Melting of the Vortex Lattice through Intermediate Hexatic Fluid in an a−MoGe Thin Film

Abstract

The hexatic fluid refers to a phase in between a solid and a liquid which has short range positional order but quasi-long range orientational order. In the celebrated theory of Berezinskii, Kosterlitz and Thouless and subsequently refined by Halperin, Nelson and Young, it was predicted that a 2-dimensional hexagonal solid can melt in two steps: first, through a transformation from a solid to a hexatic fluid which retains quasi long range orientational order and then from a hexatic fluid to an isotropic liquid. In this paper, using a combination of real space imaging and transport measurements we show that the 2dimensional vortex lattice in a-MoGe thin film follows this sequence of melting as the magnetic field is increased. Identifying the signatures of various transitions on the bulk transport properties of the superconductor, we construct a vortex phase diagram for a two dimensional superconductor.

Extreme sensitivity of the vortex state in a -MoGe films to radio-frequency electromagnetic perturbation

“…Comparing with real space STS imaging it was shown in ref. 13 that these fields corresponds to the transition from a VS to HVF state and a HVF to IVL respectively. The shallow minima in R and corresponding maxima in Ic are signatures of "peak effect" 16 which we discuss later.…”

“…The sample used in this study is a 20 nm thick a-MoGe thin film grown on surface oxidized Si substrate with Tc ~ 7.2 K, similar to that used in ref. 13 . The films were grown using pulsed laser deposition using a Mo70Ge30 target prepared by arc-melting stoichiometric amounts of Mo and Ge metals.…”

“…The vortex diffusion constant, D, which is related to the resistance observed in the mixed state of a Type II superconductor, can in-principle be obtained by tracking the motion of vortices as a function of time as was done in ref. 13. From microscopic random walk viewpoint of the Brownian motion, the Mean Square Displacement (〈d 〉) is related to the Diffusion coefficient ( ) as, 〈d 〉 = 4 -, where, t is time.…”

“…Recently, evidence of vortex liquid states existing down to very low temperatures has been obtained from detailed scanning tunneling spectroscopy (STS) imaging of the vortex lattice 13 in very weakly pinned thin films of the amorphous superconductor MoGe (a-MoGe). Here the thickness of the film is much smaller than the bending length of the vortices such that the VL is effectively 2-dimensional.…”

Experimental test of strong pinning and creep in current-voltage characteristics of type-II superconductors

“…In a similar fashion, we analyse the I-V measurements on a-MoGe films reported in Ref. [7], with data available both at different fields and temperatures. The applied magnetic field in the range H = 0.03 − 7 T (the uppercritical field is H c2 (0) = 13 T) implies a vortex lattice constant a 0 = 17 − 262 nm.…”