Anisotropic superconductivity in noncentrosymmetric BiPd

We report measurements of London penetration depth λ(T) for the noncentrosymmetric superconductor BiPd by using a tunnel diode oscillator. Pronounced anisotropic behavior is observed in the low-temperature penetration depth; the in-plane penetration depth λac(T) follows an exponential decrease, but the interplane penetration depth λb(T) shows power-law-type behavior. The superfluid density ρs(T), converted from the penetration depth λ(T), is best fitted by an anisotropic two-band BCS model. We argue that such a complex order parameter is attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of antisymmetric spin-orbit coupling in BiPd.