T. Kroll

Leibniz Institute for Solid State and Materials Research Dresden, Dresden, Saxony, Germany

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Publications (18)27.13 Total impact

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    ABSTRACT: Transition metal phthalocyanines (MPc's) are an interesting class of material, and their magnetic and electronic properties are determined by the orbital occupation of the transition metal 3d orbitals incorporated in the molecules center. Thus, the ground state configuration of the transition metal center is very important for a complete understanding of these materials. We present experimental data taken using x-ray absorption and x-ray photoemission spectroscopy together with a theoretical interpretation of MPc series with M=Zn, Cu, Ni, Co, Fe, and Mn. The combination of these methods allows us to narrow down possible dominating ground state configurations and shed a brighter light on the electronic structure of these complexes.
    The Journal of Chemical Physics 08/2012; 137(5):054306. · 3.16 Impact Factor
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    ABSTRACT: We probe the correlation regime of Ba1−xKxFe2As2 single crystals by x-ray photoemission spectroscopy. Although the data explicitly confirm the itinerant character of the electrons, the satellite features observed at the L3 threshold indicate at the same time local behavior. This dichotomy suggests that, although the pnictides appear to be unusually weakly correlated, local electronic excitations cannot be neglected for the correct description of the material.
    Physical review. B, Condensed matter 05/2010; 81(17).
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    ABSTRACT: We have studied the electronic structure of iron phthalocyanine (FePc) films at low temperature using electron energy-loss spectroscopy. The electronic excitation spectrum of FePc is rather complex and comprises both π-π* transitions of the phthalocyanine ligand and transitions that involve the Fe 3d orbitals. The C 1s core excitations provide so far unidentified information on the molecular orbitals. They demonstrate that the Fe 3d orbital with eg symmetry is energetically located in between the highest occupied and the lowest unoccupied ligand state and that it is not fully occupied.
    Physics of Condensed Matter 01/2010; · 1.28 Impact Factor
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    ABSTRACT: The 3d orbital ground state of transition-metal ions that are incorporated in a molecular matrix determines the total spin of the transition-metal ion as well as the spin anisotropy and thus the essential magnetic properties of the corresponding molecule. However, there is little known to date on the exact 3d ground state of many molecular systems, including quite complex molecular magnets as well as relatively simple systems such as, for instance, cobalt phthalocyanine (CoPc). For the latter, there are contradictory theoretical predictions with respect to the occupation of the various Co 3d electronic levels. We demonstrate that polarization-dependent X-ray absorption spectroscopy in combination with a simulation of the spectra is able to shed a brighter light on the spin and orbital ground state of the transition-metal ion in CoPc. Our results reveal a temperature-dependent ground state and emphasize the importance of taking 3d correlation effects properly into account.
    The Journal of Physical Chemistry A 08/2009; 113(31):8917-22. · 2.77 Impact Factor
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    ABSTRACT: The electronic structure of various rare-earth oxypnictides has been investigated by performing Fe L2, 3 x-ray absorption spectroscopy, and Fe 2p and valence band x-ray photoemission spectroscopy. As representative samples the non-superconducting parent compounds LnFeAsO (Ln=La, Ce, Sm and Gd) have been chosen and measured at 25 and 300 K, i.e. below and above the structural and magnetic phase transition at ~150 K. We find no significant change of the electronic structure of the FeAs layers when switching between the different rare-earth ions or when varying the temperature below and above the transition temperatures. Using a simple two-configuration model, we find qualitative agreement with the Fe 2p3/2 core-level spectrum, which allows for a qualitative explanation of the experimental spectral shapes.
    New Journal of Physics 02/2009; 11(2):025019. · 4.06 Impact Factor
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    ABSTRACT: We investigated the recently found superconductor LaFeAsO1−xFx by x-ray absorption spectroscopy. From a comparison of the O K edge with LDA calculations we find good agreement and are able to explain the structure and changes in the spectra with electron doping. From experimental Fe L2,3-edge spectra and charge-transfer multiplet calculations we gain further information on important physical values such as the hopping parameters, the charge-transfer energy Δ, and the Hubbard U. Furthermore we find the system to be very covalent with a large amount of ligand holes. A shift in the chemical potential is visible in the O K- and Fe L2,3-edge spectra which emphasizes the importance of band effects in these compounds. From the entirety of our results we conclude that LaFeAsO is a bandwidth-dominated material.
