Upper Critical Fields of the 3-K Superconducting Phase of Sr₂RuO₄

Abstract

We have investigated the normal-state and superconducting properties of the 3-K superconducting phase of Sr₂RuO₄ with the enhanced transition temperature T_c by lamellae of ruthenium metal. In contrast to the dimensional crossover in the metallic conduction in pure Sr₂RuO₄ with T_c = 1.5 K, the 3-K phase retains metallic interlayer conduction at least up to 300 K. We attribute this behavior to the interlayer metallic paths through the Ru lamellae, connected by in-plane current paths. For the superconducting properties, the phase diagram of the anisotropic upper critical fields H_c2(T) indicates that H_c2//ab is strongly suppressed in the low-temperature region. The measurements of the out-of-plane resistivity ρ_c under magnetic fields show that the finite residual resistivity recovers above H_c2(T) of pure Sr₂RuO₄. This result confirms the spatial coexistence of the enhanced superconductivity with the T_c = 1.5-K region in the 3-K superconductor. In addition, we have also observed hysteresis in ρ_c, which appear only in the 3-K phase at low temperatures and in high fields parallel to the RuO₂ plane.