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Floquet theory has spawned many exciting possibilities for electronic structure control with light, with enormous potential for future applications. The experimental demonstration in solids, however, remains largely unrealized. In particular, the influence of scattering on the formation of Floquet-Bloch states remains poorly understood. Here we combine time- and angle-resolved photoemission spectroscopy with time-dependent density functional theory and a two-level model with relaxation to investigate the survival of Floquet-Bloch states in the presence of scattering. We find that Floquet-Bloch states will be destroyed if scattering-activated by electronic excitations-prevents the Bloch electrons from following the driving field coherently. The two-level model also shows that Floquet-Bloch states reappear at high field intensities where energy exchange with the driving field dominates over energy dissipation to the bath. Our results clearly indicate the importance of long scattering times combined with strong driving fields for the successful realization of various Floquet phenomena. This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through CRC 925 (Project 170620586), CRC 1277 (Project 314695032), and the Cluster of Excellence “CUI: Advanced Imaging of Matter”. Further, the work received funding from the European Research Council (Starting Grant 851280 and Advanced Grant 694097), the European Union Graphene Flagship under Grant Agreement Nos. 785219 and 881603, Grupos Consolidados (IT1249-19), and JSPS KAKENHI (Grant Number JP20K14382). The Flatiron Institute is a division of the Simons Foundation

Physical review letters 125(26), 266402 (1-6) (2020). doi:10.1103/PhysRevLett.125.266402, Impulsive optical excitation generally results in a complex nonequilibrium electron and lattice dynamics that involves multiple processes on distinct timescales, and a common conception is that for times shorter than about 100 fs the gap in the electronic spectrum is not seriously affected by lattice vibrations. Here, however, by directly monitoring the photoinduced collapse of the spectral gap in a canonical charge-density-wave material, the blue bronze Rb$_{0.3}$MoO$_3$, we find that ultrafast (∼60 fs) vibrational disordering due to efficient hot-electron energy dissipation quenches the gap significantly faster than the typical structural bottleneck time corresponding to one half-cycle oscillation (∼315 fs) of the coherent charge-density-wave amplitude mode. This result not only demonstrates the importance of incoherent lattice motion in the photoinduced quenching of electronic order, but also resolves the perennial debate about the nature of the spectral gap in a coupled electron-lattice system., Published by APS, College Park, Md.

This corrects the article DOI: 10.1103/PhysRevLett.120.166401.

Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide $1T$-$\mathrm{Ta}{\mathrm{S}}_{2}$, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping $\ensuremath{\hbar}/J\ensuremath{\approx}14\text{ }\text{ }\mathrm{fs}$. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale.

Physical review letters 121(25), 256401 (2018). doi:10.1103/PhysRevLett.121.256401, Time- and angle-resolved photoelectron spectroscopy with 13 fs temporal resolution is used to follow the different stages in the formation of a Fermi-Dirac distributed electron gas in graphite after absorption of an intense 7 fs laser pulse. Within the first 50 fs after excitation, a sequence of time frames is resolved that are characterized by different energy and momentum exchange processes among the involved photonic, electronic, and phononic degrees of freedom. The results reveal experimentally the complexity of the transition from a nascent nonthermal towards a thermal electron distribution due to the different timescales associated with the involved interaction processes., Published by APS, College Park, Md.

Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide 1T-TaS_{2}, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping ℏ/J≈14 fs. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale.

The commensurate charge-density wave (CDW) in the layered compound $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ has hitherto mostly been treated as a quasi-two-dimensional phenomenon. Recent band structure calculations have, however, predicted that the CDW coexists with a nearly one-dimensional metallic dispersion perpendicular to the crystal planes. Using synchrotron radiation-based angle-resolved photoemission spectroscopy, we show that this metallic band does in fact exist. Its occupied band width is in excellent agreement with predictions for a simple ${\ensuremath{\tau}}_{c}$ stacking order of the CDW between adjacent layers, and its periodicity in the $c$ direction is $2\ensuremath{\pi}/c$.

The ultrafast spectral responses of a charge-ordered state in 1 T -TiSe 2 to the photo-excitation with laser-pulses of 790 nm and 395 nm center wavelength are compared. Time- and angle-resolved photoemission reveals pronounced differences in the energy-momentum distribution of the nascent electron gas; a distinct effect of the distribution character on the destruction dynamics of the charge-ordered state is, however, not observed.

