396 research outputs found

### Josephson Spin Current in Triplet Superconductor Junctions

This paper theoretically discusses the spin current in spin-triplet
superconductor / insulator / spin-triplet superconductor junctions. At low
temperatures, a midgap Andreev resonant state anomalously enhances not only the
charge current but also the spin current. The coupling between the Cooper pairs
and the electromagnetic fields leads to the Frounhofer pattern in the direct
current spin flow in magnetic fields and the alternative spin current under
applied bias-voltages.Comment: 4 pages, 2 figure

### Josephson Effect in Noncentrosymmetric Superconductor Junctions

We discuss the Josephson current between two noncentrosymmetric
superconductors. The coexistence of superconducting order parameters between
spin-singlet $\Delta_{\text{S}}$ and helical p-wave spin-triplet
$\Delta_{\text{T}}$ enriches a variety of low-temperature behavior of Josephson
current depending on their relative amplitudes. We will show that
characteristic behaviors of the Josephson current for $\Delta_{\text{S}} >
\Delta_{\text{T}}$ are clearly different from those for $\Delta_{\text{S}} <
\Delta_{\text{T}}$. The topologically protected zero-energy surface bound
states are responsible for the clear difference. We conclude that the Josephson
current well reflects character of the topological surface states and the
pairing symmetry of noncentrosymmetric superconductors.Comment: 7 pages, 6 figures embedde

### Three-dimensional symmetry breaking topological matters

We discuss topological electronic states described by the Dirac Hamiltonian
plus an additional one in three-dimension. When the additional Hamiltonian is
an element of an Abelian group, electronic states become topologically
nontrivial even in the absence of fundamental symmetries such as the
time-reversal and the particle-hole symmety. The symmetry-breaking topological
states are charercterized by the Chern number defined in the two-dimensional
partial Brillouin zone. The topological insulators under Zeeman field are an
example of the symmetry-breaking topological matters. We show the transision
from the topological insulating phase to the topological semimetal one under
the strong Zeeman field.Comment: 5 pages, 4 figure

### Stability of flat zero-energy states at the dirty surface of a nodal superconductor

We discuss the stability of highly degenerate zero-energy states tha appear
at the surface of a nodal superconductor preserving time-reversal symmetry. The
existence of such surface states is a direct consequence of the nontrivial
topological numbers defined in the restricted Brillouin zones in the clean
limit. In experiments, however, potential disorder is inevitable near the
surface of a real superconductor, which may lift the high degeneracy at zero
energy. We show that an index defined in terms of the chiral eigenvalues of the
zero-energy states can be used to measure the degree of degeneracy at zero
energy in the presence of potential disorder. We also discuss the relationship
between the index and the topological numbers.Comment: 12 pages, 7 figure

### Robustness of Gapless Interface State in a Junction of Two Topological Insulators

We theoretically study subgap states appearing at the interface between two
three-dimensional topological insulators which have different configurations in
the spin-orbit interactions from each other. The coupling of spin
$\boldsymbol{\sigma}$ with momenta $\boldsymbol{p}$ is configured by a material
dependent $3\times 3$ matrix $\boldsymbol{\Lambda}$ as ${\sigma}^\mu
{\Lambda}_\mu^\nu p_\nu$. We show that the spectra of the interface suggap
states depend strongly on the relative choices of $\boldsymbol{\Lambda}$ in the
two topological insulators. In particular, we focus on properties of gapless
states which appear when $\boldsymbol{\Lambda}$ in two topological insulators
are connected by the inversion in momentum space. We also discuss the
robustness of the gapless states under perturbations breaking the time-reversal
symmetry or the band-inversion symmetry by the numerical simulation.Comment: 13 pages, 9 figure

### Effects of surface roughness on the paramagnetic response of small unconventional superconductors

We theoretically study effects of surface roughness on the magnetic response
of small unconventional superconductors by solving the Eilenberger equation for
the quassiclassical Green function and the Maxwell equation for the vector
potential simultaneously and self-consistently. The paramagnetic phase of
spin-singlet $d$-wave superconducting disks is drastically suppressed by the
surface roughness, whereas that of spin-triplet $p$-wave disks is robust even
in the presence of the roughness. Such difference derives from the orbital
symmetry of paramagnetic odd-frequency Cooper pairs appearing at the surface of
disks. The orbital part of the paramagnetic pairing correlation is $p$-wave
symmetry in the $d$-wave disks, whereas it is $s$-wave symmetry in the $p$-wave
ones. Calculating the free-energy, we also confirm that the paramagnetic state
is more stable than the normal state, which indicates a possibility of
detecting the paramagnetic effect in experiments. Indeed our results are
consistent with an experimental finding on high-$T_c$ thin films.Comment: 11 pages, 10 figure

### Green function theory of dirty two-band superconductivity

We study the effects of random nonmagnetic impurities on the superconducting
transition temperature $T_c$ in a two-band superconductor, where we assume the
equal-time spin-singlet s-wave pair potential in each conduction band and the
hybridization between the two bands as well as the band asymmetry. In the clean
limit, the phase of hybridization determines the stability of two states:
called $s_{++}$ and $s_{+-}$. The interband impurity scatterings decrease $T_c$
of the two states exactly in the same manner when the Hamiltonian preserves
time-reversal symmetry. We find that a superconductor with larger hybridization
shows more moderate suppression of $T_c$. This effect can be explained by the
presence of odd-frequency Cooper pairs which are generated by the band
hybridization in the clean limit and are broken by impurities.Comment: 11 pages, 2 figure

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