    Physical Review B 12/2008; 78(22):220502(R). · 3.77 Impact Factor
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    ABSTRACT: We investigated the recently found superconductor LaFeAsO1-xFx by X-ray absorption spectroscopy (XAS). A shift in the chemical potential is visible in both the Fe L2,3 and O K edge spectra which emphasizes the importance of band effects and moderate correlations in these compounds. From experimental Fe L2,3 edge spectra and charge transfer multiplet calculations we gain further information on important physical values such as hopping parameters, the charge transfer energy Delta, and the on-site Hubbard U. Furthermore we find the system to be very covalent with a large amount of ligand holes.
    Int. Symp. Fe-Pnictide Superconductors, Japan; 11/2008
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    ABSTRACT: We investigated the recently found superconductor LaO_{1-x}F_xFeAs by X-ray absorption spectroscopy (XAS). From a comparison of the O K-edge with LDA calculations we find good agreement and are able to explain the structure and changes of the spectra with electron doping. An important result from this edge is a limitation of the Hubbard U to values not significantly larger than 1 eV. From experimental Fe L_2,3-edge spectra and charge transfer multiplet calculations we gain further information on important physical values such as hopping parameters, the charge transfer energy Delta, and the on-site Hubbard U. Furthermore we find the system to be very covalent with a large amount of ligand holes. A shift in the chemical potential is visible in the O K- and Fe L_2,3-edge spectra which emphasizes the importance of band effects in these compounds.
    07/2008;
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    ABSTRACT: We investigated the recently found superconductor LaFeAsO1-xFx by X-ray absorption spectroscopy (XAS). A shift in the chemical potential is visible in both the Fe L2,3 and O K edge spectra which emphasizes the importance of band effects and moderate correlations in these compounds. From experimental Fe L2,3 edge spectra and charge transfer multiplet calculations we gain further information on important physical values such as hopping parameters, the charge transfer energy Δ and the on-site Hubbard U. Furthermore we find the system to be very covalent with a large amount of ligand holes.
    01/2008;
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    ABSTRACT: We construct an effective Hamiltonian for the motion of T2g highly correlated states in NaxCoO2. We solve exactly a multiband model in a CoO6 cluster with electronic occupation corresponding to a nominal Co valence of either +3 or +4. Using the ensuing ground states, we calculate the effective O mediated hopping t=0.10 eV between many-body T2g states and estimate the direct hopping t′∼0.05 eV. The trigonal splitting 3D=0.315 eV is taken from recent quantum chemistry calculations. The resulting effective Hamiltonian is solved using a generalized slave-boson mean-field approximation. The results show a significant band renormalization and a Fermi-surface topology that agrees with experiment, in contrast to predictions using the local-density approximation.
    Physical Review B 05/2007; 75(17). · 3.77 Impact Factor
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    ABSTRACT: We have performed spectroscopic experiments and theoretical calculations on layered NaxCoO2. As an experimental tool, we used polarization dependent X-ray absorption spectroscopy in a wide doping range at various temperatures. In order to interpret these experiments, we calculated the local electronic structure around a Co atom by solving exactly a CoO6 cluster. From the results relevant aspects of the electronic structure of NaxCoO2 are disclosed.
