146 research outputs found
Frustrated multiband superconductivity
We show that a clean multiband superconductor may display one or several
phase transitions with increasing temperature from or to frustrated
configurations of the relative phases of the superconducting order parameters.
These transitions may occur when more than two bands are involved in the
formation of the superconducting phase and when the number of repulsive
interband interactions is odd. These transitions are signalled by slope changes
in the temperature dependence of the superconducting gaps.Comment: 5 pages, 3 figure
Magnetic impurities in the two-band -wave superconductors
We investigate the effects of magnetic impurities in a superconducting state
with pairing symmetry. Within a two-band model, we find that the
intra-band magnetic scattering serves as a pair breaker while the inter-band
magnetic scattering preserves pairing and hardly affects transition temperature
in the Born limit. We also show that the same physics can persist beyond the
weak scattering region. Our results coincide with recent experimental
measurements in iron-based superconductors and thus provides an indirect
evidence of the possible pairing symmetry in these materials.Comment: 5 pages,4 figure
Effect of Impurities with Internal Structure on Multiband Superconductors - Possible Enhancement of Transition Temperature -
We study inelastic (dynamical) impurity scattering effects in two-band
superconductors with the same ( wave) or different ( wave) sign
order parameters. We focus on the enhancement of the superconducting transition
temperature by magnetic interband scattering with the interchange
of crystal-field singlet ground and multiplet excited states. Either the
-wave or -wave state is favored by the impurity-mediated
pairing, which depends on the magnetic and nonmagnetic scattering strengths
derived from the hybridization of the impurity states with the conduction
bands. The details are examined for the singlet-triplet configuration that is
suggestive of Pr impurities in the skutterudite superconductor
LaOsSb.Comment: 14 pages, 5 figures, to appear in J. Phys. Soc. Jpn. Vol. 79, No. 9
(2010
Single Impurity Effects in Multiband Superconductors with Different Sign Order Parameters
A single impurity problem is investigated for multiband s-wave
superconductors with different sign order parameters (+-s-wave superconductors)
suggested in Fe-pnictide superconductors. Not only intraband but also interband
scattering is considered at the impurity. The latter gives rise to
impurity-induced local boundstates close to the impurity. We present an exact
form of the energy of the local boundstates as a function of strength of the
two types of impurity scattering. The essential role of the impurity is
unchanged in finite number of impurities. The main conclusions for a single
impurity problem help us understand effects of dense impurities in the +-s-wave
superconductors. Local density of states around the single impurity is also
investigated. We suggest impurity site nuclear magnetic resonance as a suitable
experiment to probe the local boundstates that is peculiar to the +-s-wave
state. We find that the +-s-wave model is mapped to a chiral dx2-y2+-idxy-wave,
reflecting the unconventional nature of the sign reversing order parameter. For
a quantum magnetic impurity, interband scattering destabilizes the Kondo
singlet.Comment: 23 pages, 7 figures, to be published in J. Phys. Soc. Jpn. (2009) No.
Normal State Spin Dynamics of Five-band Model for Iron-pnictides
Normal state spin dynamics of the recently discovered iron-pnictide
superconductors is discussed by calculating spin structure factor S(q, omega)
in an itinerant five-band model within RPA approximation. Due to the
characteristic Fermi surface structure of iron-pnictide, column like response
is found at (pi, 0) in extended Brillouin zone in the undoped case, which is
consistent with the recent neutron scattering experiment. This indicates that
the localized spin model is not necessary to explain the spin dynamics of this
system. Furthermore, we show that the temperature dependence of inelastic
neutron scattering intensity can be well reproduced in the itinerant model. We
also study NMR 1/T_1T in the same footing calculation and show that the
itinerant model can capture the magnetic property of iron-pnictide
superconductors.Comment: 4 page
Distinct transport behaviors of LaFe1-yCoyAsO1-xFx (x=0.11) between the superconducting and nonsuperconducting metallic y regions divided by y ~ 0.05
Electrical resistivities, Hall coefficients and thermoelectric powers have
been measured for polycrystalline samples of LaFe1-yCoyAsO1-xFx (x=0.11) with
various values of y. The results show that there exists clear distinction of
these transport behaviors between the superconducting and nonsuperconducting
metallic regions of y divided by the boundary value yc~0.05. We have found that
the behaviors in both regions are very similar to those of high-Tc Cu oxides in
the corresponding phases. If they reflect, as in the case of Cu oxides, effects
of strong magnetic fluctuations, the energy scale of the fluctuations is
considered to be smaller than that of the high Cu oxides by a factor of ~1/2.
Arguments on the electronic nature and superconducting symmetry are presented
on the basis of the observed small rate of the Tc suppression rate by the Co
doping.Comment: 8 pages, 4 figures, submitted to J. Phys. Soc. Jp
Impurity-induced in-gap state and Tc in sign-reversing s-wave superconductors: analysis of iron oxypnictide superconductors
The sign-reversing fully gapped superconducting state, which is expected to
be realized in oxypnictide superconductors, can be prominently affected by
nonmagnetic impurities due to the interband scattering of Cooper pairs. We
study this problem based on the isotropic two-band BCS model: In oxypnictide
superconductors, the interband impurity scattering is not equal to the
intraband one . In the Born scattering regime, the reduction in Tc is
sizable and the impurity-induced density of states (DOS) is prominent if , due to the interband scattering. Although impurity-induced DOS can yield a
power-law temperature dependence in , a sizable suppression in Tc is
inevitably accompanied. In the unitary scattering regime, in contrast, impurity
effect is very small for both Tc and DOS except at . By comparing theory
and experiments, we expect that the degree of anisotropy in the -wave
gap function strongly depends on compounds.Comment: 16 pages, 5 figures, to be published in New. J. Phy
Perturbation Theory of High-Tc Superconductivity in Iron Pnictides
The high-transition-temperature (high-Tc) superconductivity discovered
recently in iron pnictides is analyzed within a perturbation theory.
Specifically, the probable pairing symmetry, the doping dependence of the
transition temperature and the pairing mechanism are studied by solving the
Eliashberg equation for multi-band (2- and 5-band) Hubbard models with
realistic electronic structures. The effective pairing interaction is expanded
perturbatively in the on-site Coulomb integrals up to third order. Our
perturbative weak-coupling approach shows that sufficiently large eigenvalues
of the Eliashberg equation are obtained to explain the actual high transition
temperatures by taking realistic on-site Coulomb integrals in the 5-band model.
Thus, unconventional (non-phonon-mediated) superconductivity is highly likely
to be realized. The superconducting order parameter does not change its sign on
the Fermi surfaces, but it does change between the electron and hole Fermi
surfaces. Consequently, the probable pairing symmetry is always "a nodeless
extended s-wave symmetry (more specifically, an s_{+-}-wave symmetry)" over the
whole parameter region that we investigated. It is suggested that the 2-band
model is insufficient to explain the high values of Tc.Comment: 24 pages, 9 figure
Variation in microbial function through soil depth profiles in the Kushiro Wetland, northeastern Hokkaido, Japan
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