903 research outputs found
Dirty two-band superconductivity with interband pairing order
We study theoretically the effects of random nonmagnetic impurities on the
superconducting transition temperature in a two-band superconductor
characterized by an equal-time s-wave interband pairing order parameter. The
Fermi-Dirac statistics of electrons allows a spin-triplet s-wave pairing order
as well as a spin-singlet s-wave order parameter due to the two-band degree of
freedom. In a spin-singlet superconductor, is insensitive to the impurity
concentration when we estimate the self-energy due to the random impurity
potential within the Born approximation. On the other hand in a spin-triplet
superconductor, decreases with the increase of the impurity
concentration. We conclude that Cooper pairs belonging to odd-band-parity
symmetry class are fragile under the random impurity potential even though they
have s-wave pairing symmetry.Comment: 7 pages, 2 figures embedde
Josephson effect in two-band superconductors
We study theoretically the Josephson effect between two time-reversal
two-band superconductors, where we assume the equal-time spin-singlet -wave
pair potential in each conduction band. %as well as the band asymmetry and the
band hybridization in the normal state. The superconducting phase at the first
band and that at the second band characterize a
two-band superconducting state. We consider a Josephson junction where an
insulating barrier separates two such two-band superconductors. By applying the
tunnel Hamiltonian description, the Josephson current is calculated in terms of
the anomalous Green's function on either side of the junction. We find that the
Josephson current consists of three components which depend on three types of
phase differences across the junction: the phase difference at the first band
, the phase difference at the second band ,
and the difference at the center-of-mass phase .
A Cooper pairs generated by the band hybridization carries the last current
component. In some cases, the current-phase relationship deviates from the
sinusoidal function as a result of time-reversal symmetry breaking down.Comment: 6 page, 2 figure
Insulating Phases Induced by Crossing of Partially Filled Landau Levels in a Si Quantum Well
We study magnetotransport in a high mobility Si two-dimensional electron
system by in situ tilting of the sample relative to the magnetic field. A
pronounced dip in the longitudinal resistivity is observed during the Landau
level crossing process for noninteger filling factors. Together with a Hall
resistivity change which exhibits the particle-hole symmetry, this indicates
that electrons or holes in the relevant Landau levels become localized at the
coincidence where the pseudospin-unpolarized state is expected to be stable.Comment: 4 pages, 4 figure
Well-width dependence of valley splitting in Si/SiGe quantum wells
The valley splitting in Si two-dimensional electron systems is studied using
Si/SiGe single quantum wells (QWs) with different well widths. The energy gaps
for 4 and 5.3 nm QWs, obtained from the temperature dependence of the
longitudinal resistivity at the Landau level filling factor , are much
larger than those for 10 and 20 nm QWs. This is consistent with the well-width
dependence of the bare valley splitting estimated from the comparison with the
Zeeman splitting in the Shubnikov-de Haas oscillations.Comment: 3 pages, 2 figure
Metallic Behavior of Cyclotron Relaxation Time in Two-Dimensional Systems
Cyclotron resonance of two-dimensional electrons is studied at low
temperatures down to 0.4 K for a high-mobility Si/SiGe quantum well which
exhibits a metallic temperature dependence of dc resistivity . The
relaxation time shows a negative temperature dependence, which
is similar to that of the transport scattering time obtained from
. The ratio at 0.4 K increases as the electron
density decreases, and exceeds unity when approaches the critical
density for the metal-insulator transition.Comment: 4 pages, 3 figure
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