Showing 1–50 of 121 results for author: Knupfer, M

Optical and acoustic plasmons in the layered material Sr$_2$RuO$_4$

Authors: J. Schultz, A. Lubk, F. Jerzembeck, N. Kikugawa, M. Knupfer, D. Wolf, B. Büchner, J. Fink

Abstract: We use momentum-dependent electron energy-loss spectroscopy in transmission to study collective charge excitations in the layer metal Sr$_2$RuO$_4$. This metal has a transition from a perfect Fermi liquid below $T\approx30\,$K into a "strange" metal phase above $T\approx800\,$K. We cover a complete range between in-phase and out-of-phase oscillations. Outside the classical range of electron-hole e… ▽ More We use momentum-dependent electron energy-loss spectroscopy in transmission to study collective charge excitations in the layer metal Sr$_2$RuO$_4$. This metal has a transition from a perfect Fermi liquid below $T\approx30\,$K into a "strange" metal phase above $T\approx800\,$K. We cover a complete range between in-phase and out-of-phase oscillations. Outside the classical range of electron-hole excitations, leading to a Landau damping, we observe well-defined plasmons. The optical (acoustic) plasmon due to an in-phase (out-of-phase) charge oscillation of neighbouring layers exhibits a quadratic (linear) positive dispersion. Using a model for the Coulomb interaction of the charges in a layered system, it is possible to describe the range of optical plasmon excitations at high energies in a mean-field random phase approximation without taking correlation effects into account. In contrast, resonant inelastic X-ray scattering data show at low energies an enhancement of the acoustic plasmon velocity due to correlation effects. This difference can be explained by an energy dependent effective mass which changes from $\approx$ 3.5 at low energy to 1 at high energy near the optical plasmon energy. There are no signs of over-damped plasmons predicted by holographic theories. △ Less

Submitted 24 October, 2024; v1 submitted 11 January, 2024; originally announced January 2024.

Journal ref: Nat Commun 16, 4287 (2025)

Propagating charge carrier plasmon in Sr2RuO4

Authors: Martin Knupfer, Fabian Jerzembeck, Naoki Kikugawa, Friedrich Roth, Joerg Fink

Abstract: We report on studies of charge carrier plasmon excitations in Sr2RuO4 by transmission Electron Energy-Loss Spectroscopy. In particular, we present results on the plasmon dispersion and its width as a function of momentum transfer. The dispersion can be qualitatively explained in the framework of RPA calculations, using an unrenormalized tight-binding band structure. The constant long-wavelength wi… ▽ More We report on studies of charge carrier plasmon excitations in Sr2RuO4 by transmission Electron Energy-Loss Spectroscopy. In particular, we present results on the plasmon dispersion and its width as a function of momentum transfer. The dispersion can be qualitatively explained in the framework of RPA calculations, using an unrenormalized tight-binding band structure. The constant long-wavelength width of the plasmon indicates, that it is caused by a decay into inter-band transition and not by quantum critical fluctuations. The results from these studies on a prototypical bad metal system show that the long-wavelength plasmon excitations near 1 eV are caused by resilient quasiparticles and are not influenced by correlation effects. △ Less

Submitted 13 October, 2022; originally announced October 2022.

Comments: 6 pages, 3 figures

Investigation of potassium-intercalated bulk MoS$_2$ using transmission electron energy-loss spectroscopy

Authors: Carsten Habenicht, Axel Lubk, Roman Schuster, Martin Knupfer, Bernd Büchner

Abstract: We have investigated the effect of potassium (K) intercalation on $2H$-MoS$_2$ using transmission electron energy-loss spectroscopy. For K concentrations up to approximately 0.4, the crystals appear to be inhomogeneous with a mix of structural phases and irregular potassium distribution. Above this intercalation level, MoS$_2$ exhibits a $2a \times 2a$ superstructure in the $ab$ plane and unit cel… ▽ More We have investigated the effect of potassium (K) intercalation on $2H$-MoS$_2$ using transmission electron energy-loss spectroscopy. For K concentrations up to approximately 0.4, the crystals appear to be inhomogeneous with a mix of structural phases and irregular potassium distribution. Above this intercalation level, MoS$_2$ exhibits a $2a \times 2a$ superstructure in the $ab$ plane and unit cell parameters of a = 3.20 $\unicode{x212B}$ and c = 8.23 $\unicode{x212B}$ indicating a conversion from the $2H$ to the $1T'$ or $1T''$ polytypes. The diffraction patterns also show a $\sqrt{3}a \times \sqrt{3}a$ and a much weaker $2\sqrt{3}a \times 2\sqrt{3}a$ superstructure that is very likely associated with the ordering of the potassium ions. A semiconductor-to-metal transition occurs signified by the disappearance of the excitonic features from the electron energy-loss spectra and the emergence of a charge carrier plasmon with an unscreened plasmon frequency of 2.78 eV. The plasmon has a positive, quadratic dispersion and appears to be superimposed with an excitation arising from interband transitions. The behavior of the plasmon peak energy positions as a function of potassium concentration shows that potassium stoichiometries of less than $\sim 0.3$ are thermodynamically unstable while higher stoichiometries up to $\sim 0.5$ are thermodynamically stable. Potassium concentrations greater than $\sim 0.5$ lead to the decomposition of MoS$_2$ and the formation of K$_2$S. The real part of the dielectric function and the optical conductivity of K$_{0.41}$MoS$_2$ were derived from the loss spectra via Kramers-Kronig analysis. △ Less

Submitted 27 April, 2020; originally announced April 2020.

Comments: 7 Figures, 2 Tables

Journal ref: Phys. Rev. B 101, 155429 (2020)

Potassium-intercalated bulk HfS$_2$ and HfSe$_2$: Phase stability, structure, and electronic structure

Authors: Carsten Habenicht, Jochen Simon, Manuel Richter, Roman Schuster, Martin Knupfer, Bernd Büchner

Abstract: We have studied potassium-intercalated bulk HfS$_2$ and HfSe$_2$ by combining transmission electron energy loss spectroscopy, angle-resolved photoemission spectroscopy and density functional theory calculations. Calculations of the formation energies and the evolution of the energies of the charge carrier plasmons as a function of the potassium content show that certain, low potassium concentratio… ▽ More We have studied potassium-intercalated bulk HfS$_2$ and HfSe$_2$ by combining transmission electron energy loss spectroscopy, angle-resolved photoemission spectroscopy and density functional theory calculations. Calculations of the formation energies and the evolution of the energies of the charge carrier plasmons as a function of the potassium content show that certain, low potassium concentrations $x$ are thermodynamically unstable. This leads to the coexistence of undoped and doped domains if the provided amount of the alkali metal is insufficient to saturate the whole crystal with the minimum thermodynamically stable potassium stoichiometry. Beyond this threshold concentration the domains disappear, while the alkali metal and charge carrier concentrations increase continuously upon further addition of potassium. At low intercalation levels, electron diffraction patterns indicate a significant degree of disorder in the crystal structure. The initial order in the out-of-plane direction is restored at high $x$ while the crystal layer thicknesses expand by 33-36%. Superstructures emerge parallel to the planes which we attribute to the distribution of the alkali metal rather than structural changes of the host materials. The in-plane lattice parameters change by not more than 1%. The introduction of potassium causes the formation of charge carrier plasmons. The observation of this semiconductor-to-metal transition is supported by calculations of the density of states (DOS) and band structures as well as angle-resolved photoemission spectroscopy. The calculated DOS hint at the presence of an almost ideal two-dimensional electron gas at the Fermi level for $x<0.6$. The plasmons exhibit quadratic momentum dispersions which is in agreement with the behavior expected for an ideal electron gas. △ Less

Submitted 14 March, 2020; originally announced March 2020.

