The study of electron coherence effects in metallic systems with high-frequency AC electric fields: Weak localization and mesoscopic photovoltaic effects

Jin Liu, Purdue University

Abstract

We have studied the effect of a high-frequency microwave electric field on electron phase coherence in thin Sb films and wires. The phase coherence is monitored through the effect of weak localization on the conductance. Through careful experimental design, we were able to calibrate the high frequency electric field. The ac effect was separated from the Joule heating with either dc measurements or the application of a magnetic field. This has made it possible to make a detailed, quantitative comparison with the theory with no free parameters. We have found good agreements between the experiments and the theory for both one and two dimensional cases. We have used the simple dc heating experiment to study the electron heating effects in Sb films. The electron temperature was reflected in the resistance, as an especially striking manner, to be quite different from the lattice temperature. This experiment was also used to study the electron-phonon scattering time in thin Sb films in the temperature range 1-4K. The magnitude of the scattering time is in reasonable accord with the theory, while the temperature dependence is of the form $\tau\sb{E\sb{ph}}$ $\sim$ $T\sp{-\alpha}$, with $\alpha$ $\sim$ 1.4. The value of $\alpha$ appears to be significantly smaller than predicted by the theory, and is not understood. We have also studied the high frequency heating effects of thin AuPd, AuFe, and Au films at low temperatures. The analysis of the experiments yield consistent results with the theory for AuPd films with high values of the sheet resistance. However, for low-sheet-resistance films of AuPd, AuFe, and Au, the analysis suggests either that Joule heating is suppressed at microwave frequencies (as compared with that found for the same field strength at lower frequencies), or that a microwave field enhances the contribution of electron-electron interactions to the resistance. Either of these results would be at odds with current theories. Another experiment in which we were involved was the response of a mesoscopic film to microwave radiation. We have found that the system acts as a rectifier, resulting in a dc current whose magnitude is proportional to the microwave power. This dc current is sensitive to temperature, magnetic field, and the precise location of the scattering centers within the sample. The results are generally in good agreement with recent theoretical predictions, although the temperature dependence is somewhat weaker than expected. The symmetry of the dc current upon the reversal of the magnetic field was also studied. We have found both symmetric and antisymmetric behaviors. The symmetric behavior was thought to be from pure sample, while the antisymmetric to be due to the magnetic impurities in the junction. In any of above cases a theoretical explanation are not given.

Degree

Ph.D.

Advisors

Giordano, Purdue University.

Subject Area

Condensation|Electromagnetism

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