Critical Currents at the Bragg Glass to Vortex Glass Transition

Abstract

We present simulations of the transport properties of superconductors at the transition from the Bragg glass (BG) to the vortex glass (VG) phase. We study the frustrated anisotropic 3D XY model with point disorder, which has been shown to have a first order transition as a function of the intensity of disorder. We add an external current to the model and we obtain current-voltage curves as a function of disorder at a low temperature. We find that the in-plane critical current has a steep increase at the BG-VG transition, while the c-axis critical current has a discontinous jump down, this later result in agreement with the first-order character of the transition. The study of the vortex phase diagram of anisotropic superconductors with point disorder has been of interest since the discovery of high T c superconductivity. It is now clear that at low temperatures and low magnetic fields there is a "Bragg glass" phase (BG) with an elastically distorted vortex lattice [1]. This vortex lattice undergoes a first order melting transition to a vortex liquid (VL) when increasing the temperature T . At higher magnetic fields and low T there is a disordered vortex state, the "vortex glass" (VG) [2,3,4,5]. A disorder driven transition from the BG to the VG has been proposed [1] to occur when increasing the magnetic field H or the disorder. Experimental observations with increasing H at low T , that have been attributed to the BG-VG transition, are: the destruction of the Bragg peaks in neutron scattering [6], a dip in the differential resistance [7], the onset of the "second magnetization peak" [8,9], or a jump in the Josephson plasma resonance [10]. Numerical simulations in particle-like models with random pinning have found a transition from an ordered lattice to a disordered lattice [11,12] when increasing particle density (i.e., the magnetic field). Recently, Monte Carlo simulations in the frustrated 3D XY model with disorder have stablished that the BG-VG transition is of first-order type [13,14].Several of the experimental evidences of the BG-VG transition are directly or indirectly related to transport properties. For example, the most common determination of the BG-VG line is through the onset of a "second magnetization peak" [8,9], which is attributed to an steep increase of the in-plane critical currents. From the point of view of transport, the BG has zero linear resistivity (ρ lin = 0) [1,4]. The VG phase was originally proposed to have zero resistivity [2]. However, studies of the XY gauge glass model with finite screening (κ < ∞) have found that VG is unstable in d = 3 [3,4], and the disordered phase is a frozen vortex liquid that has a very small finite resistivity. Only for the κ → ∞ disordered XY and gauge glass models the VG phase is stable in d = 3 at low T , in which case ρ lin = 0 [5]. Therefore, it is important to understand how the the non-linear current-voltage (IV) curves and their critical currents change across a disorder driven first-order transition. We present here numerical calculations of bot...

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