626 research outputs found
Berry Phase of Dirac Nodal Line Semimetal in Single-Component Molecular Conductor
The Berry phase and curvature are studied for the Dirac nodal line semimetal
of single-component molecular conductor [Pd(dddt)]. Using two-band model on
the basis of a tight-binding model, it is shown that the Berry curvature,
, exists along a loop of the nodal line, on which the Berry
phase is obtained from surface integral of . The Berry phase,
which is called Zak phase, is also calculated from one-dimensional integral of
the Berry connection along a line between two equivalent points of boundaries
of Brillouin zone. The possible experiment is discussed in terms of the Berry
phase.Comment: 5 pages, 3 figure
Interference of Majorana fermions in NS junctions
We investigate interference of Majorana fermions (MFs) in NS junctions. A
general formula of charge conductance G for NS junctions with two MFs is
derived based on the low energy effective model. It is found that G for two MFs
takes various values 0 \leq G \leq 4e^2/h owing to interference of the MFs,
while G is quantized as G = 2e^2/h for a single MF. The value of G is
determined by symmetry of the system. As an example, we investigate the
complete destructive interference of two degenerate MFs reported by Ii et al.
[A. Ii, A. Yamakage, K. Yada, M. Sato, and Y. Tanaka, Phys. Rev. B 86, 174512
(2012)], and identify the symmetry responsible for the destructive
interference.Comment: 8 pages, 3 figure
Phase diagram of a magnetic topological nodal semimetal: Stable nodal line in an easy-plane ferromagnet
We study a topological phase diagram of a ferromagnetic topological nodal
semimetal. We consider a lattice model for three-dimensional topological
insulators with ferromagnetic ordering. The exchange coupling between the
magnetization and the electron spin leads to the nodal band structure. The
topology of the nodal band structure depends on the direction of the
magnetization and both the Weyl points and the nodal line emerge. We find that
the nodal line structure is stable under an easy-plane magnetization in
appropriate model parameters. In this case, the nodal line phase emerges as a
phase boundary between two topologically distinct Weyl semimetal phases.Comment: 6 pages, 4 figure
Mirror Majorana zero modes in spinful superconductors/superfluids -Non-Abelian braiding of integer quantum vortices-
It has been widely believed that half quantum vortices are indispensable to
realize topological stable Majorana zero modes and non-Abelian anyons in
spinful superconductors/superfluids. Contrary to this wisdom, we here
demonstrate that integer quantum vortices in spinful superconductors can host
topologically stable Majorana zero modes because of the mirror symmetry. The
symmetry protected Majorana fermions may exhibit non-Abelian anyon braiding.Comment: 6 pages, 2 figure
Quantum thermal Hall effect of Majorana fermions on the surface of superconducting topoloigcal insulators
We study the quantum anomalous thermal Hall effect in a topological
superconductor which possesses an integer bulk topological number, and supports
Majorana excitations on the surface. To realize the quantum thermal Hall
effect, a finite gap at the surface is induced by applying an external magnetic
field or by the proximity effects with magnetic materials or -wave
superconductors with complex pair-potentials. Basing on the lattice model
Hamiltonian for superconducting states in Cu-doped BiSe, we compute the
thermal Hall conductivity as a function of various parameters such as the
chemical potential, the pair-potential, and the spin-orbit coupling induced
band gap. It is argued that the bulk topological invariant corresponds to the
quantization rule of the thermal Hall conductivity induced by complex -wave
pair-potentials.Comment: 7 pages, 6 figure
Symmetry-Protected Majorana Fermions in Topological Crystalline Superconductors: Theory and Application to Sr2RuO4
Crystal point group symmetry is shown to protect Majorana fermions (MFs) in
spinfull superconductors (SCs). We elucidate the condition necessary to obtain
MFs protected by the point group symmetry. We argue that superconductivity in
Sr2RuO4 hosts a topological phase transition to a topological crystalline SC,
which accompanies a d-vector rotation under a magnetic field along the c-axis.
