Ultrafast Triggering of Insulator–Metal Transition in Two-Dimensional VSe₂

Abstract

The transition-metal dichalcogenide VSe2 exhibits an increased charge density wave transition temperature and an emerging insulating phase when thinned to a single layer. Here, we investigate the interplay of electronic and lattice degrees of freedom that underpin these phases in single-layer VSe2 using ultrafast pump–probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap, which we disentangle from the ensuing hot carrier dynamics by fitting a model spectral function to the time-dependent photoemission intensity. This procedure leads to an estimated time scale of 480 fs for the closure of the gap, which suggests that the phase transition in single-layer VSe2 is driven by electron–lattice interactions rather than by Mott-like electronic effects. The ultrafast optical switching of these interactions in SL VSe2 demonstrates the potential for controlling phase transitions in 2D materials with light.

Coexisting charge-ordered states with distinct driving mechanisms in monolayer VSe$_2$

“…The relative importance of the unscreened interactions is further enhanced within flat bands associated with a van Hove singularity, such as at the near-Γ region in ML-VSe 2 (Fig. 4) [23,30,33,34]. Indeed, the measured linewidth, or self-energy, of the band near E F is much larger than the experimental resolution [30], confirming the presence of strong electronic correlations [40].…”

“…The DFT band along Γ − M also appears more dispersive than that in ARPES, while along Γ − K the DFT band is higher (50-200 meV) than the magnitude expected from the high photoelectron count around Γ [23-26, 30, 33]. These discrepancies are likely due to the inability to duly account for electronic correlations [34]. As a result, our ab initio calculations may not capture the electronic structure near E F with sufficient quantitative accuracy to describe CDW energetics.…”

“…Indeed, such correlation-driven CDWs have been predicted to exist in TMDCs [8], including in ML-VSe 2 [35], and are consistent with the unusual characteristics of the Q 2 CDW. Crucially, a correlation-driven mechanism for the Q 2 CDW offers the only viable explanation of its prevalence over a well-nested counterpart (Q 1 ), and the complete gapping of the FS [23,30,33,34], despite the absence of any associated feature in susceptibility calculations based on models of non-interacting electrons.…”

Observation of pressure induced charge density wave order and eightfold structure in bulk VSe2

“…Layered 1 T -VSe 2 , a typical transition metal dichalcogenide (TMDCs), is an excellent candidate material for the next-generation electronic application and tunable optoelectronic device 1 – 5 . It owns many novel physical properties and exhibits a rich variety of correlated electronic phenomena, such as charge-density-wave (CDW) state 6 , high-pressure superconducting 7 , photoinduced insulator–metal transition 8 , and possible ferromagnetic order in its monolayer form 9 – 11 . Among them, the CDW is a low-temperature condensed phase that is featured by periodic modulation of charge densities accompanied with spontaneous lattice distortion 12 .…”

Switching of the electron-phonon interaction in 1T−VSe2 assisted by hot carriers

“…(k ) is the bare dispersion, is the real part, and is the imaginary part of the electronic self-energy. This framework is commonly used to analyze static ARPES data [32,[34][35][36], but here we explore its utility to describe quasithermal TR-ARPES spectra. To simulate the measured 2D image of I (E bin , k ), we use the DFT bands overlaid in Figs.…”