Local Moment and Heavy Fermion Physics

点击翻译按钮获取中文摘要

TECHNICAL SUMMARYThis award supports theoretical research and education to study the correlated, collective behavior of electrons in heavy electron materials. These are lanthanide or actinide intermetallic compounds in which the close vicinity to magnetic instability gives rise to fundamentally new kinds of electronic material behavior. Motivated by many new experimental results, a theoretical study of the break-down of conventional metallic behavior that occurs in a quantum critical heavy fermion metal will be carried out. This will involve developing theoretical models for the "intrinsic" quantum criticality recently discovered in Ytterbium rare earth heavy fermion systems YbBAl3 and YbAuAl. Specifically, the research is aimed to develop a mixed boson-fermion theoretical framework to describe the co-existence of localized magnetism and delocalized heavy electron behavior that is observed to occur in various Cerium-based heavy electron materials.A second part of the research will study the development of novel forms of electronic order in heavy-electron materials. The PI will develop theoretical models for the 115 heavy fermion superconductors to understand the entanglement of the local moment spin degrees of freedom with the pair condensate, described by "composite pairing", and the unusual robustness of these superconductors against pair breaking. This research will also study the phenomenon of hidden order URu2Si2 and analogous praseodymium heavy electron materials. NON TECHNICAL SUMMARYThis award supports theoretical research and education to study the fundamental origins of new kinds of order in heavy fermion materials. Electrons in metals interact with one another via their mutual Coulomb repulsion. When these interactions become large, the conventional metallic state becomes unstable and the electrons organize themselves via a phase transition, into new states of order. Ordered states include familiar ferro-magnets in which the intrinsic magnetism of each of the electrons is aligned, anti-ferromagnets, in which the magnetic order is staggered in space, and superconductors in which electrons form of a fluid of Cooper pairs which flows without resistance. These new states bring new kinds of materials properties, such as the fierce magnetism used for hybrid car motors, or the zero resistance of a high temperature superconductor. Phase transitions that occur at the absolute zero of temperature are entirely driven by the zero-point quantum motion of the electrons; there is no thermal motion. Through quantum phase transitions, metals develop new forms of electronic order. In many quantum phase transitions, the transition to order is preceded by a point where the quantum mechanical jiggling of the emergent order becomes very intense, spreading throughout the entire metal. This kind of "Quantum Criticality" develops when the phase transition temperature is tuned to absolute zero. This award supports research to advance current understanding of the transformations in metallic behavior induced near a magnetic quantum critical point. The research will be focused on "heavy fermion" materials, whose properties make them highly tunable for the study of quantum criticality. The PI aims to develop a new mathematical framework to describe the transformation in the magnetic character of the electrons at a quantum critical point, as they transform from mobile waves to localized magnetic moments. A second part of this research concerns the new kinds of electronic order that develop in heavy fermion materials near a magnetic instability. The PI will focus in part on heavy fermion superconductors, which display in miniature, much of the physics observed in high temperature transition metal superconductors. Normal superconductivity develops in response to the formation of Cooper pairs between electrons. This research will refine the concept of "composite pairs", whereby magnetic moments bind to electron pairs to form a "composite pair" that drives superconductivity.The PI will also investigate the phenomenon of "hidden order" observed to develop as a precursor to superconductivity in the heavy fermion compound Uranium Ruthenium-2 Silicon-2 . The PI seeks to generalize the concepts that underlie a new kind of order he has proposed, called hastatic order, and to understand its relationship to the superconductivity that develops with the hidden order. The possibility that similar types of order can occur in closely related intermetallic materials will be examined.