Ultrafast evolution and transient phases of a prototype out-of-equilibrium Mott-Hubbard material.

The study of photoexcited strongly correlated materials is attracting growing interest since their rich phase diagram often translates into an equally rich out-of-equilibrium behaviour. With femtosecond optical pulses, electronic and lattice degrees of freedom can be transiently decoupled, giving the opportunity of stabilizing new states inaccessible by quasi-adiabatic pathways. Here we show that the prototype Mott-Hubbard material V2O3 presents a transient non-thermal phase developing immediately after ultrafast photoexcitation and lasting few picoseconds. For both the insulating and the metallic phase, the formation of the transient configuration is triggered by the excitation of electrons into the bonding a1g orbital, and is then stabilized by a lattice distortion characterized by a hardening of the A1g coherent phonon, in stark contrast with the softening observed upon heating. Our results show the importance of selective electron-lattice interplay for the ultrafast control of material parameters, and are relevant for the optical manipulation of strongly correlated systems.

Surface Effects on the Mott-Hubbard Transition in Archetypal V{2}O{3}.

We present an experimental and theoretical study exploring surface effects on the evolution of the metal-insulator transition in the model Mott-Hubbard compound Cr-doped V{2}O{3}. We find a microscopic domain formation that is clearly affected by the surface crystallographic orientation. Using scanning photoelectron microscopy and x-ray diffraction, we find that surface defects act as nucleation centers for the formation of domains at the temperature-induced isostructural transition and favor the formation of microscopic metallic regions. A density-functional theory plus dynamical mean-field theory study of different surface terminations shows that the surface reconstruction with excess vanadyl cations leads to doped, and hence more metallic, surface states, which explains our experimental observations.

Tuning a Schottky barrier in a photoexcited topological insulator with transient Dirac cone electron-hole asymmetry.

The advent of Dirac materials has made it possible to realize two-dimensional gases of relativistic fermions with unprecedented transport properties in condensed matter. Their photoconductive control with ultrafast light pulses is opening new perspectives for the transmission of current and information. Here we show that the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator can control an essential parameter for photoconductivity-the balance between excess electrons and holes in the Dirac cone. This can result in a strongly out of equilibrium gas of hot relativistic fermions, characterized by a surprisingly long lifetime of more than 50 ps, and a simultaneous transient shift of chemical potential by as much as 100 meV. The unique properties of this transient Dirac cone make it possible to tune with ultrafast light pulses a relativistic nanoscale Schottky barrier, in a way that is impossible with conventional optoelectronic materials.

Circular dichroism and superdiffusive transport at the surface of BiTeI.

We investigate the electronic states of BiTeI after the optical pumping with circularly polarized photons. Our data show that photoexcited electrons reach an internal thermalization within 300 fs of the arrival of the pump pulse. Instead, the dichroic contrast generated by the circularly polarized light relaxes on a time scale shorter than 80 fs. This result implies that orbital and spin polarization created by the circular pump pulse rapidly decays via manybody interaction. The persistent dichroism at longer delay times is due to the helicity dependence of superdiffussive transport. We ascribe it to the lack of inversion symmetry in an electronic system far from equilibrium conditions.

Coherent phonon coupling to individual Bloch states in photoexcited bismuth.

We investigate the temporal evolution of the electronic states at the bismuth (111) surface by means of time- and angle-resolved photoelectron spectroscopy. The binding energy of bulklike bands oscillates with the frequency of the A(1g) phonon mode, whereas surface states are insensitive to the coherent displacement of the lattice. A strong dependence of the oscillation amplitude on the electronic wave vector is correctly reproduced by ab initio calculations of electron-phonon coupling. Besides these oscillations, all the electronic states also display a photoinduced shift towards higher binding energy whose dynamics follows the evolution of the electronic temperature.

Combined electrospray ionization source with a velocity map imaging spectrometer for studying large gas phase molecular ions.

We present a new compact and versatile experimental set-up that has been designed to perform electron and ion imaging experiments on large multiply charged gas phase molecular and cluster species. It combines an electrospray ionization source, a quadrupole mass filter guiding ion optics and a velocity map imaging spectrometer. Characterization of the spectrometer has been performed on atomic ions. Results obtained on molecular species (stilbene 420 dianions) demonstrate the possibility offered by this experimental set-up.

Ultrafast surface carrier dynamics in the topological insulator Bi2Te3.

We discuss the ultrafast evolution of the surface electronic structure of the topological insulator Bi(2)Te(3) following a femtosecond laser excitation. Using time and angle-resolved photoelectron spectroscopy, we provide a direct real-time visualization of the transient carrier population of both the surface states and the bulk conduction band. We find that the thermalization of the surface states is initially determined by interband scattering from the bulk conduction band, lasting for about 0.5 ps; subsequently, few picoseconds are necessary for the Dirac cone nonequilibrium electrons to recover a Fermi-Dirac distribution, while their relaxation extends over more than 10 ps. The surface sensitivity of our measurements makes it possible to estimate the range of the bulk-surface interband scattering channel, indicating that the process is effective over a distance of 5 nm or less. This establishes a correlation between the nanoscale thickness of the bulk charge reservoir and the evolution of the ultrafast carrier dynamics in the surface Dirac cone.

