Session J52: Buckley Prize / Apker Award Session

Buckley Prize Talk: Electrostatic Control of the Superconductor-Insulator Transition

The superconductor-insulator transitions (SITs) of ultra-thin films are among the simplest quantum phase transitions. The ground states of systems that have been studied are usually changed by adjusting the level of disorder, by the application of perpendicular and parallel magnetic fields, by altering the chemical composition, and by the seeding of the surface with pair-breaking magnetic impurities. More recently, realizations of the electric field effect have been used to tune SITs. This has been done with devices employing high dielectric constant gate insulators, as well as with electric double layer transistor devices employing ionic liquids as gate insulators. In addition to disordered ultrathin films, cuprates, and metallic interfaces between insulators have also been studied. The SITs of selected systems will be reviewed with particular attention being paid to the results of finite size scaling analyses of the transitions, and the nature of the insulating states found. In the case of the cuprates, the extent to which their phase diagrams can be traversed will be explored. Finally the potential value of electrostatic gating as a tool in the search for new superconductors will be discussed.   

Buckley Prize Talk: Bosons on the Boundaries: The magnetic field driven superconductor-insulator quantum phase transition

Experiments probing the competition between superconductivity and disorder in two-dimensional (2D) thin-film systems have provided fascinating glimpses into the physics of superconductor-insulator (S-I) quantum phase transitions (QPTs). This talk will address the use of externally applied magnetic fields to tune through the S-I transition of amorphous composite indium oxide ($\alpha $-InOx) thin films prepared at different stages of disorder. Air-stable $\alpha $-InOx films are particularly advantageous for these studies: the disorder parameter as measured by the sheet resistance can be reproducibly controlled during deposition and the films are uniformly homogeneous out to macroscopic length scales. Temperature-dependent resistance and current-voltage measurements confirm the power-law decay of the order-parameter correlation function appropriate to a Kosterlitz-Thouless description of phase transitions in 2D systems. Accordingly, the superconducting phase transition temperature Tc is related to the unbinding of vortex-antivortex pairs either by temperature and/or disorder. The application of magnetic fields unveils fundamentally different physics in which, rather than a vortex unbinding transition, a field-tuned QPT emerges with the signature of a disorder-dependent critical field Bc that identifies the delocalization and Bose condensation of field-induced vortices. The concomitant pronounced divergence in resistance, which becomes increasing sharp as the temperature is lowered, marks the boundary between a superconductor harboring both Bose condensed Cooper pairs and localized vortices and an insulator harboring both Bose condensed vortices and localized Cooper pairs. The data for this putative QPT are well described by finite temperature scaling theory with critical exponent values accurately determined. At higher fields there is a second critical field where the transverse resistance appears to diverge, signaling the unbinding of pairs with the superconducting energy gap simultaneously going to zero and localized single electrons dominating to form a Fermi glass insulator.   

Buckley Prize Talk: The Suprerconductor-(Metal)-Insulator Transition

While the classical theory of phase transitions has been extraordinarily successful, there are several reasons to exercise caution when applying this approach to the zero temperature superconducting transition. First, experimental identification of the relevant phases requires extrapolation to zero temperature, which becomes complicated, especially when one needs to identify sources of dissipation. In addition, since superconductivity may be highly inhomogeneous as appreciable superconducting order parameter may be concentrated in ``superconducting puddles'' due to disorder and/or spontaneous phase separation, the nature of the quantum phase transition to a superconducting state may be highly anomalous, where the system attempts to optimizes the formation of puddles with the Josephson coupling among them to obtain global superconductivity. In this talk we will review some of the consequences of these considerations, emphasizing the possible emergence of anomalous metallic phases close to the superconductor-insulator transition.   

Buckley Prize Talk: Theory of Quantum Phase Transitions: A 25 year purview

A 25 Purview of Quantum Phase Transitions   

Leroy Apker Award Talk: Transport measurements of a model cuprate superconductor

High-temperature superconductivity in the cuprates has been the subject of intense research since its discovery in 1986 by J. G. Bednorz and K. A. M{\"u}ller. The fundamental challenge posed by these materials is that they exhibit strong electronic correlations, giving rise to anomalous properties, such as the observation that the resistivity in optimally hole-doped samples is linear in temperature from the superconducting transition temperature up to very high temperatures. The scope of this work was to explore the phase diagram through transport measurements of a model compound. Specifically, the resistivity, magneto-resistance, Hall effect and thermoelectric power were measured for single-crystal samples of HgBa$_2$CuO$_{4+\delta}$ (Hg1201). The outcome of these measurements is highly surprising. Despite the complexity of the phase diagram, conventional Fermi-liquid metallic behavior is observed in the pseudogap regime below optimal doping.