Text
stringlengths 113
37.6k
|
---|
Submitter: Bozidar Mitrovic
Authors: B. Mitrovic and M. Castle (Brock University, Canada)
Title: Strong coupling theory of the interlayer tunneling model for high
temperature superconductors
Abstract: The interlayer pair tunneling model of Anderson et al. is generalized to
include the strong coupling effects associated with in-plane interactions. The
equations for the superconducting transition temperature T_{c} are solved
numerically for several models of electron-optical phonon coupling. The
nonmagnetic in-plane impurity scattering suppresses T_{c} in all cases
considered, and it is possible to obtain a fair agreement with experiments for
a reasonable choice of parameters. For the anisotropic electron-phonon coupling
proposed by Song and Annett we find that the interlayer pair tunneling can
stabilize the d_{x^2-y^2}-wave superconducting state with a high T_{c}.
Moreover, in this case there is a possibility of an impurity induced crossover
from the d$_{x^2-y^2}$-wave state stabilized by the interlayer tunneling to the
s-wave state at a low impurity concentration. We also calculate the isotope
effect associated with the in-plane oxygen optic mode and its dependence on the
strength of the interlayer pair tunneling. Small positive values of the isotope
exponent are obtained for strengths of pair tunneling that give high transition
temperatures.
Journal: None |
Submitter: None
Authors: Francesca Federici, Andrei A. Varlamov
Title: The Fluctuation Induced Pseudogap in the Infrared Optical Conductivity
of High Temperature Superconductors
Abstract: We study the effect of fluctuations on the {\bf ac} conductivity of a layered
superconductor both for $c$-axis and $ab$-plane electromagnetic wave
polarizations. The fluctuation contributions of different physical nature and
signs (paraconductivity, Maki-Thompson anomalous contribution, one-electron
density of states renormalization) are found to be suppressed by the external
field at different characterisitic frequencies ($ \omega_{\rm AL}\sim T-T_{\rm
c}$, $\omega_{\rm MT} \sim \max\{ T-T_{\rm c}, \tau_{\varphi}^{-1}\}$, $
\omega_{\rm DOS} \sim \min\{T,\tau ^{-1}\}$ for the $2D$ case). As a result the
appearance of the nonmonotonic frequency dependence (pseudogap) in the infrared
optical conductivity of HTS film is predicted. The effect has to be especially
pronounced in the case of the electromagnetic field polarization along
$c$-axis.
Journal: None |
Submitter: Ruslan Prozorov
Authors: R. Prozorov, M. Konczykowski, B. Schmidt, Y. Yeshurun, A. Shaulov, C.
Villard, G. Koren
Title: On the origin of the irreversibility line in thin YBaCuO7 films with and
without columnar defects
Abstract: We report on measurements of the angular dependence of the irreversibility
temperature $T_{irr}(\theta) $ in $YBa_2Cu_3O_{7-\delta }$ thin films, defined
by the onset of a third harmonic signal and measured by a miniature Hall probe.
From the functional form of $T_{irr}(\theta)$ we conclude that the origin of
the irreversibility line in unirradiated films is a dynamic crossover from an
unpinned to a pinned vortex liquid. In irradiated films the irreversibility
temperature is determined by the trapping angle.
Journal: None |
Submitter: Durga P. Choudhury
Authors: Durga P. Choudhury, Balam A. Willemsen, John S. Derov and S. Sridhar
(Physics Department, Northeastern University and Rome Laboratory, Hanscom
AFB.)
Title: Nonlinear Response of HTSC Thin Film Microwave Resonators in an Applied
DC Magnetic Field
Abstract: The non-linear microwave surface impedance of patterned YBCO thin films, was
measured using a suspended line resonator in the presence of a perpendicular DC
magnetic field of magnitude comparable to that of the microwave field.
Signature of the virgin state was found to be absent even for relatively low
microwave power levels. The microwave loss was initially found to decrease for
small applied DC field before increasing again. Also, non-linearities inherent
in the sample were found to be substantially suppressed at low powers at these
applied fields. These two features together can lead to significant improvement
in device performance.
Journal: None |
Submitter: Durga P. Choudhury
Authors: Balam A. Willemsen, J. S. Derov and S.Sridhar (Physics Department,
Northeastern University and Rome Laboratory, Hanscom AFB)
Title: Critical State Flux Penetration and Linear Microwave Vortex Response in
YBa_2Cu_3O_{7-x} Films
Abstract: The vortex contribution to the dc field (H) dependent microwave surface
impedance Z_s = R_s+iX_s of YBa_2Cu_3O_{7-x} thin films was measured using
suspended patterned resonators. Z_s(H) is shown to be a direct measure of the
flux density B(H) enabling a very precise test of models of flux penetration.
Three regimes of field-dependent behavior were observed: (1) Initial flux
penetration occurs on very low field scales H_i(4.2K) 100Oe, (2) At moderate
fields the flux penetration into the virgin state is in excellent agreement
with calculations based upon the field-induced Bean critical state for thin
film geometry, parametrized by a field scale H_s(4.2K) J_c*d 0.5T, (3) for very
high fields H >>H_s, the flux density is uniform and the measurements enable
direct determination of vortex parameters such as pinning force constants
\alpha_p and vortex viscosity \eta. However hysteresis loops are in
disagreement with the thin film Bean model, and instead are governed by the low
field scale H_i, rather than by H_s. Geometric barriers are insufficient to
account for the observed results.
Journal: None |
Submitter: Hasegawa Yasumasa
Authors: Yasumasa Hasegawa (Himeji Institute of Technology)
Title: Density of States and NMR Relaxation Rate in Anisotropic
Superconductivity with Intersecting Line Nodes
Abstract: We show that the density of states in an anisotropic superconductor with
intersecting line nodes in the gap function is proportional to $E log (\alpha
\Delta_0 /E)$ for $|E| << \Delta_0$, where $\Delta_0$ is the maximum value of
the gap function and $\alpha$ is constant, while it is proportional to $E$ if
the line nodes do not intersect. As a result, a logarithmic correction appears
in the temperature dependence of the NMR relaxation rate and the specific heat,
which can be observed experimentally. By comparing with those for the heavy
fermion superconductors, we can obtain information about the symmetry of the
gap function.
Journal: None |
Submitter: Masanori Ichioka
Authors: Naoki Enomoto, Masanori Ichioka and Kazushige Machida (Okayama Univ.)
Title: Ginzburg Landau theory for d-wave pairing and fourfold symmetric vortex
core structure
Abstract: The Ginzburg Landau theory for d_{x^2-y^2}-wave superconductors is
constructed, by starting from the Gor'kov equation with including correction
terms up to the next order of ln(T_c/T). Some of the non-local correction terms
are found to break the cylindrical symmetry and lead to the fourfold symmetric
core structure, reflecting the internal degree of freedom in the pair
potential. Using this extended Ginzburg Landau theory, we investigate the
fourfold symmetric structure of the pair potential, current and magnetic field
around an isolated single vortex, and clarify concretely how the vortex core
structure deviates from the cylindrical symmetry in the d_{x^2-y^2}-wave
superconductors.
Journal: J. Phys. Soc. Jpn. 66, 204 (1997). |