73 research outputs found
Out-of-equilibrium charge redistribution in a copper-oxide based superconductor by time-resolved X-ray photoelectron spectroscopy
Charge-transfer excitations are of paramount importance for understanding the
electronic structure of copper-oxide based high-temperature superconductors. In
this study, we investigate the response of a
BiSrCaCuO crystal to the charge
redistribution induced by an infrared ultrashort pulse. Element-selective
time-resolved core-level photoelectron spectroscopy with a high energy
resolution allows disentangling the dynamics of oxygen ions with different
coordination and bonds thanks to their different chemical shifts. Our
experiment shows that the O\, component arising from the Cu-O planes is
significantly perturbed by the infrared light pulse. Conversely, the apical
oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected.
This result highlights the peculiar behavior of the electronic structure of the
Cu-O planes. It also unlocks the way to study the out-of-equilibrium electronic
structure of copper-oxide-based high-temperature superconductors by identifying
the O\, core-level emission originating from the oxygen ions in the Cu-O
planes. This ability could be critical to gain information about the
strongly-correlated electron ultrafast dynamical mechanisms in the Cu-O plane
in the normal and superconducting phases
Tracking the surface atomic motion in a coherent phonon oscillation
X-ray photoelectron diffraction is a powerful tool for determining the
structure of clean and adsorbate-covered surfaces. Extending the technique into
the ultrafast time domain will open the door to studies as diverse as the
direct determination of the electron-phonon coupling strength in solids and the
mapping of atomic motion in surface chemical reactions. Here we demonstrate
time-resolved photoelectron diffraction using ultrashort soft X-ray pulses from
the free electron laser FLASH. We collect Se 3d photoelectron diffraction
patterns over a wide angular range from optically excited BiSe with a
time resolution of 140 fs. Combining these with multiple scattering simulations
allows us to track the motion of near-surface atoms within the first 3 ps after
triggering a coherent vibration of the A optical phonons. Using a
fluence of 4.2 mJ/cm from a 1.55 eV pump laser, we find the resulting
coherent vibrational amplitude in the first two interlayer spacings to be on
the order of 1 pm
New insights into the laser-assisted photoelectric effect from solid-state surfaces
Photoemission from a solid surface provides a wealth of information about the
electronic structure of the surface and its dynamic evolution. Ultrafast
pump-probe experiments are particularly useful to study the dynamic
interactions of photons with surfaces as well as the ensuing electron dynamics
induced by these interactions. Time-resolved laser-assisted photoemission
(tr-LAPE) from surfaces is a novel technique to gain deeper understanding of
the fundamentals underlying the photoemission process. Here, we present the
results of a femtosecond time-resolved soft X-ray photoelectron spectroscopy
experiment on two different metal surfaces conducted at the X-ray Free-Electron
Laser FLASH in Hamburg. We study photoemission from the W 4f and Pt 4f core
levels using ultrashort soft X-ray pulses in combination with synchronized
infrared (IR) laser pulses. When both pulses overlap in time and space,
laser-assisted photoemission results in the formation of a series of sidebands
that reflect the dynamics of the laser-surface interaction. We demonstrate a
qualitatively new level of sideband generation up to the sixth order and a
surprising material dependence of the number of sidebands that has so far not
been predicted by theory. We provide a semi-quantitative explanation of this
phenomenon based on the different dynamic dielectric responses of the two
materials. Our results advance the understanding of the LAPE process and reveal
new details of the IR field present in the surface region, which is determined
by the dynamic interplay between the IR laser field and the dielectric response
of the metal surfaces.Comment: 18 pages, 3 figure
Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser
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
Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO
Altermagnets are an emerging third elementary class of magnets. Unlike
ferromagnets, their distinct crystal symmetries inhibit magnetization while,
unlike antiferromagnets, they promote strong spin polarization in the band
structure. The corresponding unconventional mechanism of timereversal symmetry
breaking without magnetization in the electronic spectra has been regarded as a
primary signature of altermagnetism, but has not been experimentally visualized
to date. We directly observe strong time-reversal symmetry breaking in the band
structure of altermagnetic RuO by detecting magnetic circular dichroism in
angle-resolved photoemission spectra. Our experimental results, supported by ab
initio calculations, establish the microscopic electronic-structure basis for a
family of novel phenomena and functionalities in fields ranging from
topological matter to spintronics, that are based on the unconventional
time-reversal symmetry breaking in altermagnets
Spin texture of time reversal symmetry invariant surface states on W 110
We find in the case of W 110 previously overlooked anomalous surface states having their spin locked at right angle to their momentum using spin resolved momentum microscopy. In addition to the well known Dirac like surface state with Rashba spin texture near the point, we observe a tilted Dirac cone with circularly shaped cross section and a Dirac crossing at 0.28 amp; 8201; amp; 8201; amp; 8201; within the projected bulk band gap of tungsten. This state has eye catching similarities to the spin locked surface state of a topological insulator. The experiments are fortified by a one step photoemission calculation in its density matrix formulatio
Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens
The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e–e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from −20 to −1100 V/mm for Ekin = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 μm above the sample surface for Ekin = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at Ekin = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm2 (retarding field −21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm2, it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at Ekin = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments
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