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Tracking the surface atomic motion in a coherent phonon oscillation
Authors:
Davide Curcio,
Klara Volckaert,
Dmytro Kutnyakhov,
Steinn Ymir Agustsson,
Kevin Bühlmann,
Federico Pressacco,
Michael Heber,
Siarhei Dziarzhytski,
Yves Acremann,
Jure Demsar,
Wilfried Wurth,
Charlotte E. Sanders,
Philip Hofmann
Abstract:
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 photo…
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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 Bi$_2$Se$_3$ 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$_{1g}$ optical phonons. Using a fluence of 4.2 mJ/cm$^2$ 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.
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Submitted 26 May, 2022;
originally announced May 2022.
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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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Observation of an excitonic Mott transition through ultrafast core-$\textit{cum}$-conduction photoemission spectroscopy
Authors:
Maciej Dendzik,
R. Patrick Xian,
Enrico Perfetto,
Davide Sangalli,
Dmytro Kutnyakhov,
Shuo Dong,
Samuel Beaulieu,
Tommaso Pincelli,
Federico Pressacco,
Davide Curcio,
Steinn Ymir Agustsson,
Michael Heber,
Jasper Hauer,
Wilfried Wurth,
Günter Brenner,
Yves Acremann,
Philip Hofmann,
Martin Wolf,
Andrea Marini,
Gianluca Stefanucci,
Laurenz Rettig,
Ralph Ernstorfer
Abstract:
Time-resolved soft-X-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe$_2$. We present a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels rev…
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Time-resolved soft-X-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe$_2$. We present a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels reveals a delayed core-hole renormalization due to screening by excited quasi-free carriers, revealing an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and an ultrafast electronic phase transition.
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Submitted 28 March, 2020;
originally announced March 2020.
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An open-source, end-to-end workflow for multidimensional photoemission spectroscopy
Authors:
Rui Patrick Xian,
Yves Acremann,
Steinn Ymir Agustsson,
Maciej Dendzik,
Kevin Bühlmann,
Davide Curcio,
Dmytro Kutnyakhov,
Frederico Pressacco,
Michael Heber,
Shuo Dong,
Tommaso Pincelli,
Jure Demsar,
Wilfried Wurth,
Philip Hofmann,
Martin Wolf,
Markus Scheidgen,
Laurenz Rettig,
Ralph Ernstorfer
Abstract:
Characterization of the electronic band structure of solid state materials is routinely performed using photoemission spectroscopy. Recent advancements in short-wavelength light sources and electron detectors give rise to multidimensional photoemission spectroscopy, allowing parallel measurements of the electron spectral function simultaneously in energy, two momentum components and additional phy…
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Characterization of the electronic band structure of solid state materials is routinely performed using photoemission spectroscopy. Recent advancements in short-wavelength light sources and electron detectors give rise to multidimensional photoemission spectroscopy, allowing parallel measurements of the electron spectral function simultaneously in energy, two momentum components and additional physical parameters with single-event detection capability. Efficient processing of the photoelectron event streams at a rate of up to tens of megabytes per second will enable rapid band mapping for materials characterization. We describe an open-source workflow that allows user interaction with billion-count single-electron events in photoemission band mapping experiments, compatible with beamlines at $3^{\text{rd}}$ and $4^{\text{th}}$ generation light sources and table-top laser-based setups. The workflow offers an end-to-end recipe from distributed operations on single-event data to structured formats for downstream scientific tasks and storage to materials science database integration. Both the workflow and processed data can be archived for reuse, providing the infrastructure for documenting the provenance and lineage of photoemission data for future high-throughput experiments.
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Submitted 14 November, 2020; v1 submitted 17 September, 2019;
originally announced September 2019.
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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.
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Towards Time-Resolved Atomic Structure Determination by X-Ray Standing Waves at a Free-Electron Laser
Authors:
Giuseppe Mercurio,
Igor A. Makhotkin,
Igor Milov,
Young Yong Kim,
Ivan A. Zaluzhnyy,
Siarhei Dziarzhytski,
Lukas Wenthaus,
Ivan A. Vartanyants,
Wilfried Wurth
Abstract:
We demonstrate the structural sensitivity and accuracy of the standing wave technique at a high repetition rate free-electron laser, FLASH at DESY in Hamburg, by measuring the photoelectron yield from the surface SiO2 of Mo/Si multilayers. These experiments open up the possibility to obtain unprecedented structural information of adsorbate and surface atoms with picometer spatial accuracy and femt…
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We demonstrate the structural sensitivity and accuracy of the standing wave technique at a high repetition rate free-electron laser, FLASH at DESY in Hamburg, by measuring the photoelectron yield from the surface SiO2 of Mo/Si multilayers. These experiments open up the possibility to obtain unprecedented structural information of adsorbate and surface atoms with picometer spatial accuracy and femtosecond temporal resolution. This technique will substantially contribute to a fundamental understanding of chemical reactions at catalytic surfaces and the structural dynamics of superconductors.
