Realization of Coexisting Charge Density Wave and Quantum Spin/Anomalous Hall State in Monolayer NbTe₂

Abstract

The combination of nontrivial topology and charge density wave (CDW) has been proposed as a powerful resource for realizing novel quantum phenomena such as axion electrodynamics and the anomalous Hall effect. Hence, topological materials with CDW states attract great interest, yet they are still very rare, particularly in the 2D limit. Here, it is predicted that monolayer NbTe 2 in its high-symmetry 1T phase stabilized by anharmonicity at room temperature exhibits nontrivial topology sensitive to electronic interactions: it changes from a quantum spin Hall (QSH) state to a quantum anomalous Hall (QAH) state when the Hubbard potential U exceeds a critical value. At low temperature, a 4 × 4 CDW order emerges and coexists with the nontrivial topology. Meanwhile, the critical U increases because CDW reduces density of states at Fermi level. More interestingly, in contrast to the high-symmetry structure that actually is a topologically nontrivial metal, the CDW structure shows an insulating nontrivial gap either in the QSH or the QAH phase, indicating CDW is an effective means to modulate the topological state for developing new functions and devices. These discoveries establish NbTe 2 as a promising candidate to explore exotic quantum states at the confluence of nontrivial topology, electronic correlation, and CDW.

Realization of Multiple Charge-Density Waves in NbTe2 at the Monolayer Limit

“…Herein, we report the successful epitaxial growth of ML NbTe 2 on a bilayer graphene/SiC substrate. Using scanning tunneling microscopy/spectroscopy (STM/S), the triaxial 4 × 4 and uniaxial 4 × 1 CDW, which are absent in the bulk NbTe 2 , are found in the ML limit, in agreement with the calculations . For both phases, a correlation gap with remarkable magnitude (or an asymmetric line shape in the vicinity of Fermi level) and the intensity reversal of local density of state (DOS) maps at opposite biases are observed, which strongly support their CDW origin.…”

“…In conclusion, we reported the MBE growth and STM/S characterization of NbTe 2 thin films down to the ML limit. Multiple CDW orders are found, which are entirely different from the known bulk case and deviate from the recent theoretical predictions as well. ,, In particular, we first confirmed the 4 × 4 and 4 × 1 CDW predictions by thorough spectroscopic measurements. Moreover, we revealed unexpected CDW orders with larger length scales, including 28 × 28 phase and the 19 × 19 phase that is, surprisingly, the stable phase.…”

“…In a recent theoretical prediction, there are CDW orders forming in ML-NbTe 2 with diverse superperiodicities of 4 × 4, 4 × 1, 3 × 3, and 3 × 1. Our experimental observations of the superstructures deliver a partial agreement of the periodicities (i.e., 4 × 4 and 4 × 1) with the prediction.…”

“…It is a demanding task to be carried out in the future but is beyond the present scope of this paper. Despite the discrepancy in experiments and calculations, it is certain that the ML-NbTe 2 is endowed with rich and diversified electron/phonon correlation modes, which are greatly different from the bulk case. , In bulk NbTe 2 , the interlayer Te–Te interaction is known to be rather strong, which can even result in additional intralayer chalcogen-to-metal transfer (1/3e per unit), forming trimerized Nb ion chains. , When the thickness is down to a ML limit, the absent interlayer interaction and enhanced electron correlation may suppress the movement of electrons, leading to the emergence of new charge orders.…”

“…Therefore, the telluride layered materials could show dramatic thickness-dependent structural, electronic, magnetic, and topological phase transitions. − Recently, pioneering theoretical effort has predicted that the ML NbTe 2 may exhibit 4 × 4, 4 × 1, 3 × 3, and 3 × 1 CDW orders. The 4 × 4 phase is the most stable among them, though the differences in their formation energies are fairly small . However, despite successes in material fabrication, , the charge-density modulations in NbTe 2 at the ML limit remain experimentally unexplored.…”