The experimental progress of Yan Zhang’s group and their collaborators about the band insulator to Mott insulator transition in 1T-TaS2

Transition-metal di-chalcogenides are layered quasi two-dimension materials that not only show prominent potentials for making ultra-thin and flexible devices, but also exhibit rich electronic phases with unique properties. 1T-TaS2is one prominent example. It is structurally undistorted and electronically metallic at high temperatures. With cooling, it sequentially enters a variety of charge-density-wave (CDW) phases, and eventually ends up in an insulating state of mysterious origin. Some consider this state to be a conventional band insulator with interlayer stacking order, yet others attribute it to Mott physics that may further support a quantum spin liquid ground state of the emergent spin-1/2 degrees of freedom.

The Yan Zhang’s group and their collaborators in International Center for Quantum Materials (ICQM) at Peking University studied the phase transition of 1T-TaS2using high-resolution angle-resolved photoemission spectroscopy (ARPES) and X-Ray diffraction. They find that, at low temperatures, the band dispersion and diffraction peaks show 2p/2c and 2c periodicity, which indicates that the ground state of 1T-TaS2is a band insulator with interlayer dimerization.

More intriguingly, utilizing the self-built high-resolution ARPES system, Yan Zhang’s group succeeded in measuring the evolution of electronic structure through the phase transition with unpreceded precision. The detailed temperature dependent data resolve a Mott insulating state, which only exists in a narrow temperature window.

Fig 1: Temperature dependent XRD data.aXRD intensity of 1T-TaS2measured along c axis at 120 and 300 K. Insets show the emergence of half integer peaks using 1100 times expanded scales.bTemperature dependence of the (0, 0, 7/2) and (0, 0, 4) peaks upon heating.cSchematic illustration of the interlayer dimerization.

Fig 2: ARPES spectrum and schematic depictions of band evolution.aTemperature dependence of energy distribution curves (EDCs) taken at the Γ point upon heating.bThe merged image of the data ina.c, dBand dispersion along the kzdirection taken in the I and C-CDW phases respectively.e, fSchematic illustration of the band dispersions along kzdirection in different electronic phases.

Further analysis show that the in-plane hopping (t//), the on-site Coulomb repulsion (U), and interlayer hopping (t) share similar energy scales in 1T-TaS2. At low temperature, the system is a band insulator dominated byt. Upon heating, U become prominent. The system undergoes a band-insulator-to-Mott-insulator transition and enters the intermediate Mott insulating phase. At high temperature,t//dominates and the metallic phase recovers. Our observation of the hitherto unresolved intermediate Mott insulating state refutes the idea of searching for quantum magnetism in 1T-TaS2only at low temperatures, and it highlights the competition between on-site Column repulsion and interlayer hopping as a crucial aspect for understanding the material’s electronic properties.

This work has been published in Nature Communications on August 24, 2020 [Nature Communications 11, 4215 (2020),]. Yan Zhang at Peking University is the corresponding author. The first author of the paper is Yudi Wang, a doctoral student of Yan Zhang’s group in ICQM. The high quality single crystals are provided by Weiliang Yao from Yuan Li’s group in ICQM. Photon energy dependent experiments were conducted at BL13U beamline in National Synchrotron Radiation Laboratory (NSRL) with the help from Shengtao Cui.

This work was supported by National Natural Science Foundation of China and the National Key Research and Development Program of China.