Much of this work has industrial applications, but the most applied work is the theoretical modeling of nanoscale electronics and the nonequilibrium/nonlinear response work. One focus is on examining properties of Josephson junctions. In 1962, Brian Josephson proposed that interesting effects will be found in sandwiches made of superconductor-insulator-superconductor layers. Since superconductivity is a quantum-mechanical phenomenon, it can "leak" through the insulator (if it is thin enough) into the other superconductor. He proposed that this device could transport superconducting current with no potential difference across the device. The resulting, highly nonlinear I-V curves of Josephson junctions are what make them important for electronics applications. Josephson won the Nobel prize in 1973. My work is concentrated on understanding what happens to a Josephson junction as the properties of the barrier are tuned from a metal to an insulator. Many properties change, ranging from the current density that can pass through the junction to the voltage at which the nonlinearities occur. Of main interest to the electronics community is the speed at which the junction switches, which is maximized by maximizing the product of the critical current at zero voltage and the normal state resistance. There are current plans to build petaflop computers out of 100 GHz superconductor microprocessors (based on Rapid Single Flux Quantum logic). Such work requires improvements in the quality and reproduciblity of Josephson junctions and the digital circuit design. These problems also have applications in the power industry. High temperature superconducting wires are limited by the total critical current they can pass through them. Present technology allows YBCO tapes to be grown with domain grain boundaries of less than 10 degrees. Unfortunately, the Josephson critical current through the grain boundary depends exponentially on the angle and one needs to match the grains to better than one degree for a commercial product. Recently, Mannhart and collaborators showed that these grain boundaries are electrically active, and selective doping with Calcium can reduce the potential barrier strength and increase the current that flows across the grain boundary without requiring better alignment of the grains (this work was profiled in physicsweb). I am working on modeling these systems to understand how best to improve the critical current across a grain boundary. This work is supported by the Office of Naval Research.
Another focus of my nanoscale electronics work is to examine both thermoelectric devices and spintronics. The field of thermoelectricity was discovered hundreds of years ago by Peltier and Seebeck, but it wasn't until the work of Ioffe in the 1950's that commercial applications of thermoelectrics became possible. Thermoelectric devices have two potential applications---(i) refrigeration and cooling (with a solid state device that has no moving parts) and (ii) power generation, which converts heat energy into electricity (similar in spirit to a solar cell, but with a different operational strategy). There is a significant need for low-temperature solid state coolers, but none have yet been discovered that can operate below about 200 K. Since so-called heavy-Fermion compounds exhibit large thermoelectric response at low temperature, they may be useful in low-temperature devices, especially when layered with other metals or insulators on the nanoscale. In the case of power generation, there is a desire to improve efficiency and reduce the cost of devices. Currently, the most reliable battery is composed of radioactive materials for the heater and thermoelectric devices which convert the heat into electricity. These batteries have been used on all of the deep space probes as the power source.
Spintronic devices were proposed over a decade ago by Datta and Das. The basic idea is that we want to control the transport of both the electronic charge and the electronic spin through a device, which will allow us to make more complex circuit elements than the standard charge-based transistor. Recently there have been a number of breakthroughs in the field, including the ability to transport spin current coherently over a distance of about a micron in semiconductors, and the ability to control the spin polarization via an electrical gate voltage. The field of spintronics is too new to know exactly where the future applications will be, but they will likely be used to further improve magnetic storage (including nonvolatile computer memory) or as the fundamental elements of quantum computers (the so-called Q-bit). This work is supported by a nanotechnology interdisciplinary research team award from the NSF and is being carried out jointly with Amy Liu and Barbara Jones.
Another set of projects I am working on, in collaboration with Tom Devereaux and Andrij Shvaika (amongst others), is to examine the inelastic scattering of light with correlated electronic materials. As the electron-electron interaction is made stronger in a material, it can cause the material to exhibit strange quantum mechanical behavior such as magnetism, metal-insulator transitions, etc. It is believed that much novel behavior will be found in materials that have strong electron correlations. They are very hard to understand with theoretical models, and they are very hard to measure properties of. Our recent focus has been to examine Raman scattering and inelastic X-ray scattering in correlated materials. In the 1920's, Raman proposed that if you shine light on a material, some light will exchange energy with the excitations of the material and be emitted with a slightly different color (he won the Nobel prize in 1930). Since less than one out of a billion photons will reflect with an energy change, very bright sources of light are needed to see the inelastic scattering. With the advent of lasers (for optical light) and advanced synchrotron light sources (for X-rays), Raman scattering experiments have become feasible for a number of different materials. The experimental data in correlated materials shows a number of strange features and we have developed a series of different models that explains these different features. This work is supported, in part, by the Civilian Research and Development Fund.
I am also working on determining a new calculational formalism for nonequilibrium problems and for nonlinear response in an electric field. Most micro- and nano-scale electronics are placed in intense electrical fields and they are often driven out of equilibrium due to the large current densities that flow through the devices. We have adapted the Kadanoff-Baym and Keldysh approaches to dynamical mean field theory, which allows us to calculate the properties of systems in large electrical fields, including all nonlinear and nonequilibrium effects. We plan to generalize this work to include nonequilibrium transport in nanosctructures as well. Our goal is to try to determine how a Mott insulator responds to a strong time-dependent electrical field (pulsed or steady).
Finally, I spend some time working on rigorous and exact solution techniques for the many-body problem. One current problem involves a study of phase separation and how pervasive it is in the limit when electron-electron interactions are very strong. Phase separation has been seen in many strongly correlated systems, but the most interesting is the charge-stripe formation seen in many relatives of the high-temperature superconducting oxide materials. While most researchers believe that antiferromagnetic spin effects play a role in determining the phase separation, our perspective shows that strong electron-electron interactions in the charge sector also directly produce phase separation.
Jim Freericks, Professor of Physics