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Ultrafast recovery dynamics of dimer stripes in IrTe2
Authors:
M. Rumo,
G. Kremer,
M. Heber,
N. Wind,
C. W. Nicholson,
K. Y. Ma,
G. Brenner,
F. Pressacco,
M. Scholz,
K. Rossnagel,
F. O. von Rohr,
D. Kutnyakhov,
C. Monney
Abstract:
The transition metal dichalcogenide IrTe2 displays a remarkable series of first-order phase transitions below room temperature, involving lattice displacements as large as 20 percents of the initial bond length. This is nowadays understood as the result of strong electron-phonon coupling leading to the formation of local multicentre dimers that arrange themselves into one-dimensional stripes. In t…
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The transition metal dichalcogenide IrTe2 displays a remarkable series of first-order phase transitions below room temperature, involving lattice displacements as large as 20 percents of the initial bond length. This is nowadays understood as the result of strong electron-phonon coupling leading to the formation of local multicentre dimers that arrange themselves into one-dimensional stripes. In this work, we study the out-of-equilibrium dynamics of these dimers and track the time evolution of their population following an infrared photoexcitation using free-electron lased-based time-resolved X-ray photoemission spectroscopy. First, we observe that the dissolution of dimers is driven by the transfer of energy from the electronic subsystem to the lattice subsystem, in agreement with previous studies. Second, we observe a surprisingly fast relaxation of the dimer population on the timescale of a few picoseconds. By comparing our results to published ultrafast electron diffraction and angle-resolved photoemission spectroscopy data, we reveal that the long-range order needs tens of picoseconds to recover, while the local dimer distortion recovers on a short timescale of a few picoseconds.
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Submitted 28 October, 2025;
originally announced October 2025.
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First experiments with ultrashort, circularly polarized soft X-ray pulses at FLASH2
Authors:
S. Marotzke,
D. Gupta,
R. -P. Wang,
M. Pavelka,
S. Dziarzhytski,
C. von Korff Schmising,
S. Jana,
N. Thielemann-Kühn,
T. Amrhein,
M. Weinelt,
I. Vaskivskyi,
R. Knut,
D. Engel,
M. Braune,
M. Ilchen,
S. Savio,
T. Otto,
K. Tiedtke,
V. Scheppe,
J. Rönsch-Schulenberg,
E. Schneidmiller,
C. Schüßler-Langeheine,
H. A. Dürr,
M. Beye,
G. Brenner
, et al. (1 additional authors not shown)
Abstract:
Time-resolved absorption spectroscopy as well as magnetic circular dichroism with circularly polarized soft X-rays (XAS and XMCD) are powerful tools to probe electronic and magnetic dynamics in magnetic materials element- and site-selectively. Employing these methods, groundbreaking results have been obtained for instance for magnetic alloys, which helped to fundamentally advance the field of ultr…
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Time-resolved absorption spectroscopy as well as magnetic circular dichroism with circularly polarized soft X-rays (XAS and XMCD) are powerful tools to probe electronic and magnetic dynamics in magnetic materials element- and site-selectively. Employing these methods, groundbreaking results have been obtained for instance for magnetic alloys, which helped to fundamentally advance the field of ultrafast magnetization dynamics. At the free electron laser facility FLASH key capabilities for ultrafast XAS and XMCD experiments have recently improved: In an upgrade, an APPLE-III helical afterburner undulator was installed at FLASH2 in September 2023. This installation allows for the generation of circularly polarized soft X-ray pulses with a duration of a few tens of femtoseconds covering the L3,2-edges of the important 3d transition metal elements with pulse energies of several uJ. Here, we present first experimental results with such ultrashort X-ray pulses from the FL23 beamline employing XMCD at the L-edges of the 3d metals, Co, Fe and Ni. We obtain significant dichroic difference signals indicating a degree of circular polarization close to 100%. With the pulse-length preserving monochromator at beamline FL23 and an improved pump laser setup, FLASH can offer important and efficient experimental instrumentation for studies on ultrafast spin dynamics in 3d transition metals, multilayers, and alloys.
