Direct evidence of energy dependent Charge Density Wave State in Cu_2-xTe using Scanning Tunneling Microscopy/ Spectroscopy

Charge density wave (CDW) has been a subject of great interest in low-dimensional materials for decades. It refers to spontaneous periodic variation in electronic charge density, that often is accompanied by a corresponding distortion in the crystal lattice [1]. Layered transition metal chalcogenides have emerged as a prominent candidate for exploring correlated electronic phases, due to their diverse electronic properties and structural versatility. Cu_2-xTe is one of such materials, having hexagonal Nowotny phase with lowest ground state energy [2]. Previous studies on Cu_2-xTe provide no concrete evidence of CDW signatures and the reconstructions on the crystal surface were concluded as the consequences of the movement of liquid like copper atoms [3].

Here we report the results of comprehensive bias dependent CDW patterns of well characterized Cu_2-xTe single crystals which were grown by flux method [3] using Scanning Tunneling Microscopy/Spectroscopy (STM/S). Electric transport measurement confirms that the material shows a metal to semiconductor transition at (T_cdw) ≈ 150 K. STM/S measurements were carried out using a commercial STM (make: Unisoku, Japan) for crystals which were cleaved at room temperature and under UHV conditions. Atomically flat terraces of height ≈ 0.8 nm are observed, hexagonal atomic arrangement is observed on the sample surface. The atomic resolution topographic images at T ≈ 77 K show direct evidence of 2x2 commensurate superstructures. Fourier transform of the topographic images provide clear evidence for the superstructures. A series of other different reconstructions (3x3, √3x√3 etc.) are also observed. The interatomic distance on the surface is found to be ≈ 0.39 nm. The local STS measurements carried out on the surface reveal a gap-like feature which sustains upto T_cdw. Above T_cdw the DOS around fermi energy significantly filled up, ensuring the closure of the gap. In conclusion, we can say that our STS results on differently reconstructed surfaces at T<T_cdw may provide important insights for electronic states of Cu_2-xTe close to the Fermi level.



References:

¹M. D. Johannes and I. I. Mazin, Phys. Rev. B 77, 165135 (2008).

²W. Gao et al., Chin. Phys. Lett. 40, 017101 (2023).

³S. Liu et al., Phys. Rev. B 103, 115127 (2021).