Three-dimensional superconductivity in the infinite-layer compound Sr0.9La0.1CuO2 in entire region below Tc

The infinite-layer compound ACuO2 (A=alkaline-earth ions) is regarded as the most suitable material for exploring the fundamental nature of the CuO2 plane because it does not contain a charge-reservoir block, such as a rock-salt or a fluorite like block. We report that superconductivity in the infinite-layer compound Sr0.9La0.1CuO2 is of a three-dimensional nature, in contrast to the quasi two-dimensional superconducting behavior of all other cuprates. The key observation is that the c-axis coherence length is longer than the c-axis lattice constant even at zero temperature. This means that the superconducting order parameter of one CuO2 plane overlaps with those of neighboring CuO2 planes all the temperatures below Tc.