****NON-TECHNICAL ABSTRACT**** One of the modern challenges of science is to learn how to control and manipulate basic quantum systems in order to create electronic devices with expanded functionalities. Achieving this goal requires an improved understanding of how small systems, which are known to be strongly influenced by their environment, behave when their conditions change over time. This award supports a project to investigate time-dependent properties of an electron confined to a very small region of space and interacting with nearby macroscopic conductors. Such an arrangement possesses a striking array of highly universal static properties due to a coherent collective behavior of the confined and the delocalized electrons referred to as the Kondo effect. The project will capitalize on extensive existing knowledge of the static properties of Kondo-correlated electrons and answer fundamental questions concerning their interaction with electromagnetic fields. Such knowledge will aid further development of concepts in dynamics of simple quantum systems under practically relevant, non-equilibrium conditions. The research will benefit the community pursuing coherent control of elementary quantum states and complement recent studies of correlated dynamics in bulk materials. Students involved in the project will be trained in state-of-the-art technology, develop research and critical problem solving and decision making skills and become prepared for careers in academe, industry and government. The project will generate research opportunities for enthusiastic high-school students, which will stimulate their interest in research and invite them to pursue careers in science.
Controllable quantum dots formed by a lateral confinement of a two-dimensional electron gas in a semiconductor heterostructure have been used in recent years by several groups to test universal scaling properties and investigate non-equilibrium aspects of Kondo-correlated electrons. The latter is possible in a quantum dot because a local electric field near the magnetic "impurity", represented by an unpaired electron spin in the dot, can be created by applying a small bias between the macroscopic source and drain "leads" connected to the dot via tunnel barriers. This individual investigator award will support a project to extend such studies to a time-dependent regime and investigate dynamic properties of the Kondo state by subjecting the quantum dot to an oscillatory bias and /or gate voltage. The objective of the project is to test the predicted universality of observable properties with respect to frequency and the Kondo temperature, with the emphasis on the interplay between photon-mediated correlations and dissipation. Experiments will be performed with quantum dots made on a GaAs/AlGaAs semiconductor heterostructure in a specially constructed apparatus which permits precision measurements of the device conductance in the presence of a microwave-frequency field of tunable orientation, magnitude and frequency. Students involved in the project will be trained in state-of-the-art technology, develop research and critical problem solving and decision making skills and become prepared for careers in academe, industry and government. The project will generate research opportunities for enthusiastic high-school students, which will stimulate their interest in research and invite them to pursue careers in science.
DMR 0804199: Final report: Activities and findings The grant was used to investigate nonequilibrium behavior of correlated electronic systems using a Single-Electron transistor (SET), a specially fabricated electronic device that traps an electron in a small area and is capable of providing a tunable degree of coupling of the electron to macroscopic conductors ( contacts) nearby. In addition, the award partially supported studies of transport phenomena in semiconductor nanowires, and an investigation of the effects of electromagnetic radiation on living cells with the goal to better understand cell signaling mechanisms relevant to wound healing. Intellectual merit: Static correlation effects in mesoscopics. 1. We probed universality of the non-equilibrium Kondo effect. Universality of the Kondo physics in equilibrium is a well known and important concept, however, the extent and degree of persistence of the universal behavior as the equilibrium is distorted is not fully understood, and is significant because of the relevance of nonequilibrium correlation phenomena to operation of practical electronic devices. Our study revealed a crossover from universal to a strongly nonuniversal behavior at a certain energy scale, defined by an externally applied magnetic field. As part of the work, we also examined equilibrium scaling of the SET conductance in presence of magnetic field, and found a good agreement with recent calculations. 2. We have shown that even a small tunnel barrier --- an open system that contains no trapping island like an SET --- can nonetheless display correlation phenomena similar to the Kondo effect in SETs. The new results of our study suggest that spin-related correlation effects have a strong influence on tunneling even at very low tunneling rates, which was unexpected. 3. We investigated transport in an SET in the regime of a very low coupling to the leads, when the correlation-driven transport yields to single-particle effects. The novel aspect of this study is a complete characterization of the parameters of the SET device, which made detailed comparisons to microscopic theory possible and revealed a good overall agreement. Dynamic correlations in SETs We investigated transport in SETs in the Kondo regime in presence of the microwave-frequency signal on the device leads, and also under a simultaneous application of the microwave signal and the magnetic field. The central result is the observed agreement with published theoretical work at high frequencies, and a detailed information on the transition from the high- to low-frequency regimes. We expect these to stimulate further theoretical inquiry. Transport in Semiconductor nanowires As a step towards developing Kondo systems based on semiconductor nanostructures, we developed metal-semiconductor-metal structures based on InP and GaAs semiconductor nanowires. In particular, we have demonstrated that room temperature photocurrent can be used as a simple way to distinguish between the Zincblende lattice structure typical for bulk InP, and the Wurtzite structures sometimes found in nanostructured InP. Effects of microwave-frequency radiation on cell signaling We constructed an apparatus that makes it possible to apply microwave fields to biological samples in a way that makes the orientation and the magnitude of the field highly controllable and does not cause heating of the sample. We have shown that low-amplitude microwave electric field regulates one of the major signaling pathways (MAPK/ERK), which is a step towards development of non-farmacological wound healing therapies. Broader impacts: Four graduate students have benefitted from project activities, three of whom have now completed their PHDs. Several undergraduates, including three female physics majors, have participated in the work supported, or partially supported, by the grant. A program designed to introduce high school students to experimental research was initiated. Within the program, a teacher and several students worked the lab on educational projects related to the overall theme of low temperature physics. Several collaborations between UC Physics Department and other institutions have been established. These include both domestic ( UC engineering school, Ohio University, Xavier University, Purdue University) and international (Australia National University) collaborations, all of which have produced joint publications. Overall, the research produced 9 papers, of which 7 have been published in peer-reviewed journals and two are being prepared for submission, and resulted in multiple presentations at professional meetings. In Summary, the award supported experiments that help understand fundamental effects in nonequilibrium correlated electronic systems, and permitted to initiate research in two new directions, transport in semiconductor nanostructures and electromagnetic phenomena in cells. Funds from the award contributed to training graduate students as well as undergraduates and high school students and teachers. The research results have been published in peer reviewed journals ( 7 published papers and 2 papers in preparation) and presented at professional meetings, including multiple presentations by graduate students involved in research.Three graduate students who worked on the project supported by the award have received their PHDs and moved to postdoctoral and teaching positions.
