Subpicosecond time-resolved spectroscopy of high-transition-temperature superconductors.

Abstract

A better understanding of the electronic properties of high-transition-temperature superconductors is needed to fully exploit their current applications as well as promote the further development of new materials and devices. The goal of this work was to exploit the unique offerings of ultrafast, laser-based, time-domain techniques to study the carriers, lattice, and carrier-lattice coupling in various high-$T\sb{c}$ systems. This was accomplished in two unique ways. Through the use of short optical pulses ($\sim$100-fs) combined with the photoconductive effect, the first technique utilized the ability to generate bursts of coherent radiation. This generated radiation acted as a coherent probe of the carriers in the terahertz regime. The second technique was an all optical one, and used the short optical pulses directly, either to create and probe nonequilibrium carrier distributions and their subsequent relaxation in various high-$T\sb{c}$ systems or to impulsively excite and probe lattice vibrations, directly in the time domain. This research produced a number of significant contributions, some of which are listed below. An extension of the coherent spectroscopy technique to the study of thin-film superconductors was made. With this system, the complex conductivity of a YBa$\sb2$Cu$\sb3$O$\sb7$ thin-film was directly extracted without the need for a Kramers-Kronig analysis. The measured conductivity, exhibited a qualitatively similar behavior as that predicted for weak coupling superconductors outside the extreme clean limit. However, the reasons behind this behavior may be quite different from the coherence effects responsible for the behavior in conventional superconductors. An investigation into the nonequilibrium carrier dynamics of two cuprate superconductors (YBa$ \sb2 $Cu$ \sb3 $O$ \sb{7-x} $ and Bi$ \sb2 $Sr$ \sb2 $Ca$ \sb2 $Cu$ \sb3 $O$ \sb{10} $) was made using femtosecond optical pulses. From these experiments it was determined that the coupling parameter was too low for the observed $T\sb{c}$ in these materials (within the Eliashberg framework). With this same technique, the first observation of impulsively generated, coherent phonons in a high-$T\sb{c}$ cuprate compound was made. From temperature dependent studies, the first direct observation of an enhanced quasiparticle lifetime in high-$T\sb{c}$ materials was made. This is believed to be a direct result of relaxation of the order parameter.