****TECHNICAL ABSTRACT**** Persistent currents are one of the most dramatic phenomena of mesoscopic physics. The notion of a disipationless electrical current flowing through a resistor is quite counterintuitive, although at its simplest, it is a manifestation of the same phenomena that allows isolated atoms to posses orbital angular momentum in their electronic ground state. In mesoscopic solid-state devices, a considerable body of theoretical work suggests that persistent currents may be quite sensitive to a variety of many body effects. However until recently, the reliable measurement of persistent currents was technically very challenging. This project will use newly developed detection techniques to measure persistent currents in regimes where many body effects are expected to be dominant. Specifically, micromechanical torsional magnetometry will be used to measure the average current in diffusive metal rings at low magnetic fields, where it has been predicted that the same interaction effects that give rise to BCS superconductivity can lead to dramatically enhanced persistent current in the normal state. In addition, the same techniques will be used to measure the persistent current in high-mobility two dimensional electron gases, both in low magnetic fields (where the persistent current can be used to study the spectra of chaotic or integrable samples), and at high magnetic fields, where the persistent current can be used to probe the statistics of the quasiparticles. This project will support the training of a Ph.D. student, who will learn a variety of technical skills, including semiconductor micro- and nano-fabrication, cryogenics, fiber optics, micromechanics, and ultrasensitive measurement techniques.
One of the most striking results of applying the theory of quantum mechanics to an entire electrical circuit is the prediction that, despite the electrical resistance of the circuit, electrical current can flow perpetually through it without gaining or losing energy. This phenomenon is known as persistent current, and is directly analogous to the more familiar orbits of electrons around the nucleus of an atom. However in the setting of an electrical circuit, the orbitals which produce the persistent current are flowing through metal wires which are ten thousand times longer than a single atom. Until recently it was considered nearly impossible to measure these currents reliably, but this project will use newly-developed measurement techniques to study persistent currents in a variety of different materials. This goal is motivated by the theoretical prediction that the basic properties of the persistent current (for example, its size and the direction of its flow) can tell us a great deal about two exotic but extremely important phenomena: superconductivity (in which a metal loses all resistance to electrical currents) and the fractional quantum Hall state (in which electrons in a semiconductor bind together with a magnetic field to create particles that have no analog in nature). This project will advance our understanding of these phenomena, and will also result in new types of instruments for studying the behavior of electrons in small, isolated samples. This project will support the training of a Ph.D. student, who will learn a variety of technical skills, including semiconductor micro- and nano-fabrication, cryogenics, fiber optics, micromechanics, and ultrasensitive measurement techniques.