Browsing by Subject "Quantum phase transition"

Now showing 1 - 4 of 4
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    Electrical transport properties of ultrathin amorphous bismuth films near the superconductor-insulator transition.
    (2011-08) Lin, Yen-Hsiang
    A combination of thickness- and perpendicular magnetic field-tuning of SI transitions has been performed on quench-deposited homogeneous a-Bi thin films with a 14.67 Angstrom a-Sb underlayer. Transport properties, including measurements of resistance and of I-V characteristics have been studied in both the insulating and superconducting regimes. In the insulating regime, the resistance exhibits an Arrhenius type of conduction and the magnetoresistance (MR) exhibits a peak in perpendicular magnetic field. Furthermore, a possible quantum phase transition is found in the insulating regime. Presumably this transition is between the Bose and Fermi insulators discussed in the literature. The I-V characteristics exhibit strong non-linearities in the insulating regime at low temperatures. These non-linear curves can be well described by a heating model involving the decoupling of the electronic and phononic degrees of freedom at low temperatures. On the conductive or superconducting side of the transition, the transport properties are found to be remarkably similar to those of an overdamped random Josephson junction array, and vortex dynamics dominates the conductive behavior in both zero and non-zero magnetic fields. These observations suggest that isolated superconducting islands or localized Cooper pairs exist in both the insulating and conductive regimes. An AFM scan of the last film in the sequence has revealed that this series of films although continuous, has thickness variations on a mesoscopic length scale. Therefore, it is not surprising that there may be superconducting islands. The AFM scan also suggests that some of the thick, nominal granular films grown by quench condensed deposition are directly connected with large thickness variations. These insulating granular films also exhibit an Arrhenius type conduction at low temperatures, which reveals the existence of a hard gap in the electronic density of states, which is consistent with the theory of Feigel'man et al.. However, the activation energy of the Arrhenius type conduction found in the thickness tuning homogeneous a-Bi films doesn't follow this model. Therefore, the model may not completely explain the hard gap.
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    Electronic and Optical Properties of Quantum Dots: Metal-Insulator Transitions and Ultrafast Spectroscopy
    (2020-05) Robinson, Zachary
    In this thesis I will discuss crossing the metal-insulator transition in ZnO nanocrystal networks as well as the synthesis, electronic states and optical properties of novel infrared emitting CdSe/HgS/CdS QDs. To observe the metal-insulator transition we use a photonic sintering process to selectively increase both the inter-nanocrystal facet radius and the free electron density. This, combined with atomic layer deposition to infill the film, enables us to clear cross the metal-insulator transition. Second, I discuss the synthesis of high quality CdSe/HgS/CdS QDs. The HgS interlayer creates a 'well' for the electrons while holes are delocalized throughout the CdSe/HgS structure. This quasi-type-II system exhibits tunable emission in discrete steps as a function of the HgS interlayer thickness, which we can control with atomic level precision. We investigate the multi-excitonic properties of these dots, and also show their utility for use as near infrared single photon emitters.
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    Electrostatic modification of novel materials
    (2013-10) Lee, Yeonbae
    Electric double layer transistor configurations have been employed to electrostatically modify the physical properties of two novel materials; single crystals of insulating strontium titanate (SrTiO3) and thin films of amorphous indium oxide (a-InO). First the results of doping SrTiO3 over broad ranges of temperature and carrier concentration employing an ionic liquid as the gate dielectric are reported. The surprising results are, with increasing carrier concentration, an apparent carrier-density dependent conductor-insulator transition, a regime of the anomalous Hall effect, suggesting magnetic ordering, and finally the appearance of superconductivity. The possible appearance of magnetic order near the boundary between the insulating and superconducting regimes is reminiscent of effects associated with quantum critical behavior in some complex compounds. Secondly, the evolution with carrier concentration of the electrical properties of a-InO thin films has been studied. Carrier variations of up to 7 x 1014 carriers-cm-2 were achieved again using an ionic liquid as a gate dielectric. The superconductor-insulator transition was traversed, and both the magnitude and the position of large magnetoresistance peak found in the insulating regime were modified. The systematic variation of the magnetoresistance peak with charge concentration was found to be qualitatively consistent with a simulation based on a model involving granularity.
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    Superconductivity at a quantum critical point: A theoretical approach to the understanding of unconventional superconductors in strongly correlated systems.
    (2021-08) Wu, Yiming
    The complexity of strongly correlated electronic system is manifested by the interplay of multiple electronic orders, among which superconductivity is one of the most interesting phases. In experimentally observed phase diagrams for materials such as heavy fermion compounds, cuprates and iron based superconductors, superconductivity is close to other electronic orders such as ferromagnetism, anti-ferromagnetism and nematicity etc. This fact brings about interests of studying the role of a possible underlying quantum critical point(QCP) in determining the unusual properties of these materials. Here we consider an itinerant fermion system which is close to a QCP. Because of the closeness, the collective boson mode due to the order parameter fluctuations will couple to low energy fermions and mediate the fermion-fermion interaction. This effective interaction simultaneously gives rise to two competing fate for the fermions: On one hand it can lead to SC if the there is any pairing instability in at least one pairing channel. On the other hand, the same interaction also diminish fermion coherence and results in non Fermi liquid behavior. These two tendencies compete with each other, in a sense that SC gaps out low energy fermions and reduces the self energy , while non Fermi liquid tends to destroy fermion coherence and is detrimental to SC. In order to capture this story we adopt the approach that Eliashberg first used when he studied the electron-phonon coupling system, i.e. we approximate the fermion self energy by neglecting the vertex corrections, which is controllable when the vertex is parametrically smaller. We further assume the interaction depends only on frequency via a dynamical exponent $\gamma$, namely $V(\Omega_m)\propto 1/|\Omega_m|^\gamma$. Based on this model, we unveil many special properties of SC state on both imaginary and real frequency axis, including the ‘gap closing’ behavior observed in cuprates. As a unique feature of pairing at a QCP, we find there exists an infinite set of solutions to the gap equation, corresponding to different local minima in free energy. This set becomes a continuous one at a special case $\gamma=2$, which corresponds to phonon-mediated pairing interaction with a vanishingly small phonon frequency. We also studied the odd-frequency pairing state from this model, and find there is no zero bias peak in the quasiparticle density of states which was considered as an evidence of odd- frequency pairing. At last, in addition to mean field analysis, the superconducting phase fluctuation is also discussed.