Comments: 15 Figures

Journal ref: Phys. Rev. Materials 4, 064002 (2020)

Evidence for an orbital dependent Mott transition in the ladders of (La,Ca)$_x$Sr$_{14-x}$Cu$_{24}$O$_{41}$ derived by electron energy-loss spectroscopy

Authors: Friedrich Roth, Alexandre Revcolevschi, Bernd Büchner, Martin Knupfer, Jörg Fink

Abstract: The knowledge of the charge carrier distribution among the different orbitals of Cu and O is a precondition for the understanding of the physical properties of various Cu-O frameworks. We employ electron energy-loss spectroscopy to elucidate the charge carrier plasmon dispersion in (La, Ca)$_x$Sr$_{14-x}$Cu$_{24}$O$_{41}$ in dependency of $x$ as well as temperature. We observe that the energy of t… ▽ More The knowledge of the charge carrier distribution among the different orbitals of Cu and O is a precondition for the understanding of the physical properties of various Cu-O frameworks. We employ electron energy-loss spectroscopy to elucidate the charge carrier plasmon dispersion in (La, Ca)$_x$Sr$_{14-x}$Cu$_{24}$O$_{41}$ in dependency of $x$ as well as temperature. We observe that the energy of the plasmon increases upon increasing Ca content, which signals an internal charge redistribution between the two Cu-O subsystems. Moreover, contrary to an uncorrelated model we come to the conclusion that the holes transferred to the Cu$_2$O$_3$ ladders are mainly located in the bonding and not in the anti-bonding band. This is caused by an orbital dependent Mott transition. △ Less

Submitted 25 March, 2020; v1 submitted 7 January, 2020; originally announced January 2020.

Report number: Phys. Rev. B 101, 195132 (2020)

Journal ref: Phys. Rev. B 101, 195132 (2020)

Mapping of the energetically lowest exciton in bulk $1T$-HfS$_2$

Authors: Carsten Habenicht, Lorenzo Sponza, Roman Schuster, Martin Knupfer, Bernd Büchner

Abstract: By combining electron energy-loss spectroscopy and state-of-the-art computational methods, we were able to provide an extensive picture of the excitonic processes in $1T$-HfS$_2$. The results differ significantly from the properties of the more scrutinized group VI semiconducting transition metal dichalcogenides such as MoS$_2$ and WSe$_2$. The measurements revealed a parabolic exciton dispersion… ▽ More By combining electron energy-loss spectroscopy and state-of-the-art computational methods, we were able to provide an extensive picture of the excitonic processes in $1T$-HfS$_2$. The results differ significantly from the properties of the more scrutinized group VI semiconducting transition metal dichalcogenides such as MoS$_2$ and WSe$_2$. The measurements revealed a parabolic exciton dispersion for finite momentum $\textbf{q}$ parallel to the $Γ$K direction which allowed the determination of the effective exciton mass. The dispersion decreases monotonically for momentum exchanges parallel to the $Γ$M high symmetry line. To gain further insight into the excitation mechanisms, we solved the ab-initio Bethe-Salpeter equation for the system. The results matched the experimental loss spectra closely, thereby confirming the excitonic nature of the observed transitions, and produced the momentumdependent binding energies. The simulations also demonstrated that the excitonic transitions for $\textbf{q}$ || $Γ$M occur exactly along that particular high symmetry line. For $\textbf{q}$ || $Γ$K on the other hand, the excitations traverse the Brillouin zone crossing various high symmetry lines. A particular interesting aspect of our findings was that the calculation of the electron probability density revealed that the exciton assumes a six-pointed star-like shape along the real space crystal planes indicating a mixed Frenkel-Wannier character. △ Less

Submitted 11 October, 2018; v1 submitted 5 August, 2018; originally announced August 2018.

Comments: 12 pages, 10 figures

Journal ref: Phys. Rev. B 98, 155204 (2018)

Investigation of indirect excitons in bulk $2H$-MoS$_2$ using transmission electron energy-loss spectroscopy

Authors: Carsten Habenicht, Roman Schuster, Martin Knupfer, Bernd Büchner

Abstract: We have investigated indirect excitons in bulk $2H$-MoS$_2$ using transmission electron energy-loss spectroscopy. The electron energy-loss spectra were measured for various momentum transfer values parallel to the $Γ$K and $Γ$M directions of the Brillouin zone. The results allowed the identification of the indirect excitons between the valence band K$_{\mathrm{v}}$ and conduction band… ▽ More We have investigated indirect excitons in bulk $2H$-MoS$_2$ using transmission electron energy-loss spectroscopy. The electron energy-loss spectra were measured for various momentum transfer values parallel to the $Γ$K and $Γ$M directions of the Brillouin zone. The results allowed the identification of the indirect excitons between the valence band K$_{\mathrm{v}}$ and conduction band $Λ_{\mathrm{c}}$ points, the $Γ_{\mathrm{v}}$ and K$_{\mathrm{c}}$ points as well as adjacent K$_{\mathrm{v}}$ and K$^{\prime}_\textrm{c}$ points. The energy-momentum dispersions for the K$_{\mathrm{v}}$-$Λ_{\mathrm{c}}$, $Γ_{\mathrm{v}}$-K$_{\mathrm{c}}$ and K$_{\mathrm{v1}}$-K$^{\prime}_\textrm{c}$ excitons along the $Γ$K line are presented. The former two transitions exhibit a quadratic dispersion which allowed calculating their effective exciton masses based on the effective mass approximation. The K$_\mathrm{v1}$-K$^{\prime}_\textrm{c}$ transition follows a more linear dispersion relationship. △ Less

Submitted 4 May, 2018; originally announced May 2018.

Comments: 8 pages, 4 figures

Journal ref: Carsten Habenicht et al 2018 J. Phys.: Condens. Matter 30 205502

Nearest-neighbor Kitaev exchange blocked by charge order in electron doped $α$-RuCl$_{3}$

Authors: A. Koitzsch, C. Habenicht, E. Mueller, M. Knupfer, B. Buechner, S. Kretschmer, M. Richter, J. van den Brink, F. Boerrnert, D. Nowak, A. Isaeva, Th. Doert

Abstract: A quantum spin-liquid might be realized in $α$-RuCl$_{3}$, a honeycomb-lattice magnetic material with substantial spin-orbit coupling. Moreover, $α$-RuCl$_{3}$ is a Mott insulator, which implies the possibility that novel exotic phases occur upon doping. Here, we study the electronic structure of this material when intercalated with potassium by photoemission spectroscopy, electron energy loss spe… ▽ More A quantum spin-liquid might be realized in $α$-RuCl$_{3}$, a honeycomb-lattice magnetic material with substantial spin-orbit coupling. Moreover, $α$-RuCl$_{3}$ is a Mott insulator, which implies the possibility that novel exotic phases occur upon doping. Here, we study the electronic structure of this material when intercalated with potassium by photoemission spectroscopy, electron energy loss spectroscopy, and density functional theory calculations. We obtain a stable stoichiometry at K$_{0.5}$RuCl$_3$. This gives rise to a peculiar charge disproportionation into formally Ru$^{2+}$ (4$d^6$) and Ru$^{3+}$ (4$d^5$). Every Ru 4$d^5$ site with one hole in the $t_{2g}$ shell is surrounded by nearest neighbors of 4$d^6$ character, where the $t_{2g}$ level is full and magnetically inert. Thus, each type of Ru sites forms a triangular lattice and nearest-neighbor interactions of the original honeycomb are blocked. △ Less

