Dirac-Lifshitz tails) in the density of states and the interband absorption rate. We estimate the strength of the gap filling and discuss implications of these fluctuations on the performance as sensors for dark matter detection. The approach used in this work provides a basic framework to model the disorder by any arbitrary mechanism on the interband absorption of Dirac material sensors.

  • 66.
    Pershoguba, Sergey S.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Nakosai, Sho
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Condensed Matter Theory Laboratory, Japan.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Skyrmion-induced bound states in a superconductor2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 6, article id 064513Article in journal (Refereed)
    Abstract [en]

    We consider a superconductor proximity coupled to a two-dimensional ferromagnetic film with a skyrmion texture. Using the T-matrix calculations and numerical modeling we calculate the spin-polarized local density of states in the superconductor in the vicinity of the skyrmion. We predict the skyrmion bound states that are induced in the superconductor, similar to the well-known Yu-Shiba-Rusinov states. The bound-state wave functions have spatial power-law decay. It is suggested that superconductivity could facilitate an effective long-range interaction between skyrmions when bound-state wave functions overlap.

  • 67. Pertsova, Anna
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA; University of Connecticut, USA.
    Excitonic instability in optically pumped three-dimensional Dirac materials2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 7, article id 075109Article in journal (Refereed)
    Abstract [en]

    Recently it was suggested that transient excitonic instability can be realized in optically pumped two-dimensional (2D) Dirac materials (DMs), such as graphene and topological insulator surface states. Here we discuss the possibility of achieving a transient excitonic condensate in optically pumped three-dimensional (3D) DMs, such as Dirac and Weyl semimetals, described by nonequilibrium chemical potentials for photoexcited electrons and holes. Similar to the equilibrium case with long-range interactions, we find that for pumped 3D DMs with screened Coulomb potential two possible excitonic phases exist, an excitonic insulator phase and the charge density wave phase originating from intranodal and internodal interactions, respectively. In the pumped case, the critical coupling for excitonic instability vanishes; therefore the two phases coexist for arbitrarily weak coupling strengths. The excitonic gap in the charge density wave phase is always the largest one. The competition between screening effects and the increase of the density of states with optical pumping results in a rich phase diagram for the transient excitonic condensate. Based on the static theory of screening, we find that under certain conditions the value of the dimensionless coupling constant screening in 3D DMs can be weaker than in 2D DMs. Furthermore, we identify the signatures of the transient excitonic condensate that could be probed by scanning tunneling spectroscopy, photoemission, and optical conductivity measurements. Finally, we provide estimates of critical temperatures and excitonic gaps for existing and hypothetical 3D DMs.

  • 68.
    Pertsova, Anna
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Geilhufe, R. Matthias
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Bremholm, Martin
    Balatsky, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Connecticut, USA.
    Computational search for Dirac and Weyl nodes in f-electron antiperovskites2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 20, article id 205126Article in journal (Refereed)
    Abstract [en]

    We present the result of an ab initio search for new Dirac materials among inverse perovskites. Our investigation is focused on the less studied class of lanthanide antiperovskites containing heavy f-electron elements in the cation position. Some of the studied compounds have not yet been synthesized experimentally. Our computational approach is based on density functional theory calculations which account for spin-orbit interaction and strong correlations of the f-electron atoms. We find several promising candidates among lanthanide antiperovskites which host bulk Dirac states close to the Fermi level. Specifically, our calculations reveal massive three-dimensional Dirac states in materials of the class A(3)BO, where A=Sm, Eu, Gd, Yb, and B=Sn, Pb. In materials with finite magnetic moment, such as Eu3BO (B=Sn, Pb), the degeneracy of the Dirac nodes is lifted, leading to appearance of Weyl nodes.

  • 69. Polyakov, A.
    et al.
    Tusche, C.
    Ellguth, M.
    Crozier, E. D.
    Mohseni, K.
    Otrokov, M. M.
    Zubizarreta, X.
    Vergniory, M. G.
    Geilhufe, Matthias
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Max-Planck-Institut für Mikrostrukturphysik, Germany.
    Chulkov, E. V.
    Ernst, A.
    Meyerheim, H. L.
    Parkin, S. S. P.
    Instability of the topological surface state in Bi2Se3 upon deposition of gold2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 18, article id 180202Article in journal (Refereed)
    Abstract [en]

    Momentum-resolved photoemission spectroscopy indicates the instability of the Dirac surface state upon deposition of gold on the (0001) surface of the topological insulator Bi2Se3. Based on the structure model derived from extended x-ray absorption fine structure experiments showing that gold atoms substitute bismuth atoms, first-principles calculations provide evidence that a gap appears due to hybridization of the surface state with gold d states near the Fermi level. Our findings provide insights into the mechanisms affecting the stability of the surface state.

  • 70. Polyakov, A.
    et al.
    Tusche, C.
    Ellguth, M.
    Crozier, E. D.
    Mohseni, K.
    Otrokov, M. M.
    Zubizarreta, X.
    Vergniory, M. G.
    Geilhufe, Matthias
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Max-Planck-Institut für Mikrostrukturphysik, Germany.
    Chulkov, E. V.
    Ernst, A.
    Meyerheim, H. L.
    Parkin, S. S. P.
    Reply to Comment on 'Instability of the topological surface state in Bi2Se3 upon deposition of gold'2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 13, article id 136202Article in journal (Refereed)
    Abstract [en]

    In the Comment on our publication [Phys. Rev. B 95, 180202(R) (2017)], R. A. Gordon claims that our main conclusion is not valid, namely that gold atoms deposited in situ on the (0001) surface of single-crystalline Bi2Se3 reside in substitutional sites, i.e., replacing bismuth atoms within the topmost quintuple layer (QL). Based on x-ray absorption near-edge (XANES) spectra and a re-evaluation of extended x-ray absorption fine structure (EXAFS) data above the Au L-III edge, R. A. Gordon concludes that Au resides in a twofold environment as a result of an interface reaction leading to an Au2S-type local structure, in which gold adopts an Au(I) state and is linearly coordinated by selenium atoms. In this Reply, we will confirm the results published in the original paper and their interpretation that Au atoms reside in the substitutional site.

