Superconductivity in WP single crystals

We report the discovery of superconductivity in high-quality single crystals of transition-metal pnictides WP grown by chemical vapor transport method. Bulk superconductivity is observed at $T_{\text{c}}\sim 0.8\mathrm{K}$ at ambient pressure by electrical resistivity, ac magnetic susceptibility, and specific-heat measurements. The effects of magnetic field on the superconducting transitions are studied, leading to a large anisotropy parameter around 2 with the in-plane and out-of-plane upper critical fields of ${\mu }_0{H}_{\text{c}2,\parallel }=17.2\mathrm{mT}$ and ${\mu }_0{H}_{\text{c}2,\perp }=8.5\mathrm{mT}$, respectively. The low value of electron-phonon coupling estimated from the normal-state resistivity and specific-heat measurements suggest that WP is a weak-coupling BCS superconductor. Our finding demonstrates that WP is the first superconductor among $5d$–transition metal pnictides with MnP-type structure at ambient pressure, which will help in the search for new superconductors in transition-metal pnictides.

Figure 1

Structural characterization of WP. (a) Crystal structure of WP. (b) An optical image of the as-grown WP single crystals on a $1\mathrm{mm}\times 1\mathrm{mm}$ grid. (c) Room-temperature x-ray diffraction (XRD) spectra and the Rietveld refinement of polycrystalline WP. The open circle, solid line, and lower solid line represent experimental, calculated, and difference XRD patterns, respectively. The inset shows the XRD pattern for WP single crystals.

Figure 2

Superconductivity characterization in WP single crystals. (a) Temperature-dependent resistivity $\rho \left(T\right)$ for three typical WP single crystals (S1, S2, and S3) between 300 and 0.3 K in zero external magnetic field. The upper left inset is an enlarged view of superconducting transitions at low temperatures. The lower right inset shows the comparison between the experimental data and the fitting result with the Bloch-Gruneisen function. (b) ac magnetic susceptibility for WP single crystals measured at 0.3 K. The inset is a schematic diagram of the ac magnetic susceptibility measurement setup.

Figure 3

Low-temperature specific heat of WP single crystals plotted as $C/T\text{vs}{T}^2$ at zero field. The inset shows temperature dependence of normalized electronic specific heat $C_e/T$.

Figure 4

(a) Upper critical field $H_{\text{c}2}$ vs temperature phase diagram of WP single crystals. The dashed lines denote the Werthamer-Helfand-Hohenberg (WHH) fits to sample S1, S2, and S3, respectively. Inset: Temperature dependence of the resistance of sample S3 in different perpendicular magnetic fields. (b) Angular dependence of the upper critical field ${\mu }_0{H}_{\text{c}2}$ for sample S1. The left upper inset shows the magnetic field dependence of the resistance for sample S1 measured at $T=0.3\mathrm{K}$ with different tilt angle $\theta$. The right lower inset shows the scheme of our tilt experimental setup.

Figure 5

Cation sublattice of unit cell of WP with the indicated distances between the tungsten atoms. The components of $d_{x^2-y^2}$ along the $b$ and $c$ axis are represented by the blue and yellow lobes, respectively.