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

*技术摘要*(Ga，Mn)As是III-V稀磁半导体(DMS)中研究最多的材料。这些材料通过对其磁性的光学和电学控制，提供了可能的自旋电子应用，但实际应用还有待于设计居里温度高于室温的新材料。爱因斯坦关系允许人们根据测量的电阻率和空穴扩散系数来求出费米能级(DOS)的态密度，而与空穴密度无关。该项目将使用超快光学来测量空穴扩散系数，从而确定DOS。本实验将对(Ga，Mn)As进行瞬变光栅谱测量，从而测量自旋扩散和双极扩散，这两个过程依次给出空穴扩散。这些测量可以区分被广泛支持的(Ga，Mn)As中空穴的价带和杂质带图像。由于空穴起着调节磁交换的作用，因此对其性质的争论是理解DMS的基础，而加深对DMS的理解将有助于开发新的DMS材料。本科生将参与项目的每一个部分。他们将学习超快激光、光学对准、数据分析、磁学、半导体物理和光谱学。这些经验将使他们为各种各样的职业和研究生课程做好准备。*非技术摘要*稀磁半导体(DM)在掺入少量杂质后会变得有磁性。磁性半导体可以用于许多拟议的设备，包括将显著降低计算机功耗的非易失性晶体管，但实际应用将需要设计在室温下运行的新DMS材料，而不是现有材料的零下温度。由于缺乏对DMS如何变得有磁性的理论理解，对这类材料的探索一直受到影响。GaMnAs是一种典型的DMS，但人们对使其具有磁性的电子过程存在严重分歧，特别是关于控制其磁性的“空穴”的移动性。这个项目将使用脉冲短于十万亿分之一秒的超快激光来测量空穴的迁移率，从而提高对DMS的理解。该项目还将研究一个相关的现象，即暴露在光下会增强GaMnAs的磁性，这个现象目前还知之甚少。本科生将参与该项目的每一个部分，为实验进展做出贡献，并接受实验物理方面的培训。他们将学习使用超快激光，非常擅长光学对准，并熟悉磁学、半导体物理和光谱学。由于这些主题与科学和工业的相关性，以及超快技术的快速发展，学生们将为各种各样的职业和研究生课程做好准备。

****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.