  • 71. Principi, Alessandro
    et al.
    Bandurin, Denis
    Rostami, Habib
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Polini, Marco
    Pseudo-Euler equations from nonlinear optics: Plasmon-assisted photodetection beyond hydrodynamics2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 7, article id 075410Article in journal (Refereed)
    Abstract [en]

    A great deal of theoretical and experimental efforts have been devoted in the last decades to the study of long-wavelength photodetection mechanisms in field-effect transistors hosting two-dimensional (2D) electron systems. A particularly interesting subclass of these mechanisms is intrinsic and based on the conversion of the incoming electromagnetic radiation into plasmons, which resonantly enhance the photoresponse, and subsequent rectification via hydrodynamic nonlinearities. In this paper, we show that such a conversion and subsequent rectification occur well beyond the frequency regime in which hydrodynamic theory applies. We consider the nonlinear optical response of generic 2D electron systems and derive pseudo-Euler equations of motion for suitable collective variables. These are solved in one- and two-dimensional geometries for the case of graphene and the results are compared with those of hydrodynamic theory. Significant qualitative differences are found, which are amenable to experimental studies. Our theory expands the knowledge of the fundamental physics behind long-wavelength photodetection.

  • 72. Putnam, R.
    et al.
    Balatsky, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Connecticut, USA.
    Haraldsen, J. T.
    Spin channel induced directional dependent spin exchange interactions between divacantly substituted Fe atoms in graphene2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 12, article id 125435Article in journal (Refereed)
    Abstract [en]

    In this study, we show the divacant substitution of Fe impurities atom produces the formation of an electron spin channel along the armchair direction of graphene. This spin channel creates a directional dependent spin exchange between impurities. Using density functional theory, we simulated the electronic and magnetic properties for a supercell of graphene with spatial variation of the Fe atoms along either the armchair or zigzag directions. Overall, we find that the exchange interaction between the two Fe atoms fluctuates from ferromagnetic to antiferromagnetic as a function of the spatial distance in the armchair direction. Given the induced magnetic moment and increased density of states at the Fermi level by the surrounding carbon atoms, we conclude that an RKKY-like interaction may characterize the exchange interactions between the Fe atoms. Furthermore, we examined the same interactions for Fe atoms along the zigzag direction in graphene and found no evidence for an RKKY interaction as this system shows a standard exchange between the transition-metal impurities. Therefore, we determine that the spin channel produced through Fe-substitution in graphene induces a directional-dependent spin interaction, which may provide stability to spintronic and multifunctional devices and applications for graphene.

  • 73. Pöyhönen, Kim
    et al.
    Westström, Alex
    Pershoguba, Sergey S.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Ojanen, Teemu
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Los Alamos National Laboratory, USA.
    Skyrmion-induced bound states in a p-wave superconductor2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 21, article id 214509Article in journal (Refereed)
    Abstract [en]

    In s-wave systems, it has been theoretically shown that a ferromagnetic film hosting a skyrmion can induce a bound state embedded in the opposite-spin continuum. In this work, we consider a case of skyrmion-induced state in a p-wave superconductor. We find that the skyrmion induces a bound state that generally resides within the spectral gap and is isolated from all other states, in contrast to the case of conventional superconductors. To this end, we derive an approximate expression for the T matrix, through which we calculate the spin-polarized local density of states which is observable in scanning tunneling microscopy measurements. We find the unique spectroscopic features of the skyrmion-induced bound state and discuss how our predictions could be employed as experimental probes for p-wave superconducting states.

  • 74. Quelle, A.
    et al.
    Smith, C. Morais
    Kvorning, Thomas
    Stockholm University, Faculty of Science, Department of Physics.
    Hansson, Thors Hans
    Stockholm University, Faculty of Science, Department of Physics.
    Edge Majoranas on locally flat surfaces: The cone and the Mobius band2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 12, article id 125137Article in journal (Refereed)
    Abstract [en]

    In this paper, we investigate the edge Majorana modes in the simplest possible p(x) + ip(y) superconductor defined on surfaces with different geometries, the annulus, the cylinder, the Mobius band, and a cone (by cone we mean a cone with the tip cut away so it is topologically equivalent to the annulus and cylinder), and with different configurations of magnetic fluxes threading holes in these surfaces. In particular, we shall address two questions: Given that, in the absence of any flux, the ground state on the annulus does not support Majorana modes while the one on the cylinder does, how is it possible that the conical geometry can interpolate smoothly between the two? Given that in finite geometries edge Majorana modes have to come in pairs, how can a p(x) + ip(y) state be defined on a Mobius band, which has only one edge? We show that the key to answering these questions is that the ground state depends on the geometry, even though all the surfaces are locally flat. In the case of the truncated cone, there is a nontrivial holonomy, while the nonorientable Mobius band must necessarily support a domain wall.

  • 75. Revelli, A.
    et al.
    Loo, C. C.
    Kiese, D.
    Becker, P.
    Fröhlich, T.
    Lorenz, T.
    Sala, M. Moretti
    Monaco, G.
    Buessen, F. L.
    Attig, J.
    Hermanns, Maria
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Streltsov, S.
    Khomskii, D.
    van den Brink, I. J.
    Braden, M.
    van Loosdrecht, P. H. M.
    Trebst, S.
    Paramekanti, A.
    Grüninger, M.
    Spin-orbit entangled j=1/2 moments in Ba(2)CWeIrO(6): A frustrated fcc quantum magnet2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 8, article id 085139Article in journal (Refereed)
    Abstract [en]

    We establish the double perovskite Ba2CeIrO6 as a nearly ideal model system for j = 1/2 moments, with resonant inelastic x-ray scattering indicating that the ideal j = 1/2 state contributes by more than 99% to the ground-state wave function. The local j = 1/2 moments form an fcc lattice and are found to order antiferromagnetically at T-N = 14 K, more than an order of magnitude below the Curie-Weiss temperature. Model calculations show that the geometric frustration of the fcc Heisenberg antiferromagnet is further enhanced by a next-nearest neighbor exchange, and a significant size of the latter is indicated by ab initio theory. Our theoretical analysis shows that magnetic order is driven by a bond-directional Kitaev exchange and by local distortions via a strong magnetoelastic effect. Both, the suppression of frustration by Kitaev exchange and the strong magnetoelastic effect are typically not expected for j = 1/2 compounds making Ba2CeIrO6 a riveting example for the rich physics of spin-orbit entangled Mott insulators.

