U.S.-France Cooperative Research: Charge-Density-Wave Materials and Dynamics

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This award will support a US-French collaborative research project in condensed matter physics. The US investigators are: Dr. Robert Thorne, Cornell University, Dr. Joseph Brill, University of Kentucky, Dr. Guebre Tessema, Clemson University, and Dr. T. M. Tritt, Naval Research Laboratory. The French counterparts are: Dr. Pierre Monceau, Center for Research on Very Low Temperatures, Grenoble, and Drs. J. Rouxel and M. Sargent from the Laboratories of Solid State Chemistry in Nantes and Rennes. Charge density wave (CDW) conductors are among the richest systems in condensed matter physics, exhibiting collective charge transport with many analogies to superconductivity and remarkable dynamical effects associated with disorder and many degrees of freedom. The goals of this project are 1) to prepare new and improved materials, 2) to understand the elastic anomalies associated with CDW pinning, and 3) to understand the stress-induced density wave transition in bi- metallic selenides. The project will be divided into three parts as follows: 1) Poor material quality has been a major obstacle to progress in understanding CDW systems. The research groups at Rennes and Nantes have great expertise in preparing novel inorganic low-dimensional conductors, while the Cornell group has been developing methods for preparing higher purity crystals of the most widely studied CDW conductors. A collaboration between these laboratories is proposed to prepare crystals with improved purity and morphology, and to identify and explore other promising materials systems. 2) The highly anomalous electromechanical properties of CDW conductors are poorly understood. High- sensitivity elastic modulus measurements in high magnetic fields and ultrasound measurements are proposed by the Kentucky and Grenoble groups to examine the role of the CDW amplitude, normal carrier screening and CDW-phonon interactions in these properties. 3) A novel stress-induced metal semi-conductor phase transition and non-ohmic conduction have recently been discovered by the Clemson, Rennes and Grenoble groups in the quasi-one-dimensional conductor tantalum-molybdenum selenide. In collaboration with the Naval Research Laboratory, X-ray and magnetotransport studies in stressed crystals are proposed to establish the nature of the low-temperature density wave state.