Strong-coupling charge density wave in monolayer TiSe2

Authors gratefully acknowledge funding from The Royal Society, The Leverhulme Trust, and the European Research Council (through the ERC-714193-QUESTDO project). The MBE growth facility was funded through the Engineering and Physical Sciences Research Council (EPSRC) under Grant No. EP/M023958/1. KU and OJC acknowledge the EPSRC for PhD studentship support via Grant Nos. EP/L015110/1 and EP/K503162/1, and IM the International Max Planck Research School for Chemistry and Physics of Quantum Materials. We thank SOLEIL synchrotron for access to the CASSIOPEE beamline (proposal 20171202) and Elettra synchrotron for access to the APE-HE beamline, which both contributed to the results presented here. This work has been partially performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MIUR Italy Progetti Internazionali) facility. The research leading to this result has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020.We study the 2 × 2 charge density wave (CDW) in epitaxially-grown monolayer TiSe2. Our temperature-dependent angle-resolved photoemission spectroscopy measurements indicate a strong-coupling instability, but reveal how not all states couple equally to the symmetry-breaking distortion, with an electron pocket persisting to low temperature as a non-bonding state. We further show how the CDW order can be suppressed by a modest doping of around 0.06(2) electrons per Ti. Our results provide an opportunity for quantitative comparison with a realistic tight-binding model, which emphasises a crucial role of structural aspects of the phase transition in understanding the hybridisation in the ground state. Together, our work provides a comprehensive understanding of the phenomenology of the CDW in TiSe2 in the 2D limit.PostprintPeer reviewe