  • 76.
    Rostami, Habib
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Cappelluti, Emmanuele
    Balatsky, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Helical metals and insulators: Sheet singularity of the inflated Berry monopole2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 24, article id 245114Article in journal (Refereed)
    Abstract [en]

    We study phases of interacting Dirac matter that host Berry signatures. We predict a topological Lifshitz phase transition caused by the changes of a Dirac cone intersection from a semimetallic phase to helical insulating or metallic phases. These helical phases provide examples of a gapless topological phase where the spectral gap is not required for a topological protection. To realize nodal helical phases one would need to consider isotropic infinite-range interparticle. interaction. This interaction could emerge because of a momentum conserving scattering of electrons from a bosonic mode. For repulsive/attractive inter-particle interaction in density/pseudospin channel, the system undergoes a transition to the helical insulator phase. For an attractive density-density interaction, a metallic phase forms that hosts a nodal circle and a nodal sphere in two and three dimensions, respectively. A sheet singularity of Berry curvature is highlighted as a peculiar feature of the nodal sphere phase in three dimensions and represents the extension of the Berry monopole singularities into an inflated monopole. To illustrate the properties of these helical phases we investigate Landau levels in both metallic and insulating phases. Our study provides an extension of the paradigm in the interacting Dirac matter and makes an interesting connection to inflated topological singularities in cosmology.

  • 77.
    Rostami, Habib
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Volckaert, Klara
    Lanata, Nicola
    Mahatha, Sanjoy K.
    Sanders, Charlotte E.
    Bianchi, Marco
    Lizzit, Daniel
    Bignardi, Luca
    Lizzit, Silvano
    Miwa, Jill A.
    Balatsky, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Hofmann, Philip
    Ulstrup, Søren
    Layer and orbital interference effects in photoemission from transition metal dichalcogenides2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 23, article id 235423Article in journal (Refereed)
    Abstract [en]

    In this work, we provide an effective model to evaluate the one-electron dipole matrix elements governing optical excitations and the photoemission process of single-layer (SL) and bilayer (BL) transition metal dichalcogenides. By utilizing a k . p Hamiltonian, we calculate the photoemission intensity as observed in angle-resolved photoemission from the valence bands around the (K) over bar valley of MoS2. In SL MoS2, we find a significant masking of intensity outside the first Brillouin zone, which originates from an in-plane interference effect between photoelectrons emitted from the Mo d orbitals. In BL MoS2, an additional interlayer interference effect leads to a distinctive modulation of intensity with photon energy. Finally, we use the semiconductor Bloch equations to model the optical excitation in a time- and angle-resolved pump-probe photoemission experiment. We find that the momentum dependence of an optically excited population in the conduction band leads to an observable dichroism in both SL and BL MoS2.

  • 78. Roy, Bitan
    et al.
    Goswami, Pallab
    Juricic, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Interacting Weyl fermions: Phases, phase transitions, and global phase diagram2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 20, article id 201102Article in journal (Refereed)
    Abstract [en]

    We study the effects of short-range interactions on a generalized three-dimensional Weyl semimetal, where the band touching points act as the (anti) monopoles of Abelian Berry curvature of strength n. We show that any local interaction has a negative scaling dimension -2/n. Consequently, all Weyl semimetals are stable against weak short-range interactions. For sufficiently strong interactions, we demonstrate that the Weyl semimetal either undergoes a first-order transition into a band insulator or a continuous transition into a symmetry breaking phase. A translational symmetry breaking axion insulator and a rotational symmetry breaking semimetal are two prominent candidates for the broken symmetry phase. At the one-loop order, the correlation length exponent for continuous transitions is upsilon = n/2, indicating their non-Gaussian nature for any n > 1. We also discuss the scaling of the thermodynamic and transport quantities in general Weyl semimetals as well as inside broken symmetry phases.

  • 79. Roy, Bitan
    et al.
    Goswami, Pallab
    Juričić, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Itinerant quantum multicriticality of two-dimensional Dirac fermions2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 20, article id 205117Article in journal (Refereed)
    Abstract [en]

    We analyze emergent quantum multicriticality for strongly interacting, massless Dirac fermions in two spatial dimensions (d = 2) within the framework of Gross-Neveu-Yukawa models, by considering the competing order parameters that give rise to fully gapped (insulating or superconducting) ground states. We focus only on those competing orders which can be rotated into each other by generators of an exact or emergent chiral symmetry of massless Dirac fermions, and break O(S-1) and O(S-2) symmetries in the ordered phase. Performing a renormalization-group analysis by using the epsilon = (3 - d) expansion scheme, we show that all the coupling constants in the critical hyperplane flow toward a new attractive fixed point, supporting an enlarged O(S-1 + S-2) chiral symmetry. Such a fixed point acts as an exotic quantum multicritical point (MCP), governing the continuous semimetal-insulator as well as insulator-insulator (for example, antiferromagnet to valence bond solid) quantum phase transitions. In comparisonwith the lower symmetric semimetal-insulator quantum critical points, possessing either O(S-1) or O(S-2) chiral symmetry, the MCP displays enhanced correlation length exponents, and anomalous scaling dimensions for both fermionic and bosonic fields. We discuss the scaling properties of the ratio of bosonic and fermionic masses, and the increased dc resistivity at the MCP. By computing the scaling dimensions of different local fermion bilinears in the particle-hole channel, we establish that most of the four fermion operators or generalized density-density correlation functions display faster power-law decays at the MCP compared to the free fermion and lower symmetric itinerant quantum critical points. Possible generalization of this scenario to higher-dimensional Dirac fermions is also outlined.

  • 80. Roy, Bitan
    et al.
    Juricic, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Optical conductivity of an interacting Weyl liquid in the collisionless regime2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 15, article id 155117Article in journal (Refereed)
    Abstract [en]

    Optical conductivity (OC) can serve as a measure of correlation effects in a wide range of condensed-matter systems. We show that the long-range tail of the Coulomb interaction yields a universal correction to the OC in a three-dimensional Weyl semimetal sigma(Omega) = sigma(0)(Omega)[1 + 1/N+1], where sigma(0)(Omega) = Ne-0(2)Omega/(12hv) is the OC in the noninteracting system, with v as the actual (renormalized) Fermi velocity of Weyl quasiparticles at frequency Omega, and e(0) is the electron charge in vacuum. Such universal enhancement of OC, which depends only on the number of Weyl nodes near the Fermi level (N), is a remarkable consequence of an intriguing conspiracy among the quantum-critical nature of an interacting Weyl liquid, marginal irrelevance of the long-range Coulomb interaction, and violation of hyperscaling in three dimensions, and can directly be measured in recently discovered Weyl as well as Dirac materials. By contrast, a local density-density interaction produces a nonuniversal correction to the OC, stemming from the nonrenormalizable nature of the corresponding interacting field theory.

