Anisotropic Paramagnetic Peak Effect in Reversible Magnetization of Crystalline Miassite Superconductor Rh_17S_15

We report an unusual anisotropic paramagnetic peak effect observed in reversible magnetization of a single crystalline nodal superconductor Rh_17S_15. Both temperature- and field-dependent magnetization measurements reveal a distinct novel vortex state above approximately 1 T. This peak effect is most pronounced when the magnetic field, H, is applied parallel to the [111] direction, whereas it diminishes for H∥[110]. Intriguingly, for H∥[100], instead of a peak, we observe a step-like decrease in M(T), with the step amplitude increasing in larger applied magnetic fields. This behavior is opposite to the expectations of conventional Meissner expulsion. The magnitude of the peak effect, expressed in terms of dimensionless volume susceptibility, is on the order of Δχ=10^-5 (with full diamagnetic screening corresponding to χ=-1). The observed anisotropic paramagnetic vortex response is unusual considering the cubic symmetry of Rh_17S_15. We propose that in this distinct vortex phase, a small but finite attractive interaction between vortices below H_c2 may be responsible for this unusual phenomenon. Furthermore, the vortices seem to prefer aligning along the [111] direction, rotating toward it when the magnetic field is applied in other directions. Our findings add another item to the list of unusual properties of Rh_17S_15 that attracted recent attention as the first unconventional superconductor that has a mineral analog, miassite, found in nature.