Quantum Criticallity and Magnetic Semiconductors

这个凝聚态物理项目将研究表现出“非费米液体”特性的金属材料。在电导率等方面的定义行为，经常在零度附近发现磁相变和量子临界点。然而，对于无序和低载流子密度与附近金属到绝缘体（MI）跃迁相关的影响知之甚少。令人感兴趣的是与流动磁性相关的初始金属态。这将通过在磁性和/或MI跃迁附近的硅化物、锗化物和硫化物中进行实验来探索。这些材料的输运、磁性、光学和热力学性质将被确定，这些材料的选择是为了使载流子密度和无序度的变化能够接近有趣的量子临界点。实验解决了三个问题：(A)在剧烈波动的金属中发现的非费米液体行为是一种交叉效应，还是这种行为标志着一种新的基态的形成？(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？(C)在过渡金属硅化物或近藤绝缘体的固溶体中是否存在可能具有技术用途的室温半导体和铁磁相？本科生、研究生和博士后以及“为美国而教”项目的参与者将参加最前沿的研究和培训，这将为他们在学术界、工业界和政府部门的职业生涯做好准备。最近的许多实验表明，发生向磁有序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，这种转变温度可以向下驱动，直到它基本上处于温度的绝对零度。关键的发现是，当转变温度接近于零时，物理性质，如对电场、磁场和热能的反应，会发生变化，这与一个世纪以来对固体的研究发现不同。尽管在理解这些“量子临界”现象方面取得了进展，但仍有许多未解之谜，包括这些物理支配材料行为的温度范围和化学成分。该项目侧重于磁性系统的一个特定类别，磁性半导体，其中量子临界物理可以进行研究。这是特别重要的，因为例如，基于磁性半导体的新技术正在开发中，但它们的物理性质尚未得到充分探索。这一建议有助于本科生、研究生和博士后的发展和培养。“为美国而教”项目的参与者将被招募来参与这项研究，这项研究涉及与南方大学（HCBU）的教员以及欧洲和南部非洲的教员合作。

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.