  • 81. Roy, Bitan
    et al.
    Juricic, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Unconventional superconductivity in nearly flat bands in twisted bilayer graphene2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 12, article id 121407Article in journal (Refereed)
    Abstract [en]

    Flat electronic bands can accommodate a plethora of interaction-driven quantum phases, since kinetic energy is quenched therein and electronic interactions therefore prevail. Twisted bilayer graphene, near the so-called magic angles, features slow Dirac fermions close to the charge-neutrality point that persist up to high energies. Starting from a continuum model of slow but strongly interacting Dirac fermions, we show that with increasing chemical doping away from the charge-neutrality point, a time-reversal symmetry breaking, valley pseudospin-triplet, topological p + ip superconductor gradually sets in, when the system resides at the brink of an antiferromagnetic ordering (due to Hubbard repulsion), in qualitative agreement with recent experimental findings. The p ip paired state exhibits quantized spin and thermal Hall conductivities, and polar Kerr and Faraday rotations. Our conclusions should also be applicable for other correlated two-dimensional Dirac materials.

  • 82. Roy, Bitan
    et al.
    Juričić, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Fermionic multicriticality near Kekule valence-bond ordering on a honeycomb lattice2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 24, article id 241103Article in journal (Refereed)
    Abstract [en]

    We analyze the possibility of emergent quantum multicritical points (MCPs) with enlarged chiral symmetry, when strongly interacting gapless Dirac fermions acquire a comparable propensity toward the nucleation of Kekule valence-bond solid (KVBS) and charge-density-wave (N-b = 1) or s-wave pairing (N-b = 2) or antiferromagnet (N-b = 3) in a honeycomb lattice, where N-b counts the number of bosonic order-parameter components. Besides the cubic terms present in the order-parameter description of KVBS due to the breaking of a discrete Z(3) symmetry, quantum fluctuations generate new cubic vertices near the high-symmetry MCPs. All cubic terms are strongly relevant at the bare level near three spatial dimensions, about which we perform a leading-order renormalization group analysis of coupled Gross-Neveu-Yukawa field theory. We show that due to nontrivial Yukawa interactions among gapless bosonic and fermionic degrees of freedom, all cubic terms ultimately become irrelevant at an O(2 + N-b) symmetric MCP, at leas near two spatial dimensions, where N-b = 1, 2, 3. Therefore, MCPs with an enlarged O(2 + N-b) symmetry near KVBS ordering are stable.

  • 83. Rusz, Ján
    et al.
    Spiegelberg, Jakob
    Muto, Shunsuke
    Thersleff, Thomas
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Ohtsuka, Masahiro
    Leifer, Klaus
    Oppeneer, Peter M.
    Localization of magnetic circular dichroic spectra in transmission electron microscopy experiments with atomic plane resolution2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 17, article id 174412Article in journal (Refereed)
    Abstract [en]

    Inelastic electron scattering is a consequence of mostly Coulomb interaction between electrons in the sample and electron beam and, as such, it is a nonlocal event. In atomic resolution experiments, it thus opens the following question: How far is the origin of the inelastic scattering signal that is observed when the electron beam is passing nearby an atomic column or plane? We analyze computationally the delocalization of the magnetic signal in electron magnetic circular dichroism (EMCD) experiments in the so-called three-beam orientation, allowing one to image individual atomic planes. We compare the classical EMCD setup using the double-difference procedure (DD-EMCD) to a recently introduced atomic plane resolution EMCD (APR-EMCD) geometry, assuming the same probe size. We observe a strong localization of the EMCD signal to the closest atomic plane, confirming the potential of EMCD to study an evolution of magnetic properties near surfaces or interfaces with atomic plane resolution. The localization of the EMCD signal is remarkably higher than the localization of the nonmagnetic component of the inelastic scattering cross section. We also analyze double-channeling effects and find them particularly strong for the DD-EMCD method, while for APR-EMCD they appear to be minor. The DD-EMCD signal, on the other hand, appears to be more robust with respect to sample thickness than that of the APR-EMCD.

  • 84. Sangeetha, N. S.
    et al.
    Anand, V. K.
    Cuervo-Reyes, Eduardo
    Smetana, Volodymyr
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mudring, Anja-Verena
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Johnston, D. C.
    Enhanced moments of Eu in single crystals of the metallic helical antiferromagnet EuCo2-yAs22018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 14Article in journal (Refereed)
    Abstract [en]

    The compound EuCo2-yAs2 with the tetragonal ThCr2Si2 structure is known to contain Eu+2 ions with spin S = 7/2 that order below a temperature T-N approximate to 47 K into an antiferromagnetic (AFM) proper helical structure with the ordered moments aligned in the tetragonal ab plane, perpendicular to the helix axis along the c axis, with no contribution from the Co atoms. Here we carry out a detailed investigation of the properties of single crystals. We consistently find about 5% vacancies on the Co site from energy-dispersive x-ray analysis and x-ray diffraction refinements. Enhanced ordered and effective moments of the Eu spins are found in most of our crystals. Electronic structure calculations indicate that the enhanced moments arise from polarization of the d bands, as occurs in ferromagnetic Gd metal. Electrical resistivity measurements indicate metallic behavior. The low-field in-plane magnetic susceptibilities x(ab) (T < T-N) for several crystals are reported that are fitted well by unified molecular field theory (MFT), and the Eu-Eu exchange interactions J(ij) are extracted from the fits. High-field magnetization M data for magnetic fields H parallel to ab reveal what appears to be a first-order spin-flop transition followed at higher field by a second-order metamagnetic transition of unknown origin, and then by another second-order transition to the paramagnetic (PM) state. For H parallel to c, the magnetization shows only a second-order transition from the canted AFM to the PM state, as expected. The critical fields for the AFM to PM transition are in approximate agreement with the predictions of MFT. Heat capacity C-p measurements in zero and high H are reported. Phase diagrams for H parallel to c and H parallel to ab versus T are constructed from the high-field M(H, T) and C-p(H, T) measurements. The magnetic part C-mag(T, H = 0) of C-p(T, H = 0) is extracted and is fitted rather well below T-N by MFT, although dynamic short-range AFM order is apparent in Cmag(T) up to about 70 K, where the molar entropy attains its high-T limit of R ln 8.