    Physica C Superconductivity 01/2007; · 0.72 Impact Factor
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    ABSTRACT: We construct an effective Hamiltonian for the motion of t2g highly correlated states in NaxCoO2. This three-band model includes the indirect Co-O-Co hopping t and the crystal-field splitting 3D. Calculations in a CoO6 cluster give the effective parameters t=100 meV and 3D=315 meV. The Hamiltonian is solved using a generalized slave-boson mean-field approximation. The results show a significant band renormalization. Without any additional hypothesis, the dispersion of the bands near the Fermi energy and Fermi surface topology agrees with angle-resolved photoemission experiments, in contrast to predictions using the local-density approximation. Comment: 2 pages, 2 figures, to be published in Proceedings of M2S HTSC Dresden 2006
    Physica C Superconductivity 09/2006; · 0.72 Impact Factor
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    ABSTRACT: Measurements of polarization and temperature dependent soft x-ray absorption have been performed on NaxCoO2 single crystals with x=0.4 and x=0.6. They show a deviation of the local trigonal symmetry of the CoO6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co3+ and Co4+ sites. With the help of a cluster calculation we are able to interpret the Co L23-edge absorption spectrum and find a doping dependent energy splitting between the t2g and the eg levels (10 Dq) in NaxCoO2.
    Physical review. B, Condensed matter 06/2006; 74(11).
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    ABSTRACT: In order to shed light on the electronic structure of NaxCoO2, and motivated by recent Co L-edge x-ray absorption spectra (XAS) experiments with polarized light, we calculate the electronic spectrum of a CoO6 cluster including all interactions between 3d orbitals. We obtain the ground state for two electronic occupations in the cluster that correspond nominally to all O in the O−2 oxidation state, and Co+3 or Co+4. Then, all excited states obtained by promotion of a Co 2p electron to a 3d electron, and the corresponding matrix elements are calculated. A fit of the observed experimental spectra is good and points out a large Co-O covalency and cubic crystal field effects, that result in low spin Co 3d configurations. Our results indicate that the effective hopping between different Co atoms plays a major role in determining the symmetry of the ground state in the lattice. Remaining quantitative discrepancies with the XAS experiments are expected to come from composition effects of itineracy in the ground and excited states.
    Physical Review B 01/2006; 74(11). · 3.77 Impact Factor
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    ABSTRACT: We present magnetization data of Lax(Ca,Sr)14-xCu24O41 (xgreater-or-equal, slanted5) in high magnetic fields up to 16 T. The data reflect the magnetic response of lightly doped CuO2 spin chains containing less-than-or-equals, slant10% non-magnetic holes. Our data correspond to a strongly anisotropic magnetic phase diagram. Using Monte-Carlo simulations, we present and analyse a two-dimensional Ising model with mobile holes, describing the experimental observations qualitatively.
    jmmm. 01/2005; 290--291:306--309.
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    ABSTRACT: We present magnetization data of Lax(Ca,Sr)14-xCu24O41 (x⩾5) in high magnetic fields up to 16 T. The data reflect the magnetic response of lightly doped CuO2 spin chains containing ⩽10% non-magnetic holes. Our data correspond to a strongly anisotropic magnetic phase diagram. Using Monte-Carlo simulations, we present and analyse a two-dimensional Ising model with mobile holes, describing the experimental observations qualitatively.
    Journal of Magnetism and Magnetic Materials 01/2005; · 1.83 Impact Factor
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    ABSTRACT: Motivated by recent experiments on cuprates with low-dimensional magnetic interactions, a new class of two-dimensional Ising models with short-range interactions and mobile defects is introduced and studied. The non-magnetic defects form lines, which, as temperature increases, first meander and then become unstable. Using Monte Carlo simulations and analytical low- and high-temperature considerations, the instability of the defect stripes is monitored for various microscopic and thermodynamic quantities in detail for a minimal model, assuming some of the couplings to be indefinitely strong. The robustness of the findings against weakening the interactions is discussed as well.
