High Critical Temperature and Field Superconductivity in Nb$_{0.85}$X$_{0.15}$, (X = Ti, Zr, Hf) Alloys: Promising Candidates for Superconducting Devices

TL;DR

This study synthesizes Nb$_{0.85}$X$_{0.15}$ (X = Ti, Zr, Hf) alloys and characterizes their superconducting and normal-state properties in the α-W cubic phase. Through XRD, EDXA, magnetization, resistivity, and specific-heat measurements, the authors establish strong type-II superconductivity with $T_c$ in the 9.65–11.05 K range and high $H_{c2}(0)$ values (up to ~9.5 T), along with $J_c$ on the order of 10$^5$–10$^6$ A/cm$^2$ and substantial flux pinning. Specific-heat analysis shows a strongly coupled, isotropic single gap with $\,\Delta(0)/k_B T_c \\approx 2.15$–2.19 and $\lambda_{e-ph} \\approx 0.83$–0.92, supporting conventional electron-phonon pairing; Uemura analysis places these alloys in the conventional regime. The results suggest that Nb-rich α-W Nb$_{6}$X alloys offer a balance of robust bulk superconductivity and practical flux-pinning characteristics, making them promising candidates for thin-film superconducting devices and SRF/qubit applications, with further potential unlocked via microstructure control and film growth.

Abstract

Niobium and its alloys with early transition metals have been extensively studied for their excellent superconducting properties. They have high transition temperatures, strong upper critical fields, and high critical current densities, making them ideal for superconducting applications such as SQUIDs, MRI, NMR, particle accelerators, and Qubits. Here we report a systematic investigation of as-cast Nb-rich alloys, Nb$_{0.85}$X$_{0.15}$ (X = Ti, Zr, Hf), using magnetization, electrical transport, and specific heat measurements. They exhibit strong type-II bulk superconductivity with moderate superconducting transition temperatures and upper critical fields. The estimated magnetic field-dependent critical current density lies in the range of 10$^5$--10$^6$~A/cm$^2$ across various temperatures, while the corresponding flux-pinning force density is on the order of GNm$^{-3}$, suggesting the potential of these materials for practical applications. Electronic-specific heat data reveal a strongly coupled, single, isotropic, nodeless superconducting gap. These Nb-rich alloys, characterized by robust superconducting properties, hold significant potential for applications in superconducting device technologies.

Figures (7)

• Figure 1: (a) The bcc crystal structure of Nb$_{0.85}$X$_{0.15}$, (X = Ti, Zr, Hf) (b) Rietveld refinement of powder XRD pattern of Nb$_6$Ti alloy (c) Elemental mapping of Nb$_{0.85}$X$_{0.15}$ (X = Ti, Zr, Hf) alloys (d) Room temperature XRD patterns of Nb$_{0.85}$X$_{0.15}$, (X = Ti, Zr, Hf).
• Figure 2: Temperature-dependent electrical resistivity and the insets show zero drops in resistivity for (a) Nb$_{6}$Ti, (b) Nb$_{6}$Zr, (c) Nb$_{6}$Hf. Magnetization (corrected with demagnetization factor) in ZFCW and FCC mode at an applied field of 1 mT for (d) Nb$_{6}$Ti, (e) Nb$_{6}$Zr, and (f) Nb$_{6}$Hf, respectively.
• Figure 3: Temperature-dependent lower critical field where the solid black curves represent G-L fit using Equation\ref{['Hc1']} and the insets show the field-dependent magnetization curves for (a) Nb$_{6}$Ti, (b) Nb$_{6}$Zr, and (c) Nb$_{6}$Hf, respectively. Temperature-dependent profiles of the upper critical field, estimated from magnetization and resistivity, where the insets show magnetic field-dependent resistivity data for (d) Nb$_{6}$Ti, (e) Nb$_{6}$Zr, and (f) Nb$_{6}$Hf, respectively.
• Figure 4: (a), (b) and (c) M-H loops at different temperatures, whereas (d), (e) and (f) magnetic field dependent J$_c$ variation at different temperatures and (g), (h) and (i) Variation of flux pinning force with magnetic field for Nb$_{6}$X, (X = Ti, Zr, Hf), respectively.
• Figure 5: (a), (b) and (c) C/T vs T$^{2}$ plot for Nb$_{6}$X, (X = Ti, Zr, Hf), respectively, where the solid black lines represent the Debye-Sommerfeld fitting represented by Equation\ref{['C/T']} (d), (e) and (f) C$_{el}$ vs T plot for Nb$_{6}$X, (X = Ti, Zr, Hf), respectively, where solid black curves represent the s-wave model fit with BCS type single gap function.