Seminars

# Orbital- and kz-selective hybridisation of Se 4p and Ti 3d states at the charge density wave transition of TiSe2

## by Matthew D Watson (University St Andrews)

Tuesday, November 20, 2018 from to (Europe/Berlin)
at DESY Photon Science ( 25b/109 )
DESY Notkestraße 85 22607 Hamburg
 Description 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 : mdw5@st-andrews.ac.uk 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. [1] 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).