Detection of a Spin-Triplet Superconducting Phase in Oriented Polycrystalline ${\mathrm{U}}_2{\mathrm{PtC}}_2$ Samples Using ${}^{195}\mathrm{Pt}$ Nuclear Magnetic Resonance

Nuclear magnetic resonance (NMR) measurements on the ${}^{195}\mathrm{Pt}$ nucleus in an aligned powder of the moderately heavy-fermion material ${\mathrm{U}}_2{\mathrm{PtC}}_2$ are consistent with spin-triplet pairing in its superconducting state. Across the superconducting transition temperature and to much lower temperatures, the NMR Knight shift is temperature independent for field both parallel and perpendicular to the tetragonal $c$ axis, expected for triplet equal-spin pairing superconductivity. The NMR spin-lattice relaxation rate $1/T_1$, in the normal state, exhibits characteristics of ferromagnetic fluctuations, compatible with an enhanced Wilson ratio. In the superconducting state, $1/T_1$ follows a power law with temperature without a coherence peak giving additional support that ${\mathrm{U}}_2{\mathrm{PtC}}_2$ is an unconventional superconductor. Bulk measurements of the ac susceptibility and resistivity indicate that the upper critical field exceeds the Pauli limiting field for spin-singlet pairing and is near the orbital limiting field, an additional indication for spin-triplet pairing.

Figure 1

The ${}^{195}\mathrm{Pt}$ NMR field-swept aligned powder spectra for ${\mathbf{H}}_0\parallel \widehat{c}$ (red line, left peak) and ${\mathbf{H}}_0\perp \widehat{c}$ (blue line, right peak) normalized by the spectral area taken at a carrier frequency of ${\omega }_0\approx 23\mathrm{MHz}$, at a temperature of $T=1.8\mathrm{K}$. The ${\mathrm{U}}_2{\mathrm{PtC}}_2$ aligned powder spectra show an angular dependence consistent with an anisotropic NMR shift and successful alignment.

Figure 2

The NMR shift in both the normal and superconducting states for ${\mathbf{H}}_0\parallel \widehat{c}$ and ${\mathbf{H}}_0\perp \widehat{c}$. In the normal state, the shifts for both orientations follow the bulk susceptibility down to a characteristic temperature $T^{*}=25\mathrm{K}$. At temperatures $T^{*}>T>T_c$, the shift deviates from the bulk susceptibility, and in the superconducting state $T_c(H_0)\approx 1.1\mathrm{K}$, the shift becomes temperature independent.

Figure 3

The $K$-$\chi$ plot for both ${\mathbf{H}}_0\parallel \widehat{c}$ and ${\mathbf{H}}_0\perp \widehat{c}$. (inset) The bulk susceptibility measured by SQUID magnetometry for both orientations. $\chi$ was determined from the difference in magnetization measured in fields of 4 and 5 T.

Figure 4

The spin lattice relaxation rate in the normal and superconducting states for ${\mathbf{H}}_0\parallel \widehat{c}$ (red circles) and in the superconducting state for ${\mathbf{H}}_0\perp \widehat{c}$ (blue squares). Dotted, dashed, and dot-dashed lines indicate power-law temperature dependences.

Figure 5

The upper critical field, $H_{c2}$-superconducting transition temperature, $T_c$ phase diagram. The orientationally dependent $H_{c2}$ and $T_c$ were measured in situ using the NMR coil and turning circuit (red circles, blue squares) while the polycrystalline $T_c$ were measured as the midpoint of the resistivity superconducting transition (black triangles).