Orbital- and k_z-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 k_z-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 k_z-selective effects are absent, but orbital-selective hybridisation persists. [1] Watson et al., arXiv:1808.07141 (2018).