Superconductivity in single crystals of LaFePO

Abstract

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