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The ultrafast dynamics of excited electrons in 1T-TiSe2 after absorption of a 390 nm light pulse is probed by time- and angle-resolved photoemission spectroscopy using femtosecond XUV pulses. It is demonstrated that the experimental approach can provide a comprehensive view of hot carrier motion in momentum space during relaxation back to equilibrium. This capability opens a new avenue in the investigation of energy dissipation processes in solids after intense optical excitation.

Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to directly determine the momentum-dependent electronic structure dynamics in the layered Peierls-Mott insulators 1T-TaS(2) and 1T-TaSe(2) on the sub-300 fs time scale. Extracted spectroscopic order parameters display a global two-time-scale dynamics indicating a quasi-instantaneous loss of the electronic orders and a subsequent coherent suppression of the lattice distortion on a time scale related to the frequency of the charge-density-wave amplitude mode. After one half-cycle of coherent amplitude-mode vibration, a crossover state between insulator and metal with partially filled-in and partially closed Mott and Peierls gaps is reached. The results are discussed within the wider context of electronic order quenching in complex materials.

Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ around the high-symmetry points $\mathrm{\ensuremath{\Gamma}}$ and $M$. A global oscillation of the Fermi level at the frequency of the ${A}_{1g}(\mathrm{As})$ phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the ${A}_{1g}(\mathrm{As})$ phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling.

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.

Angle-resolved photoemission spectroscopy at a temperature of 1 K is used to determine the wave-vector dependence of the spectral gap and band renormalization due to electron-phonon coupling in the layered chargedensity-wave superconductor 2H -NbSe2. The measured gap size and coupling parameter are Fermi-surface-sheet dependent and anisotropic. The largest energy gap, highest coupling strength, and strongest variation in both quantities are found on the double-walled Nb 4d-derived Fermi-surface sheet that is centered on the corners of the hexagonal Brillouin zone. On this sheet, the spectral gap has two distinct anticorrelated components associated with superconductivity and the charge-density wave. The results establish 2H -NbSe2 as a moderately correlated intermediate-coupling anisotropic multigap superconductor.

Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics of the charge-density wave in the 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 quasiequilibrium state with domainlike 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.

Angle-resolved photoelectron spectroscopy (ARPES) is widely used to study the electronic structure of crystalline solids such as high-temperature superconductors, topological insulators and graphene-based materials. Time-resolved ARPES has opened the door to the study of the response of such electronic features on ultrafast timescales. Now Rohwer et al. add a new dimension. Using high photon energies, they are able to study ultrafast dynamics at high momenta, at which some of the most interesting fundamental phenomena occur. Applying the technique to the charge density wave material 1T-TiSe2, they obtain stroboscopic images of the electronic band structure at high momentum and show that atomic-scale periodic long-scale order collapses on a surprisingly short timescale of 20 femtoseconds. This work reveals rapid response times in photoinduced properties that could stimulate research into new types of ultrafast switching device. Angle-resolved photoemission spectroscopy (ARPES) is widely used to study the electronic structure of a wide range of correlated materials. Time-resolved ARPES allows the study of the response of such electronic features on ultrafast timescales; this paper now adds an exciting new dimension by using high photon energies that allow the study of ultrafast dynamics at high momenta, where often the most interesting fundamental phenomena occur. The technique is applied to the charge density wave material 1T-TiSe2 and it is shown with stroboscopic imaging of the electronic band structure at high momentum that atomic-scale periodic long-range order collapses on a surprisingly short timescale of 20 femtoseconds. 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 motion1,2,3. This technique is particularly useful in the study4,5 and in the control6 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 freedom7,8,9,10. 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 energy10,11. 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-TiSe2 (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 systems14, 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.