407 research outputs found
Theory of absorption spectrum in the nonequilibrium steady state of superconductors under microwave irradiation
We discuss the absorption spectrum of dirty s-wave superconductors in the
nonequilibrium steady state under a homogeneous and monochromatic microwave. In
this state, there exists a finite density of states at lower energies than the
superconducting gap, and the distribution function is also defined in this
energy region. Because of the reduction of the distribution function below the
superconducting gap, the absorption by thermally excited quasiparticles is
suppressed in the nonequilibrium state compared with that in the equilibrium
case. This is in contrast to the cases of the gap function and the absorption
by direct excitation, to which the variation of the distribution function above
the superconducting gap mainly contributes. It is also shown that amplitude
modes are generated by frequency mixing of the pump wave and the probe wave,
and two peaks appear in the absorption spectrum
A Mechanism of Spin-Triplet Superconductivity in Hubbard Model on Triangular La ttice: Application to UNi_2Al_3
We discuss the possibility of spin-triplet superconductivity in a
two-dimensional Hubbard model on a triangular lattice within the third-order
perturbation theory. When we vary the symmetry in the dispersion of the bare
energy band from D_2 to D_6, spin-singlet superconductivity in the
D_2-symmetric system is suppressed and we obtain spin-triplet superconductivity
in near the D_6-symmetric system. In this case, it is found that the vertex
terms, which are not included in the interaction mediated by the spin
fluctuation, are essential for realizing the spin-triplet pairing. We point out
the possibility that obtained results correspond to the difference between the
superconductivity of UNi_2Al_3 and that of UPd_2Al_3.Comment: 11pages, 5figure
Comparison of the phase diagram of the half-filled layered organic superconductors with the phase diagram of the RVB theory of the Hubbard-Heisenberg model
We present an resonating valence bond (RVB) theory of superconductivity for
the Hubbard--Heisenberg model on an anisotropic triangular lattice. We show
that these calculations are consistent with the observed phase diagram of the
half-filled layered organic superconductors, such as the beta, beta', kappa and
lambda phases of (BEDT-TTF)_2X [bis(ethylenedithio)tetrathiafulvalene] and
(BETS)_2X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order
transition from a Mott insulator to a d_{x^2-y^2} superconductor with a small
superfluid stiffness and a pseudogap with d_{x^2-y^2} symmetry. The
Mott--Hubbard transition can be driven either by increasing the on-site Coulomb
repulsion, U, or by changing the anisotropy of the two hopping integrals, t'/t.
Our results suggest that the ratio t'/t plays an important role in determining
the phase diagram of the organic superconductors.Comment: 4 pages, 3 figur
Relaxation Dynamics of Photoinduced Changes in the Superfluid Weight of High-Tc Superconductors
In the transient state of d-wave superconductors, we investigate the temporal
variation of photoinduced changes in the superfluid weight. We derive the
formula that relates the nonlinear response function to the nonequilibrium
distribution function. The latter qunatity is obtained by solving the kinetic
equation with the electron-electron and the electron-phonon interaction
included. By numerical calculations, a nonexponential decay is found at low
temperatures in contrast to the usual exponential decay at high temperatures.
The nonexponential decay originates from the nonmonotonous temporal variation
of the nonequilibrium distribution function at low energies. The main physical
process that causes this behavior is not the recombination of quasiparticles as
previous phenomenological studies suggested, but the absorption of phonons.Comment: 18 pages, 12 figures; to be published in J. Phys. Soc. Jpn. Vol. 80,
No.
Effect of Impurity Scattering on the Nonlinear Microwave Response in High-Tc Superconductors
We theoretically investigate intermodulation distortion in high-Tc
superconductors. We study the effect of nonmagnetic impurities on the real and
imaginary parts of nonlinear conductivity. The nonlinear conductivity is
proportional to the inverse of temperature owing to the dependence of the
damping effect on energy, which arises from the phase shift deviating from the
unitary limit. It is shown that the final-states interaction makes the real
part predominant over the imaginary part. These effects have not been included
in previous theories based on the two-fluid model, enabling a consistent
explanation for the experiments with the rf and dc fields
Effect of Umklapp Scattering on Magnetic Field Penetration Depth in High-Tc Cuprates
The renormalization of the magnetic field penetration depth owing
to the electron-electron correlation is discussed with its application to
high- cuprates. The formula for the current carried by quasiparticle
with the Umklapp scattering is derived, on the basis of which we investigate
how the value of deviates from that of where and
are the carrier density and the effective mass respectively. Although
this deviation is small in the case of weak momentum dependence of the vertex,
this is large and negative owing to the non-negligible value of the backflow in
the case of the strong antiferromagnetic spin fluctuation. The observed doping
dependence of in high- cuprates, specifically a peak
structure at the slightly overdoped region, is explained by the analytical
consideration and the numerical calculation based on the perturbation theory
and the spin fluctuation theory. The consistency between and
at absolute zero, which is the problem the
isotropic model fails to explain, is also obtained by our theory.Comment: 25 pages, 9 figures. Another version(11 pages longer) will appear in
J. Phys. Soc. Jpn (2002) No.
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