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  • 1. Atteia, J.
    et al.
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics.
    Cayssol, J.
    Ballistic transport through irradiated graphene2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 245404Article in journal (Refereed)
    Abstract [en]

    The coherent charge transport through an illuminated graphene ribbon is studied as a function of electronic doping and characteristics of the electromagnetic driving, for monochromatic circularly polarized light. We focus on the DC current carried by 2D bulk carriers which is dominant (over edge transport) for short and wide enough samples. We investigate how the ballistic conductance suppression, due to photon resonances between the valence and conduction bands, evolves when the experimentally tunable parameters are varied. The residual conductance can be associated with evanescent states and related to dynamical gaps in the Floquet quasienergy spectrum.

  • 2. Behrends, Jan
    et al.
    Rhim, Jun-Won
    Liu, Shang
    Grushin, Adolfo G.
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Max-Planck-Institut für Physik Komplexer Systeme, Germany.
    Nodal-line semimetals from Weyl superlattices2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 24, article id 245101Article in journal (Refereed)
    Abstract [en]

    The existence and topological classification of lower-dimensional Fermi surfaces is often tied to the crystal symmetries of the underlying lattice systems. Artificially engineered lattices, such as heterostructures and other superlattices, provide promising avenues to realize desired crystal symmetries that protect lower-dimensional Fermi surfaces, such as nodal lines. In this work, we investigate a Weyl semimetal subjected to spatially periodic onsite potential, giving rise to several phases, including a nodal-line semimetal phase. In contrast to proposals that purely focus on lattice symmetries, the emergence of the nodal line in this setup does not require small spin-orbit coupling, but rather relies on its presence. We show that the stability of the nodal line is understood from reflection symmetry and a combination of a fractional lattice translation and charge-conjugation symmetry. Depending on the choice of parameters, this model exhibits drumhead surface states that are exponentially localized at the surface, or weakly localized surface states that decay into the bulk at all energies.

  • 3.
    Hauschild, Johannes
    et al.
    Tech Univ Munich, Dept Phys, T42,James Franck Str 1, D-85748 Garching, Germany..
    Leviatan, Eyal
    Weizmann Inst Sci, Dept Condensed Matter Phys, IL-7610001 Rehovot, Israel..
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Altman, Ehud
    Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA..
    Zaletel, Michael P.
    Princeton Univ, Dept Phys, Princeton, NJ 08540 USA..
    Pollmann, Frank
    Tech Univ Munich, Dept Phys, T42,James Franck Str 1, D-85748 Garching, Germany..
    Finding purifications with minimal entanglement2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 23, article id 235163Article in journal (Refereed)
    Abstract [en]

    Purification is a tool that allows to represent mixed quantum states as pure states on enlarged Hilbert spaces. A purification of a given state is not unique and its entanglement strongly depends on the particular choice made. Moreover, in one-dimensional systems, the amount of entanglement is linked to how efficiently the purified state can be represented using matrix-product states (MPS). We introduce an MPS based method that allows to find the minimally entangled representation by iteratively minimizing the second Renyi entropy. First, we consider the thermofield double purification and show that its entanglement can be strongly reduced especially at low temperatures. Second, we show that a slowdown of the entanglement growth following a quench of an infinite temperature state is possible.

  • 4.
    Herviou, Loic
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. KTH Royal Inst Technol, Dept Phys, S-10691 Stockholm, Sweden..
    Regnault, Nicolas
    Univ Paris Diderot, Sorbonne Paris Cite, Sorbonne Univ, ENS,Univ PSL,CNRS,Lab Phys Ecole Normale Super, Paris, France..
    Defining a bulk-edge correspondence for non-Hermitian Hamiltonians via singular-value decomposition2019In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 99, no 5, article id 052118Article in journal (Refereed)
    Abstract [en]

    We address the breakdown of the bulk-boundary correspondence observed in non-Hermitian systems, where open and periodic systems can have distinct phase diagrams. The correspondence can be completely restored by considering the Hamiltonian's singular-value decomposition instead of its eigendecomposition. This leads to a natural topological description in terms of a flattened singular decomposition. This description is equivalent to the usual approach for Hermitian systems and coincides with a recent proposal for the classification of non-Hermitian systems. We generalize the notion of the entanglement spectrum to non-Hermitian systems, and show that the edge physics is indeed completely captured by the periodic bulk Hamiltonian. We exemplify our approach by considering the chiral non-Hermitian Su-Schrieffer-Heger and Chern insulator models. Our work advocates a different perspective on topological non-Hermitian Hamiltonians, paving the way to a better understanding of their entanglement structure.

