Dimensionality of superconductivity and vortex dynamics in the infinite-layer cuprate Sr0.9M0.1CuO2 (M=La,Gd)

Abstract

The high magnetic-field phase diagram of the electron-doped infinite layer high-temperature superconducting (high-T-c) compound Sr0.9La0.1CuO2 was probed by means of penetration depth and magnetization measurements in pulsed fields to 60 T. An anisotropy ratio of 8 was detected for the upper critical fields with H parallel (H-c2(ab)) and perpendicular (H-c2(c)) to the CuO2 planes, with H-c2(ab) extrapolating to near the Pauli paramagnetic limit of 160 T. The longer superconducting coherence length than the lattice constant along the c axis indicates that the orbital degrees of freedom of the pairing wave function are three dimensional. By contrast, low-field magnetization and specific heat measurements of Sr0.9Gd0.1CuO2 indicate a coexistence of bulk s-wave superconductivity with large moment Gd paramagnetism close to the CuO2 planes, suggesting a strong confinement of the spin degrees of freedom of the Cooper pair to the CuO2 planes. The region of the magnetic field-temperature phase diagram between H-c2(ab) and the irreversibility line in the magnetization, H-irr(ab), in Sr0.9La0.1CuO2 is anomalously large for an electron-doped high-T-c cuprate. The large reversible region even approaching zero temperature rules out thermal depinning scenarios. The temperature dependence of H-irr(ab) also differs fundamentally from those predicted for the quenched-disorder-induced vortex phase transitions for H parallel to c at low temperatures. Thus, our finding of a strongly suppressed H-irr(ab) relative to H-c2(ab) at low temperatures is suggestive of the existence of additional quantum fluctuations, possibly due to a magnetic-field-induced competing order such as the spin-density wave (SDW).