Charge density wave phenomena in trigonal transition metal dichalcogenides

Abstract

The metallic transition metal dichalcogenides (TMDs) are layered, quasi two-dimensional compounds which are host to many strongly-correlated phases of matter. Most notably, they exhibit charge density waves (CDWs); an ordered state consisting of electronic charge density modulation accompanied by a distortion of the underlying crystal lattice. Despite decades of research, there are persisting questions surrounding the origin of charge order in TMDs and numerous scenarios have been proposed. This thesis presents a study of CDW phenomena in the isostructural, selenium-based TMDs with trigonal (1T) symmetry; 1T-TiSe2, 1T-VSe2 and 1T-TaSe2. The characteristics of the CDW phase in each compound varies considerably, including; the transition temperature, distorted lattice periodicity, and commensurability. CDW phenomena are often dictated by the electronic band structure, including the shape and composition of the Fermi surface. Angle resolved photoemission spectroscopy (ARPES) and variants thereof allows direct visualisation of the electronic structure and the signatures of CDW formation, which primarily manifest as electronic gaps near the Fermi level. In order to further the understanding of CDW phenomena in these compounds, a unique experimental approach was required in each specific case. Firstly, a method was developed to grow high quality single crystal TMDs by chemical vapour transport which is discussed in detail with reference to 1T-VSe2 and the influence of growth conditions on the CDW properties. Full-wavevector ARPES measurements of the Fermi surface (FS) in 1T-VSe2 combined with DFT calculations of the electronic susceptibility revealed the importance of FS nesting. The possibility of separate 3q and 2q CDW states were considered based on the temperature and momentum dependence of the gap. The fluence dependence of photo-induced CDW suppression dynamics in 1T-TiSe2 was investigated using complimentary time- and -angle resolved photoemission spectroscopy (TR-ARPES) and time resolved reflectivity (TRR) techniques. The experimental results were compared to simulations of the quasiparticle dynamics in order to disentangle the role of excitons and phonons. Finally, investigations of coherent phonon oscillations in the low-temperature phase of 1T-TaSe2 using TR-ARPES showed a modulation of the metal-insulator transition (MIT) gap corresponding to the CDW amplitude mode frequency. Instead, multiple frequencies were triggered by a similar optical pump in TRR, suggesting a selective electron-phonon coupling in the former, shedding light on the interplay between the MIT and CDW in this compound.

Date of Award	1 May 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Enrico Da Como (Supervisor) & Alessandro Narduzzo (Supervisor)