  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 can not 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.
