Mesoscopic Quantum Critical Regimes and Disorder-Driven Deconfinement

University Of Kentucky Research Foundation, Lexington KY

Investigators

Abstract

TECHNICAL SUMMARY: This award supports theoretical research and education on materials and systems in which disorder and interactions conspire to create novel states with strong quantum fluctuations. In bulk systems strong quantum fluctuations are characteristic of the quantum critical regime near the bulk quantum phase transition. Mesoscopic systems offer zero-dimensional analogs of bulk quantum phase transitions which can be studied under controlled conditions. The research builds on the PI's past work and is focused on the nature of the ground and low-lying excited states, the signatures of quantum criticality in transport properties, and on the crossover between mesoscopic quantum critical regimes and bulk quantum criticality. The successful completion of this research will provide the fundamental understanding to enable the creation, control, and characterization of mesoscopic systems with strong quantum fluctuations. Recently, tremendous progress has been made in identifying deconfined phases and critical points in two-dimensional quantum antiferromagnets. Such deconfined regimes appear difficult to access in realistic lattice spin systems, and are probably unstable to quenched disorder. The PI finds that deconfinement is generically possible in disordered multicomponent quantum Hall systems, and is in fact driven by quenched disorder. The fundamental reason is the spin-charge relation of the lowest Landau level, which forbids hedgehogs/monopoles by local charge conservation. In turn, the suppression of these topological objects leads to deconfinement. Smooth disorder is needed to restore the broken symmetry of the quantum Hall ferromagnet and push the system into a deconfined state. The "nu" = 1 bilayer system, which experimentally shows dissipation at the lowest measured temperatures, is a good candidate for such a deconfined state. The primary focus of this research thrust will be to investigate the occurrence and properties of phases with gapped fermions and deconfined spinons. The successful completion of this research will result in a deeper understanding of both deconfined phases and multicomponent quantum Hall systems. The education of a postdoc and a graduate student in the latest techniques of mesoscopic and strongly correlated physics is an integral part of this proposal. NON-TECHNICAL SUMMARY: This award supports theoretical research and education on materials and systems that will study the nature of phase transitions which occur at the absolute zero of temperature and are believed to be able to affect the properties of materials at temperatures up to room temperature and possibly beyond. Unlike more familiar phase transitions, like the transformation of water to steam, in which thermal fluctuations are responsible for driving the system through the transformation, quantum phase transitions are driven by a fundamental principle of quantum mechanics due to Heisenberg known as the uncertainty principle. A theme of this research project is to better understand these unusual phase transitions and the affect that they have on the properties, particularly electronic properties, of materials and material systems, and the new states of matter that may occur, through the study of systems that may be particularly susceptible to the scrutiny of experiment and purposeful control. Specific mesoscopic systems involving, for example, quantum dots and nanoscale phenomena involving electrons trapped in semiconductors and exposed to large magnetic fields are identified as promising avenues of inquiry and hold potential for new discoveries. A thrust of the research is to understand how deviations from perfect order affect quantum phase transitions and the nature of the states of matter involved in the transformation. This is fundamental research that is distant from immediate technological application, but it lays the intellectual foundations that may someday support advanced technologies with devices that exploit quantum mechanical principles for their operation, for example quantum computers. The education of a postdoc and a graduate student in the latest techniques of mesoscopic and strongly correlated physics is an integral part of this proposal.