  • 85. Sangeetha, N. S.
    et al.
    Smetana, Volodymyr
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mudring, Anja-Verena
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Johnston, D. C.
    Antiferromagnetism in semiconducting SrMn2Sb2 and BaMn2Sb2 single crystals2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 1, article id 014402Article in journal (Refereed)
    Abstract [en]

    Crystals of SrMn2Sb2 and BaMn2Sb2 were grown using Sn flux and characterized by powder and single-crystal x-ray diffraction, respectively, and by single-crystal electrical resistivity rho, heat capacity C-p, and magnetic susceptibility chi measurements versus temperature T, and magnetization versus field M(H) isotherm measurements. SrMn2Sb2 adopts the trigonal CaAl2Si2-type structure whereas BaMn2Sb2 crystallizes in the tetragonal ThCr2Si2-type structure. The rho(T) data indicate semiconducting behaviors for both compounds with activation energies of greater than or similar to 0.35 eV for SrMn2Sb2 and 0.16 eV for BaMn2Sb2. The chi(T) and C-p(T) data reveal antiferromagnetic (AFM) ordering at T-N = 110 K for SrMn2Sb2 and 450 K for BaMn2Sb2. The anisotropic chi(T <= T-N) data also show that the ordered moments in SrMn2Sb2 are aligned in the hexagonal ab plane whereas the ordered moments in BaMn2Sb2 are aligned collinearly along the tetragonal c axis. The ab-plane M(H) data for SrMn2Sb2 exhibit a continuous metamagnetic transition at low fields 0 < H less than or similar to 1 T, whereas BaMn2Sb2 exhibits no metamagnetic transitions up to 5.5 T. The chi(T) and C-p(T) data for SrMn2Sb2 and BaMn2Sb2 indicate strong dynamic short-range AFM correlations above their respective TN up to at least 900 K within a local-moment picture, corresponding to quasi-two-dimensional magnetic behavior. The present results and a survey of the literature for Mn pnictides with the CaAl2Si2 and ThCr2Si2 crystal structures show that the T-N values for the CaAl2Si2-type compounds are much smaller than those for the ThCr2Si2-type materials.

  • 86. Sangeetha, N. S.
    et al.
    Smetana, Volodymyr
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mudring, Anja-Verena
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Johnston, D. C.
    Helical antiferromagnetic ordering in EuNi1.95As2 single crystals2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 9, article id 094438Article in journal (Refereed)
    Abstract [en]

    The Eu(+2 )spins-7/2 in EuNi2As2 with the body-centered tetragonal ThCr2Si2 structure order antiferromagnetically below the Ned temperature T-N = 15 K into a helical antiferromagnetic (AFM) structure with the helix axis aligned along the tetragonal c axis and the Eu ordered moments aligned ferromagnetically within the ab plane as previously reported from neutron diffraction measurements [T. Jin et al., Phys. Rev. B 99, 014425 (2019)]. Here we study the crystallographic, magnetic, thermal, and electronic transport properties of Bi flux grown single crystals using single-crystal x-ray diffraction, anisotropic magnetic susceptibility chi, isothermal magnetization M, heat capacity C-p, and electrical resistivity rho measurements versus applied magnetic field H and temperature T. Vacancies are found on the Ni sites corresponding to the composition EuNi1.95(1)As2. A good fit of the rho(T) data by the Bloch-Grijneisen theory for metals was obtained. The chi(ab )(T) data below T-N are fitted well by molecular field theory (MFT), and the helix turn angle kd and the Eu-Eu Heisenberg exchange constants are extracted from the fit parameters. The kd value is in good agreement with the neutron-diffraction result. The magnetic contribution to the zero-field heat capacity below T-N is also fitted by MFT. The isothermal in-plane magnetization M-ab exhibits two metamagnetic transitions versus H, whereas M-c(T = 2 K) is nearly linear up to H = 14 T, both behaviors being consistent with MFT. The M-c(H, T), rho(H-c, T ), and C-p(H-c, T ) data yielded aH(c)-T phase diagram separating the AFM and paramagnetic phases in good agreement with MFT. Anisotropic chi(T) literature data for the ThCr2Si2-type helical antiferromagnet EuRh2As2 were also fitted well by MFT. A comparison is made between the crystallographic and magnetic properties of ThCr2Si2-type EuM(2)Pn(2) compounds with M = Fe, Co, Ni, Cu, or Rh, and Pn = P or As, where only ferromagnetic and c-axis helical AFM structures are found.

  • 87. Sangeetha, N. S.
    et al.
    Wang, L-L
    Smirnov, A.
    Smetana, Volodymyr
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mudring, Anja-Verena
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Johnson, D. D.
    Tanatar, M. A.
    Prozorov, R.
    Johnston, D. C.
    Non-Fermi-liquid types of behavior associated with a magnetic quantum critical point in Sr(Co1-xNix)(2)As-2 single crystals2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 9, article id 094447Article in journal (Refereed)
    Abstract [en]

