March Meeting practice talks: M Watson & I Markovic
Main list: Cond Mat Seminars
Orbital- and kz-selective hybridisation of Se 4p and Ti 3d states at the CDW transition of TiSe2
We revisit the enduring problem of the 2×2×2 charge density wave (CDW) order in TiSe2, utilising photon energy-dependent angle-resolved photoemission spectroscopy (ARPES) to probe the full three-dimensional high- and low-temperature electronic structure. Our measurements demonstrate how a mismatch of dimensionality between the 3D conduction bands and the quasi-2D valence bands in this system leads to a hybridisation that is strongly kz-dependent. This 3D momentum-selective coupling shifts the strongly hybridized states well away from the Fermi level, providing the energy gain required to form the CDW. However, we show how additional ``passenger'' states remain, and dominate the low energy physics in the ordered state. In particular we find that a branch of the conduction band with 3dz2 character remains essentially unhybridised in the ordered phase, forming a coherent and ungapped electron-like Fermi surface. We conclude by making a comparison to the 2x2 CDW of monolayer TiSe2, where kz-selective effects are absent, but orbital-selective hybridisation persists.  Watson et al., Phys. Rev. Lett. 122, 076404 (2019)
Symmetry protected band crossings of spin-orbit coupled surface states of NbGeSb
The electronic structure of materials in the ZrSiS family of nonsymmorphic semimetals hosts, alongside their bulk Dirac line nodes, peculiar surface states created by the breaking of nonsymmorphic summetry at the surface layer. We study the surface electronic structure of NbGeSb (isostructural to ZrSiS) using spin- and angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT). We observe two pairs of surface states, hosting large Rashba-like spin-splittings, split off from the bulk manifold. We find how these two brances intersect each other, forming a fourfold crossing structure along the Brillouin zone boundary. Surprisingly, we find a counter-intuitive hybridisation structure around these crossings, with three remaining seemingly protected and just a single hybridised anti-crossing. We demonstrate how a mirror symmetry line provides protection along the Brillouin zone face, opening new routes to realising complex intertwined spin textures of spin-orbit coupled surface states.
February 27, 2019
From: 13h00 To: 14h00