Non-thermal separation of electronic and structural orders in a persisting charge density wave

Abstract

3Spontaneous symmetry breaking gives rise to a new quantum ground state featuring characteristic low-energy elementary excitations 3,11,14,[18][19][20][21][22] Ultrashort pulses in the terahertz (1 THz = 10 12 Hz) range have been used to trace electronic order via direct coupling to such excitations 22,23 . We demonstrate that THz pulses may simultaneously also track the crystalline order during an ultrafast phase transition.This idea is tested in a prominent reference system, 1T-TiSe 2 . Within the family of layered transition-metal dichalcogenides, this material has attracted special attention: Upon cooling below T c ≈ 200 K, it undergoes a transition into a commensurate CDW accompanied by the formation of a structural (2×2×2) superlattice 21 (Fig. 1a). In its high-temperature phase, TiSe 2 is a semimetal 20 with electron and hole pockets at the L and  points of the Brillouin zone, respectively 15,24 (Fig. 1b). The spatial reconstruction due to the CDW maps these two points on top of each other and leads to the partial opening of an electronic energy gap as well as a dramatic reduction of the density of free charge carriers 20 (Fig. 1b). Superconductivity emerges when the CDW is suppressed, e.g. by Cu intercalation 7 or pressure 25 . This discovery as well as novel chiral properties 26 have intensified the interest in the nature of the CDW in 1T-TiSe 2 . Yet, the microscopic mechanisms remain elusive 24,[27][28][29] . A first hypothesis assumes electron-phonon coupling based on a Jahn-Teller effect as the driving force 27 . A competing model suggests that the transition is purely electronically driven 24,28 . Coulomb attraction may render the system unstable against the formation of excitons between the electron-and hole-like Fermi pockets, leading to lattice deformation with the corresponding wave vector. Combinations of the two scenarios have also been proposed 29 . Time-resolved x-ray diffraction 16 and photoemission 10,15 experiments have separately tracked the dynamics of either structural or electronic orders. 4Evidence for both excitonic and phononic contributions was obtained in this way, leaving a controversial picture.Here we disentangle the two coupled components of the CDW order parameter by simultaneously tracing the ultrafast THz response of PLD-related phonons and electronic conductivity while a femtosecond pulse selectively melts the excitonic order. Our data reveal a transient phase in which the PLD persists in the absence of excitonic correlations. A quantummechanical theory 29 corroborates our conclusions.In TiSe 2 , the transition to the CDW ordered phase modifies the low-frequency optical response in three distinct ways: (i) The CDW-induced energy gap introduces a broad single- (Fig. 1d). Above T c , we observe a single TO phonon resonance at 17 meV. Below T c , back-folding of the uppermost acoustic branch from the L to the  point 21 yields an additional IR-active in-plane mode at 19 meV. The weaker peak at 22 meV likely originates from a folded optical branch at the M point 21 . 5W...

Superconductivity and hybrid soft modes inTiSe2

“…The presence and importance of excitons on the CDW order has been extensively discussed for the undoped compound 1T-TiSe 2 at ambient pressure [11,[14][15][16][43][44][45][46][47]. Below, we demonstrate theoretically that the pressure dependence of hybridazation between exciton and phonon modes can explain our experimental observations throughout the full , phase diagram.…”

“…Consistent with this observation, signatures of contributions to the CDW formation arising from both phonon-mediated and Coulomb interactions have recently been identified in time-resolved experiments. [43] Intercalation with Cu atoms shifts the chemical potential of 1T-TiSe 2 by 0.1 eV. [53,54] Since the circumstances in the pristine material are nearly ideal for the formation of excitons, they deteriorate quickly upon intercalation, leaving a dominant role for the phonons in the CDW formation in this part of the phase diagram.…”

Manipulating femtosecond spin-orbit torques with laser pulse sequences to control magnetic memory states and ringing

“…Coherent dynamics of superconducting order parameters are now beginning to be studied in condensed matter systems, 67,68 and an analogy to the magnetic order parameter studied here is clear. Other examples include quantum femtosecond magnetism in strongly-correlated manganites, 1,4 photon-dressed Floquet states in topological insulators, 69 or the existence of non-equilibrium phases in charge-density-wave 47 and other correlated systems. Femtosecond nonlinear optical spectroscopy offers the time resolution needed to disentangle different order parameters that are strongly coupled in the ground state, based on their different dynamics after "sudden" departure from equilibium.…”

“…Femtosecond nonlinear optical spectroscopy offers the time resolution needed to disentangle different order parameters that are strongly coupled in the ground state, based on their different dynamics after "sudden" departure from equilibium. 47,48 Multi-pulse switching protocols based on non-adiabatic quantum excitations can control non-equilibrium phase transitions, by initiating phase dynamics in a controllable way. fs spin-orbit torque as the net spin of the hole Fermi sea bath adjusts to the new non-equilibrium direction of S(t).…”

Nonthermal switching of charge order: Dynamical slowing down and optimal control

“…If the CDW originates from a Peierls-type lattice distortion, the lightinduced melting of the order is accompanied by coherent phonon excitations [14,[16][17][18]21,22]. In contrast, a CDW induced by electron-electron (e-e) correlations can be melted on the femtosecond timescale [10,[23][24][25], and nonthermal melting has been observed [10,25]. These developments have stimulated intensive theoretical research of the laser-driven CDW dynamics [19,[26][27][28][29][30][31].…”