Dynamics of photoinduced Charge Density Wave-metal phase transition in K0.3MoO3

We present first systematic studies of the photoinduced phase transition from the ground charge density wave (CDW) state to the normal metallic (M) state in the prototype quasi-1D CDW system K0.3MoO3. Ultrafast non-thermal CDW melting is achieved at the absorbed energy density that corresponds to the electronic energy difference between the metallic and CDW states. The results imply that on the sub-picosecond timescale when melting and subsequent initial recovery of the electronic order takes place the lattice remains unperturbed.Comment: Phys. Rev. Lett., accepted for publicatio

Gap-dependent quasiparticle dynamics and coherent acoustic phonons in parent iron pnictide CaFe2As2 across the spin density wave phase transition

We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165 K using femtosecond time-resolved pump-probe spectroscopy. The contributions in transient differential reflectivity arising from exponentially decaying QP relaxation and oscillatory coherent acoustic phonon mode show large variations in the vicinity of T_SDW. From the temperature-dependence of the QP recombination dynamics in the SDW phase, we evaluate a BCS-like temperature dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW, whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has been estimated from the linear temperature-dependence of the QP relaxation time. The long-wavelength coherent acoustic phonons with typical time-period of ~100 ps have been analyzed in the light of propagating strain pulse model providing important results for the optical constants, sounds velocity and the elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.Comment: Revised version (to appear as Full Paper in Journal of Physical Society of Japan (2013)); http://jpsj.ipap.jp/link?JPSJ/82/044715

Behaviour of neutral hydrogen atom density in the presence of a sample holder in a plasma reactor

International audienc

Dynamics of charge density wave order in the quasi one dimensional conductor (TaSe

Carrier and collective mode dynamics in the quasi one-dimensional charge density wave (CDW) system (TaSe4)2I have been investigated by means of time-resolved optical pump-probe spectroscopy. In the low excitation, linear, regime we focus on the temperature dependence of the collective amplitude modes, originating from linear coupling of the electronic modulation to phonons at qCDW. Numerous amplitude modes are observed, ranging from 100 GHz to several THz. The modes’ softening near Tc is rather weak, which could be related to strong decoupling of electronic and lattice subsystems. Alternatively, the data could be reconciled also in case the CDW phase transition is of the first-order type where a nearly constant order parameter below Tc would prevent softening. In the high excitation regime we investigated the energetics of the photoinduced CDW-normal phase transition. Similarly to the elaborately investigated one-dimensional CDW system K0.3MoO3 we observe two characteristic energy scales, related to melting the electronic modulation alone (100 meV per unit cell) and to the overall (electronic modulation and the periodic lattice distortion) collapse of the CDW (> 400 meV per unit cell). While the latter coincides with the thermal energy needed to heat the sample from 5 K above Tc the former is consistent with the mean field estimate for the electronic condensation energy, suggesting that the weak coupling description of the CDW in (TaSe4)2I is appropriate

Ultrafast Mid-infrared Spectroscopy of the Charge- and Spin-Ordered Nickelate La1.75Sr0.25NiO4

We present the first ultrafast mid-infrared study of charge and spin-ordered nickelates. A sub-picosecond modulation of the optical reflectivity is observed, indicating the filling and subsequent re-establishment of the pseudogap in the time-domain

Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale(1). The study of the interplay among these various degrees of freedom in strongly coupled electron-lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials(2), with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems(3-7). Numerous time-resolved experiments have been performed on CDWs(8-13), probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred(14). Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction(15) to study the quasi two-dimensional CDW system 1T-TaS2. Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of similar to 0.1 angstrom, is suppressed by about 20% on a timescale (similar to 250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron-phonon energy transfer (similar to 350 femtoseconds) and are followed by fast recovery of the CDW (similar to 4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems