Quantum Coherence and its Manipulation in Coupled Quantum Dots

Transient Studies of Nonequilibrium Transport in Two-Dimensional Semiconductors Research Foundation of SUNY, Principal Investigator: Jonathan Bird In the past decade or so there has been an explosion of interest in new semiconductor materials, in which the material thickness is reduced to the ultimate limit of just a few atoms(or even a single atom). These so-called "two-dimensional(2D) semiconductors" offer great promise for the development of new generations of advanced electronics (or optoelectronics), as well as for the investigation of novel condensed-matter phenomena. In this research, the overarching objective is to apply time-dependent measurement techniques to investigate aspects of nonequilibrium transport in2D semiconductors. This work builds on progress made by the PI during the prior period of DOE support, utilizing nanosecond-duration electrical pulsing to reveal novel aspects of quantum transport in different mesoscopic devices. In comparison to conventional DC measurements, transient studies provide a time-resolved picture of carrier relaxation mechanisms, allowing the fundamental interactions that govern transport to be revealed. In the current period of support, the 2D semiconductor of interest is graphene, a two-dimensional sheet of carbon in which the atoms are arranged into a honeycomb pattern. Transient pulsing is utilized in order to investigate several important aspects of the transport in this material: 1. The search for negative differential conductance in graphene. Theoretical studies of the high-field transport properties of graphene have highlighted the possibility of negative differential velocity (NDV), arising from an unusual "band dynamics" phenomenon. While there have not previously been any convincing demonstrations of this phenomenon, it could open up a new field of study of dynamical instabilities in graphene, while also possibly leading to the realization of new classes of high-frequency (terahertz) sources. In this work transient pulsing is used to search for evidence of NDV in graphene, allowing intrinsic sources of this phenomenon to be identified while excluding extrinsic factors that may erroneously generate apparent NDV. 2.Probing fundamental relaxation processes in graphene in real time.Quantum transport of carriers in mesoscopic systems is known to be governed by a number of important time scales, with prominent examples including the electron phase-breaking and energy-relaxation times. At low temperatures, around a few degrees Kelvin, typical values of these time scales are in the nanosecond range, making transient techniques ideally suited to their determination. In order to achieve this objective, in this project a novel approach is developed to determine time-resolved mesoscopic conductance fluctuations in graphene. By investigating the dependence of these fluctuations on time and transient-voltage amplitude,the fundamental mechanisms for energy relaxation and decoherence are explored. This research program yields significant impact in terms of our understanding of the fundamental condensed-matter physics of 2D semiconductors. Work on the search for negative differential conductance may open up a new field of experimental research, on the manifestations of high-field instabilities in 2D materials. It may also drive the development of new solid-state terahertz sources, which exploit the large drift velocities inherent to graphene to realize terahertz-frequency oscillators based on the driven motion of high-field domains. Additionally, the transient investigations of energy relaxation and dephasing in graphene yield important fundamental insight into the microscopic processes responsible for these processes.