The layered transition-metal dichalcogenide MoS${}_{2}$ as well as related compounds, such as WS${}_{2}$ and WSe${}_{2}$, undergo a transition from an indirect to a direct band-gap semiconductor when thinned down to a single layer. This unexpected observation, which goes along with substantial changes in electronic and optical properties, has stimulated numerous recent studies on this compound class, including pump-probe time-domain studies addressing fundamental aspects of hot carrier relaxation. So far, mainly all-optical methods have been applied to probe the ultrafast dynamics in a rather comprehensive manner. Due to intermixing of different processes and a lack of momentum-resolution, the interpretation of such data is, however, not necessarily straightforward. In order to complement these studies, the authors present here results of a time- and angle-resolved photoemission (trARPES) experiment on bulk MoS${}_{2}$, a technique that provides the most direct view onto hot electron dynamics in energy and momentum space. Surprisingly, the new data agree well with past results of monolayer MoS${}_{2}$ as opposed to bulk MoS${}_{2}$. The authors interpret this finding in terms of the high density of surface defects in MoS${}_{2}$ samples, which substantially affect hot carrier relaxation due to trapping. In contrast to all optical studies, trARPES is very efficient in probing such processes owing to the extreme surface sensitivity of photoemission.

Time-resolved hard X-ray photoelectron spectroscopy (trHAXPES) using microfocused X-ray free-electron laser (XFEL, hν = 8 keV) pulses as a probe and infrared laser pulses (hν = 1.55 eV) as a pump is employed to determine intrinsic charge-carrier recombination dynamics in La:SrTiO3. By means of a combination of experiments and numerical N-body simulations, we first develop a simple approach to characterize and decrease XFEL-induced vacuum space-charge effects, which otherwise pose a serious limitation to spectroscopy experiments. We then show that, using an analytical mean-field model, vacuum space-charge effects can be counteracted by pump laser-induced photoholes at high excitation densities. This provides us a method to separate vacuum space-charge effects from the intrinsic charge-carrier recombination dynamics in the time domain. Our trHAXPES results thus open a route to studies of intrinsic charge-carrier dynamics on picosecond time scales with lateral spatial resolution on the micrometer scale.

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 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.

An experimental setup for time- and angle-resolved photoelectron spectroscopy with sub-15 fs temporal resolution is presented. A hollow-fiber compressor is used for the generation of 6.5 fs white light pump pulses, and a high-harmonic-generation source delivers 11 fs probe pulses at a photon energy of 22.1 eV. A value of 13 fs full width at half-maximum of the pump-probe cross correlation signal is determined by analyzing a photoemission intensity transient probing a near-infrared interband transition in 1T-TiSe2. Notably, the energy resolution of the setup conforms to typical values reported in conventional time-resolved photoemission studies using high harmonics, and an ultimate resolution of 170 meV is feasible.

With the advent of ultrashort-pulsed extreme ultraviolet sources, such as free-electron lasers or high-harmonic-generation (HHG) sources, a new research field for photoelectron spectroscopy has opened up in terms of femtosecond time-resolved pump-probe experiments. The impact of the high peak brilliance of these novel sources on photoemission spectra, so-called vacuum space-charge effects caused by the Coulomb interaction among the photoemitted probe electrons, has been studied extensively. However, possible distortions of the energy and momentum distributions of the probe photoelectrons caused by the low photon energy pump pulse due to the nonlinear emission of electrons have not been studied in detail yet. Here, we systematically investigate these pump laser-induced space-charge effects in a HHG-based experiment for the test case of highly oriented pyrolytic graphite. Specifically, we determine how the key parameters of the pump pulse—the excitation density, wavelength, spot size, and emitted electron energy distribution—affect the measured time-dependent energy and momentum distributions of the probe photoelectrons. The results are well reproduced by a simple mean-field model, which could open a path for the correction of pump laser-induced space-charge effects and thus toward probing ultrafast electron dynamics in strongly excited 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.

Time- and angle-resolved photoemission spectroscopy with a high–harmonic- generation source is used to classify the potential excitonic insulator 1T -TiSe2 and the reference Peierls-Mott insulator 1T -TaS2 on the basis of the melting times of “spectroscopic order parameters”.