Electro-Optic Studies of Charge Density Wave Conductors

Intellectual Merit: Charge-density-wave (CDW) materials in which the CDW can be depinned with a small electric field exhibit some of the most unusual properties of any solids and serve as model systems for the study of the interactions of a periodic medium with quenched disorder. This proposal is to continue studying the unusual electro-optic effect, associated with CDW polarization in an applied field, discovered by the P.L's group several years ago. This effect is unlike conventional electro-optic effects in that it occurs over a wide spectral range (all energies within the CDW gap), occurs at very low electric fields (less than 0.1 V/cm, corresponding to the fields needed to depin the CDW), and varies spatially in the sample, changing sign in its center. The emphasis of the proposal will be to more fully characterize the effect with the aim of getting a better understanding of the underlying physics governing the effect and CDW dynamics and interactions in general. These unique experiments will principally be carried out in the P.I's laboratory at the University of Kentucky, using tunable diode lasers as light sources and an infrared microscope to focus and position the light on the small samples. However, some complementary measurements will also be done using a step-scan FTIR at the Advanced Light Source at Lawrence Berkeley National Laboratory (with the group of Michael Martin) when improved sensitivity is desired. Materials to be studied are the canonical quasi-one dimensional CDW conductors tantalum trisulfide, niobium triselenide, and potassium molybdate ("blue bronze"). Both changes in transmittance and reflectance will be measured. Three types of measurements will be done. 1) We will measure the frequency/spatial dependence of the electro-optic response, to better understand the dynamics ofCDW polarization. 2) We will do spectroscopic measurements, to better characterize and understand the surprising changes in phonon properties when the CDW is polarized. We will extend previous measurements in blue bronze to liquid helium temperatures to search for changes that accompany "rigid" CDW motion, in contrast to the changes we've observed at higher temperature when the CDW slides with deformations. 3) Spectroscopic measurements will also be used to search for intragap states, undetected until now but believed to be necessary for CDW motion. For these, measurements on tantalum trisulfide are very desirable because intragap states have been inferred from previous measurements, while measurements on niobium triselenide are desirable because they will present a "clean" slate with very little electro-optic response expected from phonons or uncondensed electrons. Broader Impact: The P.L has a long history of providing graduate students with research training that has allowed them to pursue a range of careers in condensed matter physics and related areas. Two Ph.D. students will carry out the proposed research and will receive thorough training in optical and cryogenic techniques. In addition to their research at the University of Kentucky, they will spend several weeks/year working with the synchrotron source at the ALS and gain experience in complementary techniques and in doing research at this major user facility. At U.K. the students will interact regularly with a growing group of students, visitors, and faculty working on collective effects in low-dimensional materials. They will also attend and make presentations at national and international meetings.