Josephson coupling in lanthanum-based cuprate superlattices

Abstract

In most anisotropic compounds such as bismuth-based layered cuprate perovskites, the supercurrent across the blocking layer is of Josephson type, and a single crystal forms a natural stack of Josephson junctions. Here, we report on the evidence of Josephson-like transport in an artificial cuprate superlattice composed of 10 LaSrCuO–LaCuO repeats, creating a superlattice of junctions, where LCO is a superconducting Mott insulator and LSCO is an overdoped metal. The superlattice has been designed with a long period of d = L + W = 5.28 nm, with L and W being the thicknesses of LCO and LSCO units, respectively, and is in the underdoped regime with an average doping level of ⟨δ⟩ = 0.11. Quantum-size effects and Rashba spin–orbit coupling are controlled by L/d = 0.75, with a quasi-2D superconducting transition temperature of 41 K and a c-axis coherence length of about 1.5 nm. Measurements at very low temperatures show evidence of Josephson phase dynamics consistent with very low Josephson coupling and a phase diffusion regime, thus explaining why Josephson coupling in LSCO superlattices has been so elusive. The tuning of LSCO superlattices in the Josephson regime enriches the phase diagram of HTS.