Coherence and Fluctuations in Novel Multicomponent Systems

Indiana University, Bloomington IN

Investigators

Abstract

TECHNICAL SUMMARY: This award supports theoretical research and education on low-dimensional, multicomponent condensed matter systems which have discrete degrees of freedom, such as spin, valley, or layer. The essential physics may often be understood in terms of effective degrees of freedom other than those of the underlying electrons, allowing for simplified descriptions of fluctuating or correlated states that cannot be understood in terms of standard Fermi liquid theory. The PI will focus on two specific systems as paradigms: graphene and quantum Hall bilayers. Graphene, a two-dimensional carbon network, is unusual among two-dimensional electron gas systems for several reasons. The electrons near the Fermi level are "massless," and obey a Dirac equation. This leads to unique physics related to the "Klein paradox" for relativistic particles, in which an electron current may penetrate a barrier via the admixture of hole states. In graphene this implies that electrons cannot be confined solely by an electrostatic potential. The consequences of this for many-body states of broken spatial symmetries, and fluctuation-induced states based upon them, will be examined. The PI will also study the impact of the valley degree of freedom which allows the possibility of unusual orderings in such states, particularly in the quantum Hall regime. In addition, the sublattice degree of freedom leads to an unusual Berry's phase with many interesting consequences for transport and interference effects, which will also be examined. The PI will investigate consequences of curvature and topological defects for the electronic properties of graphene. Quantum Hall bilayers near filling factor 1 are special in displaying an "imperfect" superfluidity - a counterflow resistance that appears to vanish in the zero temperature limit, and a strong but finite resonance in tunneling conductance. The underlying state that allows such unusual behavior to emerge remains poorly understood. The PI aims to determine how quantum fluctuations may lead to such behavior, under the premise that disorder plays a crucial role. Studies designed to understand some recent experiments within this context will also be pursued. NON-TECHNICAL SUMMARY: This award supports fundamental theoretical research and education on the electronic states of materials in two spatial dimensions. The PI will focus his research on graphene and quantum Hall bilayers - two layer systems of electrons in high magnetic fields. Each of these systems has electronic states that differ dramatically from standard textbook wisdom. For example, the electrons in graphene behave like particles of light and are governed by Einstein's theory of relativity recast in the quantum world. The quantum Hall bilayers exhibit a new state of matter that appears like an imperfect superconductor. This work represents fundamental research that seeks to understand these new states of matter. Graphene is also an important candidate for new technologies. This research contributes to understanding graphene's unusual properties and contributes to the knowledge base that will enable new technologies to keep America competitive. This research will be conducted with the full participation of graduate students, who will be trained in modern methods for analyzing condensed matter systems, preparing them for careers in science and technology. Finally the research will be performed in collaboration with scientists from the US and abroad, strengthening and enriching the US physical community.