Submitted 13 October, 2017; v1 submitted 15 September, 2017; originally announced September 2017.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Materials 1, 052001(R) (2017)

Low temperature enhancement of ferromagnetic Kitaev correlations in α-RuCl3

Authors: Andreas Koitzsch, Eric Mueller, Martin Knupfer, Bernd Buechner, Domenic Nowak, Anna Isaeva, Thomas Doert, Markus Grueninger, Satoshi Nishimoto, Jeroen van den Brink

Abstract: Kitaev-type interactions between neighbouring magnetic moments emerge in the honeycomb material $α$-RuCl3. It is debated however whether these Kitaev interactions are ferromagnetic or antiferromagnetic. With electron energy loss spectroscopy (EELS) we study the lowest excitation across the Mott-Hubbard gap, which involves a d4 triplet in the final state and therefore is sensitive to nearest-neighb… ▽ More Kitaev-type interactions between neighbouring magnetic moments emerge in the honeycomb material $α$-RuCl3. It is debated however whether these Kitaev interactions are ferromagnetic or antiferromagnetic. With electron energy loss spectroscopy (EELS) we study the lowest excitation across the Mott-Hubbard gap, which involves a d4 triplet in the final state and therefore is sensitive to nearest-neighbor spin-spin correlations. At low temperature the spectral weight of these triplets is strongly enhanced, in accordance with optical data. We show that the magnetic correlation function that determines this EELS spectral weight is directly related to a Kitaev-type spin-spin correlator and that the temperature dependence agrees very well with the results of a microscopic magnetic Hamiltonian for $α$-RuCl3 with ferromagnetic Kitaev coupling. △ Less

Submitted 8 September, 2017; originally announced September 2017.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. Materials 4, 094408 (2020)

Direct Observation of the Lowest Indirect Exciton State in the Bulk of Hexagonal Boron Nitride

Authors: R. Schuster, C. Habenicht, M. Ahmad, M. Knupfer, B. Büchner

Abstract: We combine electron energy-loss spectroscopy and first-principles calculations based on density-functional theory (DFT) to identify the lowest indirect exciton state in the in-plane charge response of hexagonal boron nitride (h-BN) single crystals. This remarkably sharp mode forms a narrow pocket with a dispersion bandwidth of $\sim 100~\text{meV}$ and, as we argue based on a comparison to our DFT… ▽ More We combine electron energy-loss spectroscopy and first-principles calculations based on density-functional theory (DFT) to identify the lowest indirect exciton state in the in-plane charge response of hexagonal boron nitride (h-BN) single crystals. This remarkably sharp mode forms a narrow pocket with a dispersion bandwidth of $\sim 100~\text{meV}$ and, as we argue based on a comparison to our DFT calculations, is predominantly polarized along the $Γ\text{K}$-direction of the hexagonal Brillouin zone. Our data support the recent report by Cassabois \emph{et al.} (Nat. Photon. \textbf{10}, 262) who indirectly inferred the existence of this mode from the photoluminescence signal, thereby establishing h-BN as an indirect semiconductor. △ Less

Submitted 15 June, 2017; originally announced June 2017.

Journal ref: Phys. Rev. B 97, 041201 (2018)

Transport band gap opening at metal-organic interfaces

Authors: Francisc Haidu, Georgeta Salvan, Dietrich R. T. Zahn, Lars Smykalla, Michael Hietschold, Martin Knupfer

Abstract: The interface formation between copper phthalocyanine (CuPc) and two representative metal substrates, i.e., Au and Co, was investigated by the combination of ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The occupied and unoccupied molecular orbitals and thus the transport band gap of CuPc are highly influenced by film thickness, i.e., molecule-substrate distance.… ▽ More The interface formation between copper phthalocyanine (CuPc) and two representative metal substrates, i.e., Au and Co, was investigated by the combination of ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The occupied and unoccupied molecular orbitals and thus the transport band gap of CuPc are highly influenced by film thickness, i.e., molecule-substrate distance. Due to the image charge potential given by the metallic substrates the transport band gap of CuPc "opens" from $(1.4 \pm 0.3)$ eV for 1 nm thickness to $(2.2 \pm 0.3)$ eV, and saturates at this value above 10 nm CuPc thickness. The interface dipoles with values of 1.2 eV and 1.0 eV for Au and Co substrates, respectively, predominantly depend on the metal substrate work functions. X-ray photoelectron spectroscopy measurements using synchrotron radiation provide detailed information on the interaction between CuPc and the two metal substrates. While charge transfer from the Au or Co substrate to the Cu metal center is present only at sub-monolayer coverages, the authors observe a net charge transfer from the molecule to the Co substrate for films in the nm range. Consequently, the Fermi level is shifted as in the case of a p-type doping of the molecule. This is, however, a competing phenomenon to the energy band shifts due to the image charge potential. △ Less

Submitted 21 February, 2017; originally announced February 2017.

Comments: 14 pages, 4 figures

Journal ref: J. Vac. Sci. Technol. A 32, 040602 (2014)

Electronic Structure of Manganese Phthalocyanine Modified via Potassium Intercalation: a Comprehensive Experimental Study

Authors: Francisc Haidu, Ovidiu D. Gordan, Dietrich R. T. Zahn, Lars Smykalla, Michael Hietschold, Boris V. Senkovskiy, Benjamin Mahns, Martin Knupfer

Abstract: Potassium (K) intercalated manganese phthalocyanine (MnPc) reveals vast changes of its electronic states close to the Fermi level. However, theoretical studies are controversial regarding the electronic configuration. Here, MnPc doped with K was studied by ultraviolet, X-ray, and inverse photoemission, as well as near edge X-ray absorption fine structure spectroscopy. Upon K intercalation the Ferm… ▽ More Potassium (K) intercalated manganese phthalocyanine (MnPc) reveals vast changes of its electronic states close to the Fermi level. However, theoretical studies are controversial regarding the electronic configuration. Here, MnPc doped with K was studied by ultraviolet, X-ray, and inverse photoemission, as well as near edge X-ray absorption fine structure spectroscopy. Upon K intercalation the Fermi level shifts toward the lowest unoccupied molecular orbital filling it up with donated electrons with the appearance of an additional feature in the energy region of the occupied states. The electronic bands are pinned 0.5 eV above and 0.4 eV below the Fermi level. The branching ratio of the Mn L3 and L2 edges indicate an increase of the spin state. Moreover, the evolution of the Mn L and N K edges reveals strong hybridization between Mn 3d and N 2p states of MnPc and sheds light on the electron occupation in the ground and n-doped configurations. △ Less

Submitted 12 February, 2017; originally announced February 2017.

Comments: 28 pages, 6 figures + abstract graphic

Semiconductor-to-Metal Transition in the Bulk of WSe2 upon Potassium Intercalation

Authors: M. Ahmad, E. Müller, C. Habenicht, R. Schuster, M. Knupfer, B. Büchner

Abstract: We present electron energy-loss spectroscopic measurements of potassium (K) intercalated tungsten diselenide (WSe2). After exposure of pristine WSe2 films to potassium, we observe a charge carrier plasmon excitation at about 0.97 eV, which indicates a semiconductor to metal transition. Our data signal the formation of one particular doped K-WSe2 phase. A Kramers-Kronig analysis (KKA) allows the de… ▽ More We present electron energy-loss spectroscopic measurements of potassium (K) intercalated tungsten diselenide (WSe2). After exposure of pristine WSe2 films to potassium, we observe a charge carrier plasmon excitation at about 0.97 eV, which indicates a semiconductor to metal transition. Our data signal the formation of one particular doped K-WSe2 phase. A Kramers-Kronig analysis (KKA) allows the determination of the dielectric function and to estimate the composition of about K0.6WSe2. Momentum dependent measurements reveal a substantial plasmon dispersion to higher energies. △ Less

Submitted 23 January, 2017; originally announced January 2017.

