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

TL;DR

This study uses femtosecond optical spectroscopy to observe rapid formation of charge-density-wave correlations in quasi-two-dimensional dichalcogenides, revealing inhomogeneous states that evolve into long-range order at low temperatures.

Contribution

It provides direct femtosecond-scale observations of gap formation and charge-density-wave correlations, highlighting inhomogeneous fluctuating states in 1T-TaS2 and 2H-TaSe2.

Findings

Large gap observed on femtosecond timescale

Slower relaxation for excitations below the gap

Inhomogeneous fluctuating states precede long-range order

Abstract

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 timescale, associated with the formation of various degrees of CDW order. In common with superconducting cuprates, excitations with energies less than the full gap show much slower relaxation. This separation of timescales cannot be explained in a quasi-2D 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.