Possible unconventional order parameter in single crystals of SrPt3P superconductor

Abstract Anisotropic properties of single crystals of SrPt 3 P were studied using London penetration depth and electrical resistivity measurements. The upper critical field, H c 2 ( T ) , was determined from four-probe electrical resistivity measurements for three orthogonal directions of a magnetic field with respect to the crystal. The London penetration depth, λ ( T ) , was determined from the magnetic susceptibility of the Meissner–London state measured using a tunnel-diode resonator technique. Whereas H c 2 ( T ) and the normal-state ρ ( T ) are practically identical for all three magnetic field orientations, the London penetration depth shows significant unidirectional anisotropy. The low-temperature λ ( T ) is exponentially attenuated when a small excitation radiofrequency magnetic field, H rf , is applied along the c ′ ′ -direction, in which case screening currents flow in the a ′ ′ b ′ ′ -plane, while for the other two orientations, H r f ∥ a ′ ′ and H r f ∥ b ′ ′ , the London penetration depth shows a much stronger, λ ( T ) ∼ T 2 , variation. Such unusual and contrasting behavior of the two anisotropies, γ H T = H c 2 , a b / H c 2 , c = ξ a b