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Jeudi 19 Octobre 2017 à 14h00
Amphi Urbain, Esc N
1ème etage
Title : Experimental investigation on low – dimensional and disordered Superconductors
Abstract : The Bardeen-Cooper-Schrieffer (BCS) theory, the fundamental microscopic theory for superconductors, does not take into account the case of phase fluctuation since for clean 3-dimensional (3d) conventional superconductors phase twisting energy is much larger than Cooper-pair binding energy. In my presentation, I shall be talking about two different systems in which superconducting transition is governed by phase fluctuations. The first one is in reduced dimension and the other one is a weakly disordered system in presence of magnetic field.
In the first part, I shall report our findings on the electro-dynamic response of a 2-dimensional NbN film across the Berezinskii-Kosterlitz-Thouless (BKT) transition for two widely separated regimes of frequencies which employ a low-frequency mutual inductance technique (10-100 kHz) and broad-band microwave spectroscopy (1-10 GHz). We see that the smooth down turn of the inductive part and huge amplitude and width of the dissipative response at low frequency, cannot be captured using standard homogeneous Bardeen-Stephen model. The reduced thickness of our sample makes the order parameter spatially non-uniform and in such a system vortex diffusion gets substantially modified than the homogeneous case.
The second part, consists of the study of the superconducting properties of a weakly disordered NbN film under the application of magnetic field, using low temperature Scanning Tunneling Microscope (STM) and transport measurements. Through real space imaging we observe that with increasing magnetic field, the superconductor develops island like structures, where superconducting regions are separated by regions with completely suppressed superconductivity. Consequently, the destruction of superconductivity with magnetic field follows a completely different route compared to conventional Type II superconductors, where phase fluctuations between the islands results in the destruction of the zero resistance state.
Reference :
1. Rini Ganguly, Dipanjan Chaudhuri, Pratap Raychaudhuri, and Lara Benfatto, Phys. Rev. B 91,054514 (2015).
2. Rini Ganguly, Indranil Roy, Anurag Banerjee, Harkirat Singh, Amit Ghosal, and Pratap Raychaudhuri, Phys. Rev. B 96, 054509 (2017).