Non-Generic Dispersion of Excitons in the Bulk of WSe$_2$

Authors: R. Schuster, Y. Wan, M. Knupfer, B. Büchner

Abstract: We combine electron energy-loss spectroscopy (EELS) and density functional theory (DFT) calculations to study the dispersion and effective mass of excitons in the bulk of WSe$_2$. Our EELS data suggest substantial deviations from the generic quadratic momentum dependence along the $ΓK$-direction. From the DFT-derived Kohn-Sham states we deduce the EELS response without the inclusion of particle-ho… ▽ More We combine electron energy-loss spectroscopy (EELS) and density functional theory (DFT) calculations to study the dispersion and effective mass of excitons in the bulk of WSe$_2$. Our EELS data suggest substantial deviations from the generic quadratic momentum dependence along the $ΓK$-direction. From the DFT-derived Kohn-Sham states we deduce the EELS response without the inclusion of particle-hole attraction to study the possible role of the single-particle band structure on the exciton behavior. Based on this analysis we argue in favor of a strongly momentum dependent particle-hole interaction in WSe$_2$ and other group VI-transition-metal dichalcogenides. △ Less

Submitted 7 April, 2016; originally announced April 2016.

Journal ref: Phys. Rev. B 94, 085201 (2016)

J$_{eff}$ description of the honeycomb Mott insulator $α$-RuCl$_3$

Authors: A. Koitzsch, C. Habenicht, E. Mueller, M. Knupfer, B. Buechner, H. Kandpal, J. van den Brink, D. Nowak, A. Isaeva, Th. Doert

Abstract: Novel ground states might be realized in honeycomb lattices with strong spin-orbit coupling. Here we study the electronic structure of $α$-RuCl$_3$, in which the Ru ions are in a d5 configuration and form a honeycomb lattice, by angle-resolved photoemission, x-ray photoemission and electron energy loss spectroscopy supported by density functional theory and multiplet calculations. We find that… ▽ More Novel ground states might be realized in honeycomb lattices with strong spin-orbit coupling. Here we study the electronic structure of $α$-RuCl$_3$, in which the Ru ions are in a d5 configuration and form a honeycomb lattice, by angle-resolved photoemission, x-ray photoemission and electron energy loss spectroscopy supported by density functional theory and multiplet calculations. We find that $α$-RuCl$_3$ is a Mott insulator with significant spin-orbit coupling, whose low energy electronic structure is naturally mapped onto Jeff states. This makes $α$-RuCl$_3$ a promising candidate for the realization of Kitaev physics. Relevant electronic parameters such as the Hubbard energy U, the crystal field splitting 10Dq and the charge transfer energy are evaluated. Furthermore, we observe significant Cl photodesorption with time, which must be taken into account when interpreting photoemission and other surface sensitive experiments. △ Less

Submitted 17 March, 2016; originally announced March 2016.

Comments: 6 pages, 6 figures

Journal ref: Phys. Rev. Lett. 117, 126403 (2016)

Electron-energy-loss and time-dependent density functional theory study on the plasmon dispersion in 2H-NbS2

Authors: Pierluigi Cudazzo, Eric Müller, Carsten Habenicht, Matteo Gatti, Helmuth Berger, Martin Knupfer, Angel Rubio, Simo Huotari

Abstract: We examine the experimental and theoretical electron-energy loss spectra in 2$H$-Cu$_{0.2}$NbS$_2$ and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range $q<0.35$ Å$^{-1}$. We argue that for a sto… ▽ More We examine the experimental and theoretical electron-energy loss spectra in 2$H$-Cu$_{0.2}$NbS$_2$ and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range $q<0.35$ Å$^{-1}$. We argue that for a stoichiometric pure 2$H$-NbS$_2$ the dispersion relation is expected to have a negative slope as is the case for other transition-metal dichalcogenides. The presence of Cu impurities, required to stabilize the crystal growth, tends to shift the negative plasmon dispersion into a positive one, but the doping level in the current system is small enough to result in a nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon dispersion is not connected with the existence of a charge-density-wave order in transition metal dichalcogenides. △ Less

Submitted 13 January, 2017; v1 submitted 10 March, 2016; originally announced March 2016.

Journal ref: New Journal of Physics 18, 103050 (2016)

Charge transfer, band-like transport, and magnetic ions at F16CoPc/rubrene interfaces

Authors: Yulia Krupskaya, Florian Rückerl, Martin Knupfer, Alberto F. Morpurgo

Abstract: Organic semiconductors offer an unprecedented flexibility to control the electronic state of interfacial electronic systems. Here we present a first step in realizing organic charge transfer interfaces that combine both a large electrical conductivity and the presence of magnetic ions. We have performed a detailed investigation of F16CoPc/rubrene interface by means of temperature dependent charge… ▽ More Organic semiconductors offer an unprecedented flexibility to control the electronic state of interfacial electronic systems. Here we present a first step in realizing organic charge transfer interfaces that combine both a large electrical conductivity and the presence of magnetic ions. We have performed a detailed investigation of F16CoPc/rubrene interface by means of temperature dependent charge transport measurements, Hall effect, scanning Kelvin probe microscopy and photoelectron spectroscopy. Our finding is that the charge transfer leads to significantly enhanced electrical conductivity and the band-like transport. We have determined the density, mobility and nature of charge carriers in the system (holes in rubrene). We have also found that the amount of charge transfer in F16CoPc/rubrene is high enough to cause the band-like transport in rubrene crystals at the interface. Finally, our XPS and UPS measurements have shown that the charge transfer in F16CoPc/rubrene involves electronic orbitals centered on the magnetic Co ions of the phthalocyanine molecules causing a change in their spin. Thus, F16CoPc/rubrene is the first organic interface where the charge transfer responsible for the interfacial conductivity fully involves the metal Co core of the phthalocyanine molecules, providing a link between charge transport and magnetic properties. △ Less

Submitted 29 February, 2016; originally announced February 2016.

Electronic properties of Mn-Phthalocyanine - C$_{60}$ bulk heterojunctions: combining photoemission and electron energy-loss spectroscopy

Authors: Friedrich Roth, Melanie Herzig, Cosmin Lupulescu, Erik Darlatt, Alexander Gottwald, Martin Knupfer, Wolfgang Eberhardt

Abstract: The electronic properties of co-evaporated mixtures (blends) of manganese phthalocyanine and the fullerene C$_{60}$ (MnPc:C$_{60}$) have been studied as a function of the concentration of the two constituents using two supplementary electron spectroscopic methods, photoemission spectroscopy (PES) as well as electron energy-loss spectroscopy (EELS) in transmission. Our PES measurements provide a de… ▽ More The electronic properties of co-evaporated mixtures (blends) of manganese phthalocyanine and the fullerene C$_{60}$ (MnPc:C$_{60}$) have been studied as a function of the concentration of the two constituents using two supplementary electron spectroscopic methods, photoemission spectroscopy (PES) as well as electron energy-loss spectroscopy (EELS) in transmission. Our PES measurements provide a detailed picture of the electronic structure measured with different excitation energies as well as different mixing ratios between MnPc and C$_{60}$. Besides a relative energy shift, the occupied electronic states of the two materials remain essentially unchanged. The observed energy level alignment is different compared to that of the related CuPc:C$_{60}$ bulk heterojunction. Moreover, the results from our EELS investigations show that despite of the rather small interface interaction the MnPc related electronic excitation spectrum changes significantly by admixing C$_{60}$ to MnPc thin films. △ Less

Submitted 3 November, 2015; originally announced November 2015.

