58 research outputs found
Transmission Coefficient as a Three-Point Retarded Function
We show that the transmission probability through a small interacting region
connected to noninteracting leads, can be written in terms of a retarded
product of a three-point correlation function defined in the real time. Our
proof is based on the Kubo formalism, and uses an Eliashberg theory for
analytic properties of vertex functions. The aim of this short report is to add
a new viewpoint to the transport theory described in the previous paper: A.O.,
J. Phys. Soc. Jpn. 70 (2001) 2666.Comment: 2 pages, 2 figures, submitted to J. Phys. Soc. Jp
Electron-phonon coupling and its evidence in the photoemission spectra of lead
We present a detailed study on the influence of strong electron-phonon
coupling to the photoemission spectra of lead. Representing the strong-coupling
regime of superconductivity, the spectra of lead show characteristic features
that demonstrate the correspondence of physical properties in the normal and
the superconducting state, as predicted by the Eliashberg theory. These
features appear on an energy scale of a few meV and are accessible for
photoemission only by using modern spectrometers with high resolution in energy
and angle.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
The motion of superconducting vortices in thin films of varying thickness
The interaction of superconducting vortices with superconductor/vacuum interfaces is considered. A vortex is first shown to intersect such an interface normally. Various thin-film models are then formulated, corresponding to different parameter regimes. A local analysis of a vortex is performed, and a law of motion for each vortex deduced. This law of motion implies that the vortex will move to the locally thinnest part of the film, and is consistent with the vortex moving under the curvature induced by being forced to intersect the boundaries of the film normall
Electrical Conductivity of Fermi Liquids. II. Quasiparticle Transport
We develop a general theory of Fermi liquids to discuss the Kadowaki-Woods
relation . We derive a formula for the ratio
which is expressed as a product of two dimensionless parameters and
, where represents a coupling constant for quasiparticle scattering
and is a geometric factor determined by the shape of the Fermi surface.
Then we argue that the universal ratio observed in heavy fermion compounds is
reproduced under the conditions and . The former is
regarded as a universality of Fermi liquids in a strong coupling regime, and
the latter is corroborated by evaluating definitely in simple cases. It is
noted that the proportional relation is just an example of the universal
phenomena to be expected for the whole class of strong coupling Fermi liquids.Comment: 28 pages, 7 figures; J. Phys. Soc. Jpn. Vol.67, No.1
The mechanism of hole carrier generation and the nature of pseudogap- and 60K-phases in YBCO
In the framework of the model assuming the formation of NUC on the pairs of
Cu ions in CuO plane the mechanism of hole carrier generation is
considered and the interpretation of pseudogap and 60 K-phases in
. is offered. The calculated dependences of hole
concentration in on doping and temperature
are found to be in a perfect quantitative agreement with experimental data. As
follows from the model the pseudogap has superconducting nature and arises at
temperature in small clusters uniting a number of
NUC's due to large fluctuations of NUC occupation. Here and
are the superconducting transition temperatures of infinite and finite
clusters of NUC's, correspondingly. The calculated and
dependences are in accordance with experiment. The area between
and corresponds to the area of fluctuations
where small clusters fluctuate between superconducting and normal states owing
to fluctuations of NUC occupation. The results may serve as important arguments
in favor of the proposed model of HTSC.Comment: 12 pages, 7 figure
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.
Electrical Conductivity of Fermi Liquids. I. Many-body Effect on the Drude Weight
On the basis of the Fermi liquid theory, we investigate the many-body effect
on the Drude weight. In a lattice system, the Drude weight is modified by
electron-electron interaction due to Umklapp processes, while it is not
renormalized in a Galilean invariant system. This is explained by showing that
the effective mass for is defined through the current, not
velocity, of quasiparticle. It is shown that the inequality is required
for the stability against the uniform shift of the Fermi surface. The result of
perturbation theory applied for the Hubbard model indicates that as a
function of the density is qualitatively modified around half filling
by Umklapp processes.Comment: 20 pages, 2 figures; J. Phys. Soc. Jpn. Vol.67, No.
Spin diffusion and relaxation in three-dimensional isotropic Heisenberg antiferromagnets
A theory is proposed for kinetic effects in isotropic Heisenberg
antiferromagnets at temperatures above the Neel point. A metod based on the
analysis of a set of Feynman diagrams for the kinetic coefficients is developed
for studying the critical dynamics. The scaling behavior of the generalized
coefficient of spin diffusion and relaxation constant in the paramagnetic phase
is studied in terms of the approximation of coupling modes. It is shown that
the kinetic coefficients in an antiferromagnetic system are singular in the
fluctuation region. The corresponding critical indices for diffusion and
relaxation processes are calculated. The scaling dimensionality of the kinetic
coefficients agrees with the predictions of dynamic scaling theory and a
renormalization group analysis. The proposed theory can be used to study the
momentum and frequency dependence of the kinetic parameters, and to determine
the form of the scaling functions. The role of nonlocal correlations and
spin-liquid effects in magnetic systems is briefly discussed.Comment: 10 pages, RevTeX, 3 EPS figures include
On the mechanisms of superfluidity in atomic nuclei
A system of equations is obtained for the Cooper gap in nuclei. The system
takes two mechanisms of superfluidity into account in an approximation
quadratic in the phonon- production amplitude : a Bardeen- Cooper- Schrieffer
(BCS) type mechanism and a quasiparticle- phonon mechanism. These equations are
solved for 120 Sn in a realistic approximation. If the simple procedures
proposed are used to determine the new particle- particle interaction and to
estimate the average effect, then the contribution of the quasiparticle- phonon
mechanism to the observed width of the pairing gap is 26% and the BCS-type
contribution is 74%. This means that at least in semimagic nuclei pairing is of
a mixed nature - it is due to the two indicated mechanisms, the first being
mainly a surface mechanism and the second mainly a volume mechanism.Comment: 6 page
What are the experimentally observable effects of vertex corrections in superconductors?
We calculate the effects of vertex corrections, of non-constant density of
states and of a (self-consistently determined) phonon self-energy for the
Holstein model on a 3D cubic lattice. We replace vertex corrections with a
Coulomb pseudopotential, mu*, adjusted to give the same Tc, and repeat the
calculations, to see which effects are a distinct feature of vertex
corrections. This allows us to determine directly observable effects ofvertex
corrections on a variety of thermodynamic properties of superconductors. To
this end, we employ conserving approximations (in the local approximation) to
calculate the superconducting critical temperatures, isotope coefficients,
superconducting gaps, free-energy differences and thermodynamic critical fields
for a range of parameters. We find that the dressed value of lambda is
significantly larger than the bare value. While vertex corrections can cause
significant changes in all the above quantities (even whenthe bare
electron-phonon coupling is small), the changes can usually be well-modeled by
an appropriate Coulomb pseudopotential. The isotope coefficient proves to be
the quantity that most clearly shows effects of vertex corrections that can not
be mimicked by a mu*.Comment: 28 pages, 12 figure
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