Schematic diagram of exciton condensation and charge density waves in single-layer HfTe2 Thin films. Credit: Gao et al.
The condensation of excitons with non-zero momentum can give rise to so-called charge density waves (CDW). This phenomenon can cause materials to transition into a fascinating new quantum phase, called an excitonic insulator.
Researchers from Shanghai Jiao Tong University and other institutes recently conducted a study exploring the possibility that this metal-to-insulator transition could occur in atomically thin semi-metallic HfTe.2. Their observations, presented in Natural physicsrevealed possible excitonic CDW and metal-insulator transitions in the atomically thin material.
“The formation of CDW in materials has various mechanisms (e.g., Fermi surface interlocking, lattice distortions, etc.) and the exclusion of other mechanisms of CDW formation is the key to identifying the existence of an excitonic insulator”, Peng Chen, corresponding author of the study. paper, told Phys.org.
“Our research team has previously conducted a series of studies on two-dimensional transition metal dichalcogenides, including TiSe.2 and ZrTe2 to explore this new phenomenon. Unfortunately, lattice distortion is still evidenced in the calculated phonon dispersions, although it may not be the main driving force in these materials.
Building on their previous work, the researchers set out to probe the existence of CDW and a metal-to-insulator transition in thin films of another material, namely HfTe.2. After successfully observing these two phenomena, they performed phonon calculations to validate their observations.
Phase diagram of HfTe transition temperature2 thin films with different carrier concentrations. Credit: Gao et al.
These calculations showed that single-layer HfTe2 does not present structural instability. Furthermore, Raman and X-ray diffraction measurements revealed no significant lattice distortion, thus providing strong evidence for the electronic origin of the metal-insulator transition in single-layer HfTe.2.
“A notable feature of exciton condensation is the sensitivity to carrier concentration near the Fermi surface,” Peng explained. “A small number of carriers and a balanced concentration of n-type and p-type carriers can in principle benefit exciton condensation. We found that a small amount of n-type doping significantly increased the transition temperature of HfTe single layer.2which is different from other types of transition mechanisms like Peierls type CDW. »
Recent findings collected by Peng and his researchers suggest that atomically thin HfTe2 could be the first known excitonic insulator in a natural solid with a purely electronic transition origin. The researchers have so far validated their results via various calculations and analyses.
ARPES spectra of HfTe2 thin films of different thicknesses and experimental evidence of network stability in single-layer films. Credit: Gao et al.
“By reducing the dimensionality of the material, screening effects around the Fermi level can be reduced, which benefits exciton condensation,” Peng said. “We have successfully prepared single-layer and multi-layer HfTe2 thin layers by molecular beam epitaxy. Angle-resolved photoemission spectroscopy measurements revealed a metal-insulator transition when the thickness was less than three layers. The top of the valence band formed a flat band at low temperatures, opening up a gap near the Fermi surface. Additionally, folded bands appeared near the tip, a typical feature of CDW formation. »
The new excitonic insulator discovered by this research team could lay the foundation for additional studies focused on exotic quantum effects arising from the interaction between excitonic insulator states and other orders (e.g., the topology and correlated states of spin). In their future work, Peng and his colleagues plan to examine the quantum insulating phase they observed in more detail, to better understand its underlying physics.
“Unlike traditional Cooper pairs in superconductors, excitons have larger binding energy, making them conducive to condensation at higher temperatures,” Peng added. “Therefore, the study of excitonic insulators is of great importance for understanding phenomena such as superconductivity and high-temperature superfluidity. As the formation of excitons is very sensitive to the number of carriers and the band gap, External stimuli such as electrical gating or deformation can be used to delicately control the carrier concentration or band structure and thus the order parameter of the electron-hole coherence.
More information:
Qiang Gao et al, Observation of possible excitonic charge density waves and metal-insulator transitions in atomically thin semimetals, Natural physics (2024). DOI: 10.1038/s41567-023-02349-0
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