What goes bump in the hall : the ghost critical field and Gaussian superconducting fluctuations
- In this thesis we study disordered two-dimensional superconductors. We examine both the nature of superconducting fluctuations in these films, and also the effect of disorder on the normal and superconducting states. In the first part of this work, we will examine in detail the growth and characterization of ultra-thin, disordered tantalum nitride (TaN) films. By tuning the film growth conditions, we are able to deposit amorphous superconducting films that lie on the edge of the metal-insulator transition. We study disorder-enhanced antilocalization at nonzero temperature, and extract electron scattering lifetimes, dephasing rates, and superconducting materials parameters. As these films become sufficiently disordered, the superconducting ground state gives way to purely insulating behavior. Using transport techniques to investigate this zero temperature superconductor-insulator phase transition, we show evidence for classical percolation scaling and a superconductor-to-Fermi insulator scenario in TaN. Finally, we compare measurements of the Hall effect across the superconductor-insulator transition with companion work on indium oxide films, and find evidence for a metal-insulator crossover in the high-field insulating state at zero temperature. The second part of this work examines superconducting fluctuations. In a superconductor above its transition temperature Tc, short-lived regions of local superconductivity known as fluctuations can dramatically affect both thermodynamic and transport properties. Fluctuations are strongly enhanced in systems of reduced dimensionality, such as in a thin films, and remain incompletely understood in both conventional superconductors as well as in more exotic materials. For example, the importance and extent of superconducting fluctuations within the ``pseudogap'' phase of high-Tc superconductors are not fully understood. Recent Nernst effect measurements of high-Tc and conventional superconductors have tentatively identified a characteristic magnetic field for the suppression of fluctuations, but such a field has not been identified in the Hall effect or explained quantitatively. We present new measurements of the fluctuation-enhanced Hall effect in a disordered thin film. Above Tc, we can describe this enhancement quantitatively using new time dependent Ginzburg Landau theory results. We can also track the fluctuations to temperatures below Tc, and I will compare this evolution with independent measurements of the upper critical field Bc2, and the ``Ghost Critical Field, '' a field analogous to Bc2 above Tc. We find that the enhancement in the Hall effect represents a distinct magnetic field scale from Bc2 and the Ghost Critical Field, and I will contrast these results with other experimental work in the field.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Breznay, Phibbs Nicholas
|Stanford University, Department of Applied Physics.
|Statement of responsibility
|Nicholas Phibbs Breznay.
|Submitted to the Department of Applied Physics.
|Thesis (Ph.D.)--Stanford University, 2013.
- © 2013 by Nicholas Breznay
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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