Parity Transition of Spin-Singlet Superconductivity Using Sublattice Degrees of Freedom

Recently, a superconducting (SC) transition from low-field (LF) to high-field (HF) SC states was reported in ${\mathrm{CeRh}}_2{\mathrm{As}}_2$, indicating the existence of multiple SC states. It has been theoretically noted that the existence of two Ce sites in the unit cell, the so-called sublattice degrees of freedom owing to the local inversion symmetry breaking at the Ce sites, can lead to the appearance of multiple SC phases even under an interaction inducing spin-singlet superconductivity. ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ is considered as the first example of multiple SC phases owing to this sublattice degree of freedom. However, microscopic information about the SC states has not yet been reported. In this study, we measured the SC spin susceptibility at two crystallographically inequivalent As sites using nuclear magnetic resonance for various magnetic fields. Our experimental results strongly indicate a spin-singlet state in both SC phases. In addition, the antiferromagnetic phase, which appears within the SC phase, only coexists with the LF SC phase; there is no sign of magnetic ordering in the HF SC phase. The present Letter reveals unique SC properties originating from the locally noncentrosymmetric characteristics.

Figure 1

Schematics of a PDW state in a two-dimensional system with locally broken inversion symmetry. (a) Schematic image of crystal structure. The unit cell contains two kinds of superconducting layers. The position of the inversion center is indicated by the arrows. (b) Theoretically predicted $H–T$ phase diagram. The position dependence of superconducting order parameters in each superconducting phase is shown by the schematic images. (c) Crystal structure of ${\mathrm{CeRh}}_2{\mathrm{As}}_2$.

Figure 2

NMR measurements in ${\mathrm{CeRh}}_2{\mathrm{As}}_2$. (a) NMR spectra at 1.2 T for $H\parallel c$ measured at various temperatures. The dashed curves indicate the results of a two-peak fitting. (b) Temperature dependence of full width at half maximum (FWHM), (c) Knight shift at the As(1) site $K^{\mathrm{As}(1)}$ and As(2) site $K^{\mathrm{As}(2)}$, determined by NMR spectrum at 1.2 T. $T_{\mathrm{SC}}$ and $T_N$ are indicated by the dashed lines. The dashed curves are guides for the eye. (d) Temperature dependence of ac magnetic susceptibility at 1.2 T for $H\parallel c$. (e) NMR spectra at 4.5 T for $H\parallel c$ measured at various temperatures. (f) Temperature dependence of FWHM, (g) Knight shift, (h) and ac magnetic susceptibility at 4.5 T for $H\parallel c$.

Figure 3

Magnetic field variation of the magnetic properties in ${\mathrm{CeRh}}_2{\mathrm{As}}_2$. The $c$-axis magnetic field dependence of FWHM at (a) 0.06 K and (b) $\mathrm{\Delta }K=K(0.06\mathrm{K})–K(0.6\mathrm{K})$. The dashed curve indicates the contribution of SC diamagnetism $K_{\mathrm{dia}}$. The dashed lines are guides for the eye. (c) The $H–T$ phase diagram in the present sample of ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ determined from the NMR and ac magnetic susceptibility measurements. Circles denote $T_{\mathrm{SC}}$ determined by the onset temperature of the SC diamagnetic signal from ac magnetic susceptibility. Crosses denote the critical magnetic field of the SC1–SC2 transition determined by the anomaly of ac magnetic susceptibility. Diamonds indicate $T_N$ determined by the increase in NMR linewidth. Inset: schematic image of the distribution of spin susceptibility and SC gap.