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Abstract:

Superconductivity is one of the fascinating gauge symmetry breaking phenomena having two observable properties : zero resistance and magnetic field expulsion. Symmetry breaking means that superconductivity is associated with a quantum phase transition. The superconducting condensate is the collection of Cooper pairs in the same quantum state. In a material, an electron's quantum state is defined from its momentum, spin, and band index if it is a multiband system. For most superconductors, each Cooper pair will have net momentum zero and net spin zero. We can differentiate classic superconductors on the basis of their microscopic length scales : penetration length and Ginzburg Landau coherence length. When the penetration length exceeds the coherence length such that the magnetic field fails to be screened out completely, it leads to intermediate-state behaviour called Type II superconductors. When magnetic field penetrates into a type-II superconductor, it brings out filamentary-alike tubes having quantized units of magnetic flux, called vortices. Besides a localized magnetic field these vortices are characterized by a depletion of the superconducting condensate. Bearing in mind the concept of "seeing is believing", scanning probe microscopy techniques plays a crucial role in probing the static vortex distribution with nanometer resolution. The motion of these fascinating nano-objects is characterized by time dependence in both the localized magnetic field profile and the condensate. Moreover, this motion is accompanied by dissipation and, as such, results in the destruction of superconductivity. Therefore, a local observation, on a relevant timescale, is of utmost importance to unravel the underlying principles of motion. Once operational, the power of these techniques will be used to investigate the dynamics of the superconducting condensate with single vortex resolution. These type of measurements will also be invaluable to validate theoretical models such as BCS and GL theory and gain new insights on the dynamics of vortex matter in a variety of interesting superconducting systems. Graphene is one of the spectacular 2D materials promising various avenues in condensed matter physics. Its impressive characteristics like ballistic transport over hundreds of nanometers, high mobility and spin polarized edge states compel us to explore the superconducting samples exploiting graphene substrates(Ref.1).This metallic-superconducting interface offers us intense possibilities for electron-hole conversion leading to Andreev reflection by combining the relativistic Dirac equation of graphene and GL equation of superconductivity. Superconducting films are highly sought after for tunable superconductivity obtaining control over various parameters such as mean free path, coherence length, superconducting gap, critical temperature etc. Inspired by theoretical and experimental evidence, we would like to fabricate the samples using experimental tools such as nanolithography, pulsed laser deposition etc. and investigate superconducting films on graphene substrates using scanning tunnelling microscopy. The overall motivation of my PhD project is to employ scanning probe microscopy technique sensitive to superconducting-graphene heterostructures investigating the global transport properties and dynamics of nanoscale entities. Vortex physics is indeed, an inseparable part of type-II superconductors visualization at nanoscale. When superconductivity is induced in graphene through proximity effect, the presence of vortices cause the variation of order parameters bringing out interesting fundamental physics. Already a lot of research had been done on vortex confinement and topographic studies of MoGe (Ref. 2) termed as Dirty superconductor (due to its penetration length surpassing coherence length). Due to its high critical field, it is a considerable candidate for investigating superconductivity-insulating quantum phase transition as superconductor-graphene hybrid structures. Of course, the challenges reside in fabrication of high quality samples needed for installing in STM. We will take advantage of thin capping layers of inert metals such as gold or platinum to substitute the uttermost requirement for in-situ preparation and characterization. 1 "Electrical control of the superconducting-to insulating transition in graphene-metal hybrids", Nature/10.1038/NMAT3335 2 "Direct observation of condensate and vortex confinement in nanostructured superconductors", doi : 10.1103/PhysRevB.93.054514