Emergent Coherence at the Edge of Magnetism: Low-Doped La2-xSrxCuO4+delta Revisited

Abstract

The La2-xSrxCuO4+delta (LSCO) system provides a unique experimental setting for exploring how magnetism, superconductivity, and disorder jointly shape charge transport in a doped Mott insulator. Transport measurements in lightly doped and oxygen-enriched LSCO reveal a strongly insulating normal state governed by variable-range hopping, accompanied by pronounced nonlinear current-voltage characteristics and, at low temperatures, current-induced negative differential resistance. With increasing carrier concentration, these features evolve into regimes characterized by granular and percolative superconductivity near the threshold of bulk superconductivity and, eventually, into a homogeneous strange-metal state close to optimal doping. Throughout this evolution, the transport response shows marked sensitivity to disorder, electronic inhomogeneity, and external control parameters, such as bias current and magnetic field. Rather than reflecting a sequence of sharply distinct phases, the observed transport regimes form a continuous crossover from a localization-dominated insulating state to granular superconductivity and further to a coherent metallic state. This crossover is driven primarily by the progressive enhancement of electronic screening, inter-region coupling, and superconducting connectivity, rather than by abrupt changes in the underlying microscopic scattering mechanisms. Taken together, the available transport data provide a coherent experimental basis for understanding how disorder and mesoscale electronic inhomogeneity organize charge transport and superconductivity across the LSCO phase diagram, underscoring the central role of percolation and nonequilibrium effects in underdoped cuprates.