Publications (129)364.22 Total impact

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ABSTRACT: The dynamic spin susceptibility (DSS) has a ubiquitous Lorentzian form in conventional materials with weak spin orbit coupling, whose spectral width characterizes the spin relaxation rate. We show that DSS has an unusual nonLorentzian form in topological insulators, which are characterized by strong SOC. At zero temperature, the high frequency part of DSS is universal and increases in certain directions as $\omega^{d1}$ with $d=2$ and 3 for surface states and Weyl semimetals, respectively, while for helical edge states, the interactions renormalize the exponent as $d=2K1$ with $K$ the Luttingerliquid parameter. As a result, spin relaxation rate cannot be deduced from the DSS in contrast to the case of usual metals, which follows from the strongly entangled spin and charge degrees of freedom in these systems. These parallel with the optical conductivity of neutral graphene. 
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ABSTRACT: We investigate the Loschmidt echo, the overlap of the initial and final wavefunctions of Luttinger liquids after a spatially inhomogeneous interaction quench. In studying the Luttinger model, we obtain an analytic solution of the bosonic Bogoliubovde Gennes equations after quenching the interactions within a finite spatial region. As opposed to the power law temporal decay following a potential quench, the interaction quench in the Luttinger model leads to a finite, hardly time dependent overlap, therefore no orthogonality catastrophe occurs. The steady state value of the Loschmidt echo after a sudden inhomogeneous quench is the square of the respective adiabatic overlaps. Our results are checked and validated numerically on the XXZ Heisenberg chain. 
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ABSTRACT: Dynamical phase transitions (DPT) are characterized by nonanalytical time evolution of the dynamical free energy. For general 2band systems in one and two dimensions (eg. SSH model, Kitaevchain, Haldane model, p+ip superconductor, etc.), we show that the time evolution of the dynamical free energy is crucially affected by the ground state topology of both the initial and final Hamiltonians, implying DPTs when the topology is changed under the quench. Similarly to edge states in topological insulators, DPTs can be classified as being topologically protected or not. In 1D systems the number of topologically protected nonequilibrium time scales are determined by the difference between the initial and final winding numbers, while in 2D no such relation exists for the Chern numbers. The singularities of dynamical free energy in the 2D case are qualitatively different from those of the 1D case, the cusps appear only in the first time derivative. 
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ABSTRACT: Topological excitations keep fascinating physicists since many decades. While individual vortices and solitons emerge and have been observed in many areas of physics, their most intriguing higher dimensional topological relatives, skyrmions (smooth, topologically stable textures) and magnetic monopoles  emerging almost necessarily in any grand unified theory and responsible for charge quantization  remained mostly elusive. Here we propose that loading a threecomponent nematic superfluid such as $^{23}$Na into a deep optical lattice and thereby creating an insulating core, one can create topologically stable skyrmion textures and investigate their properties in detail. We show furthermore that the spectrum of the excitations of the superfluid and their quantum numbers change dramatically in the presence of the skyrmion, and they reflect the presence of a trapped monopole, as imposed by the skyrmion's topology.Scientific Reports 03/2014; 5. DOI:10.1038/srep07692 · 5.08 Impact Factor 
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ABSTRACT: A (single flavor) quadratic band crossing in two dimensions is known to have a generic instability towards a quantum anomalous Hall (QAH) ground state for infinitesimal repulsive interactions. Here we introduce a generalization of a quadratic band crossing which is protected only by rotational symmetry. By focusing on the representative case of a parabolic and flat band touching, which also allows for a straightforward lattice realization, the interaction induced nematic phase is found generally to compete successfully with the QAH insulator, and to become the dominant instability in certain parts of the phase diagram already at weak coupling. The full phase diagram of the model, together with its topological properties, is mapped out using a perturbative renormalization group, strong coupling analysis, the meanfield theory. Interestingly, the Berry flux varies continuously in the single flavour limit with various control parameters.Physical Review B 02/2014; 90(4). DOI:10.1103/PhysRevB.90.045310 · 3.66 Impact Factor 
