Alberto Simoncig

Clocking the Melting Transition of Charge and Lattice Order in 1T−TaS2 with Ultrafast Extreme-Ultraviolet Angle-Resolved Photoemission Spectroscopy

Charge density waves (CDWs) underpin the electronic properties of many complex materials. Near-equilibrium CDW order is linearly coupled to a periodic, atomic-structural distortion, and the dynamics is understood in terms of amplitude and phase modes. However, at the shortest timescales lattice and charge order may become de-coupled, highlighting the electronic nature of this many-body broken symmetry ground state. Using time and angle resolved photoemission spectroscopy with sub-30-fs XUV pulses, we have mapped the time- and momentum-dependent electronic structure in photo-stimulated 1T-TaS2, a prototypical two-dimensional charge density wave compound. We find that CDW order, observed as a splitting of the uppermost electronic bands at the Brillouin zone boundary, melts well before relaxation of the underlying structural distortion. Decoupled charge and lattice modulations challenge the view of Fermi Surface nesting as a driving force for charge density wave formation in 1T-TaS2.

Possible observation of parametrically amplified coherent phasons in K0.3MoO3 using time-resolved extreme-ultraviolet angle-resolved photoemission spectroscopy

. Prompt depletion of the charge-densitywavecondensatelaunchescoherentoscillationsoftheamplitudemode,observedasa1.7-THz-frequencymodulationofthebondingbandposition.Incontrast,theantibondingbandoscillatesatabouthalfthisfrequency.Weattributetheseoscillationstocoherentexcitationofphasonsviaparametricamplificationofphasefluctuations.DOI: 10.1103/PhysRevB.88.045104 PACS number(s): 78

Soft X-Ray Second Harmonic Generation as an Interfacial Probe

Nonlinear optical processes at soft x-ray wavelengths have remained largely unexplored due to the lack of available light sources with the requisite intensity and coherence. Here we report the observation of soft x-ray second harmonic generation near the carbon K edge (∼284  eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems.

Coherent spectral enhancement of carrier-envelope-phase stable continua with dual-gas high harmonic generation

Attosecond science is enabled by the ability to convert femtosecond near-infrared laser light into coherent harmonics in the extreme ultraviolet spectral range. While attosecond sources have been utilized in experiments that have not demanded high intensities, substantially higher photon flux would provide a natural link to the next significant experimental breakthrough. Numerical simulations of dual-gas high harmonic generation indicate that the output in the cutoff spectral region can be selectively enhanced without disturbing the single-atom gating mechanism. Here, we summarize the results of these simulations and present first experimental findings to support these predictions.

Ultrafast Tr-ARPES with Artemis XUV Beamline

A new HHG XUV beamline at Artemis, user open-access facility at CLF, offers unique capabilities optimised for Tr-ARPES. Current result on ultrafast melting of Mott and charge order in TaS2 will be presented.

Monochromatised XUV Pulses for Ultrafast Science at the Artemis Facility

XUV pulses produced through high harmonic generation can probe electron dynamics in complex solid materials and in gas-phase atoms and molecules. This is demonstrated in gas-phase and condensed matter experiments at the Artemis facility.