  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.
