Obtaining insight into microscopic cooperative effects is a fascinating topic in condensed matter research because, through self-coordination and collectivity, they can lead to instabilities with macroscopic impacts like phase transitions. We used femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe3, an excellent model system with which to study these effects. We drove a transient charge density wave melting, excited collective vibrations in TbTe3, and observed them through their time-, frequency-, and momentum-dependent influence on the electronic structure. We were able to identify the role of the observed collective vibration in the transition and to document the transition in real time. The information that we demonstrate as being accessible with trARPES will greatly enhance the understanding of all materials exhibiting collective phenomena.

Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain. Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent 'order parameter' that depends exclusively on the electronic excitation.

The original observation of the Einstein-de Haas effect was a landmark experiment in the early history of modern physics that illustrates the relationship between magnetism and angular momentum 1, 2 . Today the effect is still discussed in elementary physics courses to demonstrate that the angular momentum associated with the aligned electron spins in a ferromagnet can be converted to mechanical angular momentum by reversing the direction of magnetisation using an external magnetic field. In recent times, a related problem in magnetism concerns the time-scale over which this angular momentum transfer can occur. It is known experimentally for several metallic ferromagnets that intense photoexcitation leads to a drop in the magnetisation on a time scale shorter than 100 fs, a phenomenon called ultrafast demagnetisation 3-5 . The microscopic mechanism for this process has been hotly debated, with one key question still unanswered: where does the angular momentum go on these femtosecond time scales? Here we show using femtosecond time-resolved x-ray diffraction that a majority of the angular momentum lost from the spin system on the laser-induced demagnetisation of ferromagnetic iron is transferred to the lattice on sub-picosecond timescales, manifesting as a transverse strain wave that propagates from the surface into the bulk. By fitting a simple model of the x-ray data to simulations and optical data, we estimate that the angular momentum occurs on a time scale of 200 fs and corresponds to 80% of the angular momentum lost from the spin system. Our results show that interaction with the lattice plays an essential role in the process of ultrafast demagnetisation in this system. 2Broadly speaking, proposed mechanisms for ultrafast demagnetisation fall into two categories: spin-flip scattering mechanisms and spin transport mechanisms. The first category explains the demagnetisation process as a sudden increase in scattering processes that ultimately result in a decrease of spin order. These scattering processes can include electron-electron, electron-phonon, electron-magnon and even direct spin-light interactions. On average, such scattering must necessarily involve a transfer of angular momentum from the electronic spins to some other subsystem(s). Candidates include the lattice, the electromagnetic field, and the orbital angular momentum of the electrons. Numerical estimates and experiments using circularly polarised light strongly suggest that the amount of angular momentum given to the electromagnetic field interaction is negligible 6 , and experiments using femtosecond x-ray magnetic dichroism (XMCD) indicate that the angular momentum of both electronic spins and orbitals decrease in magnitude nearly simultaneously 7-9 . The only remaining possibility for a spin-flip induced change in angular momentum therefore appears to be a transfer to the lattice via spin-orbit coupling, but this remains to be experimentally verified.The second category of proposed mechanisms relies on the idea that laser excitation causes a ...

The nonequilibrium state of the high-T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+δ) and its ultrafast dynamics have been investigated by femtosecond time- and angle-resolved photoemission spectroscopy well below the critical temperature. We probe optically excited quasiparticles at different electron momenta along the Fermi surface and detect metastable quasiparticles near the antinode, since their decay toward the nodal region through e-e scattering is blocked by phase space restrictions. The observed lack of momentum dependence in the decay rates is in agreement with relaxation dynamics dominated by Cooper pair recombination in a boson bottleneck limit.

Ultrafast non-equilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photo-induced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a non-equilibrium population of excited electronic states. Utilizing femtosecond photoemission we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photo-holes in shaping the potential energy landscape, and enables a combined momentum and real space description of PIPTs, including the ultrafast formation of chemical bonds.Artists view of the excitation and formation of chemical bonds along Indium nanowires (red balls) on a Silicon(111) surface during the ultrafast photoinduced phase transition between the 8x2 and 4x1 structures. This real space view of atoms and bonds is complemented by detailed measurememets of the electronic structure of electrons in their "momentum space" exhibiting the evolution of the band stuctrue providing a complete picture of the phase transition. 3 In/Si(111) undergoes a transition from an insulating (8x2) to a metallic (4x1) structure above 130 K (27,28), r-space schematics of which are shown in Fig. 1, C and D, respectively. The bonding motif in the insulating phase (Fig. 1C) consists of distorted hexagons, while in the conducting phase the In atoms rearrange into zig-zagging chains (Fig. 1 D).The k-space band structures of the two phases calculated within the GW approximation are given below the relevant structures in Fig. 1, E and F. In contrast to the (4x1) phase which has three metallic bands (m1 -m3) that cross EF (17), the (8x2) phase is gapped at the Γ 8x2 and X 8x2 points. Upon increasing the temperature across the (8x2) to (4x1) phase transition, the states initially lying far above EF at Γ 8x2 shift down in energy and eventually cross EF, forming the metallic m1 band of the (4x1) phase. Concurrently the energy gap in the m2 and m3 bands at the X 8x2 point closes, while at the same time the bands shift apart in momentum along the kx direction (23). We note that the three metallic bands predicted from the calculation in the (4x1) phase are clearly observed in Fig. 1B. The Fermi surface of the (4x1) phase in Fig. 1G shows the momentum cut along which our data are obtained.