Taking all three bands and spin-orbit interactions into account,
symmetry-protected MFs in the topological crystalline SC are identified.
Detection of such MFs provides evidence of the d-vector rotation in Sr2RuO4
expected from Knight shift measurements but not yet verified.Comment: 6 pages, 2 figures (proper paper)+ 6 pages, 5 figures (supplemental
material); final version accepted for publication in PR
Line-Node Dirac Semimetal and Topological Insulating Phase in Noncentrosymmetric Pnictides CaAgX (X = P, As)
Two noncentrosymmetric ternary pnictides, CaAgP and CaAgAs, are reported as
topological line-node semimetals protected solely by mirror-reflection
symmetry. The band gap vanishes on a circle in momentum space, and surface
states emerge within the circle. Extending this study to spin-orbit coupled
systems reveals that, compared with CaAgP, a substantial band gap is induced in
CaAgAs by large spin-orbit interaction. The resulting states are a topological
insulator, in which the Z2 topological invariant is given by 1; 000. To clarify
the Z2 topological invariants for time-reversal-invariant systems without
spatial-inversion symmetry, we introduce an alternative way to calculate the
invariants characterizing a line node and topological insulator for
mirror-reflection-invariant systems.Comment: 4+4 pages, 3+3 figure
Theory of tunneling spectroscopy in a superconducting topological insulator
We calculate tunneling conductance of normal metal and superconducting
topological insulator CuxBi2Se3 junctions for the possible pairing symmetries.
In the presence of gapless Andreev bound states (ABSs), the tunneling
conductance shows a zero-bias peak even for the three-dimensional full-gap
superconducting state. This zero-bias conductance peak stems from the
enhancement of the surface density of states induced by the surface-state
transition in momentum space.Comment: Proceeding of ISS201
Angular momentum and topology in semiconducting single-wall carbon nanotubes
Semiconducting single-wall carbon nanotubes are classified into two types by
means of orbital angular momentum of valley state, which is useful to study
their low energy electronic properties in finite-length. The classification is
given by an integer , which is the greatest common divisor of two integers
and specifying the chirality of nanotubes, by analyzing cutting lines.
For the case that is equal to or greater than four, two lowest subbands
from two valleys have different angular momenta with respect to the nanotube
axis. Reflecting the decoupling of two valleys, discrete energy levels in
finite-length nanotubes exhibit nearly fourfold degeneracy and its small lift
by the spin-orbit interaction. For the case that is less than or equal to
two, in which two lowest subbands from two valleys have the same angular
momentum, discrete levels exhibit lift of fourfold degeneracy reflecting the
coupling of two valleys. Especially, two valleys are strongly coupled when the
chirality is close to the armchair chirality. An effective one-dimensional
lattice model is derived by extracting states with relevant angular momentum,
which reveals the valley coupling in the eigenstates. A bulk-edge
correspondence, relationship between number of edge states and the winding
number calculated in the corresponding bulk system, is analytically shown by
using the argument principle, which enables us to estimate the number of edge
states from the bulk property. The number of edge states depends not only on
the chirality but also on the shape of boundary.Comment: 20 pages, 10 figure
Finite-size-effect-induced topological phase transition in a topological crystalline insulator
We study electronic states and topological invariants of (001)-films of
topological crystalline insulator (TCI) Pb_{x}Sn_{1-x}Te. Gapless surface Dirac
cones on bulk TCIs become gapped in thin films due to finite-size effect, which
is hybridization between those on the top and bottom surfaces. We clarify that
the TCI film has the strong finite-size effect as compared to three-dimensional
topological insulators such as Bi2Se3. Moreover, the energy gap oscillates with
the thickness of film. The oscillation stems from topological phase transitions
in two dimensions. The obtained data of the topological invariants and energy
gap serve as guide to TCI-device applications.Comment: 18 pages, 18 figure
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