Electronic Coherence Evolution at the Nearly Commensurate Incommensurate CDW Boundary of 1T-TaS2

Abstract

Transition metal dichalcogenides host a variety of charge density wave phases that couple lattice, charge, and correlation effects. In 1T-TaS2, the commensurate and nearly commensurate states are well characterized, yet the transition near 350 K into the incommensurate phase has lacked direct momentum resolved insight. Here we use temperature dependent angle resolved photoemission spectroscopy to track the electronic structure across this transition. We observe a suppression of quasiparticle spectral weight at the Brillouin zone center, coincident with the transport anomaly, but without clear evidence of a full band gap opening. The transition appears to involve momentum dependent redistribution of spectral weight, consistent with a loss of coherence that reshapes the Fermi surface while leaving conduction dispersions largely intact. These results suggest that the nearly commensurate incommensurate transition may not align with a conventional metal insulator transition picture, but rather as an electronic reconstruction driven by loss of coherence. Our work provides new microscopic insight into the resistivity anomaly near room temperature and may guide design principles for collective electronic switching in Transition metal dichalcogenides.

Figures (5)

• Figure 1: (a) Schematic of the undistorted trigonal (normal) phase. (b) Star-of-David distortion representative of the C-CDW phase. (c) Schematic phase diagram showing the evolution of CDW phases—C-CDW, NC-CDW, and IC-CDW—as a function of temperature. (d) Temperature-dependent resistivity measured during cooling and warming, displaying hysteresis between 180 K and 240 K, characteristic of the first-order NC–C transition. Inset: LEED pattern obtained at 300 K with 50 eV electrons, showing sharp diffraction spots from the bulk crystal lattice together with additional satellite peaks arising from the commensurate CDW modulation. The red hexagon outlines the primary hexagonal diffraction from the bulk lattice, while the blue hexagon highlights the CDW-induced $\sqrt{13} \times \sqrt{13}$ superlattice associated with the Star-of-David reconstruction, demonstrating the well-ordered CDW state and high crystal quality. (e) Magnified view of the resistivity near 350 K, where the slope change supports the presence of a CDW-related structural or electronic transition.
• Figure 2: ARPES measurements along the high-symmetry $M$–$\Gamma$–$M$ direction reveal the evolution of low-energy electronic structure from 300 K to 370 K. The data were acquired using 92 eV photon energies. At 300 K, the spectrum shows two dispersive bands and a ZCB (highlighted by the yellow dashed square), with local energy gaps indicative of periodic lattice distortions (green dashed square). As the temperature crosses 350 K, these energy gaps nearly diminish, and ZCB loses spectral weight. The color bar represents the ARPES intensity with endpoints marking high (H) and low (L) spectral weigh.
• Figure 3: (a) Temperature dependent EDCs at the $\Gamma$ point. The quasiparticle peak associated with ZCB is suppressed above 350 K. (b) Differential EDCs obtained by subtracting the 370 K spectrum, highlighting the disappearance of ZCB at higher temperatures. Spectra are vertically offset for clarity. (c) Temperature dependent MDCs at $E_F$ showing the development of three distinct peaks (marked by green lines) above 350 K, indicative of an electronic structure reorganization. (d) Temperature-dependent MDCs at a binding energy of 135 meV, demonstrating the disappearance of the central peak across the transition. Red lines denote the three bands. (e-f) ARPES intensity maps at 300 K and 370 K, respectively, along the $M$–$\Gamma$–$M$ direction. The color bar represents the ARPES intensity with endpoints marking high (H) and low (L) spectral weigh. (g) EDCs extracted along the yellow and cyan dashed lines shown in (e) and (f), respectively. Vertical black lines in (g) mark the shoulder peaks arising from CDW induced band hybridization. MDCs and EDCs in (c), (d), and g have been normalized to the same height to emphasize temperature-induced changes. EDCs and MDC are integrated within 0.1 Å$^{-1}$ and 20 meV, respectively. All data were acquired using 92 eV photon energies.
• Figure 4: (a), Density functional theory band structure of the normal phase. Temperature is accounted for by applying a Fermi–Dirac smearing of $\approx 0.00285$ Ry, corresponding to 450 K. (b), (c), Schematic representation of the conduction band near $\Gamma$ in the NC-CDW and IC-CDW phases, respectively. (d), ARPES constant-energy maps at $E_F$ (top row) and at 0.13 eV binding energy (bottom row), measured at 270 K and 370 K. The data were acquired using 92 eV photon energies. At 270 K, the $\Gamma$-centered band (ZCB like feature, marked by arrows) contributes strong spectral weight at $E_F$, whereas at 370 K this weight is strongly reduced and the feature appears only as a weak remnant. This temperature-driven reconstruction reflects the loss of quasiparticle coherence at the NC–IC transition while maintaining continuity with the normal-phase $\Gamma$ conduction state. The color bar represents the ARPES intensity with endpoints marking high (H) and low (L) spectral weigh.
• Figure 5: (a) Schematic of a single TaS$_2$ layer illustrating the Star-of-David distortion. Only Ta atoms are shown; S atoms are omitted. Arrows indicate in-plane radial displacements from the undistorted lattice (not to scale). The magenta rhombus denotes the commensurate CDW unit cell. The $\sqrt{13}\times\sqrt{13}$ superlattice contains three inequivalent Ta sites, labeled 'a', 'b', and 'c', within the Star-of-David cluster. (b) Large-area STM topography (15 nm $\times$ 15 nm) at Spot 1 showing a well-ordered lattice modulation. (c) High-resolution STM image (9 nm $\times$ 9 nm) at Spot 2 revealing enhanced contrast variations. The magenta rhombus marks a representative atomic arrangement, outlining the $\sqrt{13} \times \sqrt{13}$ CDW supercell with side length $\approx 12$ Å. The red line indicates the line profile in (d). STM images emphasize that the observed maxima correspond to the CDW superstructure. (d) Height profile along the red line in (c), showing atomic-scale corrugations of a few hundred picometres. STM data in (b) and (c) were acquired at different surface locations, highlighting spatial variations. Sample bias and tunneling current were 75 mV and 200 pA, respectively. Although the images in (b-c) were obtained at different surface locations, they were selected to illustrate the typical range of local CDW ordering observed in our measurements. The contrast difference therefore reflects representative spatial variations rather than imaging conditions.