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Submitted 5 February, 2025;
originally announced February 2025.
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Femtosecond charge and spin dynamics in CoPt alloys
Authors:
Martin Pavelka,
Simon Marotzke,
Ru-Pan Wang,
Mohamed F. Elhanoty,
Günter Brenner,
Siarhei Dziarzhytski,
Somnath Jana,
W. Dieter Engel,
Clemens v. Korff Schmising,
Deeksha Gupta,
Igor Vaskivskyi,
Tim Amrhein,
Nele Thielemann-Kühn,
Martin Weinelt,
Ronny Knut,
Juliane Rönsch-Schulenberg,
Evgeny Schneidmiller,
Christian Schüßler-Langeheine,
Martin Beye,
Niko Pontius,
Oscar Grånäs,
Hermann A. Dürr
Abstract:
The use of advanced X-ray sources plays a key role in the study of dynamic processes in magnetically ordered materials. The progress in X-ray free electron lasers enables the direct and simultaneous observation of the femtosecond evolution of electron and spin systems through transient X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), respectively. Such experiments…
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The use of advanced X-ray sources plays a key role in the study of dynamic processes in magnetically ordered materials. The progress in X-ray free electron lasers enables the direct and simultaneous observation of the femtosecond evolution of electron and spin systems through transient X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), respectively. Such experiments allow us to resolve the response seen in the population of the spin-split valence states upon optical excitation. Here, we utilize circularly polarized ultrashort soft X-ray pulses from the new helical afterburner undulator at the free-electron laser FLASH in Hamburg to study the femtosecond dynamics of a laser-excited CoPt alloy at the Co $L_{3}$ absorption edge. Despite employing a weaker electronic excitation level we find a comparable demagnetization for the Co $3d$-states in CoPt compared to previous measurements on CoPd. This is attributed to distinctly different orbital hybridization and spin-orbit coupling between $3d$ and $4d$ vs. $3d$ and $5d$ elements in the corresponding alloys and multilayers.
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Submitted 4 February, 2025;
originally announced February 2025.
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Simultaneous mapping of the ultrafast time and fluence dependence of the laser-induced insulator-to-metal transition in magnetite
Authors:
J. O. Schunck,
P. S. Miedema,
R. Y. Engel,
S. Dziarzhytski,
G. Brenner,
N. Ekanayake,
C. -F. Chang,
P. Bougiatioti,
F. Döring,
B. Rösner,
C. David,
C. Schüßler-Langeheine,
M. Beye
Abstract:
Pump-probe methods are a ubiquitous tool in the field of ultrafast dynamic measurements. In recent years, x-ray free-electron laser experiments have gained importance due to their ability to probe with high chemical selectivity and at atomic length scales. Measurements are typically repeated many thousands of times to collect sufficient statistics and vary parameters like delay or fluence, necessi…
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Pump-probe methods are a ubiquitous tool in the field of ultrafast dynamic measurements. In recent years, x-ray free-electron laser experiments have gained importance due to their ability to probe with high chemical selectivity and at atomic length scales. Measurements are typically repeated many thousands of times to collect sufficient statistics and vary parameters like delay or fluence, necessitating that initial conditions are restored each time. An alternative is presented by experiments which measure the relevant parameters in a single shot. Here, we present a time-to-space mapping imaging scheme that enables us to record a range of delays and laser fluences in any single shot of the x-ray probe. We demonstrate the use of this scheme by mapping the ultrafast dynamics of the optically induced insulator-to-metal Verwey transition in a magnetite thin film, probed by soft x-ray resonant diffraction. By extrapolating our results toward the conditions found at x-ray free-electron lasers with higher photon energy, we demonstrate that the presented data could be recorded in a single shot.
