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Excitons Dance the Two-Step in a 2D Material

An excitonic insulator is a material in which excitons (electron–hole pairs) all move in unison, like couples dancing to the same rhythm on a dance floor. Physicists say that excitons in this mode “condense,” meaning that they all occupy the same quantum state and move in synchrony as a single unified group.

“Excitonic insulators are a rare form of macroscopic quantum state that can be realized at a high temperature, which can be useful for quantum information science,” said Shujie Tang, who led a study on excitonic insulators as a postdoc at the Advanced Light Source (ALS) and is now a researcher at the Chinese Academy of Sciences in Shanghai. “What we found is a new ‘dance floor’—a 2D material—that could enable this coherent excitonic dance.”

In addition to searching for materials that realize the excitonic insulating state at high temperatures, scientists are also interested in how to experimentally distinguish an excitonic insulator state from a ground-state charge density wave (CDW), a puzzle that has eluded solution for decades.

Here, the researchers used angle-resolved photoemission spectroscopy (ARPES) at ALS Beamline 10.0.1 to study single-layer zirconium telluride (1T-ZrTe2), grown by molecular-beam epitaxy at the beamline. In situ growth and doping capabilities enabled tuning of the excitonic effect.

In this material, the conduction and valence bands are separated in momentum space but intersect in energy, facilitating exciton formation. Because Coulomb interactions are greatly enhanced in the monolayer limit compared to the bulk, 1T-ZrTe2 provides an ideal platform to investigate excitonic effects.

“We found that, in the low-temperature condensate state, increasing the temperature or reducing interactions by carrier injection did not restore the normal state, as expected,” said Tang. “Instead, the system entered an intermediate state with substantial electronic-structure modifications, which we confirmed to be an exciton gas state.”

The data show novel band- and energy-dependent “folding” behavior—a two-step process—prior to condensation into a final CDW state. The robustness of excitons in this system suggests opportunities to couple excitonic effects with additional properties or interactions. Specifically, polarizing electron spins in this material could lead to an unusual state known as a spin superfluid—where spin, rather than charge, flows without dissipation.

On the left, a photon (a wavy blue line) enters from upper left to probe an exciton (the letters 'h' and 'e' in an orange oval), which is superimposed over a ball-and-stick model of zirconium telluride. An emitted photoelectron (a black arrow) points to upper right toward an eye (the detector). At right, below the eye, an inset shows ARPES data labeled “Excitonic insulator.”
Excitons (electron–hole pairs) in monolayer 1T-ZrTe2 were probed using ARPES at the ALS. The energy and momentum of emitted electrons were detected and plotted, with color indicating signal intensity. The energy–momentum maps reveal unusual features, such as the replication of spectral weight from one band to another (“folding”), in this case from the “Γ” symmetry point to “M” symmetry point in the Brillouin zone. (Credit: Yekai Song/Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences)

Y.K. Song, C.J. Jia, H. Xiong, B.B. Wang, Z.C. Jiang, K. Huang, J.W. Hwang, Z.J. Li, C.G. Hwang, Z.K. Liu, D.W. Shen, J.A. Sobota, P. Kirchmann, J.M. Xue, T.P. Devereaux, S.-K. Mo, Z.-X. Shen, and S.J. Tang, “Signatures of the exciton gas phase and its condensation in monolayer 1T-ZrTe2,” Nat. Commun. 14, 1116 (2023), doi:10.1038/s41467-023-36857-7.