Unconventional superconductivity in quasi-one-dimensional ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$

Following the discovery of superconductivity in quasi-one-dimensional ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ containing ${\left[{\left({\text{Cr}}_3{\mathrm{As}}_3\right)}^{2-}\right]}_{\infty }$ chains (J. K. Bao et al., arXiv:1412.0067), we succeeded in synthesizing an analogous compound, ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, which also crystallizes in a hexagonal lattice. The replacement of K by Rb results in an expansion of the $a$ axis by 3%, indicating a weaker interchain coupling in ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. Bulk superconductivity emerges at 4.8 K, above which the normal-state resistivity shows a linear temperature dependence up to 35 K. The estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit by a factor of 2. Furthermore, the electronic specific-heat coefficient extrapolated to zero temperature in the mixed state increases with $\sqrt{H}$, suggesting the existence of nodes in the superconducting energy gap. Hence ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ manifests itself as another example of an unconventional superconductor in the ${\mathrm{Cr}}_3{\mathrm{As}}_3$-chain based system.

Figure 1

(a) Powder x-ray diffraction of the ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ polycrystalline sample protected by the Apiezon $N$ grease, which is indexed by a hexagonal unit cell. (b) Rietveld refinement profile for the powder x-ray diffraction with ${25}^{\circ }\le 2\theta \le {120}^{\circ }$. $R_{\mathrm{wp}}$ and $S$ denote the weighted profile reliable factor and “goodness-of-fit” parameter, respectively. The inset shows the crystal structure projected along the c axis.

Figure 2

(a) Temperature dependence of the electrical resistivity for ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. The inset show the linear temperature dependence from 5 to 35 K. (b) Superconducting transitions under magnetic fields up to 8 T. (c) The upper critical field as a function of temperature. The dashed line denotes a Ginzburg-Landau (GL) fit. The Pauli-limit field (${\mu }_0{H}_{\text{P}}=1.84T_{\text{c}}$) is labeled by the horizontal line.

Figure 3

(a) Temperature dependence of magnetic susceptibility (scaled by $4\pi \chi$) for the ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ polycrystals. The inset zooms in the superconducting transition in the field-cooling (FC) mode. (b) Field dependence of magnetization at 2 K. The magnetic field was applied in the order of $0\to 5\mathrm{T}\to -5\mathrm{T}\to 5$ T.

Figure 4

Low-temperature specific-heat data for ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, plotted with $C/T$ vs $T^2$. The inset of (a) shows the normalized electronic specific heat $C_{\text{e}}\left(T\right)/\left({\gamma }_{\mathrm{n}}T\right)$ as a function of temperature. (b) presents the $C\left(T\right)$ data under magnetic fields, from which the zero-temperature Sommerfeld coefficients in the superconducting mixed state $\gamma \left(H\right)$ can be extrapolated. The inset displays the $\gamma \left(H\right)$ dependence, in which the theoretical prediction for a nodal (full) superconducting gap is shown by the solid (dotted) line.