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Submitted 28 March, 2025; v1 submitted 17 January, 2025;
originally announced January 2025.
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Multimode Objective Lens for Momentum Microscopy and XPEEM: Experiments
Authors:
O. Tkach,
S. Fragkos,
D. Biswas,
J. Liu,
O. Fedchenko,
Y. Lytvynenko,
S. Babenkov,
D. Zimmer,
Q. Nguyen,
S. Chernov,
D. Kutnyakhov,
M. Scholz,
N. Wind,
A. Gloskowskii,
F. Pressacco,
J. Dilling,
L. Bruckmeier,
M. Heber,
L. Wenthaus,
G. Brenner,
D. Puntel,
P. E. Majchrzak,
D. Liu,
F. Scholz,
J. A. Sobota
, et al. (15 additional authors not shown)
Abstract:
A new type of objective lens has recently been proposed for use in X-ray photoemission electron microscopes (XPEEMs) and momentum microscopes. Adding a ring electrode concentric with the extractor allows the field in the gap between the sample and the extractor to be shaped. Forming a lens field in this gap reduces the field strength at the sample by up to an order of magnitude. This mitigates the…
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A new type of objective lens has recently been proposed for use in X-ray photoemission electron microscopes (XPEEMs) and momentum microscopes. Adding a ring electrode concentric with the extractor allows the field in the gap between the sample and the extractor to be shaped. Forming a lens field in this gap reduces the field strength at the sample by up to an order of magnitude. This mitigates the risk of field emission, particularly for cleaved samples with sharp edges. A retarding field can redirect all slow electrons, thus eliminating the primary contribution to the space-charge interaction. Here we present the first experimental investigation of the new lens, examining its performance at photon energies ranging from the extreme ultraviolet produced by a high-harmonic generation (HHG)-based source to soft and hard X-rays at two synchrotron facilities. The gap lens in a region without electrodes enables large working distances up to 23 mm. Reduced aberrations allow for larger fields of view in both k-space and real-space imaging, with resolutions comparable to those of conventional cathode lenses. However, field strengths are an order of magnitude smaller. The zero-field mode enables the study of 3D structured objects and is therefore beneficial for small cleaved samples as well as for operando devices involving top electrodes. The repeller mode reduces space-charge effects, but results in a smaller k-field diameter. This reduction ranges from 10% at hard X-ray energies to 50% in the XUV range. The usable energy interval is also reduced by a factor of two. In time-of-flight XPEEM mode the raw data show a resolution of 250 nm, which can be improved to better than 100 nm through data processing.
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Submitted 13 January, 2026; v1 submitted 18 January, 2024;
originally announced January 2024.
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New insights into the laser-assisted photoelectric effect from solid-state surfaces
Authors:
Lukas Wenthaus,
Nikolay M. Kabachnik,
Mario Borgwardt,
Steffen Palutke,
Dmytro Kutnyakhov,
Federico Pressacco,
Markus Scholz,
Dmitrii Potorochin,
Nils Wind,
Stefan Düsterer,
Günter Brenner,
Oliver Gessner,
Serguei Molodtsov,
Wolfgang Eberhardt,
Friedrich Roth
Abstract:
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…
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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.
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Submitted 14 November, 2023; v1 submitted 31 August, 2023;
originally announced September 2023.
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Out-of-equilibrium charge redistribution in a copper-oxide based superconductor by time-resolved X-ray photoelectron spectroscopy
Authors:
Denny Puntel,
Dmytro Kutnyakhov,
Lukas Wenthaus,
Markus Scholz,
Nils O. Wind,
Michael Heber,
Günter Brenner,
Genda Gu,
Robert J. Cava,
Wibke Bronsch,
Federico Cilento,
Fulvio Parmigiani,
Federico Pressacco
Abstract:
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 Bi$_2$Sr$_2$CaCu$_2$O$_{\mathrm{8}+ δ}$ crystal to the charge redistribution induced by an infrared ultrashort pulse. Element-selective time-resolved core-level photoelectron spectroscopy with a hi…
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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 Bi$_2$Sr$_2$CaCu$_2$O$_{\mathrm{8}+ δ}$ 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\,$1s$ 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\,$1s$ 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.
