Superconductivity in single crystals of LaFePO

Single crystals of the compound LaFePO were prepared using a flux growth technique at high temperatures. Electrical resistivity measurements reveal metallic behavior and a resistive transition to the superconducting state at a critical temperature T-c similar to 6.6 K. Magnetization measurements also show the onset of superconductivity near 6 K. In contrast, specific heat measurements manifest no discontinuity at Tc. These results lend support to the conclusion that the superconductivity is associated with oxygen vacancies that alter the carrier concentration in a small fraction of the sample, although superconductivity characterized by an unusually small gap value cannot be ruled out. Under applied magnetic fields, Tc is suppressed anisotropically for fields perpendicular and parallel to the ab-plane, suggesting that the crystalline anisotropy strongly influences the superconducting state. Preliminary high pressure measurements show that Tc passes through a maximum of nearly 14 K at similar to 110 kbar, demonstrating that significantly higher Tc values may be achieved in the phosphorus-based oxypnictides.