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Submitted 12 June, 2018;
originally announced June 2018.
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High resolution resonant inelastic EUV scattering from orbital excitations in a Heisenberg antiferromagnet
Authors:
Antonio Caretta,
Martina Dell'Angela,
Yi-De Chuang,
Alexandra M. Kalashnikova,
Roman V. Pisarev,
Davide Bossini,
Florian Hieke,
Wilfried Wurth,
Barbara Casarin,
Roberta Ciprian,
Fulvio Parmigiani,
Surge Wexler,
L. Andrew Wray,
Marco Malvestuto
Abstract:
We report the high resolution resonant inelastic EUV scattering study of quantum Heisenberg antiferromagnet KCoF3. By tuning the EUV photon energy to cobalt M23 edge, a complete set of low energy 3d spin-orbital excitations is revealed. These low-lying electronic excitations are modeled using an extended multiplet-based mean field calculation to identify the roles of lattice and magnetic degrees o…
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We report the high resolution resonant inelastic EUV scattering study of quantum Heisenberg antiferromagnet KCoF3. By tuning the EUV photon energy to cobalt M23 edge, a complete set of low energy 3d spin-orbital excitations is revealed. These low-lying electronic excitations are modeled using an extended multiplet-based mean field calculation to identify the roles of lattice and magnetic degrees of freedom in modifying the RIXS spectral lineshape. We have demonstrated that the temperature dependence of RIXS features upon the antiferromagnetic ordering transition enables us to probe the energetics of short-range spin correlations in this material.
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Submitted 9 October, 2017;
originally announced October 2017.
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Time-resolved observation of band-gap shrinking and electron-lattice thermalization within X-ray excited gallium arsenide
Authors:
Beata Ziaja,
Nikita Medvedev,
Victor Tkachenko,
Theophilos Maltezopoulos,
Wilfried Wurth
Abstract:
Femtosecond X-ray irradiation of solids excites energetic photoelectrons that thermalize on a timescale of a few hundred femtoseconds. The thermalized electrons exchange energy with the lattice and heat it up. Experiments with X-ray free-electron lasers have unveiled so far the details of the electronic thermalization. In this work we show that the data on transient optical reflectivity measured i…
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Femtosecond X-ray irradiation of solids excites energetic photoelectrons that thermalize on a timescale of a few hundred femtoseconds. The thermalized electrons exchange energy with the lattice and heat it up. Experiments with X-ray free-electron lasers have unveiled so far the details of the electronic thermalization. In this work we show that the data on transient optical reflectivity measured in GaAs irradiated with femtosecond X-ray pulses can be used to follow electron-lattice relaxation up to a few tens of picoseconds. With a dedicated theoretical framework, we explain the so far unexplained reflectivity overshooting as a result of band-gap shrinking. We also obtain predictions for a timescale of electron-lattice thermalization, initiated by conduction band electrons in the temperature regime of a few eVs. The conduction and valence band carriers were then strongly non-isothermal. The presented scheme is of general applicability and can stimulate further studies of relaxation within X-ray excited narrow band-gap semiconductors.
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Submitted 2 October, 2015;
originally announced October 2015.
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Trends in the magnetic properties of Fe, Co and Ni clusters and monolayers on Ir(111), Pt(111) and Au(111)
Authors:
S. Bornemann,
O. Šipr,
S. Mankovsky,
S. Polesya,
J. B. Staunton,
W. Wurth,
H. Ebert,
J. Minár
Abstract:
We present a detailed theoretical investigation on the magnetic properties of small single-layered Fe, Co and Ni clusters deposited on Ir(111), Pt(111) and Au(111). For this a fully relativistic {\em ab-initio} scheme based on density functional theory has been used. We analyse the element, size and geometry specific variations of the atomic magnetic moments and their mutual exchange interactions…
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We present a detailed theoretical investigation on the magnetic properties of small single-layered Fe, Co and Ni clusters deposited on Ir(111), Pt(111) and Au(111). For this a fully relativistic {\em ab-initio} scheme based on density functional theory has been used. We analyse the element, size and geometry specific variations of the atomic magnetic moments and their mutual exchange interactions as well as the magnetic anisotropy energy in these systems. Our results show that the atomic spin magnetic moments in the Fe and Co clusters decrease almost linearly with coordination on all three substrates, while the corresponding orbital magnetic moments appear to be much more sensitive to the local atomic environment. The isotropic exchange interaction among the cluster atoms is always very strong for Fe and Co exceeding the values for bulk bcc Fe and hcp Co, whereas the anisotropic Dzyaloshinski-Moriya interaction is in general one or two orders of magnitude smaller when compared to the isotropic one. For the magnetic properties of Ni clusters the magnetic properties can show quite a different behaviour and we find in this case a strong tendency towards noncollinear magnetism.
