Tuning Superconductivity in Sputtered W0.75Re0.25 Thin Films

TL;DR

This study demonstrates that nitrogen incorporation during UHV DC sputtering of $W_{0.75}Re_{0.25}$ thick films tunes crystal phase, driving a transition from mixed $oldsymbol{ extalpha}$-WRe/$oldsymbol{eta}$-WRe toward a predominantly amorphous or $oldsymbol{eta}$-WRe structure and substantially altering transport. The authors correlate structural disorder with higher $T_c$ in nitrogen-rich films and reveal a thickness- and disorder-dependent evolution of the upper critical field, including a 2D–3D crossover governed by an effective thickness $d_{eff}$. Fluctuation analyses above $T_c$ show 2D AL+MT behavior consistent with a disordered thin-film regime, supporting a picture of an amorphous interfacial layer influencing superconductivity. Overall, the work highlights a link between crystallinity, amorphous layering, and superconducting properties in W-based thin films, with potential relevance for SNSPD technologies and disorder-tin films.

Abstract

W0.75Re0.25, in its bulk form, has been shown to be an interesting superconducting material due to its multiple crystalline phases, each exhibiting distinct superconducting characteristics. However, little is known about how these phases manifest in thin-film form, where deposition conditions and dimensionality are critical aspects. Here, we investigate superconducting W0.75Re0.25 thin films deposited via UHV dc magnetron sputtering. In order to tune the crystalline phase of the films, we further explored the effect of incorporating N2 during the deposition. The superconducting and normal-state properties as a function of deposition conditions were investigated, revealing the role of the crystal phase on the film transport properties.

Figures (8)

• Figure 1: XRD data for W$_{0.75}$Re$_{0.25}$ 60-nm thin films grown on Si substrates. The data of samples 60P, 60N5 and 60N7.5 are reported with a blue, red and green line, respectively. The $\left(200\right)_\beta$ and $\left(210\right)_\beta$ peak of $\beta$-WRe are indicated by dashed black lines, while the $\alpha$-phase peak $\left(110\right)_\alpha$ is indicated with a purple dashed line. The peaks of the sample holder (SH) and the $\left(111\right)$ peak of W$_2$N (absent) are indicated by dashed brown and orange lines, respectively.
• Figure 2: (a) $\rho(d)$ at 10 K of the -P series, with the inset showing the trend for the -N5, and -N7.5 series; (b) $RRR$ as a function of $d$ for the four samples series. The dashed line corresponds to $RRR=1$.
• Figure 3: (a) Normalized resistive transitions of -P series samples. (b) $T_c$ and $\Delta T_c$ as functions of $d$ for the same films. No superconducting transition has been observed in sample 3P down to 1.8 K. The solid blue and orange lines are guides for the eye for the $T_c(d)$ and $\Delta T_c(d)$ data, respectively.
• Figure 4: (a) $T_c$ of the 25-nm films as a function of the N$_2$ flux percentage, with the inset showing the normalized $R(T)$ for the -N7.5 series samples; (b) $T_c$ over time for the 25P and 25N7.5 films.
• Figure 5: Fitting of the relation $d\cdot T_c = A\,R_S^{-B}$ for samples of the -P (in blue) and the -N7.5 (in green) series. The values of the fitting parameters $A$ and $B$ are reported in the panel using the same color scheme.