Electro-Optic Studies of Charge Density Wave Conductors Joseph W. Brill, University of Kentucky, DMR One-dimensional charge-density-wave (CDW) materials are prototypes of collective periodic systems interacting with a random potential. An applied electric field polarizes the CDW, changing the infrared properties of the material. We are using this electro-optic effect to study the position and voltage dependence of the dynamics of CDW polarization. Shown in the figure is the frequency dependence of the relative change in transmission of a crystal of “blue bronze” (K 0.3 MoO 3 ) with IR light focused ~ ¼ mm from a current contact. Both the response in-phase and in quadrature with an applied square- wave voltage are shown. The response is basically that of an overdamped harmonic oscillator, with low resonant frequency of ~ 1 kHz; this sluggishness, which increases with distance from a contact, is a surprising manifestation of the disordered potential. In addition, the inversion of the low frequency quadrature response indicates that the CDW polarization decays at long times, a completely unanticipated result.

Electro-Optic Studies of Charge Density Wave Conductors Joseph W. Brill, University of Kentucky, DMR A charge-density-wave (CDW) is an instability that occurs in “quasi-one-dimensional” metals (i.e. metals which conduct electrical current much better in one direction than perpendicularly) in which the electron density varies with a typical wavelength of a few atomic spacings. In a handful of CDW materials, the CDW can be made to “slide” through the crystal by application of very small voltages (less than 1 volt across a centimeter), and conduct extra current. “CDW conductors” have been the subjects of intense study for almost thirty years, on the one hand, they are prototypes for the study of how a disordered potential interacts with an intrinsically periodic system; other examples include crystal growth, magnetic properties of superconductors, and earthquake faults. In addition, CDW conductors have been found to exhibit a variety of unusual, and in some cases unique, properties. One such property, discovered in our lab, is their “electro-optic” response: their infrared properties change when the CDW slides and becomes polarized (bunches up on one side of the sample). Our research is focused on trying to understand what this electro-optic effect is telling us about how the CDW moves and interacts with the electrons and ions in the crystal. An attractive aspect of the electro-optical measurements is that multiple electrical contacts, which generally disturb the CDW motion, do not have to be applied.

Electro-Optic Studies of Charge Density Wave Conductors Joseph W. Brill, University of Kentucky, DMR This research involves the unique interfacing of optical equipment not usually used together: tunable infrared diode lasers (not generally used for solid state spectroscopy) and an infrared microscope, and so provides a rich and unusual experience in optics and cryogenics for students. During the year, three beginning graduate students and one post-doc (M. Freamat, now on the faculty at SUNY Fredonia) participated in this research. This fall, one of the students will also be working on related research at the Advanced Light Source at Lawrence Berkeley National Laboratory – the first experiments on infrared modulation spectroscopy at the ALS.

.In the course of this NSF funded research, we have adapted commercial instrumentation for new applications and developed new techniques in solid-state spectroscopy that have allowed us to observe extremely small changes in materials properties; for example, we have routinely observed 10 parts/million changes in phonon (i.e. sound wave) frequencies. Students working on this project therefore get very broad experience in experimental techniques in solid state physics; previous students have moved on to a variety of careers in research, education, and even medical technology. Electro-Optic Studies of Charge Density Wave Conductors Joseph W. Brill, University of Kentucky, DMR