Do topological semimetals NbRuSi and TaRuSi superconduct unusually?
Ruthenium silicides NbRuSi and TaRuSi, which are topological semimetals, become superconductors that spontaneously generate tiny internal magnetic fields, a hallmark of unconventional pairing.
Source
Unconventional superconductivity in topological Kramers nodal-line semimetals
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.
What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Time-reversal symmetry breaking
- A state that changes if time runs backwards; in a superconductor it shows up as spontaneous internal magnetic fields appearing at Tc.
- Muon-spin relaxation (μSR)
- A technique in which spin-polarised muons are implanted into a sample and the decay of their spin polarisation reveals very small local magnetic fields.
- Noncentrosymmetric superconductor
- A superconductor whose crystal lacks an inversion centre, allowing spin-orbit coupling to mix spin-singlet and spin-triplet pairing.
- Superfluid density
- The density of paired electrons, measured via the magnetic penetration depth; its temperature dependence reveals whether the superconducting gap has nodes.
- Kramers nodal line
- A line in momentum space along which bands stay doubly degenerate, protected by time-reversal and crystal symmetry in materials without inversion symmetry.
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Quiz yourself
What was the main evidence for time-reversal symmetry breaking in NbRuSi and TaRuSi?
Common questions
How can a superconductor, which expels magnetic fields, produce its own magnetic fields?
If the Cooper pairs carry a net spin or orbital moment, as in some mixed singlet-triplet states, tiny local fields appear inside the material even though larger external fields are still screened; μSR is sensitive enough to see them.
Why does a flat superfluid density at low temperature mean the gap is full?
If the gap closed anywhere (nodes), low-energy excitations would exist and the superfluid density would keep changing as temperature drops; a flat curve means no such excitations, so the gap is open everywhere.
Why rule out magnetic impurities as the source of extra relaxation?
The relaxation rate is flat above Tc rather than following an impurity-like Curie-Weiss trend, and a small longitudinal field fully decouples the muons, which fits weak static fields from the superconducting state.
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