Evidence of high-temperature exciton condensation in a two-dimensional semimetal

Abstract

Electrons and holes can spontaneously form excitons and condense in a semimetal or semiconductor, as predicted decades ago. This type of Bose condensation can happen at much higher temperatures in comparison with dilute atomic gases. Two-dimensional (2D) materials with reduced Coulomb screening around the Fermi level are promising for realizing such a system. Here we report a change in the band structure accompanied by a phase transition at about 180 K in single-layer ZrTe2 based on angle-resolved photoemission spectroscopy (ARPES) measurements. Below the transition temperature, gap opening and development of an ultra-flat band top around the zone center are observed. This gap and the phase transition are rapidly suppressed with extra carrier densities introduced by adding more layers or dopants on the surface. The results suggest the formation of an excitonic insulating ground state in single-layer ZrTe2, and the findings are rationalized by first-principles calculations and a self-consistent mean-field theory. Our study provides evidence for exciton condensation in a 2D semimetal and demonstrates strong dimensionality effects on the formation of intrinsic bound electron–hole pairs in solids.

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.…”

ZrTe2 Compound Dirac Semimetal Contacts for High-Performance MoS2 Transistors

“…18 Actually, we have noticed that the ARPES observation indicates the surface of bulk ZrTe 2 exhibits semimetal band structure, 24 which is similar to that of monolayer ZrTe 2 . 24,25 Meanwhile, monolayer ZrTe 2 exhibits distinct band structures on different substrates, such as semimetal on InAs 20 and semiconductor-like on graphene. 26 These observations imply that the band structure of ZrTe 2 close to the contact interface might be influenced seriously by the different contact substrates regardless of the bulk electrical transport characteristics.…”

Tungsten Nitride (W5N6): An Ultraresilient 2D Semimetal

“…Different from graphene, the conduction band minimum is not at the same momentum value as the valence band maximum, leading to an indirect electronic transition without a bandgap. This electronic structure is ideally suited for promising for the investigation of exciton condensation . To confirm the theoretical band structure predictions, we conduct spectroscopic ellipsometry measurements .…”

“…This electronic structure is ideally suited for promising for the investigation of exciton condensation. 12 To confirm the theoretical band structure predictions, we conduct spectroscopic ellipsometry measurements. 13 The optical absorption spectrum was fitted using two Lorentzian oscillators at approximately 2.47 eV (Peak 1) and 5.06 eV (Peak 2) (inset of Figure 3b).…”