Superconductivity
Do topological semimetals NbRuSi and TaRuSi superconduct unusually?
Open access · cc by · source: Europe PMC
Ruthenium silicides NbRuSi and TaRuSi, which are topological semimetals, become superconductors that spontaneously generate tiny internal magnetic fields, a hallmark of unconventional pairing.
Study at a glance
- Design
- Other — Lab measurements (susceptibility, heat capacity, resistivity, zero-, transverse- and longitudinal-field muon-spin relaxation) on arc-melted polycrystals, combined with DFT band-structure and symmetry analysis.
- N
- Compounds synthesised (TiRuSi, NbRuSi, HfRuSi, TaRuSi); only NbRuSi and TaRuSi show bulk superconductivity and were studied by muon-spin methods. No sample count applies.
- Population
- Polycrystalline TRuSi compounds (T = Ti, Nb, Hf, Ta) with a noncentrosymmetric orthorhombic structure
- Outcome
- Superconducting transition temperature, muon-spin relaxation rate below Tc (time-reversal symmetry breaking), superfluid density vs temperature (gap structure), calculated band topology
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
NbRuSi and TaRuSi become bulk superconductors at 3.1 and 4.0 K, while TiRuSi stays a normal metal. Below Tc the zero-field muon relaxation rises, implying spontaneous fields of only a few hundredths of a millitesla, which a weak longitudinal field removes, pointing to intrinsic time-reversal symmetry breaking. The superfluid density is flat at low temperature, indicating a fully open gap, and fits a mixed singlet-triplet (s + ip) pairing. Calculations show large spin-orbit band splitting and that every compound in the family hosts Kramers nodal lines, Kramers Weyl points and hourglass fermions.
Methodology
The authors made polycrystalline TRuSi samples with Ti, Nb, Hf or Ta as the transition metal and measured magnetisation, heat capacity and resistivity to find which superconduct. For the superconducting members, NbRuSi and TaRuSi, they implanted spin-polarised muons to detect spontaneous magnetic fields (zero field) and to track the superfluid density versus temperature (transverse field). They also computed the electronic band structures with density functional theory, including spin-orbit coupling, and analysed the crystal symmetry.
Limitations
The samples are polycrystalline, so direction-dependent gap structure is averaged out and the authors say single-crystal studies are needed. The superfluid density fits single-gap and multi-gap models almost equally well, and symmetric versus nonsymmetric s + ip pairing cannot be distinguished, so the exact pairing state is proposed, not proven. The topological band features come from calculations and were not directly observed (for example by ARPES), and the topological surface superconductivity is only speculated.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
Unconventional pairing is inferred from signatures, not seen directly.
Evidence for time-reversal-symmetry breaking or mixed pairing comes from indirect probes: muon-spin relaxation in NbRuSi and TaRuSi polycrystals detected tiny spontaneous fields below Tc and a fully gapped superfluid density consistent with s + ip pairing.
Evidence for the claim as stated.
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