342 research outputs found
Role of confined phonons in thin film superconductivity
We calculate the critical temperature and the superconducting energy
gaps of a thin film superconductor system, where is the
superconducting energy gap of the -th subband. Since the quantization of
both the electron energy and phonon spectrum arises due to dimensional
confinement in one direction, the effective electron-electron interaction
mediated by the quantized confined phonons is different from that mediated by
the bulk phonon, leading to the modification of in the thin film system.
We investigate the dependence of and on the film thickness
with this modified interaction.Comment: 4 pages, 2 figure
Parity-Affected Superconductivity in Ultrasmall Metallic Grains
We investigate the breakdown of BCS superconductivity in {\em ultra}\/small
metallic grains as a function of particle size (characterized by the mean
spacing between discrete electronic eigenstates), and the parity ( =
even/odd) of the number of electrons on the island. Assuming equally spaced
levels, we solve the parity-dependent BCS gap equation for the order parameter
. Both the critical level spacing and the
critical temperature at which are parity
dependent, and both are so much smaller in the odd than the even case that
these differences should be measurable in current experiments.Comment: 4 pages RevTeX, 1 encapsulated postscript figure, submitted to
Physical Review Letter
Infrared Studies of the Onset of Conductivity in Ultra-Thin Pb Films
In this paper we report the first experimental measurement of the infrared
conductivity of ultra-thin quenched-condensed Pb films. For dc sheet
resistances such that the ac conductance increases with
frequency but is in disagreement with the predictions of weak localization. We
attribute this behavior to the effects of an inhomogeneous granular structure
of these films, which is manifested at the very small probing scale of infrared
measurements. Our data are consistent with predictions of two-dimensional
percolation theory.Comment: Submitted to Physical Review Letter
Orientational Phase Transition in Na_{x}C_{60} (1
X-ray diffraction and calorimetry data on cubic NaxC60(1\u3cx60, e.g., Tm(x=1.3)=325 K. The ordered phases are the same as in pure C60: simple cubic, space group Pa3¯, but the orientations in the disordered phase are more restricted. We explain how Na stabilizes the ordered phase to rather high T, while K and Rb do not, in terms of Coulomb interactions between C60 molecules and and Na ions which we calculate from the local charge density of C60
Extracting the electron--boson spectral function F() from infrared and photoemission data using inverse theory
We present a new method of extracting electron-boson spectral function
F() from infrared and photoemission data. This procedure is
based on inverse theory and will be shown to be superior to previous
techniques. Numerical implementation of the algorithm is presented in detail
and then used to accurately determine the doping and temperature dependence of
the spectral function in several families of high-T superconductors.
Principal limitations of extracting F() from experimental
data will be pointed out. We directly compare the IR and ARPES
F() and discuss the resonance structure in the spectra in
terms of existing theoretical models
Weak Localization Effect in Superconductors
We study the effect of weak localization on the transition temperatures of
superconductors using time-reversed scattered state pairs, and find that the
weak localization effect weakens electron-phonon interactions. With solving the
BCS equation, the calculated values for are in good agreement
with experimental data for various two- and three-dimensional disordered
superconductors. We also find that the critical sheet resistance for the
suppression of superconductivity in thin films does not satisfy the universal
behavior but depends on sample, in good agreement with experiments. but depends
on sample, in good agreement with experiments.Comment: 14 pages, Revtex, 5 ps figure
Structure and Phase Transitions of the 6, 6-Cyclopropane Isomer of C_ {61} H_ {2}
We have used x-ray powder diffraction and differential scanning calorimetry to study the crystalline structures and thermal behavior of the 6,6-cyclopropane isomer of C61H2. At room temperature, the C61H2 cyclopropane molecules, like those of the 6,5-annulene isomer and C60O epoxide, are orientationally disordered and crystallize on a face-centered-cubic lattice such that their methylene groups are statistically disordered among the octahedral voids. Unlike 6,5−C61H2 and C60O, the low-temperature structure is not Pa3¯, but rather a low-symmetry orthorhombic lattice in which a≈
Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs)
X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to KxC60 with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas phase with random occupancy of tetrahedral and octahedral sites, which is thus isostructural with superconducting K3C60
Orientational Phase Transition in NaxC60 (1 \u3c x \u3c 3)
X-ray diffraction and calorimetry data on cubic NaxC60(1\u3cxTm above that of pure C60, e.g., Tm(x=1.3)=325 K. The ordered phases are the same as in pure C60: simple cubic, space group Pa3¯, but the orientations in the disordered phase are more restricted. We explain how Na stabilizes the ordered phase to rather high T, while K and Rb do not, in terms of Coulomb interactions between C60 molecules and and Na ions which we calculate from the local density approximation charge density of C60
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