Intense femtosecond (10(-15) s) light pulses can be used to transform electronic, magnetic and structural order in condensed-matter systems on timescales of electronic and atomic motion. This technique is particularly useful in the study and in the control of materials whose physical properties are governed by the interactions between multiple degrees of freedom. Time- and angle-resolved photoemission spectroscopy is in this context a direct and comprehensive, energy- and momentum-selective probe of the ultrafast processes that couple to the electronic degrees of freedom. Previously, the capability of such studies to access electron momentum space away from zero momentum was, however, restricted owing to limitations of the available probing photon energy. Here, using femtosecond extreme-ultraviolet pulses delivered by a high-harmonic-generation source, we use time- and angle-resolved photoemission spectroscopy to measure the photoinduced vaporization of a charge-ordered state in the potential excitonic insulator 1T-TiSe(2 )(refs 12, 13). By way of stroboscopic imaging of electronic band dispersions at large momentum, in the vicinity of the edge of the first Brillouin zone, we reveal that the collapse of atomic-scale periodic long-range order happens on a timescale as short as 20 femtoseconds. The surprisingly fast response of the system is assigned to screening by the transient generation of free charge carriers. Similar screening scenarios are likely to be relevant in other photoinduced solid-state transitions and may generally determine the response times. Moreover, as electron states with large momenta govern fundamental electronic properties in condensed matter systems, we anticipate that the experimental advance represented by the present study will be useful to study the ultrafast dynamics and microscopic mechanisms of electronic phenomena in a wide range of materials.

Distinguishing insulators by the dominant type of interaction is a central problem in condensed matter physics. Basic models include the Bloch-Wilson and the Peierls insulator due to electron-lattice interactions, the mott and the excitonic insulator caused by electron-electron interactions, and the Anderson insulator arising from electron-impurity interactions. In real materials, however, all the interactions are simultaneously present so that classification is often not straightforward. Here, we show that time-and angle-resolved photoemission spectroscopy can directly measure the melting times of electronic order parameters and thus identify-via systematic temporal discrimination of elementary electronic and structural processes-the dominant interaction. specifically, we resolve the debates about the nature of two peculiar charge-density-wave states in the family of transition-metal dichalcogenides, and show that Rb intercalated 1T-Tas 2 is a Peierls insulator and that the ultrafast response of 1T-Tise 2 is highly suggestive of an excitonic insulator.

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Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics in the charge-density-wave/Mott insulator 1T -TaS2. After strong photoexcitation, a prompt loss of charge order and subsequent fast equilibration dynamics of the electron-lattice system are observed. On the time scale of electron-phonon thermalization, about 1 ps, the system is driven across a phase transition from a long-range charge ordered state to a quasi-equilibrium state with domain-like short-range charge and lattice order. The experiment opens the way to study the nonequilibrium dynamics of condensed matter systems with full elemental, chemical, and atomic site selectivity.Femtosecond time-resolved spectroscopy has become a powerful tool in condensed matter research because it delivers direct dynamical information at the fundamental time scale of elementary electronic processes [1,2]. The method is particularly useful for complex materials, in which two or more of the lattice, charge, spin, and orbital degrees of freedom are strongly coupled. It allows to determine the nature and strength of the interactions between the degrees of freedom, to identify the dominant interactions, and thus to gain important insights into ground-state properties, thermally driven phase transitions, and, possibly, novel hidden phases [3][4][5][6].An exceedingly fertile ground for the combination of spectral selectivity and femtosecond time resolution has been found in materials, in which the charge and lattice degrees of freedom interact strongly to form a coupled charge-density-wave (CDW)/periodic-latticedistortion (PLD) ground state. The quasiparticle and collective mode dynamics of the CDW/PLD state, the finite electron-lattice coupling time, and the collapse of the electronic gap under strong excitation are now known [4][5][6][7][8][9]. Yet, direct dynamical information on the CDW itself, i.e., on the local charge order, is missing. Specifically, it is not clear how fast a CDW can melt and recondense after impulsive excitation. The present study provides this fundamental piece of information for the layered ref-Employing time-resolved x-ray photoemission spectroscopy (TR-XPS) with a free-electron laser [12,13], we directly measure the melting of a large-amplitude CDW in 1T -TaS 2 at atomic sites in real time. Our results show that long-range charge order collapses promptly after strong optical excitation and that a domain-like quasi-equilibrium CDW/PLD state is reached with a subpicosecond time constant.The model system 1T -TaS 2 is a complex material with a simple basic structure consisting of S-Ta-S sandwiches in which each Ta atom is octahedrally coordinated by six S atoms. The interesting physics in this compound is restricted to the hexagonal Ta layers and results from simultaneously strong electron-phonon and electron-electron interactions. The phase diagram includes a high-temperature metallic phase, incommensurate CDW and nearly commensurate CDW (NCCDW) phases...

Coherent and incoherent excitations of the Gd(0001) surface on ultrafast timescales Uwe Bovensiepen -Time-resolved HAXPES at SACLA: probe and pump pulse-induced space-charge effects L-P Oloff, M Oura, K Rossnagel et al. Abstract. The technique of time-resolved pump-probe x-ray photoelectron spectroscopy using the free-electron laser in Hamburg (FLASH) is described in detail. Particular foci lie on the macrobunch resolving detection scheme, the role of vacuum space-charge effects and the synchronization of pump and probe lasers. In an exemplary case study, the complete Ta 4f core-level dynamics in the layered charge-density-wave (CDW) compound 1T-TaS 2 in response to impulsive optical excitation is measured on the sub-picosecond to nanosecond timescale. The observed multi-component dynamics is related to the intrinsic melting and reformation of the CDW as well as to extrinsic pump-laser-induced vacuum space-charge effects.