Current Projects:   

  • New Semiconductors for Thermoelectric Applications; Vibrational and heat-transfer properties, and emerging electronic behavior.

Thermoelectric materials are solids which can generate electricity when a temperature gradient is applied. By developing new such materials we may thus be able to capture otherwise wasted heat. Since currently 60% of usable power world-wide escapes as waste heat this has the potential provide considerable energy savings. In addition, through the reverse process (Peltier cooling) thermoelectric materials have the potential to provide high-efficiency localized cooling for many applications.

Some of the ongoing work in our laboratory includes studies aimed at providing a better fundemental understanding of the atomic-scale thermal and vibrational properties of such materials. We are using NMR spectroscopy to study anharmonic vibrational behavior of encapsulated ions, and the corresponding effect on the thermoelectric properties. Modeling by ab initio methods also helps us to understand the physical structure of substituted materials, and we are studying thermoelectric, transport, and magnetic behavior of these materials as well. Some of this work has focused on group-IV clathrate materials, with fullerene-type cages encapsulating rare earth and alkaline earth ions. The encapsulated ions can contribute to glass-like vibrational properties as well as enhanced magnetic and superconducting behavior. In addition to these materials we are currently studying a wide range of thermoelectrics and thermoelectric materials, including skutterudites, chalcogenides such as Cu2Se and CuAgSe, and delafossite oxides.
We are also interested in electronic behavior in these and related materials. This includes identifying signatures of charge carriers as well as their local magnetic susceptibilies, using mostly NMR as a primary tool to probe the atomic-scale behavior. Our interests include identifying spin and orbital effects as well as new topological electronic properties of these materials, based on fundamental physical interest in these emerging properties as well as the potential for next-generation electronic devices.
  • New Metallic Materials; shape memory materials and new alloys for magnetocaloric applications

Our work has also been focused on the magnetic and vibrational behavior of new magnetic materials, including iron alumindes, Heusler alloys, and a number of Rare-earth-based materials. For example see our recent work on Ni2MnIn-related magnetocaloric materials, and related work on Magnetic Shape Memory-type Alloys. We are interested in elucidating the multiferroic properties of these materials, such as the coupled structual-magnetic transformations in the Heusler alloys, and in understanding the interplay of itinerant and localized magnetic moments that underlies the magnetic properties of these materials. Techniques include NMR spectroscopy, magnetic susceptibility, specific heat and transport, thermopower, and ab initio computational methods.

NMR-specific notes:   

  • Notes on NMR shifts and symmetries:

These notes may be generally useful:

I. General notes on NMR shifts in solids, and Crystallographic tensors: determining the shift tensor symmetries based on crystal point groups vs. shift tensor symmetry.
II. Some notes on common shift notation conventions: including standard notations for chemical shifts, also Knight shifts and quadrupole shifts.
III. Notes on NMR Knight shift and Chemical shift mechanisms, and computational notes.