Researchers at Gdansk University of Technology have synthesized a family of superconducting alloys, including the medium-entropy alloy Re0.56Nb0.19Ta0.19V0.06, that exhibit bulk type-II superconductivity, suggesting a wider range of structurally complex materials could host this property. Pavan Kumar Meena and Tomasz Klimczuk report that alloys of rhenium with vanadium, niobium and tantalum, specifically formulated to remain within the narrow stability range of the σ phase, demonstrate superconductivity despite exhibiting an unconventional negative temperature coefficient in their normal-state resistivity. Transition temperatures determined by electrical resistivity measurements were higher than those found using magnetization and specific heat measurements, indicating a complex interplay between measurement technique and the superconducting state in these materials.

σ-Phase Alloy Synthesis: Re-X (X=V, Nb, Ta) and Medium-Entropy Approach

The synthesis of a medium-entropy alloy, Re0.56Nb0.19Ta0.19V0.06, expands the range of known superconducting sigma (σ)-phase alloys. Precise compositional control proved essential, as even slight deviations from the optimized composition resulted in the formation of secondary phases. Energy-dispersive X-ray spectroscopy confirmed the alloy’s composition, revealing 1.6 at.% variation for V, 1.1 at.% for Nb, and 1.3 at.% for Ta, highlighting the sensitivity of σ-phase stability. Researchers established the formation of a single-phase tetragonal σ-phase crystal structure through powder X-ray diffraction.

The study’s findings are consistent with a fully gapped, weak-coupling Bardeen-Cooper-Schrieffer (BCS) superconducting state, as evidenced by analysis of the electronic heat capacity. Further microscopic investigations could clarify the superconducting gap structure and pairing symmetry.

The stabilization of the σ-phase medium-entropy alloy suggests a pathway toward predictive compositional tuning and data-driven design of materials. This sensitivity highlights the challenges in synthesizing these materials and the need for refined control over alloy composition. The authors acknowledge the contribution of Oliwia Barra, who provided support during the experiment, and state that the data are available upon reasonable request.

Tetragonal σ-Phase Structure Confirmed via Powder X-ray Diffraction

Powder X-ray diffraction analysis confirmed that Re0.76V0.24, Re0.56Nb0.44, and Re0.60Ta0.40 all crystallize within a tetragonal CrFe-type structure, establishing a foundational structural characteristic for subsequent investigation of their superconducting properties. This structural confirmation was critical, as the stability of the tetragonal phase is highly sensitive to composition, with even minor deviations potentially leading to the formation of undesirable secondary phases like α-Mn or W-type cubic structures.

The precise control over stoichiometry required to stabilize the σ phase is highlighted by energy-dispersive X-ray spectroscopy (EDS) measurements, which revealed compositions of V = 26.0 ± 1.6 at.% and Re = 74.0 ± 1.6 at.% for Re0.76V0.24, Nb = 42.7 ± 1.1 at.% and Re = 57.3 ± 1.1 at.% for Re0.56Nb0.44, and Ta = 38.2 ± 1.3 at. % and Re = 61.8 ± 1.3 at. % for Re0.60Ta0.40.

Magnetization, electrical resistivity, and specific-heat measurements subsequently established bulk type-II superconductivity in all three compounds, with transition temperatures of Tc = 4.46 K for Re0.76V0.24, 2.28 K for Re0.56Nb0.44, and 1.76 K for Re0.60Ta0.40. Further analysis extended to the medium-entropy alloy Re0.56Nb0.19Ta0.19V0.06, which also exhibited bulk type-II superconductivity with a transition temperature of 1.64 K. Thermodynamic and magnetic data analysis indicates weakly coupled, conventional type-II superconductivity within the phenomenological Ginzburg-Landau interpolation framework, providing a preliminary understanding of the superconducting mechanism at play.

The consistency of superconducting behavior across these alloys, despite variations in composition and degree of disorder, highlights the robustness of the superconducting state within the σ-phase structure. A clear bifurcation between zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves was observed below Tc in the polycrystalline samples, a phenomenon attributed to the material’s microstructure.

“This difference may be associated with surface superconductivity, which can persist to somewhat higher temperatures than bulk superconductivity,” the paper states, acknowledging that factors beyond bulk superconductivity may influence the observed magnetic response. The results demonstrate that both the σ-phase Re-X alloys, incorporating 3d, 4d and 5d transition-metal substitutions, and their medium-entropy counterparts represent a promising family of materials for investigating the complex relationship between structural complexity, chemical disorder, spin-orbit coupling and superconductivity in Re-based materials.

Bulk Type-II Superconductivity Characterized by Magnetization and Specific Heat

Analysis of the electronic heat capacity reveals a superconducting state consistent with weakly coupled, fully gapped BCS behavior in all tested compounds, providing insight into the mechanism driving superconductivity within these materials. Magnetization, electrical resistivity, and specific-heat measurements establish bulk type-II superconductivity in all compounds.

Corresponding values from heat capacity measurements are 6.57(7), 2.50(5), and 1.92(2) Tesla at 4.52(1), 4.44(2), and 1.76(1) Kelvin for Re0.56Nb0.44, and Re0.60Ta0.40. Powder X-ray diffraction confirms that all synthesized compounds crystallize in a single-phase tetragonal σ-phase structure, an essential aspect for understanding the material’s properties and ruling out the presence of confounding secondary phases.

Demagnetization factors, estimated from low-temperature isothermal magnetization measurements, are 0.49 and 0.46 for Re0.76V0.24 and Re0.56Nb0.44, respectively, and were used to correct susceptibility data. “These values were estimated from low-temperature isothermal M(H) measurements [insets of and (f)] and are consistent with those expected from the sample geometry,” the researchers state, validating the accuracy of the magnetic measurements.

The observed superconductivity is not limited to the binary Re-X alloys. This finding broadens the scope for designing novel superconducting materials with tailored properties through compositional engineering.

Researchers at Gdansk University of Technology synthesized the medium-entropy alloy Re₀.56Nb₀.19Ta₀.19V₀.06, derived from binary Re-X (X = V, Nb, Ta) compounds, all of which also display superconductivity. Despite the strong spin-orbit coupling associated with rhenium and the inherent structural disorder, the superconducting state remains consistent with conventional weak-coupling behavior.

👉 More information
🗞 Superconductivity in structurally complex σ -phase Re- X ( X = V, Nb, Ta) and derived medium-entropy alloys
✍️ Pavan Kumar Meena and Tomasz Klimczuk
🧠 DOI: http://link.aps.org/doi/10.1103/qcpr-m6qx

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