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ABSTRACT: Dynamical phase transitions (DPT) occur after quenching some global parameters in quantum systems and are signalled by the nonanalytical time evolution of the dynamical free energy, which is calculated from the Loschmidt overlap between the initial and time evolved states. In a recent letter (M. Heyl et al., Phys. Rev. Lett. \textbf{110}, 135704 (2013)), it was suggested that DPTs are closely related to equilibrium phase transitions (EPT) for the transverse field Ising model. By studying a minimal model, the XY chain in transverse magnetic field, we show analytically that this connection does not hold generally. We present examples where DPT occurs without crossing any equilibrium critical lines by the quench, and a nontrivial example with no DPT but crossing a critical line by the quench. Albeit the nonanalyticities of the dynamical free energy on the real time axis do not indicate the presence or absence of an EPT, the structure of Fisherlines for complex times reveal a qualitative difference.Physical Review B 01/2014; 89(16). DOI:10.1103/PhysRevB.89.161105 · 3.66 Impact Factor 
Article: A unified theory of spinrelaxation due to spinorbit coupling in metals and semiconductors.
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ABSTRACT: Spintronics is an emerging paradigm with the aim to replace conventional electronics by using electron spins as information carriers. Its utility relies on the magnitude of the spinrelaxation, which is dominated by spinorbit coupling (SOC). Yet, SOC induced spinrelaxation in metals and semiconductors is discussed for the seemingly orthogonal cases when inversion symmetry is retained or broken by the socalled ElliottYafet and D'yakonovPerel' spinrelaxation mechanisms, respectively. We unify the two theories on general grounds for a generic twoband system containing intra and interband SOC. While the previously known limiting cases are recovered, we also identify parameter domains when a crossover occurs between them, i.e. when an inversion symmetry broken state evolves from a D'yakonovPerel' to an ElliottYafet type of spinrelaxation and conversely for a state with inversional symmetry. This provides an ultimate link between the two mechanisms of spinrelaxation.Scientific Reports 11/2013; 3:3233. DOI:10.1038/srep03233 · 5.08 Impact Factor 
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ABSTRACT: We study the nonequilibrium dynamics of the Luttinger model after a quantum quench, when the initial state is a finite temperature thermal equilibrium state. The diagonal elements of the density matrix in the steady state show thermal features for high temperature initial states only, otherwise retain highly nonthermal character. The time evolution of Uhlmann fidelity, which measures the distance between the time evolved and initial states, is evaluated for arbitrary initial temperatures and quench protocols. In the long time limit, the overlap between the time evolved and initial system decreases exponentially with the temperature with a universal prefactor. Within perturbation theory, the statistics of final total energy and work are numerically evaluated in the case of a sudden quench, which yield identical distributions at zero temperature. In both statistics, temperature effects are more significant in small systems. The Diracdelta peak at the adiabatic ground state energy remains present in the probability distribution of the total energy, but disappears from the work distribution at nonzero initial temperatures.Physical Review B 07/2013; 88(15). DOI:10.1103/PhysRevB.88.155115 · 3.66 Impact Factor 
Article: Loschmidt Echo and the ManyBody Orthogonality Catastrophe in a QubitCoupled Luttinger Liquid
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ABSTRACT: We investigate the manybody generalization of the orthogonality catastrophe by studying the generalized Loschmidt echo of Luttinger liquids (LLs) after a global change of interaction. It decays exponentially with system size and exhibits universal behavior: the steady state exponent after quenching back and forth n times between 2 LLs (bangbang protocol) is 2n times bigger than that of the adiabatic overlap and depends only on the initial and final LL parameters. These are corroborated numerically by matrixproduct state based methods of the XXZ Heisenberg model. An experimental setup consisting of a hybrid system containing cold atoms and a flux qubit coupled to a Feshbach resonance is proposed to measure the Loschmidt echo using rf spectroscopy or Ramsey interferometry.Physical Review Letters 07/2013; 111(4):046402. DOI:10.1103/PhysRevLett.111.046402 · 7.73 Impact Factor 
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ABSTRACT: The persistent current in strictly onedimensional Dirac systems is investigated within two different models, defined in the continuum and on a lattice, respectively. The object of the study is the effect of a single magnetic or nonmagnetic impurity in the two systems. In the continuum Dirac model, an analytical expression for the persistent current flowing along a ring with a single deltalike magnetic impurity is obtained after regularization of the unbounded negative energy states. The predicted decay of the persistent agrees with the lattice simulations. The results are generalized to finite temperatures. To realize a single Dirac massless fermion, the lattice model breaks the timereversal symmetry, and, in contrast with the continuum model, a pointlike nonmagnetic impurity can lead to a decay in the persistent current.Physical Review B 07/2013; 88(20). DOI:10.1103/PhysRevB.88.205401 · 3.66 Impact Factor 
