Femtosecond low-energy dynamics of a charge density wave in TiSe2

Summary form only given. The ultrafast interplay between charge and lattice in correlated electron systems often leads to spontaneous collective order. The charge density wave (CDW) in 1T-TiSe<;sub>2<;/sub> (T<;sub>c<;/sub> ~ 200 K) represents a prominent reference. The microscopic driving mechanism has been a topic of a long-standing debate, promoting a Jahn-Teller effect, an excitonic origin, or a crossover of both. Time-resolved optical reflectivity, x-ray diffraction [1] and photo-emission [2] studies offer elegant means to separate various degrees of freedom via their intrinsic dynamics.Here we track ultrafast optical melting and recovery of a CDW in 1T-TiSe2 in a qualitatively new way via its low-energy collective plasma response. Single-cycle multi-THz pulses and field-sensitive electro-optic detection allow us to directly map out the evolution of the energy loss function -Im(1/ε) in the mid infrared, while the system is excited by a 12-fs optical pump [3]. Poles of -Im(1/ε) indicate longitudinal electromagnetic eigenmodes. For temperatures T <; Tc, we observe a sharp plasmon resonance at a frequency of ωp/2π = 14.5 THz (Fig. 1(a)), implying a free carrier density of n0 = 1.5 × 1020/cm3. Its narrow width (scattering time τ = 0.2 ps) is characteristic of the CDW state [4]. The photoinduced dynamics depends sensitively on the pump fluence Φ. Below a critical value Φth = 0.1 mJ/cm2, femtosecond photodoping rapidly up shifts the plasma resonance followed by a full relaxation after tD = 1 ps (Fig. 1(a) and (c)). The line width is only weakly affected (Fig. 1(d)), confirming that the charge order remains intact while the pump pulse injects additional carriers. Accordingly we find (not shown) that the dynamics is weakly modulated with the coherent A1g amplitude mode of the CDW at 3.5 THz. In contrast, a pump fluence of 400 μJ/cm2 fully destroys the density wa- e (Fig. 1(b)): The carrier density is increased by one order of magnitude shifting the plasma frequency above 50 THz while the scattering rate rises by a factor of 4, indicating the transition to the normal phase [4]. The subsequent relaxation of the free carrier density proceeds on the same timescale observed for low Φ, whereas the evolution of τ follows a very unusual trace (Fig. 1(d)): τ remains well below 60 fs up to a delay time tD = 1.2 ps when the line width suddenly collapses within 0.2 ps due to ultrafast locking to the CDW phase. Importantly the buildup of the ordered phase out of the normal state occurs only after the carrier density has decayed close to its equilibrium value. This scenario attests to a high susceptibility of the formation process of the charge order to the excess carrier density, as expected within an exciton-based model [5]. In contrast a pre-existing charge-ordered state appears to be more robust against carrier injection. Our unprecedented low-energy view of ultrafast melting and recovery of a charge order provides an important new benchmark for state-of-the-art models of this fascinating class of phenomena.