Using phonon imaging to probe anisotropy in the superconducting energy gap of lead.

摘要

A new application of phonon-imaging techniques is demonstrated to probe the superconducting state of Pb. I have observed highly selective attenuation of phonons with wavevectors in {lcub}111{rcub} planes, inspiring an investigation of the superconducting properties of Pb. This thesis will consist of several parts that investigate and develop a possible explanation of the attenuation of {lcub}111{rcub} phonons.; Most probes of the superconducting state used for Pb have been either insensitive to directions of reduced gap or limited to probing the gap along a few symmetry directions. In this work, I will attempt to use the scanning nature of phonon-imaging experiments to overcome this limitation and determine if an anisotropic gap explains the phonon data in Pb.; A crucial observation is that the anisotropic attenuation increases with increasing sample temperature. This temperature dependence is consistent with the earlier specific-heat and ultrasound-attenuation experiments that suggest regions of reduced gap for Pb. However, a significant difference from these earlier experiments is that the directional dependence of phonon attenuation in these experiments will be used to gain insight into the locations of reduced gap.; I will show that the shape of the Fermi surface has profound consequences on phonon scattering and may have important implications on a successful theory of a highly anisotropic energy gap in Pb. The only theory, so far, to explain the anomalous superconducting properties of Pb postulates the presence of a spin-density-wave (SDW) ground state. I will show that, at present, most of the experimental data can be explained by assuming that the ground state of Pb is described by a family of twelve [211] SDWs.; I will show that the short laser pulses used to generate phonons create a localized phonon source with a lifetime up to 50 times longer than the laser duration. In investigating the temporal characteristics of the phonon source in Pb, an unexplained phonon pulse arriving after the ballistic propagation time is discussed. A new method for low-pass filtering phonons arriving at the detector is demonstrated in the current attempts to explain this anomalous pulse. (Abstract shortened by UMI.)