Atomically Thin Hafnium Telluride: Uncovering its Role as an Excitonic Insulator

The date is February 9, 2024.

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Article written by Ingrid Fadelli, Phys.org

This article explores the condensation of excitons with non-zero momentum, which can result in charge density waves (CDW). This occurrence can induce the evolution of materials into an exceptional new quantum phase, referred to as an excitonic insulator.

A group of researchers from Shanghai Jiao Tong University and other institutions have conducted a study to examine if this metal-insulator transition could transpire in the atomically thin semi-metallic HfTe2. They documented potential excitonic CDW and metal-insulator transitions in this super-thin material in their report featured in Nature Physics.

Peng Chen, the paper's corresponding author, comments on the formation of CDW. He states that various mechanisms, like Fermi surface nesting and lattice distortions, drive the formation process. The absence of other CDW formation mechanisms is crucial to confirm the presence of an excitonic insulator.

The team carried out multiple studies on two-dimensional transition metal dichalcogenides such as TiSe2 and ZrTe2 in their previous research efforts. They wanted to investigate this novel occurrence. However, they found signs of lattice distortion in calculated phonon dispersions, even though it might not be the primary driving force in these materials.

Following their previous research work, the investigators went on to examine the presence of CDW and a metal-insulator transition in thin films of a different material, named HfTe2. After successfully identifying these phenomena, they executed phonon calculations to confirm their observations.

Their calculations showed no sign of structural instability in single-layer HfTe2. Raman and X-ray diffraction measurements didn't reveal significant lattice distortions, lending robust evidence to the electronic origin of the metal-insulator transition in single-layer HfTe2.

According to Peng, the exciton condensation is prone to carrier concentration near the Fermi surface. A small number of carriers and a balanced n-type and p-type carrier concentration, in theory, can enhance exciton condensation. Their experiment confirmed that a minor quantity of n-type doping escalated the transition temperature of single-layer HfTe2, an unusual event compared to other transition mechanisms like Peierls-type CDW.

The recent study's findings suggest the first known excitonic insulator in a natural solid to originate purely from electronic transitions could be atomically thin HfTe2. The scientists have so far validated their results with diverse calculations and analyses.

Peng explains that decreasing the dimensionality of the material helps to reduce the screening effects around the Fermi level, thus aiding exciton condensation. The team prepared single-layer and multi-layer HfTe2 thin films using molecular beam epitaxy. They saw a metal-insulator transition when the thickness of the band was less than three layers. The valence band displayed a flat band at low temperatures, which opened a gap near the Fermi surface. In addition, folded bands surfaced near the point, typical of CDW formation.

The discovery of this new excitonic insulator could act as a foundation for future research focusing on the unique quantum effects that originate from the interaction between excitonic insulating states and other states, such as topology and spin-correlated states. Peng and his team aim to examine this quantum insulator phase further to understand its underlying physics.

Concluding the discussion, Peng stated, unlike typical Cooper pairs in superconductors, excitons have a larger binding energy that supports condensation at higher temperatures. This makes the investigation of excitonic insulators critical to comprehend phenomena such as high-temperature superconductivity and superfluidity. He added that the exciton formation is susceptible to the number of carriers and band gap, and hence, external stimuli like electric gating or strain can finely adjust the carrier concentration or band structure and thereby the electron-hole coherence's order parameter.

Journal information: Provided by Nature Physics

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