Femtosecond snapshots of gap-forming charge-density-wave correlations in quasi-two-dimensional dichalcogenides 1T−TaS2 and 2H−TaSe2

Time-resolved optical spectroscopy of collective and single-particle excitations of 1T-TaS₂ and 2H-TaSe₂ reveals the presence of a large gap in the excitation spectrum on the femtosecond time scale, associated with the formation of various degrees of charge-density-wave order. In common with superconducting cuprates, excitations with energies less than the full gap show much slower relaxation. This separation of time scales cannot be explained in a quasi-two-dimensional Fermi-liquid picture with an anisotropic gap but rather suggests the formation of a fluctuating spatially inhomogeneous state eventually forming a long-range ordered state at low temperatures.