A groundbreaking study led by Prof. He Junfeng from the University of Science and Technology of China (USTC) has made significant advancements in understanding the relationship between electronic and lattice structural instability and their effects on charge density waves (CDW) in kagome metals. The team, in collaboration with domestic and international researchers, focused on the novel Ti-based kagome metal CsTi3Bi5 and discovered that the energy level of the van Hove singularity (VHS) can be manipulated without causing lattice structural instability. This research opens up new possibilities for modulating electronic instabilities independently of structural instabilities.
Kagome metals are known to exhibit electronic instabilities that often coincide with lattice structural instabilities, making it challenging to differentiate their individual effects on CDW. To gain a deeper understanding, the team conducted extensive research on CsTi3Bi5, a Ti-based kagome metal with a similar lattice structure to AV3Sb5 (A=K, Rb, Cs). Notably, CsTi3Bi5 does not possess charge density wave states, providing an intriguing platform for investigation.
The researchers began by studying the electronic structure of pristine CsTi3Bi5 using high-resolution angle-resolved photoemission spectroscopy. The results were in excellent agreement with first-principles calculations, indicating no electronic instability caused by the energy position of the VHS, which was significantly higher than the Fermi level. Furthermore, through first-principles calculations and low-temperature X-ray diffraction measurements, the team confirmed the absence of lattice instability in CsTi3Bi5.
Through Cs surface doping, the researchers successfully introduced electrons into CsTi3Bi5, enabling the modulation of the VHS within a broad energy range. When the VHS approached the Fermi level, it triggered electronic instabilities as confirmed by first-principles calculations. Importantly, the researchers observed that this manipulation of the VHS did not induce lattice structural instability in CsTi3Bi5. This groundbreaking outcome demonstrated the decoupling of electronic and lattice instabilities, establishing CsTi3Bi5 as a unique platform for independent modulation of electronic instabilities.
Despite the introduction of electronic instability near the Fermi energy level, the researchers discovered that it did not generate the necessary energy gap to enable charge density waves in CsTi3Bi5. This finding indicated that electronic instability alone was insufficient to trigger CDW in CsTi3Bi5.
Through further first-principles calculations, the team investigated the evolution from CsV3Sb5 to CsTi3Bi5 and its correlation with CDW phase transition. The study revealed that the appearance of CDW is directly associated with the change in the system’s total energy. CDW phase transition occurs only when the corresponding crystal structure possesses the lowest total energy. Thus, lattice structural instability plays a crucial role in CDW phase transition in kagome metals.
This groundbreaking study led by Prof. He Junfeng and his team sheds light on the intricate relationship between electronic and lattice structural instability and their impact on charge density waves in kagome metals. The discovery of independent modulation of electronic instabilities in CsTi3Bi5 paves the way for further understanding and manipulation of CDW in similar materials. By decoupling these instabilities, researchers can explore new avenues for advancing the field of electronic materials and their applications.