Collective transition of the vortex lattice from Mott-like to metal-like state in a superconducting film with periodic array of holes

We observe that under an a.c. magnetic field excitation, vortex lattice in a superconductor with artificial array of holes undergoes transition from Mott-like state, with every vortex localized in a hole, to a metal-like state, where the vortices are delocalized.In our experiment we use two-coil mutual inductance measurement technique where the sample is sandwiched between the two coils. Sending an a.c. current in the primary coil we record the mutual inductance obtained from the induced voltage in the secondary. The real part of this mutual inductance in our experiment measures the compressibility of the vortex lattice while the imaginary part measures the dissipation in the system.

Varying the amplitude of the a.c. current in the primary coil we measure the shielding response of the vortex lattice in different regimes. At low amplitude the vortex lattice is in its Mott state, characterized by a low compressibility and low dissipation. Above a threshold of the a.c. amplitude, we observe series of sharp jumps in compressibility and dissipation signaling the transition from Mott-state to a metallic state.The onset of this transition point depends upon the filling fraction of vortices in the holes and we observe that the transition a.c. amplitude is lowest for integer filling of holes with vortices.This along with quantitative analysis confirm that the transition is a collective phenomenon.

References:
[1]	Indranil Roy, Prashant Chauhan, Harkirat Singh, Sanjeev Kumar, John Jesudasan, Pradnya Parab, Rajdeep Sensarma, Sangita Bose, Pratap Raychaudhuri (arXiv:1609.05752v2) (2016). Physical Review B (Preprint).
[2] 	Sanjeev Kumar, Chandan Kumar, John Jesudasan, Vivas Bagwe, Pratap Raychaudhuri, and Sangita Bose, Applied Physics Letters 103, 262601 (2013).
[3] 	Sanjeev Kumar, Chandan Kumar, John Jesudasan, VivasBagwe, PradnyaParab, PratapRaychaudhuri and Sangita Bose, Supercond. Sci. Technol. 28, 055007 (2015).