Ultrafast charge ordering by self-amplified exciton–phonon dynamics in TiSe2

Abstract

The origin of charge density waves (CDWs) in TiSe has long been debated, mainly due to the difficulties in identifying the timescales of the excitonic pairing and electron–phonon coupling (EPC). Without a time-resolved and microscopic mechanism, one has to assume simultaneous appearance of CDW and periodic lattice distortions (PLD). Here, we accomplish a complete separation of ultrafast exciton and PLD dynamics and unravel their interplay in our real-time time-dependent density functional theory simulations. We find that laser pulses knock off the exciton order and induce a homogeneous bonding–antibonding transition in the initial 20 fs, then the weakened electronic order triggers ionic movements antiparallel to the original PLD. The EPC comes into play after the initial 20 fs, and the two processes mutually amplify each other leading to a complete inversion of CDW ordering. The self-amplified dynamics reproduces the evolution of band structures in agreement with photoemission experiments. Hence we resolve the key processes in the initial dynamics of CDWs that help elucidate the underlying mechanism.

Attractive Coulomb interaction, temperature-dependent hybridization, and natural circular dichroism in 1T−TiSe2

“…The results showed that the PLD and excitonic orders are coupled in 1T-TiSe 2 , consistent with theoretical results which invoked a cooperative J-T effect and excitonic order [22]. In fact, in a very recent theoretical study using real-time time-dependent density functional calculations, it was concluded that the excitonic order and PLD mutually amplify each other, resulting in the CDW transition [23].…”

First‐principles dynamics of photoexcited molecules and materials towards a quantum description

“…The major disagreement between the two mechanisms previously proposed: electron–EPC and excitonic paring, lies on the explicit roles of periodic lattice distortions (PLD) in forming the CDW. We adopt the newly developed rt‐TDDFT method to perform quantum photoexcitation simulation of TiSe 2 under laser pulses, in order to specify a detailed atomic mechanism for the ultrafast CDW dynamics 39 …”

“…The laser pulse melts charge order within 20 fs, producing the forces that trigger the ionic movements. The self‐amplified dynamics is assisted by electron–phonon couplings after initial excitation (Reprinted with permission from Reference 39. Copyright 2019 Springer Nature)…”

Electronic and crystal structure of bi-intercalated titanium diselenide CuxNiyTiSe2

“…Titanium dichalcogenides TiCh 2 (Ch = S, Se, Te) have been studied for a long time. [1][2][3][4][5][6] The intercalation of 3d transition metals into TiCh 2 alters significantly the physical properties and electronic structure of these materials. [7][8][9][10] To date, the effect of intercalation on the electronic structure of layered transition metal dichalcogenides is well understood.…”