Browsing by Author "Du, Rui-rui"

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    Hydrogen doping and the metal-insulator phase transition in vanadium dioxide
    (2015-04-22) Ji, Heng; Natelson, Douglas; Du, Rui-rui; Biswal, Sibani L
    Strongly correlated systems represent a major topic of study in condensed matter physics. Vanadium dioxide, a strongly correlated material, has a sharp metal-to-insulator phase transition at around 340 K (67 °C), a moderate temperature which can be easily achieved. Its potential as a functional material in optical switches and semiconductor applications has attracted a great deal of attention in recent years. In this thesis, after a detailed introduction of this material and the methods we used to grow VO2 in our lab (Chapter 1), I will discuss our attempts to modulate the electronic properties and phase transition of single-crystal VO2 samples. It started with a plan to use ionic liquid to apply an electrostatic gate to this material. Although modulation of the resistance was observed, we also discovered an unexpected electrochemical reaction, leading to a suspicion that hydrogen doping is the reason for the change of properties of VO2 (Chapter 2). Next, a series of experiments were performed to systematically study the mechanism of this hydrogen doping process and to characterize the hydrogenated VO2. Our collaborators also provided supporting simulation results to interpret these phenomena from a theoretical point of view, as well as results from synchrotron x-ray diffraction and neutron diffraction experiments. From all these studies, we confirmed the existence of the hydrogen intercalation in VO2 (Chapter 3), and further, plotted the phase diagram as a function of temperature and hydrogen concentration (Chapter 5). We also found that this diffusion process prefers the rutile crystal structure of VO2 (i.e. metallic phase) and specifically, its c-axis (Chapter 4). Finally, the low-temperature electric transport properties of the hydrogenated VO2 material have been studied for the first time, and interesting magneto-resistance responses will be discussed (chapter 6).
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    Nonequilibrium Dynamics of Quantum-Degenerate Fermionic and Bosonic Gases in Semiconductors Probed by Coherent Terahertz Magneto-optics
    (2015-10-21) Zhang, Qi; Kono, Junichiro; Du, Rui-rui; Natelson, Doug
    Quantum-confined semiconductor structures are ideal systems in which to study non-equilibrium and coherent dynamics of interacting many particles in a highly controllable fashion. In particular, two-dimensional (2D) semiconductor systems in a strong perpendicular magnetic field provide one of the cleanest condensed matter systems with ultralong coherence times, allowing us to excite and control macroscopic coherent phenomena. When doped, either electrically or optically, such systems can accommodate quantum-degenerate fermions (electrons) and/or bosons (excitons). In this dissertation, we studied the coherent terahertz (THz) dynamics of 2D gases of electrons and excitons in GaAs quantum wells in magnetic fields with time-domain THz magneto-spectroscopy. In high-mobility 2D electron gases, we made the first observation of collective radiative decay, or superradiance, of cyclotron resonance (CR). The decay rate of coherent CR oscillations increased linearly with the electron density in a wide range, which is a hallmark of superradiant damping. Our fully quantum mechanical theory provided a universal formula for the decay rate. We further achieved ultrastrong coupling of coherent CR with THz photons in a high quality factor 1D photonic crystal cavity. We directly observed time-domain vacuum Rabi oscillations, and the square root of N dependence of collective Rabi splitting with respect to the carrier density. Superradiance decay of CR was significantly suppressed in the cavity, and an intrinsic CR linewidth as sharp as 5.6~GHz was resolved. In undoped GaAs quantum wells, we systematically investigated the nonequilibrium dynamics of electron-hole pairs using ultrafast optical-pump THz-probe spectroscopy. We simultaneously monitored the intraexcitonic 1s-2p transition, which splits into the 1s-2p+ and 1s-2p- transitions in a magnetic field, and the CR of unbound carriers as a function of pair density, temperature, magnetic field, and probe delay time. We found that the 1s-2p- feature is robust at high magnetic fields even under high excitation fluences, indicating magnetically enhanced stability of excitons. While mimicking some of the well-known phenomena in quantum optics of atomic and molecular gases, these results highlight some of the unique features of condensed matter systems due to strong many-body Coulomb interactions among carriers and open a door to the novel physics of THz many-body electrodynamics.