Ultrafast Melting of Charge-Density-Wave Order and a Transient Metastable State in 4Hb-TaS2

Abstract

4Hb-TaS2 is a natural heterostructure of correlated (T) and superconducting (H) layers. Using time- and angle-resolved photoemission spectroscopy, we observe a narrow feature slightly above the Fermi level on the T-layer termination, corresponding to the band edge of a charge-density-wave (CDW)-gapped electron-like dispersion. Above a characteristic incident fluence, the fitted CDW gap becomes indistinguishable from zero, while enhanced occupied-state spectral weight persists for tens of picoseconds after hot carriers decay. Together, these spectral and dynamical signatures identify a transient metastable electronic state. Its much shorter lifetime than that of the persistent hidden state in bulk 1T-TaS2 is consistent with a role for interrupted direct T–T stacking, although relaxation channels involving the H layers may also contribute. These results highlight the influence of the interlayer environment on nonequilibrium-state stability and establish 4Hb-TaS2 as a platform for ultrafast control of correlated states.