Optical spectra of high-transition-temperature superconductors in the mid-infrared display a gap of in-plane conductivity whose role for superconductivity remains unresolved. Femtosecond measurements of the mid-infrared reflectivity of YBa(2)Cu(3)O(7-delta) after nonequilibrium optical excitation are used to demonstrate the ultrafast fill-in of this gap and reveal two gap constituents: a picosecond recovery of the superconducting condensate in underdoped and optimally doped material and, in underdoped YBa(2)Cu(3)O(7-delta), an additional subpicosecond component related to pseudogap correlations. The temperature-dependent amplitudes of both contributions correlate with the antiferromagnetic 41-millielectronvolt peak in neutron scattering, supporting the coupling between charges and spin excitations.

No abstract

We introduce a simple empirical rule wherein the pairing interaction in superconductors is cancelled when normal and umklapp phonon scattering coexist. Superconductivity then arises solely from the residual umklapp contribution. As a result the deduced electron-phonon interactions in niobium, tantalum, lead and aluminum become virtually identical in the normal and superconducting states. Transition temperatures calculated under the rule are accurate within a few per cent when compared with experimental data. Features of the Matthias relations are also explained. The high T c so far predicted for metallic hydrogen is probably overly optimistic.

Experimental evidence for flux-line cutting in superconductors (intersection and cross-joining of singly quantized vortices) is briefly reviewed. The interaction energy between two straight vortices tilted at an angle a (~ O)is thenshown to be finite in the London model, i.e., in the limit of vanishing core radius. Next, the activation energy and maximum interaction force are calculated for the vortices in an analytic approximation to the GinzburgLandau theory. Here two competing interactions determine the behavior. Electromagnetic repulsion (0 < a < 7r/2) varies as cos a and decays over distances scaled by the penetration depth A, while core attraction is independent of a and varies over distances scaled by the coherence length ~.The force is always repulsive at large flux-line separation (19 < ~ < Ir/2) and its maximum decreases rapidly as K decreases, so that flux-line cutting is expected to be more probable in low-K materials. The calculations provide a basis for explaining longitudinal flux-flow resistance as well as some intriguing magnetization behavior in the same configuration.

No abstract

No abstract

Coulomb repulsion among the many electrons in a metal is in a balance, which can be toppled by even a weak electron-phonon attractive interaction. Therefore neglecting the Coulomb term from the BCS reduced Hamiltonian has little effect on T c . This is shown by a field-theoretic argument, an analysis based on the Bogoliubov model potential and a direct numerical calculation. Detailed knowledge about electrons and phonons for various materials can be incorporated into the BCS theory through a refined treatment of the self-consistent gap equation. Consequently the universal ratio 3.5 in the BCS theory is replaced by a range of values varying from 3.51 for Ga to 4.76 for Hg. It is found that the phonon cutoff frequency is much lower than the Debye frequency. Extraordinarily high T c could be expected if all phonons were involved in pairing electrons in a BCS superconductor.

A study of the inelastic electron tumelhg spectra GETS) of formic acid, [2H2]formic acid, acetic acid, [2H4]acetic acid and trifiuoroacetic acid shows that these chemisorb from the vapour phase on to plasma-grown aluminium oxide as surface-bound formates and acetates. Detailed vibrational mode band assignments are made and the carboxylates are deduced to be adsorbed symmetrically on, though not necessarily perpendicular to, the oxide surface. The plasma-grown oxide shows enhanced reactivity to acetaldehyde over bulk y-alumina behaviour. Evidence is obtained for the involvement of two surface-site types in reaction with trifluoroacetic acid.