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Possible unconventional order parameter in single crystals of SrPt$_3$P superconductor

Anisotropic properties of single crystals of SrPt$_{3}$P were studied using London penetration depth and electrical resistivity measurements. The upper critical field, $H_{c2}(T)$, 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 $H_{c2}(T)$ 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, $H_{rf}\parallel a"$ and $H_{rf}\parallel b"$, the London penetration depth shows a much weaker, $λ(T)\sim T^{2}$, variation. Such unusual and contrasting behavior of the two anisotropies, $γ_{H}\left(T\right)=H_{c2,ab}/H_{c2,c}=ξ_{ab}/ξ_{c}$ and $γ_λ\left(T\right)=λ_{c}/λ_{ab}$, imposes significant constraints on the possible order parameter. Although our measurements are insufficient to derive conclusively the superconducting gap anisotropy, qualitatively, order parameter with two point nodes and a modulation in the perpendicular direction is consistent with the experimental observations.