New Applications of Strongly Correlated Materials James K. Freericks, Georgetown University, DMR Strongly correlated materials are materials in which the electrons interact with each other so that the motion of one electron affects the motion of the other ones. They often have highly tunable properties. In this work, we show a new effect in sandwiches of metals and Mott insulators, which is one class of strongly correlated material that does not allow current to flow. We find that the ability to move current through these devices is a highly nonlinear function of the temperature, as the Mott insulator becomes conducting in steps from the outermost planes of the barrier inward. Quantum effects force the insulator to become metallic regardless of the thickness. Metal Insulator Metal Stack of metallic and insulating planes. Current flow A normal metal has its resistance increase as T 2, and insulator decreases as exp(-C/T). Here the device has a totally different and novel T dependence.

International undergraduate research experience James K. Freericks, Georgetown University, DMR This year, rising junior Jonathan Balloch was sent to the Institute of Physics in Zagreb, Croatia for an international research experience in the laboratory of Duro Drobac. He measured the temperature dependence of the magnetic susceptibility of different amorphous materials and compared the measured curves to scaling theory for the critical point. He has written a report on this work, and we are looking into the possibility of it becoming a part of a future publication from Drobac’s lab. Undergraduate rising junior Jonathan Balloch spent 7 weeks at the Institute for Physics in Zagreb, Croiatia working in the lab of Duro Drobac. Here he is attaching a cable to the dewar. The full group of Drobac’s lab.

Quicker than a flash James K. Freericks, Georgetown University, DMR Lasers have been created that can produce ultrafast and ultrapowerful pulses of light. In a new twist on the work that won Albert Einstein his Nobel prize, these ultrafast laser pulses are directed onto materials releasing electrons via the photoelectric effect. If one first “pumps” the system, by hitting it with an intense infrared laser, and then delays the time between the pump and the ultraviolet “probe” pulse, then one can measure properties about the material on times scales where electrons move and scatter in the material. In our work, we derived a full and complete theory to describe these experiments, and evaluated it by using high performance computers to solve the complex set of equations that result in the analysis. These series of images show the intensity of the electrons (as a function of their energy) in a false color plot as the time delay between the pump and the probe are varied. The oscillations are due to the electrical current that flows through the material

Patterns and sorting in physics research James K. Freericks, Georgetown University, DMR The PI visits a kindergarten class and shows different classes of patterns and has the class determine the sorting rules. The PI visited a kindergarten classroom at Barrett elementary school in Arlington, VA (which has a high minority population) and discussed patterns that arise as ground states in the one- dimensional Falicov-Kimball model. Using the sorting rule of the most homogeneous states, he had the students identify new properties in the patterns. The classroom teacher indicated that students were much more interested in exploring longer patterns after the class visit.