Journal ref: J. Appl. Phys., 118, 185310 (2015)

Low energy exciton pocket at finite momentum in tetracene molecular solids

Authors: Friedrich Roth, Markus Nohr, Silke Hampel, Martin Knupfer

Abstract: The excited state dynamics in organic semiconductors plays an important role for many processes associated with light absorption and emission. We have studied the momentum dependence of the lowest singlet excitons in tetracene molecular solids, an archetype system for other organic semiconductors. Our results reveal an anisotropic bandstructure of these excitons with an energy minimum at finite mo… ▽ More The excited state dynamics in organic semiconductors plays an important role for many processes associated with light absorption and emission. We have studied the momentum dependence of the lowest singlet excitons in tetracene molecular solids, an archetype system for other organic semiconductors. Our results reveal an anisotropic bandstructure of these excitons with an energy minimum at finite momentum, i. e., a low energy exciton pocket. The existence of such low energy states might have important consequences for the photophysical behavior, also in view of applications in, e. g., organic solar cells. Our studies stress the importance of momentum dependent considerations in organic systems. △ Less

Submitted 29 October, 2015; originally announced October 2015.

Journal ref: Europhys. Lett. 112, 37004 (2015)

Investigation of the dispersion and the effective masses of excitons in bulk 2H-MoS2 using transition electron energy-loss spectroscopy

Authors: Carsten Habenicht, Martin Knupfer, Bernd Büchner

Abstract: We have investigated the electronic excitations in bulk 2H-MoS2 using electron energy-loss spectroscopy. The electron energy-loss spectra in the ΓM and ΓK directions were measured for various momentum transfer values. The results allow the identification of the A1 and B1 exciton peaks and in particular their energy-momentum dispersion. The dispersions exhibit approximately quadratic upward trends… ▽ More We have investigated the electronic excitations in bulk 2H-MoS2 using electron energy-loss spectroscopy. The electron energy-loss spectra in the ΓM and ΓK directions were measured for various momentum transfer values. The results allow the identification of the A1 and B1 exciton peaks and in particular their energy-momentum dispersion. The dispersions exhibit approximately quadratic upward trends and slight anisotropies in the ΓM and ΓK directions. The fitted energy-momentum transfer functions allow the estimation of the effective masses of the excitons which are in close proximity to predicted values. △ Less

Submitted 17 June, 2015; v1 submitted 23 May, 2015; originally announced May 2015.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. B 91, 245203 (2015)

Surface properties of SmB6 from x-ray photoelectron spectroscopy

Authors: N. Heming, U. Treske, M. Knupfer, B. Büchner, D. S. Inosov, N. Y. Shitsevalova, V. B. Filipov, S. Krause, A. Koitzsch

Abstract: We have investigated the properties of cleaved SmB$_6$ single crystals by x-ray photoelectron spectroscopy. At low temperatures and freshly cleaved samples a surface core level shift is observed which vanishes when the temperature is increased. A Sm valence between 2.5 - 2.6 is derived from the relative intensities of the Sm$^{2+}$ and Sm$^{3+}$ multiplets. The B/Sm intensity ratio obtained from t… ▽ More We have investigated the properties of cleaved SmB$_6$ single crystals by x-ray photoelectron spectroscopy. At low temperatures and freshly cleaved samples a surface core level shift is observed which vanishes when the temperature is increased. A Sm valence between 2.5 - 2.6 is derived from the relative intensities of the Sm$^{2+}$ and Sm$^{3+}$ multiplets. The B/Sm intensity ratio obtained from the core levels is always larger than the stoichiometric value. Possible reasons for this deviation are discussed. The B $1s$ signal shows an unexpected complexity: an anomalous low energy component appears with increasing temperature and is assigned to the formation of a suboxide at the surface. While several interesting intrinsic and extrinsic properties of the SmB$_6$ surface are elucidated in this manuscript no clear indication of a trivial mechanism for the prominent surface conductivity is found. △ Less

Submitted 10 March, 2015; originally announced March 2015.

Journal ref: Phys. Rev. B 90, (2014), 195128

Anisotropic Particle-Hole Excitations in Black Phosphorus

Authors: R. Schuster, J. Trinckauf, M. Knupfer, B. Büchner

Abstract: We report about the energy and momentum resolved optical response of black phosphorus (BP) in its bulk form. Along the armchair direction of the puckered layers we find a highly dispersive mode that is trongly suppressed in the perpendicular (zig-zag) direction. This mode emerges out of the single-particle continuum for finite values of momentum and is therefore interpreted as an exciton. We argue… ▽ More We report about the energy and momentum resolved optical response of black phosphorus (BP) in its bulk form. Along the armchair direction of the puckered layers we find a highly dispersive mode that is trongly suppressed in the perpendicular (zig-zag) direction. This mode emerges out of the single-particle continuum for finite values of momentum and is therefore interpreted as an exciton. We argue that this exciton, which has already been predicted theoretically for phosphorene -- the monolayer form of BP -- can be detected by conventional optical spectroscopy in the two-dimensional case and might pave the way for optoelectronic applications of this emerging material. △ Less

Submitted 9 March, 2015; originally announced March 2015.

Electronic excitation spectrum of doped organic thin films investigated using electron energy-loss spectroscopy

Authors: Friedrich Roth, Martin Knupfer

Abstract: The electronic excitation spectra of undoped, and potassium as well as calcium doped phenantrene-type hydrocarbons have been investigated using electron energy-loss spectroscopy (EELS) in transmission. In the undoped materials, the lowest energy excitations are excitons with a relatively high binding energy. These excitons also are rather localized as revealed by their vanishing dispersion. Upon d… ▽ More The electronic excitation spectra of undoped, and potassium as well as calcium doped phenantrene-type hydrocarbons have been investigated using electron energy-loss spectroscopy (EELS) in transmission. In the undoped materials, the lowest energy excitations are excitons with a relatively high binding energy. These excitons also are rather localized as revealed by their vanishing dispersion. Upon doping, new low energy excitation features appear in the former gaps of the materials under investigation. In K$_3$picene and K$_3$chrysene they are characterized by a negative dispersion while in Ca$_3$picene they are dispersionless. △ Less

Submitted 25 February, 2015; originally announced February 2015.

Journal ref: J. Electron. Spectrosc. Relat. Phenom. (2015)

Electron Energy-Loss Spectroscopy: A versatile tool for the investigations of plasmonic excitations

Authors: Friedrich Roth, Andreas König, Jörg Fink, Bernd Büchner, Martin Knupfer

Abstract: The inelastic scattering of electrons is one route to study the vibrational and electronic properties of materials. Such experiments, also called electron energy-loss spectroscopy, are particularly useful for the investigation of the collective excitations in metals, the charge carrier plasmons. These plasmons are characterized by a specific dispersion (energy-momentum relationship), which contain… ▽ More The inelastic scattering of electrons is one route to study the vibrational and electronic properties of materials. Such experiments, also called electron energy-loss spectroscopy, are particularly useful for the investigation of the collective excitations in metals, the charge carrier plasmons. These plasmons are characterized by a specific dispersion (energy-momentum relationship), which contains information on the sometimes complex nature of the conduction electrons in topical materials. In this review we highlight the improvements of the electron energy-loss spectrometer in the last years, summarize current possibilities with this technique, and give examples where the investigation of the plasmon dispersion allows insight into the interplay of the conduction electrons with other degrees of freedom. △ Less

Submitted 14 May, 2014; originally announced May 2014.

