Electro-Optic Studies of Charge Density Wave Conductors

University Of Kentucky Research Foundation, Lexington KY

Investigators

Abstract

Charge-density-wave (CDW) materials in which the CDW can be depinned with a small electric field serve as model systems studies of the interactions of a periodic medium with quenched disorder. This single-investigator award supports the continued investigation of an unusual electro-optic effect, associated with CDW polarization in an applied field. The effect was discovered by the P.I.'s group several years ago. This effect is unlike conventional electro-optic effects. It occurs over a wide spectral range (at infrared energies below 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 within the sample, changing sign in the sample's center. The emphasis of this project will be to more fully characterize the effect with the aim of obtaining a better understanding of its underlying physics. In the process an increased understanding of CDW interactions with phonons and uncondensed electrons, and the dynamics of CDW polarization will be obtained. Materials to be studied are the quasi-one dimensional CDW conductors tantalum trisulfide, niobium triselenide, and potassium molybdate ("blue bronze"). Electro-reflectance and electro-transmittance experiments at the University of Kentucky, carried out with tunable diode lasers, will be complemented with measurements at the Advanced Light Source, providing a broad educational experience for the students working on the project. Materials whose optical properties are modified by application of a voltage are known as electro-optic materials. They are the basis of optical communications and therefore are of increasing technological importance. Recently, the P.I.'s group at the University of Kentucky discovered that "charge-density-wave" (CDW) conductors, materials in which the electron density oscillates with a typical wavelength of one nanometer (one billionth of a meter), exhibit a number of very unusual electro-optic properties. In particular, voltage dependent changes in the transmittance and reflectance of electro-magnetic waves occur over a very wide infrared spectral range. These changes occur at very small voltages (e.g. 0.01 V across a 1 mm long sample), and vary with position within the sample.. This award supports a project to continue characterization of the effect in a few CDW materials, with the goal of better understanding its underlying physics and the properties of CDW's in general. Experiments will be carried out using tunable infrared diode lasers, representing one of the few applications of these devices for solid state spectroscopy. These studies will be complemented with experiments at the Advanced Light Source at Lawrence Berkeley National Laboratory, providing a broad instructional experience for the students working on the project.