Speaker
Description
Spontaneous rotational-symmetry breaking (RSB) in the amplitude of the superconducting gap is a necessary condition for “nematic” superconductivity. This was evidenced in the topological superconductor Cu$_x$Bi$_2$Se$_3$ where, despite the threefold symmetry of its lattice, a twofold symmetry of electronic properties emerged from nuclear magnetic resonance$^1$, transport$^2$, and specific-heat$^3$ measurements, when the applied magnetic field is rotated in the Se planes. This is also the case of CaSn$_3$ semimetal with the cubic AuCu$_3$-type structure: we prove a spontaneous RSB below Tc$^4$ by magnetotransport- and muon-spectroscopy (μSR) measurements.
Particularly meaningful are the transverse-field (TF)- μSR results in the mixed superconducting phase of CaSn$_3$, where the muon-depolarization rate depends on the magnetic field direction (here, applied along the [110] or [001] crystal directions). The absence of any additional muon depolarization along [110] suggests that an unconventional vortex lattice (VL) sets in. Conversely, in the [001] case, a VL encompassing at least 52% of the sample volume indicates the bulk nature of superconductivity.
Similarly, by scanning tunnelling spectroscopy in Cu$_x$Bi$_2$Se$_3$, vortices exhibit an elliptical shape within stretched VLs for applied fields H orthogonal to the Se planes, whereas “no obvious in-plane vortices” could be observed for H parallel to the Se layers$^5$.
Such evidence and our current experimental results on CaSn$_3$ seriously question the pertinence of the conventional Abrikosov model to the superconducting mixed state of nematic superconductors since multi-component order parameter superconductors may exhibit unusual vortex structures (fractional and/or non-axial vortices)$^6$.
Finally, the superfluid density in the (001) planes, extracted from TF-µSR data, shows a fully gapped low-temperature behaviour, with $\Delta$(0)=0.61(7) meV. Additional zero-field μSR results indicate that the superconducting state is time-reversal invariant. This fact and the RSB in a fully-gapped superconductor suggest CaSn$_3$ as nematic superconductor with an unconventional pairing state in a multidimensional representation.
$^1$https://doi.org/10.1038/nphys3781
$^2$https://doi.org/10.1038/s41467-019-14126-w
$^3$https://doi.org/10.1038/nphys3907
$^4$https://doi.org/10.1103/PhysRevB.105.094508
$^5$https://doi.org/10.1103/PhysRevX.8.041024
$^6$https://doi.org/10.1103/RevModPhys.63.239