Journal ref: J. Electron. Spectrosc. Relat. Phenom. 195, 85 (2014)

Observation of strontium segregation in LaAlO$_{3}$/SrTiO$_{3}$ and NdGaO$_{3}$/SrTiO$_{3}$ oxide heterostructures by X-ray photoemission spectroscopy

Authors: Uwe Treske, Nadine Heming, Stefan Krause, Martin Knupfer, Bernd Büchner, Emiliano Di Gennaro, Umberto Scotti di Uccio, Fabio Miletto Granozio, Andreas Koitzsch

Abstract: LaAlO$_{3}$ and NdGaO$_{3}$ thin films of different thickness have been grown by pulsed laser deposition on TiO$_2$-terminated SrTiO$_{3}$ single crystals and investigated by soft X-ray photoemission spectroscopy. The surface sensitivity of the measurements has been tuned by varying photon energy $hν$ and emission angle $Θ$. In contrast to the core levels of the other elements, the Sr $3d$ line sh… ▽ More LaAlO$_{3}$ and NdGaO$_{3}$ thin films of different thickness have been grown by pulsed laser deposition on TiO$_2$-terminated SrTiO$_{3}$ single crystals and investigated by soft X-ray photoemission spectroscopy. The surface sensitivity of the measurements has been tuned by varying photon energy $hν$ and emission angle $Θ$. In contrast to the core levels of the other elements, the Sr $3d$ line shows an unexpected splitting for higher surface sensitivity, signaling the presence of a second strontium component. From our quantitative analysis we conclude that during the growth process Sr atoms diffuse away from the substrate and segregate at the surface of the heterostructure, possibly forming strontium oxide. △ Less

Submitted 31 January, 2014; originally announced January 2014.

Journal ref: APL Mat. 2, 012108 (2014)

Photoemission study of PDI8-CN2 /Au, PDI8-CN2 / Si and PDI8-CN2 - T6 interfaces

Authors: Flavia Viola Di Girolamo, Benjamin Mahns, Martin Knupfer

Abstract: We report on the study of the electronic properties in PDI8-CN2 / Au, PDI8-CN2 / Si, PDI8-CN2 - T6 interfaces investigated by X-Ray (XPS) and Ultraviolet (UPS) Photoemission Spectroscopy. In order to verify the consistency with the results on field effect devices, heated substrates have been considered, while only in the case of PDI8-CN2 / Au interface also sputter-cleaned Au has been used as a re… ▽ More We report on the study of the electronic properties in PDI8-CN2 / Au, PDI8-CN2 / Si, PDI8-CN2 - T6 interfaces investigated by X-Ray (XPS) and Ultraviolet (UPS) Photoemission Spectroscopy. In order to verify the consistency with the results on field effect devices, heated substrates have been considered, while only in the case of PDI8-CN2 / Au interface also sputter-cleaned Au has been used as a reference. We found a small interface dipole between PDI8-CN2 and sputter-cleaned Au, which reverses its sign as a consequence of the 'pillow effect' due to the presence of the contamination layer in the heated substrates. This result is in agreement with the transport measurements on PDI8-CN2 field effect transistors, which show an 'always on' behavior. We also found evidences of the formation of a built-in potential in T6 / PDI8-CN2 and PDI8-CN2 / T6 interfaces, mainly from the T6 side. This also well agrees with previous results on T6 / PDI8-CN2 heterostructures devices, exhibiting transport properties explainable with the presence of an accumulation heterojunction at the interface. △ Less

Submitted 18 November, 2013; originally announced November 2013.

Comments: 18 pages, 8 figures

Band dependent emergence of heavy quasiparticles in CeCoIn5

Authors: A. Koitzsch, T. K. Kim, U. Treske, M. Knupfer, B. Büchner, M. Richter, I. Opahle, R. Follath, E. D. Bauer, J. L. Sarrao

Abstract: We investigate the low temperature (T $<$ 2 K) electronic structure of the heavy fermion superconductor CeCoIn5 (T$_c$ = 2.3 K) by angle-resolved photoemission spectroscopy (ARPES). The hybridization between conduction electrons and f-electrons, which ultimately leads to the emergence of heavy quasiparticles responsible for the various unusual properties of such materials, is directly monitored an… ▽ More We investigate the low temperature (T $<$ 2 K) electronic structure of the heavy fermion superconductor CeCoIn5 (T$_c$ = 2.3 K) by angle-resolved photoemission spectroscopy (ARPES). The hybridization between conduction electrons and f-electrons, which ultimately leads to the emergence of heavy quasiparticles responsible for the various unusual properties of such materials, is directly monitored and shown to be strongly band dependent. In particular the most two-dimensional band is found to be the least hybridized one. A simplified multiband version of the Periodic Anderson Model (PAM) is used to describe the data, resulting in semi-quantitative agreement with previous bulk sensitive results from de-Haas-van-Alphen measurements. △ Less

Submitted 23 July, 2013; originally announced July 2013.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. B 88, 035124 (2013)

Doping Dependence of the Plasmon Dispersion in 2H-Tantalum-Diselenide

Authors: Andreas König, Roman Schuster, Martin Knupfer, Bernd Büchner, Helmuth Berger

Abstract: Inelastic electron scattering is applied to investigate the impact of potassium intercalation on the charge carrier plasmon energy and dispersion in the charge-density wave (CDW) bearing compound 2H-tantalum-diselenide. We observe an unususal doping dependence of the plasmon dispersion, which even changes sign upon alkali addition. In contrast to the continous energy shift of the plasmon position… ▽ More Inelastic electron scattering is applied to investigate the impact of potassium intercalation on the charge carrier plasmon energy and dispersion in the charge-density wave (CDW) bearing compound 2H-tantalum-diselenide. We observe an unususal doping dependence of the plasmon dispersion, which even changes sign upon alkali addition. In contrast to the continous energy shift of the plasmon position upon doping at lowest momentum transfer, its dispersion changes in a rather discontinuous manner. We argue that the observed dynamics can only be explained in a picture, where complex phenomena are taken into account including the suppression of the CDW upon doping as well as the interplay of the CDW and the plasma resonance. △ Less

Submitted 16 May, 2013; v1 submitted 22 April, 2013; originally announced April 2013.