    Physics of Condensed Matter 10/2002; 30(1):83-92. · 1.28 Impact Factor
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    Thomas Kroll
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    ABSTRACT: Die Entdeckung unerwartet großer Thermokraft bei gleichzeitigem niedrigem Widerstand und niedriger thermischen Leitfähigkeit in NaxCoO2 führte zu einem großen wissenschaftlichem Interesse an diesen Materialien und zu einer großen Anzahl an experimentellen und theoretischen Arbeiten. Dieses Interesse steigerte sich noch einmal nach der Entdeckung von Supraleitung in der hydrierten Verbindung Na0.35CoO2 •1.3H2O im Jahre 2003. Die Ähnlichkeit der Na Kobaltate zu den Hochtemperatur-Supraleitern (HTSL) – beides sind Übergangsmetalloxide mit einer geschichteten Kristallstruktur in der quasi zwei dimensionale (Cu,Co)O2 Ebenen enthalten sind – ist ein wichtiger Aspekt moderner wissenschaftlicher Arbeiten. Im Gegensatz zu den HTSL Kupraten bestehen die CoO2 Ebenen aus CoO6 Oktaedern die über ihre Kanten verbunden sind und in der Art verzerrt sind, dass die resultierende Symmetrie trigonal ist. Die trigonale Anordnung der Co Plätze führt zu einer geometrischen Frustration und unkonventionellen elektronischen Grundzuständen. Diese geometrisch frustrierten CoO2 Untergitter existieren ebenfalls in den nicht hydrierten Mutterverbindungen NaxCoO2, welche in dieser Arbeit untersucht wurden. Interkalierung von Wasser in die Mutterverbindung hat nur einen kleinen Einfluss auf die Fermi Oberfläche und führt zu einem Anstieg des zwei dimensionalen Charakters durch den Effekt der Ausdehnung. Durch Verminderung der Symmetrie von kubisch zu trigonal splitten die vormals entarteten t2g Zustände auf in Zustände mit eg und a1g Symmetrie. Zur Bestimmung der elektronischen Struktur von NaxCoO2 wurden polarisations- und temperaturabhängige Messungen der Röntgenabsorption im weichen Röntgenbereich für einen großen Dotierungsbereich durchgeführt. Neben den Co L_2,3-Kanten wurden auch die O K-Kante und die Na K-Kante gemessen. Sie zeigen eine starke Polarisationsabhängigkeit speziell für Anregungen in die niederenergetischen a1g Level. Zusätzlich wurde eine unerwartete Polarisationsabhängigkeit bei höheren Energien gefunden, die für trigonalen Symmetrie so nicht auftauchen dürfte. Diese Ergebnisse weisen auf eine Abweichung von der lokalen trigonalen Symmetrie der CoO6 Oktaeder hin, welche Temperatur unabhängig ist in einem Temperaturbereich zwischen 25 und 372 Kelvin. Diese Abweichung ist unterschiedlich für Co3+ und Co4+ Ionen was wiederum auf einen polaronischen Transport hinweist. Zusätzlich wird deutlich, dass eine starke Co-O Hybridisierung existieren muss. Um weiteres Informationen über die elektronische Struktur von NaxCoO2 zu erhalten, wurde das elektronische Spektrum eines CoO6 Oktaeders berechnet in dem alle Wechselwirkungen zwischen 3d Orbitalen sowie Hüpfprozesse zwischen Co und O sowie O und O Ionen enthalten sind. Der Grundzustand für zwei elektronische Besetzungen in einem Cluster (d.h. Co3+ und Co4+) wurde bestimmt für O Ionen mit einer nominellen O-2 Oxidation sowie Co3+ und Co4+ Ionen. Im angeregten Zustand werden die entsprechenden Anregungen eines Co 2p Elektrons in einen unbesetzten 3d Zustand berücksichtigt und die entsprechenden Matrixelemente berechnet. Ein Fit an den experimentellen Daten ist gut und weist auf eine starke Co-O Kovalenz und auf einen starken Einfluss des kubischen Kristallfeldes hin, der zu einer Low-Spin Co 3d Konfiguration führt. Die Ergebnisse zeigen, dass ein effektives Hüpfen zwischen benachbarter Co Ionen eine große Rolle für die Symmetrie des Grundzustandes im Gitter spielt. Quantitative Unterschiede zwischen den experimentellen und theoretischen Daten kommen anscheinend von itineranten Effekten im Grund- und angeregten Zustand. Zusätzlich zu den oben kurz beschriebenen Ergebnissen werden in dieser Arbeit weitere Ergebnisse der Photoemissionsspektroskopie, der Magnetisierung sowie aus resonanter und nicht resonanter Röntgenbeugung mit harter Röntgenstrahlung gezeigt und diskutiert.