Signatures of the exciton gas phase and its condensation in monolayer 1T-ZrTe2

Abstract

The excitonic insulator (EI) is a Bose-Einstein condensation (BEC) of excitons bound by electron-hole interaction in a solid, which could support high-temperature BEC transition. The material realization of EI has been challenged by the difficulty of distinguishing it from a conventional charge density wave (CDW) state. In the BEC limit, the preformed exciton gas phase is a hallmark to distinguish EI from conventional CDW, yet direct experimental evidence has been lacking. Here we report a distinct correlated phase beyond the 2×2 CDW ground state emerging in monolayer 1T-ZrTe2 and its investigation by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The results show novel band- and energy-dependent folding behavior in a two-step process, which is the signatures of an exciton gas phase prior to its condensation into the final CDW state. Our findings provide a versatile two-dimensional platform that allows tuning of the excitonic effect.

Controlling the Charge Density Wave Transition in Single-Layer TiTe2xSe2(1–x) Alloys by Band Gap Engineering

“…This explains its relatively strong intensity in the ARPES measurement and indicates that a similar phenomenology underpins band hybridization at the CDW transition across the entire alloy series. Indeed, we note that a similar band flattening has been recently observed in the (2 × 2) CDW phase of ML-ZrTe 2 , where the flattening of the backfolded valence band top has been ascribed as the signature of an excitonic condensation occurring at low temperatures. , In our model, however, the band top flattening observed in the 2D alloy can be explained by considering a simple hybridization between the conduction and valence states allowed by the (2 × 2) periodic lattice distortion, and irrespective of its microscopic driving mechanism. Thus, we conclude that spectral signatures like this do not, by themselves, allow us to draw any conclusion about the nature of the coupling.…”

Charge density waves in two-dimensional transition metal dichalcogenides

“…The gap size shrinks gradually and band folding intensity reduces. However, ARPES finds a significant amount of spectral weight is still transferred from the Γ point to the M point, as shown in figure 7(b) [224]. Concurrently, the superlattice contrast in STM becomes diffusive and finally disappears above the transition temperature T CDW .…”

“…Monolayer 1T-ZrTe 2 has been grown by MBE on the graphitized SiC [224][225][226][227] and InAs (111) substrates [228]. At low temperatures, monolayer 1T-ZrTe 2 enters a CDWordered state, evidenced by several clear experimental signatures [224,225].…”

“…Monolayer 1T-ZrTe 2 has been grown by MBE on the graphitized SiC [224][225][226][227] and InAs (111) substrates [228]. At low temperatures, monolayer 1T-ZrTe 2 enters a CDWordered state, evidenced by several clear experimental signatures [224,225]. The ARPES measurements show the folding of the valence band and the opening of an energy gap between valence and conduction bands.…”

“…The peculiarity of the CDW transition in monolayer 1T-ZrTe 2 is that the system resides in an 'intermediate' state at temperatures much higher than the CDW transition temperature, where only the top valence band at the Γ point shows folding and flattening and a significant amount of spectral weight is still transferred to the M-point to be the brightest part of the spectra [224] (figure 7). The second valence band at the higher energy around the Γ point follows the conventional CDW behavior, i.e.…”