    The compound SrCo2As2 with the body-centered tetragonal ThCr2Si2 structure is known to remain paramagnetic down to a temperature T = 0.05 K, but inelastic neutron scattering studies have shown that both ferromagnetic (FM) and antiferromagnetic (AFM) fluctuations occur in single crystals. Thus it is of interest to study how the magnetism evolves on doping SrCo2As2. Previous work on polycrystalline samples of Sr(Co1-xNix)(2)As-2 indicated the development of AFM order for 0 < x less than or similar to 0.3. Here we studied single crystals of Sr(Co1-xNix)(2)As-2 for 0 < x <= 1 and confirmed the occurrence of AFM order which we deduce to have a c-axis helix structure. We also find evidence for an unusual composition-induced magnetic quantum critical point at x approximate to 0.3 where non-Fermi-liquid types of behavior were revealed by heat capacity and electrical resisitivity measurements at low T. Electron-doped Sr (Co1-xNix)(2)As-2 single crystals with compositions x = 0 to 0.9 were grown out of self-flux and SrNi2As2 single crystals out of Bi flux. The crystals were characterized using single-crystal x-ray diffraction (XRD) at room temperature, and magnetic susceptibility chi (H, T), isothermal magnetization M(H, T), heat capacity C-p (H, T), and electrical resistivity rho(H, T) measurements versus applied magnetic field H and T. The XRD studies show that the system undergoes a continuous structural crossover from the uncollapsed-tetragonal (ucT) structure to the collapsed tetragonal (cT) structure with increasing Ni doping. The chi (T) data show that SrCo2As2 exhibits an AFM ground state almost immediately upon Ni doping on the Co site. Ab initio electronic-structure calculations for x = 0 and 0.15 indicate that a flat band with a peak in the density of states just above the Fermi energy is responsible for this initial magnetic-ordering behavior on Ni doping. The AFM ordering is observed in the range 0.013 <= x <= 0.25 with the ordered moments aligned in the ab plane and with a maximum ordering temperature T-N = 26.5 K at x = 0.10. The Curie-Weiss-like T dependence of chi in the paramagnetic (PM) state indicates dominant FM interactions. The behavior of the anisotropic susceptibilities below T-N suggests a planar helical magnetic ground state with a composition-dependent pitch based on a local-moment molecular-field-theory model, with FM interactions in the ab plane and weaker AFM interactions along the helix c axis. However, the small ordered (saturation) moments similar to 0.1 mu(B) per transition metal atom, where mu(B) is the Bohr magneton, and the values of the Rhodes-Wohlfarth ratio indicate that the magnetism is itinerant. The high-field M(H) isotherms and the low-field chi(-1) (T > T-N) data were successfully analyzed within the framework of Takahashi's theory of FM spin fluctuations. The C-p (T) at low T exhibits Fermi-liquid behavior for 0 <= x <= 0.15, whereas an evolution to a logarithmic non-Fermi-liquid (NFL) behavior is found for x = 0.2 to 0.3. The logarithmic dependence is suppressed in an applied magnetic field. The low-T rho(H = 0, T) data show a T-2 dependence for 0 <= x <= 0.20 and a power-law dependence rho(H = 0, T) = rho(0) + AT(n) with n < 2 for x = 0.20 and 0.30. The exponent n shows a notable field dependence, suggesting both doping- and magnetic-field-tuned quantum critical phenomena. These low-T NFL types of behavior observed in the C-p and rho measurements are most evident near the quantum critical concentration x approximate to 0.3 at which a T = 0 composition-induced transition from the AFM phase to the PM phase occurs.

  • 88. Sbierski, Bjoern
    et al.
    Trescher, Maximilian
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics. Freie Universität Berlin, Germany.
    Brouwer, Piet W.
    Disordered doubleWeyl node: Comparison of transport and density of states calculations2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 11, article id 115104Article in journal (Refereed)
    Abstract [en]

    Double Weyl nodes are topologically protected band crossing points which carry chiral charge +/- 2. They are stabilized by C-4 point-group symmetry and are predicted to occur in SrSi2 or HgCr2Se4. We study their stability and physical properties in the presence of a disorder potential. We investigate the density of states and the quantum transport properties at the nodal point. We find that, in contrast to their counterparts with unit chiral charge, double Weyl nodes are unstable to any finite amount of disorder and give rise to a diffusive phase, in agreement with the predictions of Goswami and Nevidomskyy [Phys. Rev. B 92, 214504 (2015)] and Bera, Sau, and Roy [Phys. Rev. B 93, 201302 (2016)]. However, for finite system sizes a crossover between pseudodiffusive and diffusive quantum transport can be observed.

  • 89.
    Sen, K.
    et al.
    University of Fribourg, Switzerland.
    Marsik, P.
    University of Fribourg, Switzerland.
    Das, S.
    University of Fribourg, Switzerland.
    Perret, E.
    University of Fribourg, Switzerland.
    de Andrés Prada, Roberto
    University of Fribourg, Switzerland.
    Alberca, A.
    University of Fribourg, Switzerland; Paul Scherrer Institut, Switzerland.
    Biskup, N.
    Universidad Complutense, Spain.
    Varela, M.
    Universidad Complutense, Spain.
    Bernhard, C.
    University of Fribourg, Switzerland.
    Superconductivity and charge-carrier localization in ultrathin La1.85Sr0.15CuO4/La2CuO4 bilayers2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 21, article id 214506Article in journal (Refereed)
    Abstract [en]

    La1.85Sr0.15CuO4/La2CuO4 (LSCO15/LCO) bilayers with a precisely controlled thickness of N unit cells (UCs) of the former and M UCs of the latter ([LSCO15_N/LCO_M]) were grown on (001)-oriented SrLaAlO4 (SLAO) substrates with pulsed laser deposition (PLD). X-ray diffraction and reciprocal space map (RSM) studies confirmed the epitaxial growth of the bilayers and showed that a [LSCO15_2/LCO_2] bilayer is fully strained, whereas a [LSCO15_2/LCO_7] bilayer is already partially relaxed. The in situ monitoring of the growth with reflection high energy electron diffraction (RHEED) revealed that the gas environment during deposition has a surprisingly strong effect on the growth mode and thus on the amount of disorder in the first UC of LSCO15 (or the first two monolayers of LSCO15 containing one CuO2 plane each). For samples grown in pure N2O gas (growth type B), the first LSCO15 UC next to the SLAO substrate is strongly disordered. This disorder is strongly reduced if the growth is performed in a mixture of N2O and O2 gas (growth type A). Electric transport measurements confirmed that the first UC of LSCO15 next to the SLAO substrate is highly resistive and shows no sign of superconductivity for growth type B, whereas it is superconducting for growth type A. Furthermore, we found, rather surprisingly, that the conductivity of the LSCO15 UC next to the LCO capping layer strongly depends on the thickness of the latter. A LCO capping layer with 7 UCs leads to a strong localization of the charge carriers in the adjacent LSCO15 UC and suppresses superconductivity. The magnetotransport data suggest a similarity with the case of weakly hole doped LSCO single crystals that are in a so-called ‘cluster-spin-glass state.’ We discussed several mechanisms that could lead to such a localization of holes that are embedded in a short-range ordered antiferromagnetic background for the case of a thick LCO capping layer with M=7 but not for a thin one with M=2.

