Orbital- and kz-selective hybridisation of Se 4p and Ti 3d states at the charge density wave transition of TiSe2
Matthew D Watson
School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, UK
Email : firstname.lastname@example.org
The transition metal dichalcogenide material TiSe2 exhibits a well-known phase transition at Tc = 200 K, which involves the hybridization of valence and conduction band states across a narrow indirect band gap. While often described as a charge density wave, this transition has also been linked to a possible condensation of excitons, and the mechanism of the transition remains under debate.
In this talk I will present a new perspective on the problem by utilizing photon energy-dependent angle-resolved photoemission spectroscopy (ARPES), which probes the full three-dimensional high- and low-temperature electronic structure. The measurements, performed at the I05-ARPES beamline at Diamond Light Source, 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 additional ``passenger'' states remain, and dominate the low energy physics in the ordered state. In particular, 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. Counter-intuitively, the band gap is found to be smaller in the ordered phase of TiSe2. I will conclude the talk by discussing the case of monolayer TiSe2, which undergoes a similar CDW order.
 Matthew D. Watson, Oliver J. Clark, Federico Mazzola, Igor Marković, Veronika Sunko, Timur K. Kim, Kai Rossnagel, Philip D. C. King, arXiv:1808.07141 (2018).