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 ∼6.6 K. Magnetization measurements also show the onset of superconductivity near 6 K. In contrast, specific heat measurements manifest no discontinuity at T c . 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, T c 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 T c passes through a maximum of nearly 14 K at ∼110 kbar, demonstrating that significantly higher T c values may be achieved in the phosphorus-based oxypnictides.