[Paper Review] Superconductivity in new family of Rhenium-based binary alloys: Re$_{7}$X$_{3}$ (X = Nb, Ta, Ti, Zr, Hf)

R. K. Kushwaha, P. K. Meena+7 more|arXiv (Cornell University)|Feb 12, 2024

Rare-earth and actinide compounds4 citations

TL;DR

This study reports the synthesis and comprehensive characterization of a new family of rhenium-based binary superconductors, Re₇X₃ (X = Nb, Ta, Ti, Zr, Hf), which exhibit superconductivity in both centrosymmetric (CS) and non-centrosymmetric (NCS) crystal structures. The key finding is that NCS Re₇X₃ compounds, especially Re₇Nb₃ and Re₇Ta₃, display upper critical fields approaching the Pauli limit, indicating strong spin-orbit coupling and unconventional superconductivity, while CS compounds show significantly lower H_C2, highlighting the critical role of crystal symmetry and antisymmetric spin-orbit coupling in determining superconducting properties.

ABSTRACT

Rhenium-based superconductors have recently attracted significant interest due to their unconventional superconducting properties. In this work, we report the synthesis and properties of new superconducting Re$_{7}$X$_{3}$ (X = Nb, Ta, Ti, Zr, Hf) binary alloys which maintain a fixed composition of rhenium while crystallizing in centrosymmetric to non-centrosymmetric crystal structures, depending on the elements of the X site. Comprehensive structural and superconducting properties were investigated using powder x-ray diffraction, AC transport, magnetization, and specific heat measurements, and on the basis of these measurements, the superconducting phase diagram was constructed. The results suggest a complex interplay of crystal structure and the Re/X ratio, which governs the strength of spin-orbital coupling and controls the unconventional superconducting behavior in Re-based superconductors.

Motivation & Objective

• To investigate the superconducting properties of a new family of Re₇X₃ binary alloys with varying X-site elements (Nb, Ta, Ti, Zr, Hf) across both centrosymmetric and non-centrosymmetric crystal structures.
• To determine how crystal structure, Re/X ratio, and antisymmetric spin-orbit coupling (ASOC) influence superconducting transition temperature (T_C), upper critical field (H_C2), and gap symmetry.
• To clarify the role of non-centrosymmetry and spin-orbit coupling in enabling time-reversal symmetry breaking and unconventional superconductivity in Re-based materials.
• To construct a superconducting phase diagram linking elemental composition, crystal structure, and superconducting parameters for Re-based systems.

Proposed method

• Polycrystalline Re₇X₃ samples were synthesized via arc melting of high-purity metals under argon to ensure stoichiometric homogeneity.
• Powder X-ray diffraction (XRD) with Fullprof refinement was used to determine crystal structure, phase purity, and lattice parameters.
• AC susceptibility, DC magnetization, and specific heat measurements were performed to determine T_C, H_C2, and superconducting gap symmetry.
• Energy-dispersive X-ray spectroscopy (EDAX) confirmed elemental composition and homogeneity of the polycrystalline samples.
• A superconducting phase diagram was constructed by correlating T_C and H_C2 with crystal structure and X-site element across the Re₇X₃ series.
• Comparative analysis with known Re-based superconductors (e.g., Re₆Zr, Re₇B₃, Re₃Ta) was used to isolate the effects of crystal symmetry and Re concentration.

Experimental results

Research questions

• RQ1 How does the crystal structure (centrosymmetric vs. non-centrosymmetric) influence the superconducting transition temperature (T_C) and upper critical field (H_C2) in Re₇X₃ alloys?
• RQ2 To what extent does the choice of X-site element (3d, 4d, 5d transition metals) modulate antisymmetric spin-orbit coupling (ASOC) and thus unconventional superconducting behavior in Re₇X₃ compounds?
• RQ3 Why do some Re-based superconductors exhibit time-reversal symmetry breaking (TRSB) while others do not, despite similar Re concentrations and crystal structures?
• RQ4 How does the Re concentration in Re₇X₃ compare to other Re-based systems (e.g., Re₁₋ₓTₓ) in terms of T_C and H_C2, and what does this reveal about the role of Re content?
• RQ5 What is the nature of the superconducting gap in Re₇X₃ compounds, and does it support nodeless s-wave pairing with weak electron-phonon coupling?

Key findings

• All Re₇X₃ compounds exhibit superconductivity with T_C values ranging from 1.5 K (Re₇Ti₃) to 8.5 K (Re₇Nb₃), with the highest T_C observed in Re₇Nb₃.
• The upper critical field (H_C2) of NCS Re₇X₃ compounds (Re₇Nb₃ and Re₇Ta₃) approaches the Pauli limit, indicating strong spin-orbit coupling and potential spin-triplet pairing components.
• In contrast, CS Re₇X₃ compounds (Re₇Zr₃ and Re₇Hf₃) exhibit significantly lower H_C2 values, well below the Pauli limit, highlighting the critical role of non-centrosymmetry in enhancing H_C2.
• Specific heat measurements confirm a nodeless s-wave superconducting gap with weak electron-phonon coupling across all Re₇X₃ compounds.
• The presence of time-reversal symmetry breaking in both NCS and CS Re-based superconductors suggests that crystal symmetry alone is insufficient to explain TRSB, implicating local microstructure and Re concentration as key factors.
• Comparative studies with Re₇B₃ and other Re-based superconductors reveal that the X-site element and Re concentration are critical in tuning ASOC strength and superconducting anisotropy.