Quantum Criticallity and Magnetic Semiconductors

Louisiana State University, Baton Rouge LA

Investigators

Abstract

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.