Full characterization and optimization of a femtosecond ultraviolet laser source for time and angle-resolved photoemission on solid surfaces.

A novel experimental apparatus for time and angle-resolved photoemission on solid surfaces is presented. A 6.28 eV laser source operating at 250 kHz repetition rate is obtained by frequency mixing in nonlinear beta barium borate crystals. This UV light source has a high photon flux of 10(13) photons/s with relatively low number of photons/pulse so that Fermi surface mapping over a wide region of the Brillouin zone is possible while mitigating space charge effects. The UV source has been fully characterized spatially, spectrally, and temporally. Its potential for time and angle-resolved photoemission is demonstrated through Fermi surface mapping and photoexcited electron dynamics in Bismuth. True femtosecond time resolution <65 fs is obtained while the energy resolution of 70 meV appears to be mainly limited by the laser bandwidth.

Giant anisotropy of spin-orbit splitting at the bismuth surface.

We investigate the bismuth (111) surface by means of time and angle resolved photoelectron spectroscopy. The parallel detection of the surface states below and above the Fermi level reveals a giant anisotropy of the spin-orbit spitting. These strong deviations from the Rashba-like coupling cannot be treated in k·p perturbation theory. Instead, first principles calculations could accurately reproduce the experimental dispersion of the electronic states. Our analysis shows that the giant anisotropy of the spin-orbit splitting is due to a large out-of plane buckling of the spin and orbital texture.

A microscopic view on the Mott transition in chromium-doped V(2)O(3).

V(2)O(3) is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal-to-insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator. This or related MITs have a high technological potential, among others, for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V(2)O(3) with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure, that promotes a genuine Mott transition to an eventually homogeneous metallic state.

Transfer-matrix-based method for an analytical description of velocity-map-imaging spectrometers.

We propose a simple and general analytical model describing the operation of a velocity-map-imaging spectrometer. We show that such a spectrometer, possibly equipped with a magnifying lens, can be efficiently modeled by combining analytical expressions for the axial potential distributions along with a transfer matrix method. The model leads transparently to the prediction of the instrument's operating conditions as well as to its resolution. A photoelectron velocity-map-imaging spectrometer with a magnifying lens, built and operated along the lines suggested by the model has been successfully employed for recording images at threshold photoionization of atomic lithium. The model's reliability is demonstrated by the fairly good agreement between experimental results and calculations. Finally, the limitations of the analytical method along with possible generalizations, extensions, and potential applications are also discussed. The model may serve as a guide for users interested in building and operating such spectrometers as well as a tutorial tool.

Quasiparticles at the Mott transition in V2O3: wave vector dependence and surface attenuation.

We present an angle resolved photoemission study of V2O3, a prototype system for the observation of Mott transitions in correlated materials. We show that the spectral features corresponding to the quasiparticle peak in the metallic phase present a marked wave vector dependence, with a stronger intensity along the GammaZ direction. The analysis of their intensity for different probing depths shows the existence of a characteristic length scale for the attenuation of coherent electronic excitations at the surface. This length scale, which is larger than the thickness of the surface region as normally defined for noncorrelated electronic states, is found to increase when approaching the Mott transition. These results are in agreement with the behavior of quasiparticles at surfaces as predicted by Borghi et al.

Non-thermal desorption/ablation of molecular solids induced by ultra-short soft x-ray pulses.

We report the first observation of single-shot soft x-ray laser induced desorption occurring below the ablation threshold in a thin layer of poly (methyl methacrylate)--PMMA. Irradiated by the focused beam from the Free-electron LASer in Hamburg (FLASH) at 21.7 nm, the samples have been investigated by atomic-force microscope (AFM) enabling the visualization of mild surface modifications caused by the desorption. A model describing non-thermal desorption and ablation has been developed and used to analyze single-shot imprints in PMMA. An intermediate regime of materials removal has been found, confirming model predictions. We also report below-threshold multiple-shot desorption of PMMA induced by high-order harmonics (HOH) at 32 nm. Short-time exposure imprints provide sufficient information about transverse beam profile in HOH's tight focus whereas long-time exposed PMMA exhibits radiation-initiated surface ardening making the beam profile measurement infeasible.

Spectral phase distribution retrieval through coherent control of harmonic generation.

The temporal intensity distribution of the third harmonic of a Ti:sapphire laser generated in Xe gas is fully reconstructed from its spectral phase and amplitude distributions. The spectral phases are retrieved by cross correlating the fundamental laser frequency field with that of the third harmonic, in a three laser versus one harmonic photon coupling scheme. The third harmonic spectral amplitude distribution is extracted from its field autocorrelation. The measured pulse duration is found to be in agreement with that expected from lowest order perturbation theory both for unstretched and chirped pulses.