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ABSTRACT: The manipulation and movement of Dirac points in the Brillouin zone by the electronelectron interaction is considered within leading order perturbation theory. At the merging point, an infinitesimal interaction is shown to cause opening of the gap or splitting of the Dirac points, depending on the inter or intrasublattice nature of the merging and the sign of the interaction. The topology of the spectrum can therefore be efficiently changed by simply tuning the interaction between particles, as opposed to the usual careful band structure engineering. This is illustrated around the merging transition of one, two, and three dimensional DiracWeyl fermions. A simple Weyllike Hamiltonian that describes the quadratic bandcrossing in three dimensions is also proposed, and its stability under interactions is addressed.Physical review. B, Condensed matter 05/2013; 88(7). DOI:10.1103/PhysRevB.88.075126 · 3.66 Impact Factor 
Article: Diverging dc conductivity due to a flat band in disordered pseudospin1 DiracWeyl fermions
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ABSTRACT: Several lattices, such as the dice or the Lieb lattice, possess Dirac cones and a flat band crossing the Dirac point, whose effective model is the pseudospin1 DiracWeyl equation. We investigate the fate of the flat band in the presence of disorder by focusing on the density of states (DOS) and dc conductivity. While the central hubsite does not reveal the presence of the flat band, the sublattice resolved DOS on the noncentral sites exhibits a narrow peak with height ~ 1/\sqrt{g} with g the dimensionless disorder variance. Although the group velocity is zero on the flat band, the dc conductivity diverges as ln(1/g) with decreasing disorder due to interband transitions around the band touching point between the propagating and the flat band. Generalizations to higher pseudospin are given.Physical Review B 05/2013; 88(16). DOI:10.1103/PhysRevB.88.161413 · 3.66 Impact Factor 
Article: Floquet topological insulators
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ABSTRACT: Topological insulators represent unique phases of matter with insulating bulk and conducting edge or surface states, immune to small perturbations such as backscattering due to disorder. This stems from their peculiar band structure, which provides topological protections. While conventional tools (pressure, doping etc.) to modify the band structure are available, time periodic perturbations can provide tunability by adding time as an extra dimension enhanced to the problem. In this short review, we outline the recent research on topological insulators in non equilibrium situations. Firstly, we introduce briefly the Floquet formalism that allows to describe steady states of the electronic system with an effective timeindependent Hamiltonian. Secondly, we summarize recent theoretical work on how light irradiation drives semimetallic graphene or a trivial semiconducting system into a topological phase. Finally, we show how photons can be used to probe topological edge or surface states.physica status solidi (RRL)  Rapid Research Letters 02/2013; 7(12). DOI:10.1002/pssr.201206451 · 2.39 Impact Factor 
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ABSTRACT: We study variablerate linear quenches in the anisotropic Heisenberg (XXZ) chain, starting at the XX point. This is equivalent to switching on a nearest neighbour interaction for hardcore bosons or an interaction quench for free fermions. The physical observables we investigate are: the energy pumped into the system during the quench, the spinflip correlation function, and the bipartite fluctuations of the z component of the spin in a box. We find excellent agreement between exact numerics (infinite system timeevolving block decimation, iTEBD) and analytical results from bosonization, as a function of the quench time, spatial coordinate and interaction strength. This provides a stringent and muchneeded test of Luttinger liquid theory in a nonequilibrium situation.Physical review. B, Condensed matter 11/2012; 87(4). DOI:10.1103/PhysRevB.87.041109 · 3.66 Impact Factor 
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ABSTRACT: We present a unified treatment of the ElliottYafet (EY) and the D'yakonovPerel' (DP) spinrelaxation mechanisms using the MoriKawasaki formula, which gives the spinrelaxation rate to lowest order in the spinorbit coupling (SOC) but to infinite order in the quasiparticle scattering rate, Gamma. We consider a fourstate Hamiltonian of the conduction and a nearby (valence) band with spin degeneracy, including SOC between adjacent bands (interSOC) and within the same band (intraSOC). We find in agreement with the expectations that intraSOC yields the DP whereas the interSOC the EYlike result. However, we identify parameter domains of Gamma and the band structure where a crossover occurs between the two types of spinrelaxation mechanisms. The result ultimately connects the EY and the DP spinrelaxation mechanisms into a unified description and it leads to a better understanding of spinrelaxation in strongly correlated systems and where band degeneracy plays a role such as e.g. in graphene. 
