Visualization of Tunable Electronic Structure of Monolayer TaIrTe$_4$

TL;DR

This work directly visualizes the electronic structure of a hole-doped, charge-neutral, and electron-doped monolayer TaIrTe4 using in-operando microARPES on graphene-encapsulated devices, complemented by SOC-enabled DFT-HSE calculations. The study finds an insulating ground state consistent with a QSHI, and reveals a non-rigid-band response to doping where valence bands renormalize before the conduction band fills, with Cs decoration introducing a graphene-derived electron pocket. The combination of high-spatial-resolution ARPES and first-principles modeling demonstrates that induced charges reshape the band topology rather than simply rigidly shifting bands, highlighting how gating and adsorption can tune correlated topological phases in 2D TTMCs. The approach establishes a framework for probing and controlling topological and correlated states in air-sensitive layered materials, with implications for designing tunable topological devices.

Abstract

Monolayer TaIrTe$_4$ has emerged as an attractive material platform to study intriguing phenomena related to topology and strong electron correlations. Recently, strong interactions have been demonstrated to induce strain and dielectric screening tunable topological phases such as quantum spin Hall insulator (QSHI), trivial insulator, higher-order topological insulator, and metallic phase, in the ground state of monolayer TaIrTe$_4$. Moreover, charge dosing has been demonstrated to convert the QSHI into a dual QSHI state. Although the band structure of monolayer TaIrTe$_4$ is central to interpreting its topological phases in transport experiments, direct experimental access to its intrinsic electronic structure has so far remained elusive. Here we report direct measurements of the monolayer TaIrTe$_4$ band structure using spatially resolved micro-angle-resolved photoemission spectroscopy (microARPES) with micrometre-scale resolution. The observed dispersions show quantitative agreement with density functional theory calculations using the Heyd-Scuseria-Ernzerhof hybrid functional, establishing the insulating ground state and revealing no evidence for strong electronic correlations. We further uncover a pronounced electron-hole asymmetry in the doping response. Whereas hole doping is readily induced by electrostatic gating, attempts to introduce electrons via gating or alkali metal deposition do not yield a rigid upward shift of the Fermi level. Fractional charge calculations demonstrate that added electrons instead drive band renormalization and shrink the band gap. Taken together, our experimental and theoretical results identify the microscopic mechanism by which induced charges reshape the band topology of monolayer TaIrTe$_4$, showing that doping can fundamentally alter the electronic structure beyond the rigid band behaviour that is typically assumed.

Figures (11)

• Figure 1: Overview of the in operando microARPES setup of a monolayer TaIrTe$_4$ micro-electronic device. a. Top view schematic of TaIrTe$_4$ device integrated with ARPES measurements. b. Crystallographic orientation of the monolayer TaIrTe$_4$ relative to graphene layer in the measured device. c. Fermi surface of the TaIrTe$_4$ with its Brillouin zone and the graphene Brillouin zone shown. d. Isoenergy surface of the TaIrTe$_4$ at $\mathrm{-0.3~eV}$. Asterisk marks indicate the calculated bandstructure from DFT calculation with integration energy window of $\mathrm{0.05~eV}$ (purple lines). e. TaIrTe$_4$ ARPES dispersion along the $\mathrm{\bar{Y}\bar{\Gamma}}$ line, f. the $\mathrm{\bar{\Gamma}\bar{X}}$ line, and g. the $\mathrm{\bar{Y}\bar{S}}$ line with the respective calculated bandstructures (blue lines) showing good agreement. The momentum scale-bar is $0.2\,\text{\AA}$.
• Figure 2: Evolution of the gate-dependent band structure of graphene and monolayer TaIrTe$_4$ Gate dependent bandstructure of a. Graphene, b. TaIrTe$_4$$\mathrm{\bar{X}\bar{\Gamma}\bar{X}}$ cut, and c. TaIrTe$_4$$\mathrm{\bar{S}\bar{Y}\bar{S}}$ cut with (i) as $-7.8\mathrm{V}$, (ii) as zero bias $\mathrm{0V}$, and (iii) as $\mathrm{8V}$. The symbols * and ** indicate the two hole pocket bands. In general, upon increasing the gate voltage from negative to positive, the band shifts from the hole-doped to the electron-doped regime. The momentum scale-bar is $0.2\text{\AA}$.
• Figure 3: Cesium dosed bandstructure of graphene and TaIrTe$_4$ revealing a cesium induced electron pocket around the $\mathrm{\Gamma}$ point. a. Fermi level after cesium dosing revealing the Fermi surface of the cesium induced electron pocket decorating the pristine TaIrTe$_4$ band, and the isoenergy of the cesium dosed system at $\mathrm{-0.4~eV}$ from the new Fermi level. Cesium dosed bandstructure of b. graphene, c. TaIrTe$_4$$\mathrm{\bar{X}\bar{\Gamma}\bar{X}}$ cut from the first Brillouin zone and second Brillouin zone, d. TaIrTe$_4$$\mathrm{\bar{S}\bar{Y}\bar{S}}$ cut. The graphene's Fermi level rises significantly as compared to the Fermi level of the TaIrTe$_4$. The momentum scale-bar is $0.2\,\text{\AA}$
• Figure 4: Simulated band structure of cesium decorated graphene and charged monolayer TaIrTe$_4$ a. Cesium decorating graphene with a $2\times2$ pattern. The pink rectangle is the unit cell of TaIrTe$_4$ (not included in the simulations), which is not commensurate with the cesium dosed graphene unit cells. b. HSE bandstructure of the cesium decorated graphene (without the TaIrTe$_4$) showing the emergence of cesium-derived electron pockets at the $\Gamma$ and M point, with a significantly smaller spectral weight in the projected band structure. c. HSE band structures of monolayer TaIrTe$_4$ with varying fractional charges showing band renormalization followed by band gap closing. The corresponding additional electron density are 0.03,0.05 and 0.25 e/unit cell.
• Figure S1: Optical and photoemission characterization of monolayer TaIrTe$_4$ a. Optical image of the monolayer TaIrTe$_4$ flake; the inset shows an exfoliated TaIrTe$_4$ flake with different thicknesses from a separate region. b. Core-level photoemission spectrum of the constituent elements in the TaIrTe$_4$ device. c. Optical image of the TaIrTe$_4$ device, with different layers of the device highlighted with colored dashed lines. The orange circle marks the spatial location on the device where all the band structure measurements were performed. The top and bottom gray regions correspond to the gate and ground electrodes, respectively, and the inverted L-shaped purple region denotes the SiO$_2$/Si substrate. d. Photoemission emission map of the device.