Nodal-line superconductor PbTaSe₂
PbTaSe₂A superconductor whose normal state is topological. Inserting a lead layer into TaSe2 breaks inversion symmetry, and the lead sheet contributes a Dirac cone that spin–orbit coupling gaps by about 0.8 eV. The result carries topological nodal lines – closed loops in momentum space where bands touch, protected by a mirror symmetry – and then superconducts at 3.7 K, which is why it keeps being examined as a place where topology and superconductivity coexist in one crystal.
Key properties
- Type-II BCS superconductor with a transition at 3.72 K and a Ginzburg–Landau parameter of about 17
- Electron–phonon coupling constant of ~0.74, so the pairing itself looks conventional
- The lead layer supplies a bulk Dirac cone at K, similar to graphene but with a 0.8 eV gap opened by spin–orbit coupling
- Topological nodal lines protected by reflection symmetry and carrying an integer topological invariant, confirmed by photoemission
- Broken inversion symmetry also produces large Rashba splitting
How it is made
- Single crystals by vapour transport or flux growth from the elements
- Mechanical exfoliation into flakes for transport
- Photoemission on fresh cleaves to map the nodal lines
Uses, and how close they are
- Model system for topological superconductivity and Majorana searcheslab
Readiness runs lab → prototype → pilot → deployed.
Open problems
- Does the superconducting state inherit anything topological from the nodal lines, or is it conventional pairing in a topological metal?
- Are the zero-bias features reported in vortex cores Majorana modes, or ordinary bound states?
- How do the nodal lines evolve as crystals are thinned to a few layers?
Going deeper
Short notes for specialists. Choose a lens in the header and yours comes first.
A clean test of symmetry bookkeeping: the lead layer removes inversion, mirror symmetry protects the nodal lines, and strong spin–orbit coupling sets their size. Because the pairing looks conventional, any topological superconductivity would come from the normal-state topology rather than from an exotic order parameter, which makes the surface states and their response to disorder the interesting quantity to compute.
Report the residual resistivity ratio and the transition width with any claim about vortex-core states, since disorder generates bound states that look like Majorana signatures. Photoemission needs fresh cleaves and careful alignment, because the nodal lines live on specific mirror planes.
No application role: a 3.7 K superconductor studied for physics rather than for devices.
In the research tracks
Theory frontiers
Key references
- Noncentrosymmetric superconductor with a bulk three-dimensional Dirac cone gapped by strong spin-orbit couplingcited by 170doi:10.1103/PhysRevB.89.020505
- Topological nodal-line fermions in spin-orbit metal PbTaSe2cited by 860doi:10.1038/ncomms10556
- Topological Dirac surface states and superconducting pairing correlations in PbTaSe2cited by 105doi:10.1103/PhysRevB.93.245130
- Superconducting topological surface states in the noncentrosymmetric bulk superconductor PbTaSe2cited by 187doi:10.1126/sciadv.1600894