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ABSTRACT: The subtle interplay of randomness and quantum fluctuations at low temperatures gives rise to a plethora of unconventional phenomena in systems ranging from quantum magnets and correlated electron materials to ultracold atomic gases. Particularly strong disorder effects have been predicted to occur at zerotemperature quantum phase transitions. Here, we demonstrate that the compositiondriven ferromagnetictoparamagnetic quantum phase transition in Sr(1x)Ca(x)RuO3 is completely destroyed by the disorder introduced via the different ionic radii of the randomly distributed Sr and Ca ions. Using a magnetooptical technique, we map the magnetic phase diagram in the compositiontemperature space. We find that the ferromagnetic phase is significantly extended by the disorder and develops a pronounced tail over a broad range of the composition x. These findings are explained by a microscopic model of smeared quantum phase transitions in itinerant magnets. Moreover, our theoretical study implies that correlated disorder is even more powerful in promoting ferromagnetism than random disorder.Physical Review Letters 05/2012; 108(18):185701. DOI:10.1103/PhysRevLett.108.185701 · 7.73 Impact Factor 
Article: Testing the ElliottYafet spinrelaxation mechanism in KC8; a model system of biased graphene
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ABSTRACT: Temperature dependent electron spin resonance (ESR) measurements are reported on stage 1 potassium doped graphite, a model system of biased graphene. The ESR linewidth is nearly isotropic and although the gfactor has a sizeable anisotropy, its majority is shown to arise due to macroscopic magnetization. Albeit the homogeneous ESR linewidth shows an unusual, nonlinear temperature dependence, it appears to be proportional to the resistivity which is a quadratic function of the temperature. These observations suggests the validity of the ElliottYafet relaxation mechanism in KC8 and allows to place KC8 on the empirical BeuneuMonod plot among ordinary elemental metals.Physical review. B, Condensed matter 03/2012; 85(23). DOI:10.1103/PhysRevB.85.235405 · 3.66 Impact Factor 
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ABSTRACT: We analyze the probability distribution function (PDF) of work done on a Luttinger liquid for an arbitrary finite duration interaction quench and show that it can be described in terms a generalized Gibbs ensemble. We construct the corresponding density matrix with explicit intermode correlations, and determine the duration and interaction dependence of the probability of an adiabatic transition and the PDF of nonadiabatic processes. In the thermodynamic limit, the PDF of work exhibits a nonGaussian maximum around the excess heat, carrying almost all spectral weight. In contrast, in the small system limit most spectral weight is carried by a delta peak at the energy of the adiabatic process, and an oscillating PDF with dips at energies commensurate to the quench duration and with an exponential envelope develops. Relevance to cold atom experiments is also discussed.Physical review. B, Condensed matter 03/2012; 86(16). DOI:10.1103/PhysRevB.86.161109 · 3.66 Impact Factor 
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ABSTRACT: Timeperiodic perturbations can be used to engineer topological properties of matter by altering the Floquet band structure. This is demonstrated for the helical edge state of a spin Hall insulator in the presence of monochromatic circularly polarized light. The inherent spin structure of the edge state is influenced by the Zeeman coupling and not by the orbital effect. The photocurrent (and the magnetization along the edge) develops a finite, helicitydependent expectation value and turns from dissipationless to dissipative with increasing radiation frequency, signalling a change in the topological properties. The connection with Thouless' charge pumping and nonequilibrium zitterbewegung is discussed, together with possible experiments.Physical Review Letters 02/2012; 108(5):056602. DOI:10.1103/PhysRevLett.108.056602 · 7.73 Impact Factor 
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ABSTRACT: Timeperiodic perturbations can be used to engineer topological properties of matter by altering the Floquet band structure. This is demonstrated for the helical edge state of a spin Hall insulator in the presence of monochromatic circularly polarized light. We first demonstrate that the inherent spin structure of the edge state is influenced by the Zeeman coupling and not by the orbital effect. The photocurrent (and the magnetization along the edge) develops a finite, helicity dependent expectation value and turns from dissipationless to dissipative with increasing radiation frequency, signalling a change in the topological properties. The connection with Thouless' charge pumping and nonequilibrium Zitterbewegung is discussed, together with possible experiments. B. Dora, J. Cayssol, F. Simon, and R. Moessner, Optically engineering the topological properties of a spin Hall insulator, arXiv:1105.5963
Publication Stats
970  Citations  
364.22  Total Impact Points  
Top Journals
Institutions

2000–2015

Budapest University of Technology and Economics
 Department of Physics
Budapeŝto, Budapest, Hungary


2007–2012

Hungarian Academy of Sciences
 Institute for Solid State Physics and Optics
Budapeŝto, Budapest, Hungary


2011

University of Vienna
 Faculty of Physics
Vienna, Vienna, Austria


2009

Tokyo Institute of Technology
 Department of Physics
Tokyo, Tokyoto, Japan


2008–2009

Max Planck Institute of Physics
München, Bavaria, Germany


2006–2009

Max Planck Institute for Dynamics of Complex Technical Systems
Magdeburg, SaxonyAnhalt, Germany 
Hallym University
Sŏul, Seoul, South Korea


2004

Abdus Salam International Centre for Theoretical Physics
Trst, Friuli Venezia Giulia, Italy


2003

Max Planck Institute for Chemical Physics of Solids
Dresden, Saxony, Germany
