Excitonic effects in two-dimensional TiSe2 from hybrid density functional theory

Transition metal dichalcogenides (TMDs), whether in bulk or in monolayer form, exhibit a rich variety of charge-density-wave (CDW) phases and stronger periodic lattice distortions. While the actual role of nesting has been under debate, it is well understood that the microscopic interaction responsible for the CDWs is the electron-phonon coupling. The case of 1T-TiSe2 is, however, unique in this family in that the normal state above the critical temperature T-CDW is characterized by a small quasiparticle band gap as measured by angle-resolved photoemission spectroscopy, so that no nesting-derived enhancement of the susceptibility is present. It has therefore been argued that the mechanism responsible for this CDW should be different and that this material realizes the excitonic insulator phase proposed by W. Kohn. On the other hand, it has also been suggested that the whole phase diagram can be explained by a sufficiently strong electron-phonon coupling. In this paper, in order to estimate how close this material is to the pure excitonic insulator instability, we quantify the strength of electron-hole interactions by computing the exciton band structure at the level of hybrid density functional theory, focusing on the monolayer. We find that in a certain range of parameters the indirect gap at q(CDW) is significantly reduced by excitonic effects. We also stress the important role of the spin-orbit coupling in significantly reducing the band gap. We discuss the consequences of those results regarding the debate on the physical mechanism responsible for this CDW. Based on the dependence of the calculated exciton binding energies as a function of the mixing parameter of hybrid density functional theory, we conjecture that a necessary condition for a pure excitonic insulator is that its noninteracting electronic structure is metallic.

2D transition metal dichalcogenides

Andras Kis, Sajedeh Manzeli, Dmitry Ovchinnikov, Diego José Pasquier, Oleg Yazyev

Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin-orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examined and their properties are discussed, with particular attention to their charge density wave, superconductive and topological phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties.

Nature Publishing Group2017

Splitting in the Fermi surface of ZrTe3: A surface charge density wave system

Corsin Battaglia, Helmuth Berger

The electronic band structure and Fermi surface of ZrTe3 was determined by angle-resolved photoemission spectroscopy. Several bands and a large part of the Fermi surface are found to be split by 100-200 meV into two parallel dispersions. Calculations of the bulk band structure cannot reproduce this splitting even if spin-orbit interaction is taken into account. A bilayer model representing the top layers of a surface-relaxed structure without reconstruction introduces the observed splitting and reproduces most features observed in the data thus suggesting a surface relaxation of the freshly cleaved ZrTe3. The dispersion of the highly nested small electron pocket that gives rise to the charge density wave is traceable even in the low-temperature gapped state and the gap energy is determined as epsilon(g)=65+/-10 meV.

2009

Three-dimensional momentum-resolved electronic structure of 1T-TiSe2: A combined soft-x-ray photoemission and density functional theory study

Philipp Aebi, Helmuth Berger, Elia Razzoli, Thorsten Schmitt

1T-TiSe2 is a quasi-two-dimensional transition metal dichalcogenide, which exhibits a charge density wave transition at a critical temperature of similar to 200 K as well as low-temperature superconductivity induced by pressure or intercalation. The electronic energy dispersion measured by soft x-ray angle-resolved photoemission is not only momentum resolved parallel to the surface but also perpendicular to it. Experiments are compared to density functional theory based band structure calculations using different exchange-correlation functionals. The results reveal the importance of including spin-orbit coupling for a good description of the experimental bands. Compared to calculations within the local density approximation, the use of the modified Becke-Johnson (mBJ) exchange functional leads to a band structure that does not need an artificial downwards shift of the valence band to fit the experiment. The mBJ functional thus clearly appears as the most adapted functional for the theoretical description of the 1T-TiSe2 band structure within the DFT framework.

Amer Physical Soc2015