No abstract

Large polarons have been of significant recent technological interest as they screen and protect electrons from point-scattering centers. Anatase TiO2 is a model system for studying large polarons as they can be studied systematically over a wide range of temperature and carrier density. The electronic and magneto transport properties of reduced anatase TiO2 epitaxial thin films are analyzed considering various polaronic effects. Unexpectedly, with increasing carrier concentration, the mobility increases, which rarely happens in common metallic systems. We find that the screening of the electron–phonon (e–ph) coupling by excess carriers is necessary to explain this unusual dependence. We also find that the magnetoresistance could be decomposed into a linear and a quadratic component, separately characterizing the carrier transport and trapping as a function of temperature, respectively. The various transport behaviors could be organized into a single phase diagram, which clarifies the evolution of large polaron in this material.

No abstract

The asymmetric response of the counter and base electrodes of x-ray detectors employing polycrystalline niobium tunnel junctions has been studied by low-temperature scanning electron microscopy ͑LTSEM͒. LTSEM has revealed that the base electrode produces a signal more than two times larger than the counter electrode, and effective quasiparticle lifetimes are 100 ns in the counter and 280 ns in the base. Based on the I-V characteristics and the measured effective quasiparticle lifetimes, we propose a model structure for a spatial variation of the gap parameter ⌬, which involves proximity quasiparticle trapping layers. The small counter signal is caused by the shorter quasiparticle lifetime and the trapping effect. The LTSEM results are consistent with x-ray spectra for a radioactive 55 Fe source. ͓S0163-1829͑96͒06437-5͔

Nb-based superconducting tunnel junctions are being developed as high energy resolution X-ray detectors. Unfortunately, loss of excess quasiparticles at the edges, combined with lateral diffusion, results in an inhomogeneous response. To study this degradation of energy resolution, we manufactured detectors with a Ta trap in the top or bottom electrode away from the tunneling barrier. Excess quasiparticles in this so-called killed electrode will be trapped effectively and thus removed from the tunneling process. The X-ray spectra of the active electrode can be fitted with a model based on classical diffusion of quasiparticles. On junctions with a killed bottom electrode also Low Temperature Scanning Electron Microscopy (LTSEM) measurements have been performed. The X-ray spectra and the LTSEM scans are consistent with each other and with the model. The energy resolution of the junctions presented here is limited by loss of quasiparticles at the edges.