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Submitted 27 April, 2012;
originally announced April 2012.
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Ultrafast melting of a charge-density wave in the Mott insulator 1T-TaS2
Authors:
S. Hellmann,
M. Beye,
C. Sohrt,
T. Rohwer,
F. Sorgenfrei,
H. Redlin,
M. Kalläne,
M. Marczynski-Bühlow,
F. Hennies,
M. Bauer,
A. Föhlisch,
L. Kipp,
W. Wurth,
K. Rossnagel
Abstract:
Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics in the charge-density-wave/Mott insulator 1T-TaS2. After strong photoexcitation, a prompt loss of charge order and subsequent fast equilibration dynamics of the electron-lattice system are observed. On the time scale of electron-phonon thermal…
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Femtosecond time-resolved core-level photoemission spectroscopy with a free-electron laser is used to measure the atomic-site specific charge-order dynamics in the charge-density-wave/Mott insulator 1T-TaS2. After strong photoexcitation, a prompt loss of charge order and subsequent fast equilibration dynamics of the electron-lattice system are observed. On the time scale of electron-phonon thermalization, about 1 ps, the system is driven across a phase transition from a long-range charge ordered state to a quasi-equilibrium state with domain-like short-range charge and lattice order. The experiment opens the way to study the nonequilibrium dynamics of condensed matter systems with full elemental, chemical, and atomic site selectivity.
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Submitted 4 August, 2010; v1 submitted 27 April, 2010;
originally announced April 2010.
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Non-collinear magnetism of Cr nanostructures on Fe$_{3ML}$/Cu(001): first--principles and experimental investigations
Authors:
Samir Lounis,
Matthias Reif,
Phivos Mavropoulos,
Leif Glaser,
Peter H. Dederichs,
Michael Martins,
Stefan Blügel,
Wilfried Wurth
Abstract:
A combined experimental, using X-ray Magnetic Circular Dichroism and theoretical investigation, using full-potential Korringa-Kohn-Rostoker (KKR) Green function method, is carried out to study the spin structure of small magnetic Cr adatom--clusters on the surface of 3 monolayers of $fcc$ Fe deposited on Cu(001). The exchange interaction between the different Cr adatoms as well as between the Cr…
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A combined experimental, using X-ray Magnetic Circular Dichroism and theoretical investigation, using full-potential Korringa-Kohn-Rostoker (KKR) Green function method, is carried out to study the spin structure of small magnetic Cr adatom--clusters on the surface of 3 monolayers of $fcc$ Fe deposited on Cu(001). The exchange interaction between the different Cr adatoms as well as between the Cr atoms and the Fe atoms is of antiferromagnetic nature and of comparable magnitude, leading due to frustration to complex non-collinear magnetic configurations. The presence of non-collinear magnetic coupling obtained by {\it ab initio} calculations is confirmed by the experimental results.
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Submitted 2 August, 2006;
originally announced August 2006.
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Surface Core Level Shifts of Clean and Oxygen Covered Ru(0001)
Authors:
S. Lizzit,
A. Baraldi,
A. Groso,
K. Reuter,
M. V. Ganduglia-Pirovano,
C. Stampfl,
M. Scheffler,
M. Stichler,
C. Keller,
W. Wurth,
D. Menzel
Abstract:
We have performed high resolution XPS experiments of the Ru(0001) surface, both clean and covered with well-defined amounts of oxygen up to 1 ML coverage. For the clean surface we detected two distinct components in the Ru 3d_{5/2} core level spectra, for which a definite assignment was made using the high resolution Angle-Scan Photoelectron Diffraction approach. For the p(2x2), p(2x1), (2x2)-3O…
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We have performed high resolution XPS experiments of the Ru(0001) surface, both clean and covered with well-defined amounts of oxygen up to 1 ML coverage. For the clean surface we detected two distinct components in the Ru 3d_{5/2} core level spectra, for which a definite assignment was made using the high resolution Angle-Scan Photoelectron Diffraction approach. For the p(2x2), p(2x1), (2x2)-3O and (1x1)-O oxygen structures we found Ru 3d_{5/2} core level peaks which are shifted up to 1 eV to higher binding energies. Very good agreement with density functional theory calculations of these Surface Core Level Shifts (SCLS) is reported. The overriding parameter for the resulting Ru SCLSs turns out to be the number of directly coordinated O atoms. Since the calculations permit the separation of initial and final state effects, our results give valuable information for the understanding of bonding and screening at the surface, otherwise not accessible in the measurement of the core level energies alone.
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Submitted 20 February, 2001;
originally announced February 2001.