Journal ref: Phys. Rev. B 87, 195119 (2013)

Challenging the nature of low energy plasmon excitations in CaC$_6$ using electron energy-loss spectroscopy

Authors: Friedrich Roth, Andreas König, Christian Kramberger, Thomas Pichler, Bernd Büchner, Martin Knupfer

Abstract: The nature of low energy plasmon excitations plays an important role in understanding the low energy electronic properties and coupling mechanism of different superconducting compounds such as CaC$_6$. Recent ab-initio studies predict a charge carrier intraband plasmon in keeping with a low energy acoustic plasmon. Here, we have studied the low-energy electronic excitations of CaC$_6$ using high-r… ▽ More The nature of low energy plasmon excitations plays an important role in understanding the low energy electronic properties and coupling mechanism of different superconducting compounds such as CaC$_6$. Recent ab-initio studies predict a charge carrier intraband plasmon in keeping with a low energy acoustic plasmon. Here, we have studied the low-energy electronic excitations of CaC$_6$ using high-resolution electron energy-loss spectroscopy in transmission at low temperatures. The analysis of the core-level excitations leads to the conclusion that hybridization between graphite and calcium states plays an essential role in this graphite intercalated compound. Regarding the low energy plasmon excitation, we observe the formation of an intraband (charge carrier) plasmon with a negative dispersion at about 3.5 eV in sound agreement with the theory. Finally, a weak excitation around 1.2 eV with an almost linear dispersion relation can be observed as predicted for an acoustic plasmon that may mediate the superconducting coupling in CaC$_6$. However its optical limit at ~1 eV challenges the theoretical predictions and safely rules out an electronic superconducting coupling mechanism in CaC$_6$. △ Less

Submitted 20 March, 2013; originally announced March 2013.

Comments: 5 figures

Journal ref: EPL 102 17001 (2013)

Exciton properties of selected aromatic hydrocarbon systems

Authors: Friedrich Roth, Benjamin Mahns, Silke Hampel, Markus Nohr, Helmuth Berger, Bernd Büchner, Martin Knupfer

Abstract: We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increas… ▽ More We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules. △ Less

Submitted 9 January, 2013; originally announced January 2013.

Journal ref: Eur. Phys. J. B 86, 66 (2013)

Angular Dependent Spectral-Weight Transfer and Evidence for Symmetry Broken In-Plane Charge Response in Ca$_{1.9}$Na$_{0.1}$CuO$_2$Cl$_2$

Authors: R. Schuster, S. Pyon, M. Knupfer, M. Azuma, M. Takano, H. Takagi, B. Büchner

Abstract: We report about the energy and momentum dependent charge response in Ca$_{1.9}$Na$_{0.1}$CuO$_2$Cl$_2$ employing electron energy-loss spectroscopy. Along the diagonal of the Brillouin zone (BZ) we find a plasmon peak---indicating the presence of metallic states in this momentum region---which emerges as a consequence of substantial spectral-weight transfer from excitations across the charge-transf… ▽ More We report about the energy and momentum dependent charge response in Ca$_{1.9}$Na$_{0.1}$CuO$_2$Cl$_2$ employing electron energy-loss spectroscopy. Along the diagonal of the Brillouin zone (BZ) we find a plasmon peak---indicating the presence of metallic states in this momentum region---which emerges as a consequence of substantial spectral-weight transfer from excitations across the charge-transfer (CT) gap and is the two-particle manifestation of the small Fermi pocket or arc observed with photoemission in this part of the BZ. In contrast, the spectrum along the [100]-direction is almost entirely dominated by CT excitations, reminiscent of the insulating parent compound. We argue that the observed polarization dependent shape of the spectrum is suggestive of a breaking of the underlying tetragonal lattice symmetry, possibly due to fluctuating nematic order in the charge channel. In addition we find the plasmon bandwidth to be suppressed compared to optimally doped cuprates. △ Less

Submitted 21 September, 2012; originally announced September 2012.

Journal ref: Phys. Rev. B 86, 245112, (2012)

Plasmon Evolution and Charge-Density Wave Suppression in Potassium Intercalated Tantalum Diselenide

Authors: A. König, K. Koepernik, R. Schuster, R. Kraus, M. Knupfer, B. Büchner, H. Berger

Abstract: We have investigated the influence of potassium intercalation on the formation of the charge-density wave (CDW) instability in 2H-tantalum diselenide by means of Electron Energy-Loss Spectroscopy and density functional theory. Our observations are consistent with a filling of the conduction band as indicated by a substantial decrease of the plasma frequency in experiment and theory. In addition, e… ▽ More We have investigated the influence of potassium intercalation on the formation of the charge-density wave (CDW) instability in 2H-tantalum diselenide by means of Electron Energy-Loss Spectroscopy and density functional theory. Our observations are consistent with a filling of the conduction band as indicated by a substantial decrease of the plasma frequency in experiment and theory. In addition, elastic scattering clearly points to a destruction of the CDW upon intercalation as can be seen by a vanishing of the corresponding superstructures. This is accompanied by a new superstructure, which can be attributed to the intercalated potassium. Based on the behavior of the c-axis upon intercalation we argue in favor of interlayer-sites for the alkali-metal and that the lattice remains in the 2H-modification. △ Less

Submitted 9 October, 2012; v1 submitted 12 September, 2012; originally announced September 2012.

Evidence for phase formation in potassium intercalated 1,2;8,9-dibenzopentacene

Authors: Friedrich Roth, Andreas König, Benjamin Mahns, Bernd Büchner, Martin Knupfer

Abstract: We have prepared potassium intercalated 1,2;8,9-dibenzopentacene films under vacuum conditions. The evolution of the electronic excitation spectra upon potassium addition as measured using electron energy-loss spectroscopy clearly indicate the formation of particular doped phases with compositions K$_x$dibenzopentacene ($x$ = 1,2,3). Moreover, the stability of these phases as a function of tempera… ▽ More We have prepared potassium intercalated 1,2;8,9-dibenzopentacene films under vacuum conditions. The evolution of the electronic excitation spectra upon potassium addition as measured using electron energy-loss spectroscopy clearly indicate the formation of particular doped phases with compositions K$_x$dibenzopentacene ($x$ = 1,2,3). Moreover, the stability of these phases as a function of temperature has been explored. Finally, the electronic excitation spectra also give insight into the electronic ground state of the potassium doped 1,2;8,9-dibenzopentacene films. △ Less

Submitted 4 June, 2012; originally announced June 2012.

Comments: 6 pages, 5 figures. arXiv admin note: text overlap with arXiv:1201.2002

Journal ref: Eur. Phys. J. B 85, 242 (2012)

Momentum dependence of the excitons in pentacene

Authors: Friedrich Roth, Roman Schuster, Andreas König, Martin Knupfer, Helmuth Berger

Abstract: We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*,b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) feature… ▽ More We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*,b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function. △ Less

Submitted 22 May, 2012; originally announced May 2012.

Comments: 13 pages, 4 figures

Journal ref: J. Chem. Phys. 136, 204708 (2012)

Absence of photoemission from the Fermi level in potassium intercalated picene and coronene films: structure, polaron or correlation physics?

Authors: Benjamin Mahns, Friedrich Roth, Martin Knupfer

Abstract: The electronic structure of potassium intercalated picene and coronene films has been studied using photoemission spectroscopy. Picene has additionally been intercalated using sodium. Upon alkali metal addition core level as well as valence band photoemission data signal a filling of previously unoccupied states of the two molecular materials due to charge transfer from potassium. In contrast to t… ▽ More The electronic structure of potassium intercalated picene and coronene films has been studied using photoemission spectroscopy. Picene has additionally been intercalated using sodium. Upon alkali metal addition core level as well as valence band photoemission data signal a filling of previously unoccupied states of the two molecular materials due to charge transfer from potassium. In contrast to the observation of superconductivity in K_xpicene and K_xcoronene (x ~ 3), none of the films studied shows emission from the Fermi level, i.e. we find no indication for a metallic ground state. Several reasons for this observation are discussed. △ Less

Submitted 21 March, 2012; originally announced March 2012.

