99 research outputs found
Majorana bound states in two-channel time-reversal-symmetric nanowire systems
We consider time-reversal-symmetric two-channel semiconducting quantum wires
proximity coupled to an s-wave superconductor. We analyze the requirements for
a nontrivial topological phase and find that necessary conditions are 1) the
determinant of the pairing matrix in channel space must be negative, 2)
inversion symmetry must be broken, and 3) the two channels must have different
spin-orbit couplings. The first condition can be implemented in semiconducting
nanowire systems where interactions suppress intra-channel pairing, while the
inversion symmetry can be broken by tuning the chemical potentials of the
channels. For the case of collinear spin-orbit directions, we find a general
expression for the topological invariant by block diagonalization into two
blocks with chiral symmetry only. By projection to the low-energy sector, we
solve for the zero modes explicitly and study the details of the gap closing,
which in the general case happens at finite momenta.Comment: 6 pages. Corrected versio
Interplay between Magnetic and Vestigial Nematic Orders in the Layered - Classical Heisenberg Model
We study the layered - classical Heisenberg model on the square
lattice using a self-consistent bond theory. We derive the phase diagram for
fixed as a function of temperature , and interplane coupling
. Broad regions of (anti)ferromagnetic and stripe order are found, and are
separated by a first-order transition near (in units of
). Within the stripe phase the magnetic and vestigial nematic
transitions occur simultaneously in first-order fashion for strong . For
weaker there is in addition, for , an intermediate
regime of split transitions implying a finite temperature region with nematic
order but no long-range stripe magnetic order. In this split regime, the order
of the transitions depends sensitively on the deviation from and
, with split second-order transitions predominating for . We find that the value of depends weakly on the
interplane coupling and is just slightly larger than for . In contrast the value of increases quickly from at
as the interplane coupling is further reduced. In
addition, the magnetic correlation length is shown to directly depend on the
nematic order parameter and thus exhibits a sharp increase (or jump) upon
entering the nematic phase. Our results are broadly consistent with predictions
based on itinerant electron models of the iron-based superconductors in the
normal-state, and thus help substantiate a classical spin framework for
providing a phenomenological description of their magnetic properties.Comment: 13 pages, 20 figure
Cotunneling renormalization in carbon nanotube quantum dots
We determine the level-shifts induced by cotunneling in a Coulomb blockaded
carbon nanotube quantum dot using leading order quasi-degenerate perturbation
theory within a single nanotube quartet. It is demonstrated that otherwise
degenerate and equally tunnel-coupled and states are mixed by
cotunneling and therefore split up in energy except at the
particle/hole-symmetric midpoints of the Coulomb diamonds. In the presence of
an external magnetic field, we show that cotunneling induces a gate-dependent
-factor renormalization, and we outline different scenarios which might be
observed experimentally, depending on the values of both intrinsic
splitting and spin-orbit coupling.Comment: 12 pages, 7 figure
Sources of negative tunneling magneto-resistance in multilevel quantum dots with ferromagnetic contacts
We analyze distinct sources of spin-dependent energy level shifts and their
impact on the tunneling magnetoresistance (TMR) of interacting quantum dots
coupled to collinearly polarized ferromagnetic leads. Level shifts due to
virtual charge fluctuations can be quantitatively evaluated within a
diagrammatic representation of our transport theory. The theory is valid for
multilevel quantum dot systems and we exemplarily apply it to carbon nanotube
quantum dots, where we show that the presence of many levels can qualitatively
influence the TMR effect.Comment: 4 pages, 2 figures, supplemental materia
Current-induced gap opening in interacting topological insulator surfaces
Two-dimensional topological insulators (TIs) host gapless helical edge states
that are predicted to support a quantized two-terminal conductance.
