Unconventional and High-Temperature Superconductivity
There exists a large array of fascinating materials that manifest superconducting phases whose properties depart radically from the expectations of conventional, single-phonon, based BCS theory. Complex, highly correlated, electronic phenomena such as the coupling to proximate magnetic instabilities or self-organized electronic order are thought to comprise several alternative, unconventional, routes to the formation of superconducting condensate within many unusual superconductors. One of the most striking examples of anomalous superconducting phases appeared with the discovery of the high-temperature superconducting (high-Tc) phase within the lamellar copper oxides. Despite several decades of intensive study, the microscopic mechanism responsible for the formation of the superconducting condensate within the lamellar copper oxides remains unresolved. A recent boon to the field of high-Tc has been provided through the discovery of a separate, robust, class of high temperature superconductors based on iron. Experimental exploration of universalities within the electronic behavior between these different classes of high-Tc superconductors along with the continued discovery of novel magnetic phenomena coupled to the high-Tc superconducting condensate generates a framework for the theoretical understanding of the microscopic pairing mechanism both within the cuprates and other unconventional superconductors.
Our studies focus on both the static and dynamic lattice and spin properties within the high temperature superconducting copper oxides and iron pnictides explored through neutron scattering and x-ray techniques. Current avenues of interest are the fundamental interactions governing the coupled magnetic and structural phase transitions within the parent and lightly doped iron pnicides. Another area of interest is investigation into the effect of isovalent impurity substitution into both the high-Tc cuprates and iron pnictides as a probe of the perturbed high-Tc groundstate.
Strongly Correlated Electron Materials
The broader field of strongly correlated electron materials constitutes a fertile regime of condensed matter physics where electrons, often through multiple degrees of freedom, strongly couple to one another resulting in novel collective behavior and, frequently, fundamentally different electronic ground states. Studying the interplay between these electronic degrees of freedom, in particular the coupling between charge and spin behaviors, often reveals fundamental insights into the mechanisms driving the formation of these novel ground states.
One interest of our group's research is probing the influence of quantum critical fluctuations on the magnetic properties of materials in proximity to quantum critical points within their respective phase diagrams. We are also beginning a program of exploring the evolution of spin behavior in strongly correlated ruthenium oxide and iridium oxide compounds as they are tuned across their respective phase diagrams via carrier doping. The existence of a number of interesting phenomena such as novel 5d Mott insulating states and spin triplet superconductivity can be probed in this manner.
Lattice Dynamics and Emergent Phenomena in Nanocomposite Materials
There exist a large array of systems whose properties are drastically renormalized when these materials are reduced to sizes comparable to the wavelengths of fundamental excitations (electronic- or lattice-based) or characteristic length scales of static order within these solids. Spatial confinement effects within these nanocrystalline systems along with newly relevant degrees of freedom such as surface modes or collective intergranular vibrations can render enhanced materials properties or emergent new collective behavior. In many cases, such as in nanocomposite thermoelectrics, these new properties present significant potential for future applications as they can be harnessed by low-cost, functional nanocomposites.
Our current research involves probing the microscopic means through which lattice vibrations couple into the reduced thermal transport characteristics of nanocomposite thermoelectric materials. Additionally, we are studying the interplay between spatial confinement, surface states, and phase separation within nanocrystalline manganite materials. We are also beginning a program exploring the coupling of phonon frequencies to phonon-assisted luminescence in nanocrystalline, optical ceramics.