The topological semimetal \(\beta\)-Ag2Se features a Kramers Weyl node at the origin in momentum space and a quadruplet of spinless Weyl nodes, which are annihilated by spin-orbit coupling. We show ...that single crystalline \(\beta\)-Ag2Se manifests giant Shubnikov-de Haas oscillations in the longitudinal magnetoresistance which stem from a small electron pocket that can be driven beyond the quantum limit by a field less than 9 T. This small electron pocket is a remainder of the spin-orbit annihilatedWeyl nodes and thus encloses a Berry-phase structure. Moreover, we observed a negative longitudinal magnetoresistance when the magnetic field is beyond the quantum limit. Our experimental findings are complemented by thorough theoretical band structure analyses of this Kramers Weyl semimetal candidate, including first-principle calculations and an effective k*p model.
A Weyl semimetal is a new state of matter that host Weyl fermions as quasiparticle excitations. The Weyl fermions at zero energy correspond to points of bulk band degeneracy, Weyl nodes, which are ...separated in momentum space and are connected only through the crystal's boundary by an exotic Fermi arc surface state. We experimentally measure the spin polarization of the Fermi arcs in the first experimentally discovered Weyl semimetal TaAs. Our spin data, for the first time, reveal that the Fermi arcs' spin polarization magnitude is as large as 80% and possesses a spin texture that is completely in-plane. Moreover, we demonstrate that the chirality of the Weyl nodes in TaAs cannot be inferred by the spin texture of the Fermi arcs. The observed non-degenerate property of the Fermi arcs is important for the establishment of its exact topological nature, which reveal that spins on the arc form a novel type of 2D matter. Additionally, the nearly full spin polarization we observed (~80%) may be useful in spintronic applications.
Recent \(\mu\)SR measurements on SrPtAs revealed time-reversal-symmetry breaking with the onset of superconductivity Biswas et al., Phys. Rev. B 87, 180503(R) (2013), suggesting an unconventional ...superconducting state. We investigate this possibility via functional renormalization group and find a chiral \((d+\mathrm{i}d)\)-wave order parameter favored by the multiband fermiology and hexagonal symmetry of SrPtAs. This \((d+\mathrm{i}d)\)-wave state exhibits significant gap anisotropies as well as gap differences on the different bands, but only has point nodes on one of the bands at the Brillouin zone corners. We study the topological characteristics of this superconducting phase, which features Majorana-Weyl nodes in the bulk, protected surface states, and an associated thermal Hall response. The lack of extended nodes and the spontaneously broken time-reversal symmetry of the \((d+\mathrm{i}d)\)-wave state are in agreement with the \(\mu\)SR experiments. Our theoretical findings together with the experimental evidence thus suggests that SrPtAs is the first example of chiral \(d\)-wave superconductivity.
