Superconducting Properties of the Noncentrosymmetric Superconductor LaPtSi Across a Broad Pressure Range

The structural and superconducting properties of the noncentrosymmetric superconductor LaPtSi, prepared using an arc-melting method, were investigated across a broad pressure range. Noncentrosymmetric crystals, which break spatial inversion symmetry, are known to exhibit fascinating physical phenomena, specifically the magnetoelectric effect and the Edelstein effect (electrical-current-induced magnetization). The pairing mechanism for superconductors lacking spatial inversion symmetry cannot be featured by a specified spin state (or a parity of orbital) owing to an indistinguishable spin state, where singlet and triplet states are mixed. This leads to unconventional superconductivity, which cannot be explained by the conventional BCS theory, where a spin-singlet s-wave electron pairing (s-wave pairing) is postulated. We examined the crystal structure of the noncentrosymmetric superconductor LaPtSi, known to lack spatial inversion symmetry at ambient pressure, over a wide pressure range. Our findings confirmed that LaPtSi is a superconductor with a superconducting transition temperature (T_c) of 3.95 K at ambient pressure and exhibits a tetragonal structure (space group I4₁md (No. 109)) across a wide pressure range of 0–23.2 GPa. The space group does not possess spatial inversion symmetry. LaPtSi is an analogue of the noncentrosymmetric superconductor LaPtGe, which has a T_c of approximately 3.0 K. Thus, LaPtSi represents a potential unconventional superconductor at least up to ∼23 GPa. The pressure dependence of the superconducting properties was investigated at 0–17.5 GPa, based on the temperature (T) dependence of electrical resistance R, revealing a robust T_c against pressure that remains independent of pressure. The R–T plots under different applied magnetic fields (applied magnetic field (H) of 0–2 T) were measured at 5.21 GPa, yielding various characteristic parameters such as the orbital limiting field, $H_{c2}^{\mathrm{orbital}}\left(0\right)$, the Ginzburg–Landau upper critical field, H_c2(0), and the Pauli limiting field, $H_{c2}^P$. The value of $H_{c2}^{\mathrm{orbital}}\left(0\right)$ (= 1.44(5) T) was found to be smaller than H_c2(0) (= 1.75(3) T), while H_c2(0) was significantly smaller than $H_{c2}^P$ (= 6.34 T). These results suggest simple s-wave electron pairing in LaPtSi under pressure, despite the absence of spatial inversion symmetry. Notably, we have observed that H_c2(0) exceeds the orbital limiting field at high pressure, indicating that LaPtSi does not follow the conventional Werthamer–Helfand–Hohenberg theory. In this study, we thoroughly discussed the electron pairing mechanism and the origin of H_c2(0) > $H_{c2}^{\mathrm{orbital}}\left(0\right)$ obtained in LaPtSi, along with the robust T_c against pressure.