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Submitted 22 June, 2023;
originally announced June 2023.
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Ultrafast manipulation of the NiO antiferromagnetic order via sub-gap optical excitation
Authors:
Xiaocui Wang,
Robin Y. Engel,
Igor Vaskivskyi,
Diego Turenne,
Vishal Shokeen,
Alexander Yaroslavtsev,
Oscar Grånäs,
Ronny Knut,
Jan O. Schunck,
Siarhei Dziarzhytski,
Günter Brenner,
Ru-Pan Wang,
Marion Kuhlmann,
Frederik Kuschewski,
Wibke Bronsch,
Christian Schüßler-Langeheine,
Andriy Styervoyedov,
Stuart S. P. Parkin,
Fulvio Parmigiani,
Olle Eriksson,
Martin Beye,
Hermann A. Dürr
Abstract:
Wide-band-gap insulators such as NiO offer the exciting prospect of coherently manipulating electronic correlations with strong optical fields. Contrary to metals where rapid dephasing of optical excitation via electronic processes occurs, the sub-gap excitation in charge-transfer insulators has been shown to couple to low-energy bosonic excitations. However, it is currently unknown if the bosonic…
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Wide-band-gap insulators such as NiO offer the exciting prospect of coherently manipulating electronic correlations with strong optical fields. Contrary to metals where rapid dephasing of optical excitation via electronic processes occurs, the sub-gap excitation in charge-transfer insulators has been shown to couple to low-energy bosonic excitations. However, it is currently unknown if the bosonic dressing field is composed of phonons or magnons. Here we use the prototypical charge-transfer insulator NiO to demonstrate that 1.5 eV sub-gap optical excitation leads to a renormalised NiO band-gap in combination with a significant reduction of the antiferromagnetic order. We employ element-specific X-ray reflectivity at the FLASH free-electron laser to demonstrate the reduction of the upper band-edge at the O 1s-2p core-valence resonance (K-edge) whereas the antiferromagnetic order is probed via X-ray magnetic linear dichroism (XMLD) at the Ni 2p-3d resonance (L2-edge). Comparing the transient XMLD spectral line shape to ground-state measurements allows us to extract a spin temperature rise of 65 +/- 5 K for time delays longer than 400 fs while at earlier times a non-equilibrium spin state is formed. We identify transient mid-gap states being formed during the first 200 fs accompanied by a band-gap reduction lasting at least up to the maximum measured time delay of 2.4 ps. Electronic structure calculations indicate that magnon excitations significantly contribute to the reduction of the NiO band gap.
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Submitted 10 January, 2022;
originally announced January 2022.
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Optical control of 4f orbital state in rare-earth metals
Authors:
N. Thielemann-Kühn,
T. Amrhein,
W. Bronsch,
S. Jana,
N. Pontius,
R. Y. Engel,
P. S. Miedema,
D. Legut,
K. Carva,
U. Atxitia,
B. E. van Kuiken,
M. Teichmann,
R. E. Carley,
L. Mercadier,
A. Yaroslavtsev,
G. Mercurio,
L. Le Guyader,
N. Agarwal,
R. Gort,
A. Scherz,
S. Dziarzhytski,
G. Brenner,
F. Pressacco,
R. Wang,
J. O. Schunck
, et al. (6 additional authors not shown)
Abstract:
A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth (RE) elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafas…
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A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth (RE) elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time-resolved X-ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4f-electronic excitations out of the ground state multiplet occur after optical pumping. These excitations are driven by inelastic 5d-4f-electron scattering, alter the 4f-orbital state and consequently the MCA with important implications for magnetization dynamics in 4f-metals, and more general for the excitation of localized electronic states in correlated materials.
