68 research outputs found
Strongly angle-dependent magnetoresistance in Weyl semimetals with long-range disorder
The chiral anomaly in Weyl semimetals states that the left- and right-handed
Weyl fermions, constituting the low energy description, are not individually
conserved, resulting, for example, in a negative magnetoresistance in such
materials. Recent experiments see strong indications of such an anomalous
resistance response; however, with a response that at strong fields is more
sharply peaked for parallel magnetic and electric fields than expected from
simple theoretical considerations. Here, we uncover a mechanism, arising from
the interplay between the angle-dependent Landau level structure and long-range
scalar disorder, that has the same phenomenology. In particular, we ana-
lytically show, and numerically confirm, that the internode scattering time
decreases exponentially with the angle between the magnetic field and the Weyl
node separation in the large field limit, while it is insensitive to this angle
at weak magnetic fields. Since, in the simplest approximation, the internode
scattering time is proportional to the anomaly-related conductivity, this
feature may be related to the experimental observations of a sharply peaked
magnetoresistance.Comment: 8 pages, 4 figure
Quantized Fermi-arc-mediated transport in Weyl semimetal nanowires
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-P\'erot 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 the 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.Comment: 13 pages, 6 figure
Defining a bulk-edge correspondence for non-Hermitian Hamiltonians via singular-value decomposition
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.Comment: 6+5 pages, 8 figure
Anomalous Nernst and Thermal Hall Effects in Tilted Weyl Semimetals
We study the anomalous Nernst and thermal Hall effects in a linearized
low-energy model of a tilted Weyl semimetal, with two Weyl nodes separated in
momentum space. For inversion symmetric tilt, we give analytic expressions in
two opposite limits: for a small tilt, corresponding to a type-I Weyl
semimetal, the Nernst conductivity is finite and independent of the Fermi
level, while for a large tilt, corresponding to a type-II Weyl semimetal, it
acquires a contribution depending logarithmically on the Fermi energy. This
result is in a sharp contrast to the nontilted case, where the Nernst response
is known to be zero in the linear model. The thermal Hall conductivity
similarly acquires Fermi surface contributions, which add to the Fermi level
independent, zero tilt result, and is suppressed as one over the tilt parameter
at half filling in the Type-II phase. In the case of inversion breaking tilt,
with the tilting vector of equal modulus in the two Weyl cones, all Fermi
surface contributions to both anomalous responses cancel out, resulting in zero
Nernst conductivity. We discuss two possible experimental setups, representing
open and closed thermoelectric circuits
Unbounded growth of entanglement in models of many-body localization
An important and incompletely answered question is whether a closed quantum
system of many interacting particles can be localized by disorder. The time
evolution of simple (unentangled) initial states is studied numerically for a
system of interacting spinless fermions in one dimension described by the
random-field XXZ Hamiltonian. Interactions induce a dramatic change in the
propagation of entanglement and a smaller change in the propagation of
particles. For even weak interactions, when the system is thought to be in a
many-body localized phase, entanglement shows neither localized nor diffusive
behavior but grows without limit in an infinite system: interactions act as a
singular perturbation on the localized state with no interactions. The
significance for proposed atomic experiments is that local measurements will
show a large but nonthermal entropy in the many-body localized state. This
entropy develops slowly (approximately logarithmically) over a diverging time
scale as in glassy systems.Comment: 4 pages, 2 figures, v2. added more dat
Robust Transport Signatures of Topological Superconductivity in Topological Insulator Nanowires
Finding a clear signature of topological superconductivity in transport
experiments remains an outstanding challenge. In this work, we propose
exploiting the unique properties of three-dimensional topological insulator
nanowires to generate a normal-superconductor junction in the single-mode
regime where an exactly quantized zero-bias conductance can be
observed over a wide range of realistic system parameters. This is achieved by
inducing superconductivity in half of the wire, which can be tuned at will from
trivial to topological with a parallel magnetic field, while a perpendicular
field is used to gap out the normal part, except for two spatially separated
chiral channels. The combination of chiral mode transport and perfect Andreev
reflection makes the measurement robust to moderate disorder, and the
quantization of conductance survives to much higher temperatures than in tunnel
junction experiments. Our proposal may be understood as a variant of a Majorana
interferometer which is easily realizable in experiments.Comment: 5 pages, 3 figure
Many-body localization in a disordered quantum Ising chain
Many-body localization occurs in isolated quantum systems when Anderson
localization persists in the presence of finite interactions. Despite strong
evidence for the existence of a many-body localization transition a reliable
extraction of the critical disorder strength is difficult due to a large drift
with system size in the studied quantities. In this work we explore two
entanglement properties that are promising for the study of the manybody
localization transition: the variance of the half-chain entanglement entropy of
exact eigenstates and the long time change in entanglement after a local quench
from an exact eigenstate. We investigate these quantities in a disordered
quantum Ising chain and use them to estimate the critical disorder strength and
its energy dependence. In addition, we analyze a spin-glass transition at large
disorder strength and provide evidence for it being a separate transition. We
thereby give numerical support for a recently proposed phase diagram of
many-body localization with localization protected quantum order [Huse et al.
Phys. Rev. B 88, 014206 (2013)].Comment: 4+ pages + 1.5 pages appendix, 5 figure
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