Study of Vortex Dynamics and Phase Transitions in Superconducting Thin Films

TL;DR

This thesis presents an experimental investigation of vortex behavior and phase transitions in various superconducting thin films, revealing new insights into vortex lattice dynamics, disorder effects, and quantum fluctuations.

Contribution

It introduces detailed experimental analysis of vortex phases, including vortex lattice melting, pseudogap formation, and quantum fluctuations in superconducting thin films.

Findings

Observation of vortex lattice melting and hexatic vortex fluid phase.

Identification of pseudogap phase related to superconductor-insulator transition.

Evidence of quantum vortex fluctuations and crossover phenomena.

Abstract

The work reported in my doctoral thesis is an experimental study of vortex dynamics and phase transitions in thin films of type II superconductors using scanning tunneling spectroscopy, low frequency ac susceptibility measurements and complimentary transport measurements. Chapter I and II cover basics of superconductivity and methodologies used in the thesis. Chapter III discusses the effect of periodic pinning centers on the geometry and dynamics of vortex lattice in NbN thin films. Consequent study of dynamic transition of vortex Mott-like to vortex metal-like state is described. In Chapter IV, effect of strong disorder on vortex lattice in NbN thin films is studied. Here we show magnetic field induced granularity gives rise to pseudogap phase which is utilized to explain superconductor to insulator-like transition in stronger disorder. Chapter V contains the study of 2-dimensional vortex lattice melting in a-MoGe thin films. We discuss the observation of hexatic vortex fluid phase and the BKTHNY two-step melting in this scenario. Effect of sample thickness on this phases is also described. In Chapter VI, we study the possibility of quantum fluctuation of vortices in weakly pinned a-MoGe thin films. Effect of pinning on this fluctuation and a possibility of a quantum to thermal crossover is also explored.