What the resonance peak cannot do

In certain cuprates, a spin 1 resonance mode is prominent in the magnetic structure measured by neutron scattering. It has been proposed that this mode is responsible for significant features seen in other spectroscopies, such as photoemission and optical absorption, which are sensitive to the charge dynamics, and even that this mode is the boson responsibile for ``mediating'' the superconducting pairing. We show that its small (measured) intensity and weak coupling to electron-hole pairs (as deduced from the measured lifetime) disqualifies the resonant mode from either proposed role.Comment: 4 pages, no figur

Carrier relaxation, pseudogap, and superconducting gap in high-Tc cuprates: A Raman scattering study

We describe results of electronic Raman-scattering experiments in differently doped single crystals of Y-123 and Bi-2212. The comparison of AF insulating and metallic samples suggests that at least the low-energy part of the spectra originates predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach. Dynamical scattering rates and mass-enhancement factors for the carriers are obtained. In B2g symmetry the Raman data compare well to the results obtained from ordinary and optical transport. For underdoped materials the dc scattering rates in B1g symmetry become temperature independent and considerably larger than in B2g symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in B2g symmetry in the range between the superconducting transition at Tc and a characteristic temperature T* of order room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity related features in the spectra can be observed only in B2g symmetry. The peak frequencies scale with Tc. We do not find a direct relation between the pseudogap and the superconducting gap.Comment: RevTeX, 21 pages, 24 gif figures. For PostScript with embedded eps figures, see http://www.wmi.badw-muenchen.de/~opel/k2.htm

AOT microemulsions: droplet size and clustering in the temperature range between the supercooled state and the upper phase boundary

The main focus of this work is the investigation of the mechanism of droplet aggregation and the droplet structure of water-AOT (sodium bis(2-ethyl hexyl) sulfosuccinate)-decane microemulsions between the supercooled state and the upper phase boundary by means of SAXS. We study the temperature dependent droplet structure and size in detail at compositions of droplet volume fractions curly or open phi = 0.1-0.4 and water/AOT molar ratios of omega = 5-40. We found a change in droplet size at varying temperatures and explain our results by the change of the effective droplet surface covered by an AOT molecule (between the two phase boundaries) and by macro-phase separated water ice below a certain supercooling temperature. We monitor the continuous change of droplet interaction by means of a phenomenological structure factor combining hard sphere interactions with a Lorentzian function accounting for droplet density fluctuations in the system. We analyse the droplet aggregates in dependency on phi omega leading to a better understanding of droplet aggregation with increasing temperature