RUI: Measurement of Density of States of (Ga,Mn)As and Diffusion of Photoinduced Order by Ultrafast Transient-Grating Spectroscopy

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****Technical Abstract****(Ga,Mn)As is the best-studied of the III-V dilute magnetic semiconductors (DMS). These materials offer possible spintronic applications through the optical and electrical control of their magnetic properties, but practical application awaits the design of new materials with Curie temperature above room temperature. The Einstein relation allows one to find the density of states at the Fermi level (DOS) based on measurements of the resistivity and the hole diffusion coefficient, independent of hole density. This project will use ultrafast optics to measure the hole diffusion coefficient, and thereby determine the DOS. The experiment will apply transient-grating spectroscopy to (Ga,Mn)As, thus measuring the spin diffusion and the ambipolar diffusion; these two in turn, give the hole diffusion. These measurements could distinguish between the widely supported valence-band and impurity-band pictures of holes in (Ga,Mn)As. Since the holes mediate magnetic exchange, the debate over their properties is fundamental to understanding DMS's, and improved understanding could aid the development of new DMS materials. Undergraduate students will participate in every part of the project. They will learn about ultrafast lasers, optical alignment, data analysis, magnetism, semiconductor physics, and spectroscopy. The experience will prepare them for a wide variety of careers and graduate programs.****Non-Technical Abstract****The dilute magnetic semiconductors (DMS's) become magnetic when they are "doped" with small quantities of impurities. Magnetic semiconductors could be used in many proposed devices, including nonvolatile transistors that would dramatically reduce computers' power consumption, but practical use will require the design of new DMS materials that operate at room temperature, rather than the subzero temperatures of current materials. The search for such materials has suffered from a lack of theoretical understanding of how a DMS becomes magnetic. GaMnAs is an archetypal DMS, but there is sharp disagreement over the electronic processes that make it magnetic, particularly over how mobile the "holes" are that control its magnetism. This project will use ultrafast lasers, with pulses shorter than one ten-trillionth of a second, to measure the holes' mobility, providing an improved understanding of DMS's. The project will also investigate a related phenomenon, as yet poorly understood, in which exposure to light strengthens the magnetism of GaMnAs. Undergraduate students will participate in every part of the project, contributing to experimental progress and being trained in experimental physics. They will learn to use ultrafast lasers, get very good at optical alignment, and become familiar with magnetism, semiconductor physics, and spectroscopy. Because of the scientific and industrial relevance of these topics, and the rapid growth of ultrafast technology, the students will be prepared for a wide variety of careers and graduate programs.