Study of Vortex Dynamics and Phase Transitions in Superconducting Thin Films

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.
Comment: 149 pages, A thesis submitted to the Tata Institute of Fundamental Research, Mumbai for the PhD degree in Physics