Vorticity, phase stiffness and the cuprate phase diagram

The vortex-Nernst effect is a highly sensitive probe for detecting vortex motion in a type II superconductor [1]. In the past 3 years, we have used it to map out the region in the fieldtemperature (H-T ) plane in which vorticity may be observed [2,3,4,5]. The results provide a fresh perspective on the cuprate phase diagram which we sketch here. When a superconductor (in the vortex-liquid state) is exposed to a weak gradient −∇T ||x̂ in a field H||ẑ, vortices diffuse down the gradient with velocity v||x̂. As each vortex core crosses the line between a pair of transverse voltage electrodes, the 2π phase slip of the condensate phase leads to a Josephson E field given by E = B × v. The Nernst signal is defined as ey = Ey/|∇T |. In cuprates, Nernst experiments were initially conducted on optimally-doped samples [6]. In extending the experiments to underdoped La2−xSrxCuO4 (LSCO), Xu et al. [2] observed that ey persists to temperatures 50-100 K above Tc0. Figure 1 shows several curves of ey in underdoped LSCO (x = 0.12). Below the zero-field transition temperature Tc0 ≃ 29 K, ey(T,H) is initially zero until the melting field line Hm(T ) is exceeded (for e.g., at 5 T in the curve at 10 K).