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Submitted 8 April, 2024; v1 submitted 18 June, 2021;
originally announced June 2021.
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Ultrafast electronic line width broadening in the C 1s core level of graphene
Authors:
Davide Curcio,
Sahar Pakdel,
Klara Volckaert,
Jill A. Miwa,
Søren Ulstrup,
Nicola Lanatà,
Marco Bianchi,
Dmytro Kutnyakhov,
Federico Pressacco,
Günter Brenner,
Siarhei Dziarzhytski,
Harald Redlin,
Steinn Agustsson,
Katerina Medjanik,
Dmitry Vasilyev,
Hans-Joachim Elmers,
Gerd Schönhense,
Christian Tusche,
Ying-Jiun Chen,
Florian Speck,
Thomas Seyller,
Kevin Bühlmann,
Rafael Gort,
Florian Diekmann,
Kai Rossnagel
, et al. (9 additional authors not shown)
Abstract:
Core level binding energies and absorption edges are at the heart of many experimental techniques concerned with element-specific structure, electronic structure, chemical reactivity, elementary excitations and magnetism. X-ray photoemission spectroscopy (XPS) in particular, can provide information about the electronic and vibrational many-body interactions in a solid as these are reflected in the…
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Core level binding energies and absorption edges are at the heart of many experimental techniques concerned with element-specific structure, electronic structure, chemical reactivity, elementary excitations and magnetism. X-ray photoemission spectroscopy (XPS) in particular, can provide information about the electronic and vibrational many-body interactions in a solid as these are reflected in the detailed energy distribution of the photoelectrons. Ultrafast pump-probe techniques add a new dimension to such studies, introducing the ability to probe a transient state of the many-body system. Here we use a free electron laser to investigate the effect of a transiently excited electron gas on the core level spectrum of graphene, showing that it leads to a large broadening of the C 1s peak. Confirming a decade-old prediction, the broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron system, rather than by vibrational excitations. This interpretation is supported by a line shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in agreement with electronic temperature values obtained from valence band data. Furthermore, making use of time- and momentum-resolved C 1s spectra, we illustrate how the momentum change of the outgoing core electrons leads to a small but detectable change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the narrow $σ$-band in the C 1s state. The results demonstrate that the XPS line shape can be used as an element-specific and local probe of the excited electron system and that X-ray photoelectron diffraction investigations remain feasible at very high electronic temperatures.
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Submitted 21 May, 2021;
originally announced May 2021.
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Subpicosecond metamagnetic phase transition driven by non-equilibrium electron dynamics
Authors:
Federico Pressacco,
Davide Sangalli,
Vojtěch Uhlíř,
Dmytro Kutnyakhov,
Jon Ander Arregi,
Steinn Ymir Agustsson,
Günter Brenner,
Harald Redlin,
Michael Heber,
Dmitry Vasilyev,
Jure Demsar,
Gerd Schönhense,
Matteo Gatti,
Andrea Marini,
Wilfried Wurth,
Fausto Sirotti
Abstract:
Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study com…
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Femtosecond light-induced phase transitions between different macroscopic orders provide the possibility to tune the functional properties of condensed matter on ultrafast timescales. In first-order phase transitions, transient non-equilibrium phases and inherent phase coexistence often preclude non-ambiguous detection of transition precursors and their temporal onset. Here, we present a study combining time-resolved photoelectron spectroscopy and ab-initio electron dynamics calculations elucidating the transient subpicosecond processes governing the photoinduced generation of ferromagnetic order in antiferromagnetic FeRh. The transient photoemission spectra are accounted for by assuming that not only the occupation of electronic states is modified during the photoexcitation process. Instead, the photo-generated non-thermal distribution of electrons modifies the electronic band structure. The ferromagnetic phase of FeRh, characterized by a minority band near the Fermi energy, is established 350+- 30 fs after the laser excitation. Ab-initio calculations indicate that the phase transition is initiated by a photoinduced Rh-to-Fe charge transfer.