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å−1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å−1, and a system response function of 150 fs.

Time-and angle-resolved photoemission spectroscopy (trARPES) employing a 500 kHz extreme-ultraviolet (XUV) light source operating at 21.7 eV probe photon energy is reported. Based on a high-power ytterbium laser, optical parametric chirped pulse amplification (OPCPA), and ultraviolet-driven high-harmonic generation, the light source produces an isolated high-harmonic with 110 meV bandwidth and a flux of more than 10 11 photons/second on the sample. Combined with a stateof-the-art ARPES chamber, this table-top experiment allows high-repetition rate pump-probe experiments of electron dynamics in occupied and normally unoccupied (excited) states in the entire Brillouin zone and with a temporal system response function below 40 fs. function A(k,ω) and a matrix element between the initial and final state |M k if | 2 ; here k and ω denote the electron's wavevector and angular frequency, respectively. Many-body effects are encoded in the spectral function A(k,ω) and manifest themselves in renormalization of the bare electronic bands and in the observed lineshape 1 . In a trARPES experiment, the distribution I(k,ω) is collected for a series of delays (τ ) between pump and probe pulses: after perturbation, the population distribution f(k,ω,τ ) evolves towards a quasi-thermal distribution and energetically relaxes on femto-to picosecond timescales 2 . During relaxation, the concomitant many-body interactions affect the transient spectral function A(k,ω,τ ) and even the photoemission matrix elements might change, if the final state's orbital symmetry is altered 3 . trARPES accesses at once the population dynamics, the evolution of the spectral function and the evolution of matrix elements. trARPES has found increasingly successful applications in the past few decades 4-6 : among many examples, trARPES was used to study photo-induced phase transitions 7-11 and to observe electronic states above the Fermi level, unoccupied under equilibrium conditions [12][13][14][15][16] . Energy conservation in the photoemission processes imposes that a femtosecond light source for trARPES must possess a photon energy ω ph exceeding the work function Φ, which in most materials lies in the range between 4 to 6 eV. Ultraviolet femtosecond light sources are thus required for these experiments.The conservation of the electrons' in-plane momentum ( k ) in the photoemission process allows reciprocal space resolution. The advantage of a probe with high photon energy is Journal of Electron Spectroscopy and Related Phenomena 200, 15 (2015). 79 SPECS Surface Nano Analysis GmbH, product spectrometer PHOIBOS TM 150 (2013), see

Quasiparticle lifetimes in metals as described by Fermi-liquid theory 1 are essential in surface chemistry 2 and determine the mean free path of hot carriers 3. Relaxation of hot electrons is governed by inelastic electron-electron scattering, which occurs on femtosecond timescales owing to the large scattering phase space competing with screening effects 4. Such lifetimes are widely studied by time-resolved two-photon photoemission 5,6 , which led to understanding of electronic decay at surfaces 6-8. In contrast, quasiparticle lifetimes of metal bulk 5,9-12 and films 11,13-15 are not well understood because electronic transport 10,16,17 leads to experimental lifetimes shorter than expected theoretically 13,15,18. Here, we lift this discrepancy by investigating Pb quantum-well structures on Si(111), a two-dimensional model system 19-29. For electronic states confined to the film by the Si bandgap we find quantitative agreement with Fermi-liquid theory and ab initio calculations 4,7 for bulk Pb, which we attribute to efficient screening. For states resonant with Si bands, extra decay channels open for electron transfer to Si, resulting in lifetimes shorter than expected for bulk. Thereby we demonstrate that for understanding electronic decay in nanostructures coupling to the environment is essential, and that even for electron confinement to a few ångströms Fermi-liquid theory for bulk can remain valid. We begin the discussion by analysing the quantized electronic structure in epitaxially grown Pb films. Figure 1a shows the photoemission intensity in a colour map as a function of Pb film thickness. We probe occupied states at energies below the Fermi level E F by one-photon photoemission (1PPE) with a photon energy hν larger than the work function E F − E vac and unoccupied states above E F by two-photon photoemission (2PPE), where the sum of both photon energies hν 1 + hν 2 is larger than the work function. These linear and nonlinear methods of photoemission are illustrated in Fig. 1, left. Symbols denote the peak positions of Lorentzian line fits. In Fig. 1b the fitting results are shown for three exemplary Pb thicknesses; details are given in the Supplementary Information. The data exhibit three branches of occupied quantumwell states (QWSs) and five branches of unoccupied QWSs that disperse towards E F with increasing coverage. The binding energies (E −E F) of the lowest unoccupied QWS (luQWS) oscillate between 1.65 and 0.6 eV by 45-165% on variation by one monolayer (1 ML) in thickness from N to N +1 ML and form the two lowest branches of unoccupied QWSs. Simultaneously, the QWS peaks exhibit a LETTERS NATURE PHYSICS