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. 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, 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. Numerous time-resolved experiments have been performed on CDWs, probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred. Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction to study the quasi two-dimensional CDW system 1T-TaS(2). 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 ∼0.1 Å, is suppressed by about 20% on a timescale (∼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 (∼350 femtoseconds) and are followed by fast recovery of the CDW (∼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.

The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields or pressure. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe(2)As(2) following excitation with a femtosecond optical pulse. When starting in the low-temperature ground state, optical excitation results in a melting of the SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced when we excite the normal state above the transition temperature. Very surprisingly, the transient ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a pronounced spin-phonon coupling in pnictides that supports rapid development of a macroscopic order on small vibrational displacement even without breaking the symmetry of the crystal.

We present the first systematic studies of the photoinduced phase transition from the ground charge density wave (CDW) state to the normal metallic state in the prototype quasi-1D CDW system K0.3MoO3. Ultrafast nonthermal 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 subpicosecond time scale when melting and subsequent initial recovery of the electronic order takes place the lattice remains unperturbed.

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.

We report on the high resolution studies of the temperature (T) dependence of the q = 0 phonon spectrum in the quasi one-dimensional charge density wave (CDW) compound K0.3MoO3 utilizing time-resolved optical spectroscopy. Numerous modes that appear below T c show pronounced Tdependences of their amplitudes, frequencies and dampings. Utilizing the time-dependent GinzburgLandau theory we show that these modes result from linear coupling of the electronic part of the order parameter to the 2kF phonons, while the (electronic) CDW amplitude mode is overdamped.Femtosecond optical spectroscopy is becoming an important tool for investigation of the so called strongly correlated systems due to its intrinsic ability to determine the interaction strengths between various degrees of freedom which lead to fascinating phenomena like superconductivity or giant magnetoresistance. Low dimensional charge density wave (CDW) systems, with their inherently multi-component order parameter (modulation of carrier density is accompanied by the modulation of the underlying lattice) present no exception. In the past decade or so various one and two dimensional CDWs have been studied by time-resolved optical [1][2][3][4][5][6] as well as photoemission [7, 8] techniques. The initial focus of research was in identifying various components in the observed photoinduced transients with the corresponding ones obtained by standard time-averaging spectroscopic techniques, as well as in coherent control of the collective modes [9]. Recently it was shown that photoexcitation with an intense optical pulse can nonthermally drive the phase transition from the low temperature CDW state to a metastable state, characterized by a suppressed carrier modulation with the lattice remaining nearly frozen [6]. This observation has an important implication for the understanding of ultrafast relaxation processes in this class of materials, and, as we will show, for the general understanding of the cooperative phenomena leading to the appearance of the CDW state and the nature of their collective excitations.In this Letter we present high resolution studies of Tdependent time-resolved reflectivity dynamics in a prototype quasi-1D CDW material K 0.3 MoO 3 . The high sensitivity achieved in this experiment enabled us to measure the T-evolution of the low frequency phonons with unprecedented resolution. We were able to show that not only the 1.68 THz (57 cm −1 ) mode, that is commonly assigned to the collective amplitude mode (AM) of the CDW, shows softening upon increasing T towards T c = 183 K, the phase transition to the normal metallic state. Qualitatively the same softening is observed also for a number of phonon modes that appear below T c . The frequencies of these modes correspond well to the phonon frequencies at the 2k F modulation vector as observed by neutron experiments [10,11], and are shown to result from the linear coupling of the electronic part of the order parameter (EOP), ∆, to 2k F phonons. Utilizing the time-dependent Ginzburg-Landau (TDGL) mod...