Enhancement of Superconductivity in WP via Oxide-Assisted Chemical Vapor Transport

Abstract

Tungsten monophosphide (WP) has been reported to superconduct below 0.8 K, and theoretical work has predicted an unconventional Cooper pairing mechanism. Here we present data for WP single crystals grown by means of chemical vapor transport (CVT) of WO3, P, and I2. In comparison to synthesis using WP powder as a starting material, this technique results in samples with substantially decreased low-temperature scattering and favors a more three-dimensional morphology. We also find that the resistive superconducting transitions in these samples begin above 1 K. Variation in Tc is often found in strongly correlated superconductors, and its presence in WP could be the result of influence from a competing order and/or a non-s-wave gap.

Keywords: quantum materials; superconductivity.

Figure 1

(a) The different materials that form at the cold end of a sealed quartz ampule when starting with ${\mathrm{WO}}_3$, P, and ${\mathrm{I}}_2$ at the hot end. Pieces marked A are chunks of ${\mathrm{W}}_8{\mathrm{P}}_4{\mathrm{O}}_{32}$, those marked B are WP (either single crystals or multiple intergrown crystals), and C is a piece of WP fused to a piece of ${\mathrm{W}}_8{\mathrm{P}}_4{\mathrm{O}}_{32}$. (b) A comparison of a (top) needlelike and (bottom) platelike crystal of WP after initial polishing of only a single pair of opposing faces. The latter morphology was much more common in oxide-assisted growth. The grid paper in both pictures is composed of $1\times 1{\mathrm{mm}}^2$ squares.

Figure 2

(a) Resistivity as a function of temperature for a WP single crystal with current along the b-axis in zero field and with a 9 T applied field. (b) The Hall coefficient (based on temperature sweeps in ±14 T) for a WP single crystal. (c) Low-temperature specific heat of a different WP single crystal. The green line is a fit to the Debye model, with extracted parameters noted. (d) Magnetic susceptibility of WP powder as a function of temperature. Data shown are field cooled, but there was no difference with zero-field cooling. Inset: An inverse susceptibility plot of the same data with a Curie–Weiss fit (maroon line) over the data from 100 to 250 K (see text for details). For (a,b,d), the field was applied parallel to [101].

Figure 3

(a) Magnetoresistance (defined as ${\displaystyle \frac{\rho \left(B\right)-{\rho }_0}{{\rho }_0}}$, where ${\rho }_0$ is the lowest resistance above the superconducting transition) of a WP sample with H ‖ a-axis at 0.40 K up to 41.5 T. Data up to 3 K were practically indistinguishable, aside from superconducting transitions visible at a very low field in the data below 1.3 K. The red line is a linear fit to normal-state data from $H_{c2}$ (determined by the change in slope; see inset) to 20 T. (b) MR of a different WP crystal with H ‖ [011] at multiple temperatures. Data showed little variation from base temperature to 8 K, and the next lowest temperature was 15.4 K. The light-blue line is a power-law fit of the base temperature data, yielding n = 2.27. The inset shows MR at 34.5 T (the maximum field of the higher temperature measurements) as a function of temperature, including temperatures not shown in the main plot.

Figure 4

(a) Zero-field temperature sweeps, showing superconducting transitions, of five different WP single crystals. Sample D has had its resistivity reduced by a factor of 10 to fit the plot scale, while for Sample B, measured at the NHMFL, resistance was not converted to resistivity, so the units are arbitrary. The lower two panels are temperature sweeps in various fields for WP samples (b) D and (c) E, which were measured simultaneously. All lines are guides to the eye.

Figure 5

The critical field for the same WP samples shown in Figure 4a with matching colors and symbols, as well as the sample from the work of Liu et al. [12] with the highest critical field. The criteria for our samples was a 10% drop in resistivity; for the reference samples the transitions are sharp enough that different criteria would not impact the appearance of the data. Error bars mark uncertainty in either temperature (horizontal) or field (vertical) sweeps during measurements when the other variable was held constant. D and F have H ‖ c-axis, and for E and the reference sample, the orientations are unknown. Fits are made according to a Ginzburg–Landau formula, $H_{c2}\left(\mathit{T}\right)=H_{c2,0}\left[{\displaystyle \frac{1-{\left(\frac{T}{T_c}\right)}^2}{1+{\left(\frac{T}{T_c}\right)}^2}}\right]$.