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Submitted 18 February, 2021;
originally announced February 2021.
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Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS
Authors:
Friedrich Roth,
Mario Borgwardt,
Lukas Wenthaus,
Johannes Mahl,
Steffen Palutke,
Günter Brenner,
Giuseppe Mercurio,
Serguei Molodtsov,
Wilfried Wurth,
Oliver Gessner,
Wolfgang Eberhardt
Abstract:
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, h…
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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:C$_{60}$. 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.
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Submitted 5 January, 2021; v1 submitted 18 September, 2020;
originally announced September 2020.
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Ultrafast molecular orbital tomography of a pentacene thin film using time-resolved momentum microscopy at a free-electron laser
Authors:
K. Baumgärtner,
M. Reuner,
C. Metzger,
D. Kutnyakhov,
M. Heber,
C. H. Min,
T. R. F. Peixoto,
M. Reiser,
C. Kim,
W. Lu,
R. Shayduk,
W. M. Izquierdo,
G. Brenner,
F. Roth,
F. Pressacco,
A. Schöll,
S. Molodtsov,
W. Wurth,
F. Reinert,
A. Madsen,
D. Popova-Gorelova,
M. Scholz
Abstract:
We use time-resolved momentum microscopy at a free-electron laser (FEL) and extend orbital tomography into the time domain to image the electronic wave functions of excited molecular orbitals. This technique provides unprecedented insight into the ultrafast interplay between structural and electronic dynamics. In this work we prove general applicability and establish the experimental conditions at…
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We use time-resolved momentum microscopy at a free-electron laser (FEL) and extend orbital tomography into the time domain to image the electronic wave functions of excited molecular orbitals. This technique provides unprecedented insight into the ultrafast interplay between structural and electronic dynamics. In this work we prove general applicability and establish the experimental conditions at FEL sources to minimize space charge effects and radiation damage. We investigate a bilayer pentacene film on Ag(110) by optical laser pump and FEL probe experiments. From the momentum microscopy signal, we obtain time-dependent momentum maps of the excited-state dynamics of both pentacene layers separately. Combining experimental observations with a theoretical study, we interpret the observed signal for the bottom layer as resulting from the charge redistribution between the molecule and the substrate induced by excitation. We identify that the dynamics of the top pentacene layer resembles excited-state molecular dynamics.
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Submitted 3 December, 2021; v1 submitted 24 July, 2019;
originally announced July 2019.
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Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser
Authors:
Dmytro Kutnyakhov,
Rui Patrick Xian,
Maciej Dendzik,
Michael Heber,
Federico Pressacco,
Steinn Ymir Agustsson,
Lukas Wenthaus,
Holger Meyer,
Sven Gieschen,
Giuseppe Mercurio,
Adrian Benz,
Kevin Bühlman,
Simon Däster,
Rafael Gort,
Davide Curcio,
Klara Volckaert,
Marco Bianchi,
Charlotte Sanders,
Jill Atsuko Miwa,
Søren Ulstrup,
Andreas Oelsner,
Christian Tusche,
Ying-Jiun Chen,
Dmitrii Vasilyev,
Katerina Medjanik
, et al. (16 additional authors not shown)
Abstract:
Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of non-equilibrium 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 photoelectro…
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Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of non-equilibrium 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 analysis 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. The PG2 beamline at FLASH (DESY, Hamburg) provides a high pulse rate of 5000 pulses/s, 60 fs pulse duration and 40 meV bandwidth in an energy range of 25-830 eV with a photon beam size down to 50 microns in diameter. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines FEL 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 ($k_x$, $k_y$, $E$) 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 $2.5\times10^5$ data voxels. As an example, we present results for the ultrafast excited state dynamics in the model van der Waals semiconductor WSe$_2$.
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Submitted 18 September, 2019; v1 submitted 28 June, 2019;
originally announced June 2019.