The search for anyon superconductivity: Do high-temperature superconductors exhibit a spontaneous T-violating circular dichroism?

摘要

The subject of this thesis is an investigation of whether a class of cuprate compounds having superconducting transition temperatures as high as 100 K change the optical properties of reflected light in such a way that would indicate the violation of time-reversal symmetry (T-violation), analogous to that observed in ferromagnets. Such an effect could be interpreted as evidence that anyons are the fundamental microscopic charge carriers responsible for the superconductivity. An "anyon" is a new, fundamental, excitation of a strongly-correlated two-dimensional electron gas which can obey particle-exchange statistics anywhere between the Fermi-Dirac or Bose-Einstein limits.; In this thesis, I report on an experiment in which I attempted to reproduce the data originally reported by K. B. Lyons, et al., of AT&T Bell Laboratories. Their data purportedly showed that certain high temperature superconducting compounds exhibit a spontaneous circular dichroism in reflection, below about 200 K. I show a simple extension to their original experiment, which I discovered, that discriminates against an important spurious term, and makes my apparatus exclusively sensitive to T-violating circular dichroism. I found no evidence for a temperature dependent signal in both thin films and an untwinned single crystal of YBa{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb{lcub}7-delta{rcub}{dollar} (T{dollar}sb{lcub}rm c{rcub}{dollar} {dollar}approx{dollar} 90 K), to an accuracy approaching 10{dollar}sp{lcub}-6{rcub}.{dollar} However, my data does show a temperature independent "background" signal which is correlated with the optical quality of the sample's surface. I offer a detailed explanation of the origins of this background signal, and show how it can become a temperature dependent signal when there are changes to the surface properties of the sample due to condensation of residual gases in a poor vacuum.; Although this experiment shows no evidence for the macroscopic breakdown of time-reversal symmetry, anyons could still be at the heart of high temperature superconductivity, since there are numerous mechanisms that can hide microscopic T-violation. Even though I did not find evidence for the elusive anyon superconductor, I did achieve an unprecedented level of accuracy in measuring minute changes in the polarization state of light.