Adrian L. Cavalieri

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.

Driving magnetic order in a manganite by ultrafast lattice excitation

Femtosecond midinfrared pulses are used to directly excite the lattice of the single-layer manganite La0.5Sr1.5MnO4. Magnetic and orbital orders, as measured by femtosecond resonant soft x-ray diffraction with an x-ray free-electron laser, are reduced within a few picoseconds. This effect is interpreted as a displacive exchange quench, a prompt shift in the equilibrium value of the magnetic- and orbital-order parameters after the lattice has been distorted. Control of magnetism through ultrafast lattice excitation may be of use for high-speed optomagnetism.

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

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.

Probing ultrafast electron dynamics in condensed matter with attosecond photoemission

We discuss experiments that address the ultrafast dynamics inherent to the photoemission process in condensed matter. In our experimental approach, an extreme ultraviolet attosecond light pulse launches photoelectron wave packets inside a solid. The subsequent emission dynamics of these photoelectrons is probed with the light field of a phase-stabilized near-infrared laser pulse. This technique is capable of resolving subtle emission delays of only a few attoseconds between electron wave packets that are released from different energy levels of the crystal. For simple metals, we show that these time shifts may be interpreted as the real-time observation of photoelectrons propagating through the crystal lattice prior to their escape into vacuum. The impact of adsorbates on the observed emission dynamics is also investigated.