302 research outputs found
Ballistic quantum spin Hall state and enhanced edge backscattering in strong magnetic fields
The quantum spin Hall (QSH) state, observed in a zero magnetic field in HgTe
quantum wells, respects the time-reversal symmetry and is distinct from quantum
Hall (QH) states. We show that the QSH state persists in strong quantizing
fields and is identified by counter-propagating (helical) edge channels with
nonlinear dispersion inside the band gap. If the Fermi level is shifted into
the Landau-quantized conduction or valence band, we find a transition between
the QSH and QH regimes. Near the transition the longitudinal conductance of the
helical channels is strongly suppressed due to the combined effect of the
spectrum nonlinearity and enhanced backscattering. It shows a power-law decay
1/B^2N with magnetic field B, determined by the number of backscatterers on the
edge, N. This suggests a rather simple and practical way to probe the quality
of recently realized quasiballistic QSH devices using magnetoresistance
measurements.Comment: 4 pages, 3 figures, minor changes, accepted for publication in PR
New type of antiferromagnetic polaron and bipolaron in HTc - superconductors
The possibility of formation of a new type of polaron based on the quantum
aniferromagnet (AF) model is reported. We take into account exchange
interactions between localized d-d spins of the AF, as well as the p-d
interaction of the AF with p-carriers.
The energy minimum is found when maximum charge density occurs on every
second spin. The formation of such ``comb''-like polarons results from the
damping of quantum fluctuations and the appearance of Van Vleck-like staggered
magnetization. Such polarons tend to form pairs coupled by an AF ``glue''.Comment: 3 pages 2 figure
Weak Localization and Antilocalization in Topological Materials with Impurity Spin-Orbit Interactions
Topological materials have attracted considerable experimental and
theoretical attention. They exhibit strong spin-orbit coupling both in the band
structure (intrinsic) and in the impurity potentials (extrinsic), although the
latter is often neglected. Here we discuss weak localization and
antilocalization of massless Dirac fermions in topological insulators and
massive Dirac fermions in Weyl semimetal thin films taking into account both
intrinsic and extrinsic spin-orbit interactions. The physics is governed by the
complex interplay of the chiral spin texture, quasiparticle mass, and scalar
and spin-orbit scattering. We demonstrate that terms linear in the extrinsic
spin-orbit scattering are generally present in the Bloch and momentum
relaxation times in all topological materials, and the correction to the
diffusion constant is linear in the strength of the extrinsic spin-orbit. In
TIs, which have zero quasiparticle mass, the terms linear in the impurity
spin-orbit coupling lead to an observable density dependence in the weak
antilocalization correction. They produce substantial qualitative modifications
to the magnetoconductivity, differing greatly from the conventional HLN formula
traditionally used in experimental fits, which predicts a crossover from weak
localization to antilocalization as a function of the extrinsic spin-orbit
strength. In contrast, our analysis reveals that topological insulators always
exhibit weak antilocalization. In WSM thin films having intermediate to large
values of the quasiparticle mass extrinsic spin-orbit scattering strongly
affects the boundary of the weak localization to antilocalization transition.
We produce a complete phase diagram for this transition as a function of the
mass and spin-orbit scattering strength. We discuss implications for
experiments and provide a brief comparison with transition metal
dichalcogenides.Comment: arXiv admin note: text overlap with arXiv:1705.0761
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