Resonant Ultrasound Studies of Complex Materials: Clathrates and Transition Metal Oxides

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*** NON-TECHNICAL ABSTRACT ***One of the most exciting themes of modern condensed matter physics is the discovery and investigation of novel phenomena and new forms of order in materials systems of ever increasing complexity. It is one of the goals of the proposed program to study the elastic properties (i.e., how a material responds to strain) of novel, complex materials. Two kinds of complex materials will be studied: transition metal oxides and so-called inclusion compounds (compounds with a cage-like structure). The latter are promising for the development of better thermoelectric devices, which convert heat to electricity (and vice-versa) without moving parts or chloroflurocarbon (CFC)-containing refrigerants. The elastic behavior of these complex materials can be quite exotic, and this research program will explore to what extent the behavior can be understood with recently developed theories. Both senior undergraduate and graduate students will gain in-depth experience in the field of elastic properties and low temperature physics, and will acquire skills in various techniques in materials science. This proposal includes the development of outreach activities to undergraduate students in the department. Special efforts will be made to reach female students.*** TECHNICAL ABSTRACT ***This research program is designed to systematically study the elastic response of complex materials, including transition metal oxides (TMOs) and inclusion compounds. The materials that will be investigated are at the forefront of condensed matter physics and include the layered perovskite ruthenates Ca_(2-x)Sr_(x)RuO_(4), the triangular lattice compound Na_(x)CoO_(2), and inclusion compounds such as YbFe_(4)Sb_(12) and type-II clathrates such as Rb_(8)Na_(16)Si_(136). Resonant Ultrasound Spectroscopy will be routinely used from 2-400 K to obtain the elastic moduli. Pulse-echo and attenuation measurements in a He3-cryostat will be performed as appropriate. Scientifically, there are a number of issues that need to be resolved. One issue involves the relationship between inclusion compounds and structural glasses. Specifically, are tunneling states intrinsic to inclusion compounds? If so, can the tunneling entities be identified? Another issue involves structure-property relationships. To what extent can various structural features be used to predict the properties of the material? Can the elastic properties of TMOs be understood with recently developed ideas involving geometric and other types of frustration, nano-scale phase separation, and quantum-criticality? Both senior undergraduate and graduate students will gain in-depth experience in the field of elastic properties and low temperature physics, and will acquire skills in various techniques in materials science. This proposal includes the development of outreach activities to undergraduate students in the department. Special efforts will be made to reach female students.