16 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
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
Quantum Imaging with Incoherently Scattered Light from a Free-Electron Laser
The advent of accelerator-driven free-electron lasers (FEL) has opened new
avenues for high-resolution structure determination via diffraction methods
that go far beyond conventional x-ray crystallography methods. These techniques
rely on coherent scattering processes that require the maintenance of
first-order coherence of the radiation field throughout the imaging procedure.
Here we show that higher-order degrees of coherence, displayed in the intensity
correlations of incoherently scattered x-rays from an FEL, can be used to image
two-dimensional objects with a spatial resolution close to or even below the
Abbe limit. This constitutes a new approach towards structure determination
based on incoherent processes, including Compton scattering, fluorescence
emission or wavefront distortions, generally considered detrimental for imaging
applications. Our method is an extension of the landmark intensity correlation
measurements of Hanbury Brown and Twiss to higher than second-order paving the
way towards determination of structure and dynamics of matter in regimes where
coherent imaging methods have intrinsic limitations
Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO 2
Altermagnets are an emerging elementary class of collinear 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 time-reversal 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 RuO2 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 interesting phenomena and functionalities in fields ranging from topological matter to spintronics, which are based on the unconventional time-reversal symmetry breaking in altermagnets
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Dynamical Nonlinear Inversion of the Surface Photovoltage at Si(100)
A surface photovoltage (SPV) is observed whenever a doped semiconductor with non-negligible band bending is illuminated by light and charge carriers are excited across the band gap. The sign of the SPV depends on the nature of the doping, the amplitude of the SPV increases with the fluence of the light illumination up to a saturation value, which is determined by the doping concentration. We have investigated Si(100) samples with well-characterized doping levels over a wide range of illumination fluences. Surprisingly, the sign of the SPV upon illumination with 532 nm photons reverses for some p-doping concentrations at high fluences. This is a new effect associated with a crossover between electronic excitations in the bulk and at the surface of the semiconductor
Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy
Progress in the development of plasmon-enabled light-harvesting technologies requires a better understanding of their fundamental operating principles and current limitations. Here, we employ picosecond time-resolved X-ray photoemission spectroscopy to investigate photoinduced electron transfer in a plasmonic model system composed of 20 nm sized gold nanoparticles (NPs) attached to a nanoporous film of TiO2. The measurement provides direct, quantitative access to transient local charge distributions from the perspectives of the electron donor (AuNP) and the electron acceptor (TiO2). On average, approximately two electrons are injected per NP, corresponding to an electron injection yield per absorbed photon of 0.1%. Back electron transfer from the perspective of the electron donor is dominated by a fast recombination channel proceeding on a time scale of 60 ± 10 ps and a minor contribution that is completed after ∼1 ns. The findings provide a detailed picture of photoinduced charge carrier generation in this NP–semiconductor junction, with important implications for understanding achievable overall photon-to-charge conversion efficiencies
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Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS.
The ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C60. A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions
Incentive system and its evaluation in automotive industry
Devletler ekonomik ve sosyal sorunların çözümüne yönelik olarak sektörlere yönelik yardımlarda bulunurlar. Devletler bu yardımları, sanayi ve ticaret politikaları doğrultusunda bölgesel adaletsizliklerin azaltılması, yatırımın desteklenmesi, rekabet gücünün korunması ve sosyal hedeflere ulaşılması amacıyla, teşvik araçlarını kullanarak gerçekleştirirler. Otomotiv sektöründeki gibi, projelerin etkin işlediği, iş gücü gereksiniminin yüksek olduğu firmalar için proje yönetiminin önemi büyüktür. Proje yönetimi, projenin hedeflerine ulaşılması için gerekli ihtiyaçları karşılamak üzere ilgili tüm bilgi, beceri, araç ve tekniklerin proje faaliyetlerine uygulanmasıdır. Otomotiv sektörü gibi ekonomide büyük yeri olan sektörler için, yatırım yapılacak uygun projenin seçilmesi açısından proje yönetiminin önemi giderek artmaktadır.States are involved in sectoral aid to address economic and social problems. States use these aids for the reduction of regional injustices towards industrial and commercial policies, investment support, protection of competitive power and to reach social goals using incentive tools. As seen in the automotive sector, project management is a great asset for firms which projects are processed efficiently and that require high workforce. The purpose of project management is to meet the needs to reach the project objectives requiring the application of all relevant knowledge, skills, tools and techniques on project activities. The prominence of project management is increasing in terms of selecting the appropriate project to be invested in sectors such as the automotive sector that have a great deal in the economy
Photoinduced dynamics at the water/TiO(101) interface
We present a femtosecond time-resolved optical pump-soft x-ray probe photoemission study in which we follow the dynamics of charge transfer at the interface of water and anatase TiO(101). By combining our observation of transient oxygen O 1s core level peak shifts at submonolayer water coverages with Ehrenfest molecular dynamics simulations we find that ultrafast interfacial hole transfer from TiO to molecularly adsorbed water is completed within the 285 fs time resolution of the experiment. This is facilitated by the formation of a new hydrogen bond between an Oc site at the surface and a physisorbed water molecule. The calculations fully corroborate our experimental observations and further suggest that this process is preceded by the efficient trapping of the hole at the surface of TiO by hydroxyl species (-OH), that form following the dissociative adsorption of water. At a water coverage exceeding a monolayer, interfacial charge transfer is suppressed. Our findings are directly applicable to a wide range of photocatalytic systems in which water plays a critical role