22 research outputs found
Electric-Field Induced Majorana Fermions in Armchair Carbon Nanotubes
We consider theoretically an armchair Carbon nanotube (CNT) in the presence
of an electric field and in contact with an s-wave superconductor. We show that
the proximity effect opens up superconducting gaps in the CNT of different
strengths for the exterior and interior branches of the two Dirac points. For
strong proximity induced superconductivity the interior gap can be of the
p-wave type, while the exterior gap can be tuned by the electric field to be of
the s-wave type. Such a setup supports a single Majorana bound state at each
end of the CNT. In the case of a weak proximity induced superconductivity, the
gaps in both branches are of the p-wave type. However, the temperature can be
chosen in such a way that the smallest gap is effectively closed. Using
renormalization group techniques we show that the Majorana bound states exist
even after taking into account electron-electron interactions
Structure factor of interacting one-dimensional helical systems
We calculate the dynamical structure factor S(q, {\omega}) of a weakly
interacting helical edge state in the presence of a magnetic field B. The
latter opens a gap of width 2B in the single-particle spectrum, which becomes
strongly nonlinear near the Dirac point. For chemical potentials |{\mu}| > B,
the system then behaves as a nonlinear helical Luttinger liquid, and a
mobile-impurity analysis reveals interaction-dependent power-law singularities
in S(q,{\omega}). For |{\mu}| < B, the low-energy excitations are gapped, and
we determine S(q,{\omega}) by using an analogy to exciton physics.Comment: 5 pages, 3 figure
Majorana states in inhomogeneous spin ladders
We propose an inhomogeneous open spin ladder, related to the Kitaev honeycomb
model, which can be tuned between topological and nontopological phases. In
extension of Lieb's theorem, we show numerically that the ground state of the
spin ladder is either vortex free or vortex full. We study the robustness of
Majorana end states (MES) which emerge at the boundary between sections in
different topological phases and show that while the MES in the homogeneous
ladder are destroyed by single-body perturbations, in the presence of
inhomogeneities at least two-body perturbations are required to destabilize
MES. Furthermore, we prove that x, y, or z inhomogeneous magnetic fields are
not able to destroy the topological degeneracy. Finally, we present a
trijunction setup where MES can be braided. A network of such spin ladders
provides thus a promising platform for realization and manipulation of MES
Effect of topological length on Bound states signatures in a Topological nanowire
Majorana bound states (MBS) at the end of nanowires have been proposed as one
of the most important candidate for the topological qubits. However, similar
tunneling conductance features for both the MBS and Andreev bound states (ABS)
have turned out to be a major obstacle in the verification of the presence of
MBS in semiconductor-superconductor heterostructures. In this article, we use a
protocol to probe properties specific to the MBS and use it to distinguish the
topological zero-bias peak (ZBP) from a trivial one. For a scenario involving
quantized ZBP in the nanowire, we propose a scheme wherein the length of the
topological region in the wire is altered. The tunneling conductance signatures
can then be utilized to gauge the impact on the energy of the low-energy
states. We show that the topological and trivial ZBP behave differently under
our protocol, in particular, the topological ZBP remains robust at zero bias
throughout the protocol, while the trivial ZBP splits into two peaks at finite
bias. This protocol probes the protection of near zero energy states due to
their separable nature, allowing us to distinguish between topological and
trivial ZBP.Comment: 35 figure
Spin-orbit mediated anisotropic spin interaction in interacting electron systems
We investigate interactions between spins of strongly correlated electrons
subject to the spin-orbit interaction. Our main finding is that of a novel,
spin-orbit mediated anisotropic spin-spin coupling of the van der Waals type.
Unlike the standard exchange, this interaction does not require the wave
functions to overlap. We argue that this ferromagnetic interaction is important
in the Wigner crystal state where the exchange processes are severely
suppressed. We also comment on the anisotropy of the exchange between spins
mediated by the spin-orbital coupling.Comment: 4.1 pages, 1 figure; (v2) minor changes, published versio