  • 90. Shahnazaryan, V.
    et al.
    Kyriienko, Oleksandr
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). ITMO University, Russia; University of Exeter, United Kingdom.
    Rostami, Habib
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Exciton routing in the heterostructure of a transition metal dichalcogenide monolayer on a paraelectric substrate2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 16, article id 165303Article in journal (Refereed)
    Abstract [en]

    We propose a scheme for the spatial exciton energy control and exciton routing in a transition-metal dichalcogenide (TMD) monolayer which lies on a quantum paraelectric substrate. It relies on the ultrasensitive response of the substrate dielectric permittivity to temperature changes, allowing for spatially inhomogeneous screening of Coulomb interaction in a monolayer. As an example, we consider the heterostructure of TMD and strontium titanate oxide SrTiO3, where large dielectric screening can be attained. We study the impact of substrate temperature on the characteristic electronic features of TMD monolayers such as the particle band gap and exciton binding energy, Bohr radius, and nonlinearity (an exciton-exciton interaction). The combination of particle band gap and exciton binding energy modulation results in the shift of the exciton resonance energy. Applying local heating, we create spatial patterns with varying exciton resonant energy and an exciton flow toward the energetically lower region of the sample.

  • 91. Shiranzaei, Mahroo
    et al.
    Fransson, Jonas
    Cheraghchi, Hosein
    Parhizgar, Fariborz
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Uppsala University, Sweden; Institute for Research in Fundamental Sciences (IPM), Iran.
    Nonlinear spin susceptibility in topological insulators2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 18, article id 180402Article in journal (Refereed)
    Abstract [en]

    We revise the theory of the indirect exchange interaction between magnetic impurities beyond the linear response theory to establish the effect of impurity resonances in the surface states of a three-dimensional topological insulator. The interaction is composed of isotropic Heisenberg, anisotropic Ising, and Dzyaloshinskii-Moriya types of couplings. We find that all three contributions are finite at the Dirac point, which is in stark contrast to the linear response theory which predicts a vanishing Dzyaloshinskii-Moriya-type contribution. We show that the spin-independent component of the impurity scattering can generate large values of the Dzyaloshinskii-Moriya-type coupling in comparison with the Heisenberg and Ising types of couplings, while these latter contributions drastically reduce in magnitude and undergo sign changes. As a result, both collinear and noncollinear configurations are allowed magnetic configurations of the impurities.

  • 92. Slager, Robert-Jan
    et al.
    Juričić, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Roy, Bitan
    Dissolution of topological Fermi arcs in a dirty Weyl semimetal2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 20, article id 201401Article in journal (Refereed)
    Abstract [en]

    Weyl semimetals (WSMs) have recently attracted a great deal of attention as they provide a condensed matter realization of chiral anomaly, feature topologically protected Fermi arc surface states, and sustain sharp chiral Weyl quasiparticles up to a critical disorder at which a continuous quantum phase transition (QPT) drives the system into a metallic phase. We here numerically demonstrate that with increasing strength of disorder, the Fermi arc gradually loses its sharpness, and close to the WSM-metal QPT it completely dissolves into the metallic bath of the bulk. The predicted topological nature of the WSM-metal QPT and the resulting bulk-boundary correspondence across this transition can be directly observed in angle-resolved photoemission spectroscopy (ARPES) and Fourier transformed scanning tunneling microscopy (STM) measurements by following the continuous deformation of the Fermi arcs with increasing disorder in recently discovered Weyl materials.

  • 93. Smylie, M. P.
    et al.
    Willa, K.
    Bao, J.-K.
    Ryan, K.
    Islam, Z.
    Claus, H.
    Simsek, Y.
    Diao, Zhu
    Stockholm University, Faculty of Science, Department of Physics. Halmstad University, Sweden.
    Rydh, Andreas
    Stockholm University, Faculty of Science, Department of Physics.
    Koshelev, A. E.
    Kwok, W.-K.
    Chung, D. Y.
    Kanatzidis, M. G.
    Welp, U.
    Anisotropic superconductivity and magnetism in single-crystal RbEuFe4As42018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 10, article id 104503Article in journal (Refereed)
    Abstract [en]

    We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe(4)As(4 )using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu moments of T-m = 15 K and a superconducting transition temperature of T-c = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of Gamma = 1.7. Ginzburg-Landau parameters of K-c similar to 67 and K-ab similar to 108 indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu magnetism is quasi-two dimensional (2D), as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and <0.04 K. A consequence of the quasi-2D nature of the Eu magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a kinklike anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 and 4 kG.

  • 94.
    Spånslätt, Christian
    Stockholm University, Faculty of Science, Department of Physics.
    Geometric Josephson effects in chiral topological nanowires2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 5, article id 054508Article in journal (Refereed)
    Abstract [en]

    One of the salient signatures of Majorana zero modes and topological superconductivity is a 4 pi-periodic Josephson effect due to the combination of fermion parity conservation and the presence of a topologically protected odd number of zero-energy crossings in the Andreev spectrum. In this paper, we study this effect in Josephson junctions composed of two semiconducting nanowires with Rashba spin-orbit coupling and induced superconductivity from the proximity effect. For certain orientations of the externalmagnetic field, such junctions possess a chiral symmetry, and we show how this symmetry allows the Andreev spectrum and the protected crossings to be shifted by introducing a relative angle between the two wires. The junction then displays a geometrically induced anomalous Josephson effect, the flow of a supercurrent in the absence of external phase bias. Furthermore, we derive a proportionality relation between the local current density and the local curvature for a single curvedwire. This result can be viewed as a one-dimensional analog of the recently proposed geo-Josephson effect [T. Kvorning et al., Phys. Rev. Lett. 120, 217002 (2018)]. Our two proposed effects can, in principle, be used as signatures of topological superconductivity in one dimension.

