This individual investigator award will support advanced optical spectroscopy studies of low-lying excitation modes of two-dimensional electron liquids in quantum Hall regimes. Collective excitations in the spin or charge degrees of freedom will be accessed by inelastic light scattering methods developed by the PI and collaborators. The primary interests are on the diverse and competing quantum phases of many particle systems that emerge at very low temperatures when electrons populate higher Landau levels in highly perfect systems. Being composed solely of electrons, the electron fluids and solids in the quantum Hall regimes are unique phases of matter that emerge at very low temperatures and high magnetic fields and that arise from the impact of fundamental Coulomb interactions among electrons. In these intriguing phases small changes in magnetic field may alter the statistics of quasiparticles from fermions to bosons and even to anyons. The goals of this body of research are the discovery and elucidation of the physics in these competing quantum phases of matter. The broader impacts stem from prospects of advancing the field by offering previously unavailable insights on striking quantum phases of matter. The group of the PI has strong representation of women and minorities, and significant links to leading scientists in the US and abroad.
Non Technical When the spin, which is analogous to a small magnet attached to the electron, or electrical charge of electrons of systems containing many electrons existing in a certain state called the "quantum Hall regime" new electronic properties of the material containing them emerge. These properties will be measured using methods involving the reflection of light that were developed by the PI and his collaborators. The quantum Hall regimes occur at very low temperatures when electrons in nearly perfect man-made semiconductor structures are restricted to move on a flat plane with a magnetic field applied perpendicular to this plane. This group of electrons makes up a fluid or solid system of particles and is in a unique state of matter that emerges from the fact that each electron has a charge causing interactions to occur among them. In these intriguing electron systems under study, the collective motion of electrons is represented by what is known as ?quasiparticles? that may assume different characteristics that can be altered by applying small magnetic field changes to the system. The goal of the projected research is the discovery and elucidation of the physics that is at the origin of the competing electron states that emerge in these many electron systems. The research is of interest to an international community of scientists and to the general public that is amazed by manifestations of basic electron properties and their potential applications. The group of the PI offers significant opportunities to diverse groups of students and junior scientists to participate in world-class research.
Intellectual Merit The research focus in this project has been the study in emergent quantum phases of two-dimensional electron systems that occur in man-made semiconductor structures. These structures are constructed with semiconductor layers of nanoscale thickness that are created with a highest perfection that is reached with fine atomic control of growth. Fabrication of the layered semiconductors in this project is at the moving frontiers of the state-of-the-art. When embedded in high magnetic fields electrons in the artificial structures occupy quantized states called Landau levels. The quantized levels can have full or partial population. Electrons in partially populated Landau levels collectively arrange into 2D exotic phases that manifest the striking weirdness of quantum mechanics. The research in this project is unique in that it relies on measurements of low-lying collective excitations modes of the new quantum phases. Of primary interest are the phases that emerge in partially populated lowest and in the next (or second) Landau levels. Examples of intriguing quantum phases of interest are the fractional quantum Hall states that occur even- and odd-denominator fractional population in the second Landau level. Much of the research by others in this exciting field is carried out by studies of electrical transport in the high magnetic field. Studies in this project can be regarded as a quest in which measurements of low-lying excitation modes in partially populated Landau level uncover exciting new electron physics from quantum interactions in reduced dimensionality semiconductors. The experiments employ advanced light scattering methods at very low temperatures (reaching to below 50millikelvin degrees) created in the group of the PI. Among several key discoveries is the extensive evidence of anomalies in the collective states of spin in the quantum phases of the partially populated second Landau level. Understanding of the roles of the spin degree of freedom in the second Landau level is particularly significant. One of the reasons is that the predicted non-Abelian characteristics of fractional quantum Hall states, with fundamental roles in quantum computation with these states, occur for quantum phases that have spin polarization. The capability to monitor the intriguing evolution of the collective states of spin in the second Landau level is among the unique advantages of the experimental methods in this project. Broader Impacts Studies of low-dimensional electron systems attract wide attention beyond progress in understanding of their fundamental physics. The highly significant advances in materials science expand the state-of-the-art in technologies of contemporary and future electronics and optoelectronics. In fundamental science, the research appeals to world-class scientists working on topics of interaction physics that seek answers to questions that are at current frontiers. The milestones achieved in this project created experimental methodologies that provide fruitful and surprising insights. The appeal of this research is greatly enhanced by collaborative efforts of scientists in the group of the PI with colleagues in leading national and international research groups that share similar interests in studies of key science topics. Studies of electrons in greatly reduced dimensions cover major segments of contemporary science. The project offered opportunities undergraduate students, graduate students, and postdoctoral scientists interested in participation in research at frontiers of condensed matter science. Women and minorities are well represented in the group of the PI. Opportunities for junior scientists are magnified by the links with other scientists at Columbia and at leading national and international institutions. Such research networking ensures that the studies in this project have significant impacts in broad and diverse communities.