Quantization is protected by time-reversal symmetry, which forbids elastic
backscattering. Paradoxically, the current-carrying state itself breaks the
time-reversal symmetry that protects it. Here we show that the combination of
electron-electron interactions and momentum-dependent spin polarization in
helical edge states gives rise to feedback through which an applied current
opens a gap in the edge state dispersion, thereby breaking the protection
against elastic backscattering. Current-induced gap opening is manifested via a
nonlinear contribution to the system's characteristic, which persists
down to zero temperature. We discuss prospects for realizations in recently
discovered large bulk band gap TIs, and an analogous current-induced gap
opening mechanism for the surface states of three-dimensional TIs.Comment: 6 pages, 2 figures, published versio
Symmetry analysis of strain, electric and magnetic fields in the -class of topological insulators
Based on group theoretical arguments we derive the most general Hamiltonian
for the -class of materials including terms to third
order in the wave vector, first order in electric and magnetic fields, first
order in strain and first order in both strain and wave vector. We determine
analytically the effects of strain on the electronic structure of
. For the most experimentally relevant surface
termination we analytically derive the surface state spectrum, revealing an
anisotropic Dirac cone with elliptical constant energy counturs giving rise to
different velocities in different in-plane directions. The spin-momentum
locking of strained is shown to be modified and for
some strain configurations we see a non-zero spin component perpendicular to
the surface. Hence, strain control can be used to manipulate the spin degree of
freedom via the spin-orbit coupling. We show that for a thin film of
the surface state band gap induced by coupling between
the opposite surfaces changes opposite to the bulk band gap under strain.
Tuning the surface state band gap by strain, gives new possibilities for the
experimental investigation of the thickness dependent gap and optimization of
optical properties relevant for, e.g., photodetector and energy harvesting
applications. We finally derive analytical expressions for the effective mass
tensor of the BiSe class of materials as a function of strain and
electric field
The antiferromagnetic phase of the Floquet-driven Hubbard model
A saddle point plus fluctuations analysis of the periodically driven
half-filled two-dimensional Hubbard model is performed. For drive frequencies
below the equilibrium gap, we find discontinuous transitions to time-dependent
solutions. A highly excited, generically non-thermal distribution of magnons
occurs even for drive frequencies far above the gap. Above a critical drive
amplitude, the low-energy magnon distribution diverges as the frequency tends
to zero and antiferromagnetism is destroyed, revealing the generic importance
of collective mode excitations arising from a non-equilibrium drive
Incipient nodal pairing in planar d-wave superconductors
The possibility of a second pairing transition () in
planar -wave superconductors which occurs in the absence of external
magnetic field, magnetic impurities or boundaries is established in the
framework of the non-perturbative phenomenon of dynamical chiral symmetry
breaking in the system of -dimensional Dirac-like nodal quasiparticles. We
determine the critical exponents and quasiparticle spectral functions that
characterize the corresponding quantum critical behavior and discuss some of
its potentially observable spectral and transport features
Inelastic scattering rates in d-wave superconductors
The inelastic scattering rates of quasiparticles in a two-dimensional d-wave superconductor, which arise from interactions with either acoustic phonons or other quasiparticles, are calculated within second order perturbation theory. We discover a strong enhancement of scattering with collinear momenta, brought about by the special kinematics of the two-dimensional fermions with Dirac-like spectrum near the nodes of the d-wave order parameter. In the case of a local instantaneous interparticle potential we find that either an RPA-type resummation of the perturbation series or an inclusion of non-linear corrections to the Dirac spectrum is called for in order to obtain a finite scattering rate in the limit ω/T→0. In either way, we find drastic changes in the scattering rate, as compared to the naively expected cubic temperature dependence
Yu-Shiba-Rusinov states in phase-biased S-QD-S junctions
We study the effects of a phase difference on Yu-Shiba-Rusinov (YSR) states
in a spinful Coulomb-blockaded quantum dot contacted by a superconducting loop.
In the limit where charging energy is larger than the superconducting gap, we
determine the subgap excitation spectrum, the corresponding supercurrent, and
the differential conductance as measured by a normal-metal tunnel probe. In
absence of a phase difference only one linear combination of the superconductor
lead electrons couples to the spin, which gives a single YSR state. With finite
phase difference, however, it is effectively a two-channel scattering problem
and therefore an additional state emerges from the gap edge. The energy of the
phase-dependent YSR states depend on the gate voltage and one state can cross
zero energy twice inside the valley with odd occupancy. These crossings are
shifted by the phase difference towards the charge degeneracy points,
corresponding to larger exchange couplings. Moreover, the zero-energy crossings
give rise to resonant peaks in the differential conductance with magnitude
. Finally, we demonstrate that the quantum fluctuations of the dot spin
do not alter qualitatively any of the results.Comment: 13 pages, 7 figure
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