ELECTRONIC TRANSPORT IN MOLYBDENUM-SESQUISULFIDE

This dissertation presents the results of detailed investigations of the conductivity, thermoelectric power, and conductivity fluctuations in Mo(,2)S(,3). In particular, a very unusual time-dependent conductivity is observed. Single crystals of Mo(,2)S(,3) exist as long, needle-shaped fibers which have a metallic appearance. At room temperature, the crystal structure is monoclinic, with zig-zag molybdenum chains which run parallel to the crystalline b-axis. The electrical resistivity of Mo(,2)S(,3) shows the existence of two first-order phase transitions at 182 K and 145 K on cooling. In addition, a large peak in the resistivity is observed at 80 K. Below 80 K, the resistivity is metal-like, decreasing with decreasing temperature. The time-dependent conductivity measurements were made at temperatures between 75 K and 300 K first by heating the Mo(,2)S(,3) sample a few degrees and then rapidly quenching (in a few milliseconds). For temperatures below 110 K, the sample conductivity was observed to slowly decrease after thermal quenching, with a characteristic time constant varying between 10 mS (110 K) and several minutes (75 K). These results show that a relatively high conductivity metastable state exists for the charge carriers. All of the time-dependent conductivity measurements can be explained by a double-well potential model for the carriers. The conductivity fluctuations (electrical noise) were measured by passing a constant electric current through the sample, and measuring the electrical noise thereby generated. The frequency dependence of the noise is completely consistent with the double-well potential model used to describe the time-dependent conductivity measurements. The thermoelectric power measurements show that the dominant charge carriers in Mo(,2)S(,3) are holes. The time-dependent thermoelectric power measurements show a behavior similar to the time-dependent conductivity. Two physical models for the charge carriers in Mo(,2)S(,3) appear most likely to be able to explain these experimental results. Both a charge-density wave model, and an acoustic polaron model are discussed at some length, and further investigations to help determine which, if either, of these models is correct are suggested