  • 5.
    Herviou, Loic
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Bera, Soumya
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Multiscale entanglement clusters at the many-body localization phase transition2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 13, article id 134205Article in journal (Refereed)
    Abstract [en]

    We numerically study the formation of entanglement clusters across the many-body localization transition. We observe a crossover from strong many-body entanglement in the ergodic phase to weak local correlations in the localized phase, with continuous clusters throughout the phase diagram. Critical states close to the transition have a structure compatible with fractal or multiscale-entangled states, characterized by entanglement at multiple levels: small strongly entangled clusters are weakly entangled together to form larger clusters. The critical point therefore features subthermal entanglement and a power-law distributed cluster size. Upon entering the localized phase, the power-law distribution seems to persist with a varying power that crosses over into a stretched exponent before eventually becoming exponential deep in the localized phase. These results are in agreement with some of the recently proposed phenomenological renormalization-group schemes characterizing the many-body localized critical point, and serve to constrain other such schemes.

  • 6.
    Javanmard, Younes
    et al.
    Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany..
    Trapin, Daniele
    Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany..
    Bera, Soumya
    Indian Inst Technol, Dept Phys, Bombay 400076, Maharashtra, India..
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Heyl, Markus
    Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany..
    Sharp entanglement thresholds in the logarithmic negativity of disjoint blocks in the transverse-field Ising chain2018In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 20, article id 083032Article in journal (Refereed)
    Abstract [en]

    Entanglement has developed into an essential concept for the characterization of phases and phase transitions in ground states of quantum many-body systems. In this work we use the logarithmic negativity to study the spatial entanglement structure in the transverse-field Ising chain both in the ground state and at nonzero temperatures. Specifically, we investigate the entanglement between two disjoint blocks as a function of their separation, which can be viewed as the entanglement analog of a spatial correlation function. We find sharp entanglement thresholds at a critical distance beyond which the logarithmic negativity vanishes exactly and thus the two blocks become unentangled, which holds even in the presence of long-ranged quantum correlations, i.e., at the system's quantum critical point. Using time-evolving block decimation, we explore this feature as a function of temperature and size of the two blocks and present a simple model to describe our numerical observations.

  • 7.
    Kaladzhyan, Vardan
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Quantized Fermi arc mediated transport in Weyl semimetal nanowires2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 8, article id 085424Article in journal (Refereed)
    Abstract [en]

    We study longitudinal transport in Weyl semimetal nanowires, both in the absence and in the presence of a magnetic flux threading the nanowires. We identify two qualitatively different regimes of transport with respect to the chemical potential in the nanowires. In the "surface regime," for low doping, most of the conductance occurs through the Fermi arc surface states, and it rises in steps of one quantum of conductance as a function of the chemical potential; furthermore, with varying flux the conductance changes in steps of one quantum of conductance with characteristic Fabry-Perot interference oscillations. In the "bulk-surface regime," for highly doped samples, the dominant contribution to the conductance is quadratic in the chemical potential, and mostly conditioned by the bulk states; the flux dependence shows clearly that both the surface and the bulk states contribute. The two aforementioned regimes prove that the contribution of Fermi arc surface states is salient and, therefore, crucial for understanding transport properties of finite-size Weyl semimetal systems. Last but not least, we demonstrate that both regimes are robust to disorder.

  • 8.
    Lezama, Talia L. M.
    et al.
    Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany..
    Bera, Soumya
    Indian Inst Technol, Dept Phys, Mumbai 400076, Maharashtra, India..
    Bardarson, Jens H.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory. Max Planck Inst Phys Komplexer Syst, D-01187 Dresden, Germany..
    Apparent slow dynamics in the ergodic phase of a driven many-body localized system without extensive conserved quantities2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 16, article id 161106Article in journal (Refereed)
    Abstract [en]

    We numerically study the dynamics on the ergodic side of the many-body localization transition in a periodically driven Floquet model with no global conservation laws. We describe and employ a numerical technique based on the fast Walsh-Hadamard transform that allows us to perform an exact time evolution for large systems and long times. As in models with conserved quantities (e.g., energy and/or particle number) we observe a slowing down of the dynamics as the transition into the many-body localized phase is approached. More specifically, our data are consistent with a subballistic spread of entanglement and a stretched-exponential decay of an autocorrelation function, with their associated exponents reflecting slow dynamics near the transition for a fixed system size. However, with access to larger system sizes, we observe a clear flow of the exponents towards faster dynamics and cannot rule out that the slow dynamics is a finite-size effect. Furthermore, we observe examples of nonmonotonic dependence of the exponents with time, with the dynamics initially slowing down but accelerating again at even larger times, consistent with the slow dynamics being a crossover phenomenon with a localized critical point.

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