  • 95.
    Sukhachov, P. O.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Juričić, Vladimir
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Connecticut, USA.
    Odd-frequency Berezinskii superconductivity in Dirac semimetals2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 18, article id 180502Article in journal (Refereed)
    Abstract [en]

    We formulate a general framework for addressing both odd- and even-frequency superconductivity in Dirac semimetals and demonstrate that the odd-frequency or the Berezinskii pairing can naturally appear in these materials because of the chirality degree of freedom. We show that repulsive frequency-dependent interactions favor the Berezinskii pairing while an attractive electron-electron interaction allows for the BCS pairing. In the case of Dirac and Weyl semimetals at the charge neutrality point, both the conventional BCS and odd-frequency Berezinskii pairings require critical coupling. Since these pairings could originate from physically different mechanisms, our findings pave the way for controlling the realization of the Berezinskii superconductivity in topological semimetals. We also present the density of states with several cusplike features that can serve as an experimentally verifiable signature of the odd-frequency gap.

  • 96.
    Sukhachov, Pavlo O.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Balatsky, Alexander
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Connecticut, USA.
    Spectroscopic and optical response of odd-frequency superconductors2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 13, article id 134516Article in journal (Refereed)
    Abstract [en]

    The optical response of superconductors with odd-frequency Berezinskii pairing is studied. By using a simple model with a parabolic dispersion law and a nonmagnetic disorder, the spectral function, the electron density of states, and the optical conductivity are calculated for a few gap Ansatze. The spectral function and the electron density of states clearly reveal the gap for the Berezinskii pairing for the sufficiently strong frequency dependence of the order parameters. It is found that, similarly to the conventional BCS pairing, the odd-frequency gaps induce peaks in the real part of the conductivity, which, however, are sharper than in the BCS case. The magnitude and position of these peaks are determined by the frequency profile of the gap. The imaginary part of the optical conductivity for the Berezinskii pairing demonstrates sharp cusps that are absent in the case of the BCS superconductors. The corresponding results suggest that the Berezinskii pairing might allow for the optical transparency windows related to the onsets of the attenuation peaks in the real part of the conductivity. Thus, the study of the optical response not only provides an alternative way to probe the odd-frequency gaps, but can also reveal additional features of the dynamic superconducting pairing.

  • 97. Trescher, Maximilian
    et al.
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Knolle, Johannes
    Quantum oscillations and magnetoresistance in type-II Weyl semimetals: Effect of a field-induced charge density wave2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 12, article id 125304Article in journal (Refereed)
    Abstract [en]

    Recent experiments on type-II Weyl semimetals such as WTe2 , MoTe2 , MoxW1-xTe2 , and WP2 reveal remarkable transport properties in the presence of a strong magnetic field, including an extremely large magnetoresistance and an unusual temperature dependence. Here, we investigate magnetotransport via the Kubo formula in a minimal model of a type-II Weyl semimetal taking into account the effect of a charge density wave (CDW) transition, which can arise even at weak coupling in the presence of a strong magnetic field because of the special Landau level dispersion of type-II Weyl systems. Consistent with experimental measurements we find an extremely large magnetoresistance with close to B-2 scaling at particle-hole compensation, while in the extreme quantum limit there is a transition to a qualitatively new scaling with approximately B-0.75 . We also investigate the Shubnikov-de Haas effect and find that the amplitude of the resistivity quantum oscillations are greatly enhanced below the CDW transition temperature which is accompanied by an unusual nonmonotonous (non-Lifshitz-Kosevich) temperature dependence.

  • 98. Trescher, Maximilian
    et al.
    Bergholtz, Emil J.
    Stockholm University, Faculty of Science, Department of Physics.
    Udagawa, Masafumi
    Knolle, Johannes
    Charge density wave instabilities of type-II Weyl semimetals in a strong magnetic field2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 20, article id 201101Article in journal (Refereed)
    Abstract [en]

    Shortly after the discovery of Weyl semimetals, properties related to the topology of their bulk band structure have been observed, e.g., signatures of the chiral anomaly and Fermi arc surface states. These essentially single particle phenomena are well understood, but whether interesting many-body effects due to interactions arise in Weyl systems remains much less explored. Here, we investigate the effect of interactions in a microscopic model of a type-II Weyl semimetal in a strong magnetic field. We identify a charge density wave (CDW) instability even for weak interactions stemming from the emergent nesting properties of the type-II Weyl Landau level dispersion. We map out the dependence of this CDW on magnetic field strength. Remarkably, as a function of decreasing temperature, a cascade of CDW transitions emerges and we predict characteristic signatures for experiments.

  • 99. Trescher, Maximilian
    et al.
    Sbierski, Björn
    Brouwer, Piet W.
    J. Bergholtz, Emil
    Stockholm University, Faculty of Science, Department of Physics. Freie Universität Berlin, Germany.
    Tilted disordered Weyl semimetals2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 4, p. 1-8, article id 045139Article in journal (Refereed)
    Abstract [en]

    Although Lorentz invariance forbids the presence of a term that tilts the energy-momentum relation in the Weyl Hamiltonian, a tilted dispersion is not forbidden and, in fact, generic for condensed matter realizations of Weyl semimetals. We here investigate the combined effect of such a tilted Weyl dispersion and the presence of potential disorder. In particular, we address the influence of a tilt on the disorder-induced phase transition between a quasiballistic phase at weak disorder, in which the disorder is an irrelevant perturbation, and a diffusive phase at strong disorder. Our main result is that the presence of a tilt leads to a reduction of the critical disorder strength for this transition or, equivalently, that increasing the tilt at fixed disorder strength drives the system through the phase transition to the diffusive strong-disorder phase. Notably this obscures the tilt-induced Lifshitz transition to an overtilted type II Weyl phase at any finite disorder strength. Our results are supported by analytical calculations using the self-consistent Born approximation and numerical calculations of the density of states and of transport properties.

  • 100. Triola, Christopher
    et al.
    Balatsky, Alexander V.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Center for Quantum Materials (CQM), Sweden; Los Alamos National Laboratory, USA.
    Odd-frequency superconductivity in driven systems2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 9, article id 094518Article in journal (Refereed)
    Abstract [en]

    We show that Berezinskii's classification of the symmetries of Cooper pair amplitudes holds for driven systems even in the absence of translation invariance. We then consider a model Hamiltonian for a superconductor coupled to an external driving potential and, treating the drive as a perturbation, we investigate the corrections to the anomalous Green's function, density of states, and spectral function. We find that in the presence of an external drive the anomalous Green's function develops terms that are odd in frequency and that the same mechanism responsible for these odd-frequency terms generates additional features in the density of states and spectral function.

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