Coherent Structural Dynamics of a Prototypical Charge-Density-Wave-to-Metal Transition

Abstract

Using femtosecond time-resolved x-ray diffraction, we directly monitor the coherent lattice dynamics through an ultrafast charge-density-wave-to-metal transition in the prototypical Peierls system K(0.3)MoO(3) over a wide range of relevant excitation fluences. While in the low fluence regime we directly follow the structural dynamics associated with the collective amplitude mode; for fluences above the melting threshold of the electronic density modulation we observe a transient recovery of the periodic lattice distortion. We can describe these structural dynamics as a motion along the coordinate of the Peierls distortion triggered by the prompt collapse of electronic order after photoexcitation. The results indicate that the dynamics of a structural symmetry-breaking transition are determined by a high-symmetry excited state potential energy surface distinct from that of the initial low-temperature state.

Ultrafast Formation of a Charge Density Wave State in 1T−TaS2 : Observation at Nanometer Scales Using Time-Resolved X-Ray Diffraction

“…A number of photo-induced phase transitions that have been achieved in CDW compounds correspond to a suppression of the CDW order, i.e. a transition between a CDW state and a metallic state free of any structural modulation [5,[9][10][11][12][13][14][15]. Among those, the photo-induced suppression of the CDW state in blue bronze was shown to involve a coherent motion of atoms along the normal coordinates of the CDW amplitude mode [5].…”

“…In the low fluence regime, laser pulses are known to coherently excite the amplitude mode of the NC-CDW [36][37][38], which results in periodic oscillations of the modulus of the order parameter p N C in time. This behavior can be modeled assuming a displacive excitation mechanism [5,39]. For t ≥ 0, one writes …”

“…At thermodynamic equilibrium, the corresponding many-body interactions lead to rich phase diagrams as a function of temperature, pressure or doping. Such compounds also display fascinating out-of-equilibrium physics, in the form of ultra-fast symmetry changes known as photoinduced phase transitions [4][5][6], and occurrence of new, transient states [6][7][8].Charge density wave (CDW) states are broken symmetry states of metals arising from electron-phonon interactions. They are characterized by a periodic modulation of both atomic positions and electron density.…”

“…Diffraction techniques are especially well adapted to study CDW compounds [5,10,12,14,15,22,[24][25][26][27][28][29]. Indeed, a structural modulation with wavevector q gives rise to satellite peaks located at positions ± q with respect to each regular lattice peak (Fig.…”

Photoinduced Enhancement of the Charge Density Wave Amplitude

“…This surprisingly long time scale indicates anharmonic phonon-phonon interaction as the dominant decay channel. Electron-hole pair excitation, which typically leads to strong damping of order-parameter oscillations in strongly correlated electron systems [32,40,41], is expected to be ineffective here because of the SDW gap in the electronic spectrum [42]. Because of ultrafast carrier cooling and recondensation in the lattice distortion potential, this gap will quickly reopen, even after a complete quench of the electronic order [17].…”