Quantum Criticallity and Magnetic Semiconductors

这个凝聚态物理项目将研究具有“非费米液体”特性的金属材料。导电性等方面的定义行为，在零温度磁相变和量子临界点附近经常发现。然而，很少有人知道与附近的金属到绝缘体（MI）的过渡的无序和低载流子密度的影响。感兴趣的是与巡回磁性相关的初始金属状态。这将通过硅化物，锗化物，硫化物在接近磁性和/或MI过渡的实验进行探测。运输，磁性，光学和热力学性质将在这些材料中确定，这些材料已被选择，使载流子密度和无序的变化将允许访问有趣的量子临界点。实验解决了三个问题：（A）在强烈波动的金属中发现的非费米液体行为是交叉效应，还是这种行为标志着新基态的形成？(B)与附近MI跃迁相关的无序和低载流子浓度对量子临界点附近材料的性质有什么影响？以及（C）在可能具有技术用途的过渡金属硅化物或近藤绝缘体的固溶体中，是否存在室温半导体相和铁磁相？本科生，研究生和博士后学生，以及为美国而教的参与者将参加尖端的研究和培训，这将为他们在商业，工业和政府的职业生涯做好准备。最近的许多实验表明，发生磁有序状态转变的温度可以随着材料化学成分的变化或外部压力的施加而变化。对于各种磁系统，这个转变温度可以被向下驱动，直到它基本上处于绝对零度。关键的发现是，当转变温度接近零时，物理性质，如对电场和磁场以及热能的响应，会发生变化，从而与世纪以来固体研究中发现的任何物理性质都不同。虽然在理解这些“量子临界”现象方面取得了进展，但仍有许多谜团，包括这些物理学控制材料行为的温度和化学成分范围。该项目的重点是磁系统的一个特定类别，磁性半导体，在那里量子临界的物理可以进行调查。这一点特别重要，因为，例如，基于磁性半导体的新技术一直在开发中，但其物理特性尚未得到充分探索。该提案有助于本科生、研究生和博士后学生的发展和培训。“为美国而教”的参与者将被招募参加这项研究，这项研究涉及与南方大学、HCBU以及欧洲和南部非洲的教师合作。

This condensed matter physics project will investigate metallic materials that exhibit "non-Fermi-liquid" properties. The defining behavior in terms of conductivity et al., is frequently found near zero temperature magnetic phase transitions and quantum critical points. However, little is known about the effect of disorder and low carrier density associated with a nearby metal-to-insulator (MI) transition. Of interest is the incipient metallic state that is correlated with itinerant magnetism. This will be probed via experiments in silicides, germanides, and sulfides in proximity to magnetic and/or MI transitions. Transport, magnetic, optical, and thermodynamic properties will be determined in these materials, which have been chosen so that variations in carrier densities and disorder will allow access to interesting quantum critical points. The experiments address three questions: (A) Is the non-Fermi-liquid behavior discovered in strongly fluctuating metals a crossover effect, or does this behavior signal the formation of a novel ground state? (B) What effect does the disorder and low carrier concentration associated with a nearby MI transition have on the properties of materials near quantum critical points? and, (C) Are there room temperature semiconducting and ferromagnetic phases among the solid solutions of transition metal silicides or Kondo insulators that may have technological uses? Undergraduate, graduate and postdoctoral students, as well as Teach for America participants will be take part in cutting edge research and training that will prepare them for careers in academe, industry and government..Many recent experiments have shown that the temperature for which a transition to a magnetically ordered state occurs can be varied with either a change in chemical composition of the material or the application of external pressure. For a variety of magnetic systems this transition temperature can be driven downward until it is essentially at the absolute zero of temperature. The key discovery is that when the transition temperature is close to zero, the physical properties, such as the response to electric and magnetic fields and thermal energy, change such that they differ from any found in a century of research in solids. Although progress has been made toward understanding these "quantum critical" phenomena, many mysteries remain, including the range of temperatures and chemical compositions where these physics dominates the behavior of materials. This project focuses on a particular category of magnetic systems, magnetic semiconductors, where the physics of quantum criticality can be investigated. This is of particular importance since, for example, new technologies based on magnetic semiconductors have been under development, yet their physical properties have not been fully explored. This proposal contributes to the development and training of undergraduate, graduate and post-doctoral students. Teach for America participants will be recruited to take part in the research, which involves collaborations with faculty at Southern University, an HCBU, and with faculty in Europe and Southern Africa.