Evidence for a Novel State of Superconductivity in Noncentrosymmetric ${\mathrm{C}\mathrm{e}\mathrm{P}\mathrm{t}}_3\mathrm{S}\mathrm{i}$: A ${}^{195}\mathrm{P}\mathrm{t}$-NMR Study

We report on novel antiferromagnetic (AFM) and superconducting (SC) properties of noncentrosymmetric ${\mathrm{C}\mathrm{e}\mathrm{P}\mathrm{t}}_3\mathrm{S}\mathrm{i}$ through measurements of the ${}^{195}\mathrm{P}\mathrm{t}$ nuclear spin-lattice relaxation rate $1/T_1$. In the normal state, the temperature ($T$) dependence of $1/T_1$ unraveled the existence of low-lying levels in crystal-electric-field multiplets and the formation of a heavy-fermion (HF) state. The coexistence of AFM and SC phases that emerge at $T_{\mathrm{N}}=2.2\mathrm{K}$ and $T_{\mathrm{c}}=0.75\mathrm{K}$, respectively, takes place on a microscopic level. ${\mathrm{C}\mathrm{e}\mathrm{P}\mathrm{t}}_3\mathrm{S}\mathrm{i}$ is the first HF superconductor that reveals a peak in $1/T_1$ just below $T_{\mathrm{c}}$ and, additionally, does not follow the $T^3$ law that used to be reported for most unconventional HF superconductors. We remark that this unexpected SC characteristic may be related to the lack of an inversion center in its crystal structure.

Figure 1

The ${}^{195}\mathrm{P}\mathrm{t}$-NMR spectrum for the oriented powder along the $c$ axis parallel to magnetic field ($H$) at 4.2 K and $f=8.9\mathrm{M}\mathrm{H}\mathrm{z}$ ($H\sim 1\mathrm{T}$).

Figure 2

The $T$ dependence of $1/T_{1}T$ at site I at $f=8.9$ (circles), 18.1 (squares), and 46.2 MHz (triangles) for ${\mathrm{C}\mathrm{e}\mathrm{P}\mathrm{t}}_3\mathrm{S}\mathrm{i}$ together with the result of ${\mathrm{L}\mathrm{a}\mathrm{P}\mathrm{t}}_3\mathrm{S}\mathrm{i}$ (crosses). A dashed line shows a Korringa law with $1/T_{1}T=12.34{sec}^{-1}{\mathrm{K}}^{-1}$ for ${\mathrm{L}\mathrm{a}\mathrm{P}\mathrm{t}}_3\mathrm{S}\mathrm{i}$ in the measured $T$ range.

Figure 3

The $T$ dependence of $1/T_{1}T$ at site II for ${\mathrm{C}\mathrm{e}\mathrm{P}\mathrm{t}}_3\mathrm{S}\mathrm{i}$ at $f=8.9$ (circles), 18.1 (squares), and 46.2 MHz (triangles) together with the result of ${\mathrm{L}\mathrm{a}\mathrm{P}\mathrm{t}}_3\mathrm{S}\mathrm{i}$ (crosses). The dashed line shows a Korringa law with $1/T_{1}T=17.03{sec}^{-1}{\mathrm{K}}^{-1}$. The solid lines are least-square fits to the data below $T_N$ using Eq. (1). The inset presents the $T$ dependence of $1/T_{1}T$ of ${}^{27}\mathrm{A}\mathrm{l}$ for the antiferromagnetic HF superconductor ${\mathrm{U}\mathrm{P}\mathrm{d}}_2{\mathrm{A}\mathrm{l}}_3$ with $T_{\mathrm{N}}=14.5\mathrm{K}$ and $T_{\mathrm{c}}=2\mathrm{K}$ [10].

Figure 4

The plot of $(1/T_{1}T{)}_{\mathrm{S}\mathrm{C}}/(1/T_{1}T{)}_{\mathrm{N}}$ vs $T/T_{\mathrm{c}}$ at 8.9 (circles) and 18.1 MHz (triangles). Here $(1/T_{1}T{)}_{\mathrm{N}}$ corresponds to the value at the normal state at 8.9 ($H\sim 1\mathrm{T}$) and 18.1 MHz ($H\sim 2\mathrm{T}$). The solid line is a tentative fit calculated by applying the Balian-Werthamer model (BW isotropic triplet SC state) with a value of $2\Delta /k_{\mathrm{B}}{T}_{\mathrm{c}}=4$ [19]. The dashed line indicates a fit by a line-node gap model with $2\Delta /k_{\mathrm{B}}{T}_{\mathrm{c}}=5.1$.