Comments: 15 pages, 6 figures

Journal ref: J. Chem. Phys. 136, 134503 (2012)

Electronic structure of undoped and potassium doped coronene investigated by electron energy-loss spectroscopy

Authors: Friedrich Roth, Johannes Bauer, Benjamin Mahns, Bernd Büchner, Martin Knupfer

Abstract: We performed electron energy-loss spectroscopy studies in transmission in order to obtain insight into the electronic properties of potassium intercalated coronene, a recently discovered superconductor with a rather high transition temperature of about 15\,K. A comparison of the loss function of undoped and potassium intercalated coronene shows the appearance of several new peaks in the optical ga… ▽ More We performed electron energy-loss spectroscopy studies in transmission in order to obtain insight into the electronic properties of potassium intercalated coronene, a recently discovered superconductor with a rather high transition temperature of about 15\,K. A comparison of the loss function of undoped and potassium intercalated coronene shows the appearance of several new peaks in the optical gap upon potassium addition. Furthermore, our core level excitation data clearly signal filling of the conduction bands with electrons. △ Less

Submitted 10 January, 2012; originally announced January 2012.

Comments: 15 pages, 5 figures. arXiv admin note: substantial text overlap with arXiv:1102.3286

Journal ref: Phys. Rev. B 85, 014513 (2012)

Unoccupied electronic structure of TiOCl studied by x-ray absorption near-edge spectroscopy

Authors: Sebastian Glawion, Maurits W. Haverkort, Götz Berner, Markus Hoinkis, Gianina Gavrila, Roberto Kraus, Martin Knupfer, Michael Sing, Ralph Claessen

Abstract: We study the unoccupied electronic structure of the spin-1/2 quantum magnet TiOCl by x-ray absorption near-edge spectroscopy (XANES) at the Ti L and O K edges. Data is acquired both in total electron and fluorescence yield mode (TEY and FY, respectively). While only the latter allows to access the unconventional low-temperature spin-Peierls (SP) phase of TiOCl the signal is found to suffer from si… ▽ More We study the unoccupied electronic structure of the spin-1/2 quantum magnet TiOCl by x-ray absorption near-edge spectroscopy (XANES) at the Ti L and O K edges. Data is acquired both in total electron and fluorescence yield mode (TEY and FY, respectively). While only the latter allows to access the unconventional low-temperature spin-Peierls (SP) phase of TiOCl the signal is found to suffer from significant self-absorption in this case. Nevertheless, we conclude from FY data that effects of the SP distortion on the electronic structure are absent within experimental accuracy. The similarity of room-temperature FY and TEY data, the latter not being obscured by self-absorption, allows us to use TEY spectra for comparison with simulations. These are performed by cluster calculations in D$_{4h}$ and D$_{2h}$ symmetries using two different codes. We extract values of the crystal-field splitting (CFS) and parametrize our results in often found notations by Slater, Racah and Butler. In all cases, good agreement with published values from other studies is found. △ Less

Submitted 25 October, 2011; originally announced October 2011.

Journal ref: J. Phys.: Condens. Matter 24, 255602 (2012)

The Effect of Charge Order on the Plasmon Dispersion in Transition-Metal Dichalcogenides

Authors: Jasper van Wezel, Roman Schuster, Andreas König, Martin Knupfer, Jeroen van den Brink, Helmuth Berger, Bernd Büchner

Abstract: We investigate the dispersion of the charge carrier plasmon in the three prototypical charge-density wave bearing transition-metal dichalcogenides 2H-TaSe2, 2H-TaS2 and 2H-NbSe2 employing electron energy-loss spectroscopy. For all three compounds the plasmon dispersion is found to be negative for small momentum transfers. This is in contrast to the generic behavior observed in simple metals as wel… ▽ More We investigate the dispersion of the charge carrier plasmon in the three prototypical charge-density wave bearing transition-metal dichalcogenides 2H-TaSe2, 2H-TaS2 and 2H-NbSe2 employing electron energy-loss spectroscopy. For all three compounds the plasmon dispersion is found to be negative for small momentum transfers. This is in contrast to the generic behavior observed in simple metals as well as the related system 2H-NbS2, which does not exhibit charge order. We present a semiclassical Ginzburg-Landau model which accounts for these observations, and argue that the vicinity to a charge ordered state is thus reflected in the properties of the collective excitations. △ Less

Submitted 5 July, 2011; originally announced July 2011.

Comments: 4 pages, 3 figures

Journal ref: Phys. Rev. Lett., 107, 176404 (2011)

Electronic confinement and ordering instabilities in colossal magnetoresistive bilayer manganites

Authors: J. Trinckauf, T. Hänke, V. Zabolotnyy, T. Ritschel, M. O. Apostu, R. Suryanarayanan, A. Revcolevschi, K. Koepernik, T. K. Kim, M. v. Zimmermann, S. V. Borisenko, M. Knupfer, B. Büchner, J. Geck

Abstract: We present angle-resolved photoemission studies of (La1-zPrz)2-2xSr1+2xMn2O7 with x=0.4 and z=0.1,0.2 and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO2-layer, which in tu… ▽ More We present angle-resolved photoemission studies of (La1-zPrz)2-2xSr1+2xMn2O7 with x=0.4 and z=0.1,0.2 and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO2-layer, which in turn results in a strongly nested Fermi surface. In addition to this, the spectral function also displays clear signatures of an electronic ordering instability well below the Fermi level. The increase of the corresponding interaction strength with z and its magnitude of ~400 meV make the coupling to a bare phonon highly unlikely. Instead we conclude that fluctuating order, involving electronic and lattice degrees of freedom, cause the observed renormalisation of the spectral features. △ Less

Submitted 4 April, 2012; v1 submitted 26 May, 2011; originally announced May 2011.

Comments: 4 pages, 3 figures, if not displayed correctly try Adobe Reader

Journal ref: Phys. Rev. Lett. 108, 016403 (2012)

Electronic excitations of potassium intercalated manganese phthalocyanine investigated by electron energy-loss spectroscopy

Authors: Benjamin Mahns, Friedrich Roth, Mandy Grobosch, Dietrich R. T. Zahn, Martin Knupfer

Abstract: The electronic excitations of manganese phthalocyanine (MnPc) films were studied as a function of potassium doping using electron energy-loss spectroscopy in transmission. Our data reveal doping induced changes in the excitation spectrum, and they provide evidence for the existence of three doped phases: K$_1$MnPc, K$_2$MnPc, and K$_4$MnPc. Furthermore, the addition of electrons first leads to a f… ▽ More The electronic excitations of manganese phthalocyanine (MnPc) films were studied as a function of potassium doping using electron energy-loss spectroscopy in transmission. Our data reveal doping induced changes in the excitation spectrum, and they provide evidence for the existence of three doped phases: K$_1$MnPc, K$_2$MnPc, and K$_4$MnPc. Furthermore, the addition of electrons first leads to a filling of orbitals with strong Mn 3d character, a situation which also affects the magnetic moment of the molecule. △ Less

Submitted 28 April, 2011; originally announced April 2011.

Comments: 12 pages, 8 figures

Journal ref: J. Chem. Phys. 134, 194504 (2011)

Investigation of the In-Gap Electronic Structure of LaAlO3 - SrTiO3 Heterointerfaces by Soft X-ray Spectroscopy

Authors: A. Koitzsch, J. Ocker, M. Knupfer, M. C. Dekker, K. Dörr, B. Büchner, P. Hoffmann

Abstract: We investigated \LAO\ - \STO\ heterointerfaces grown either in oxygen rich or poor atmosphere by soft x-ray spectroscopy. Resonant photoemission across the Ti L$_{2,3}$ absorption edge of the valence band and Ti 2p core level spectroscopy directly monitor the impact of oxygen treatment upon the electronic structure. Two types of Ti$^{3+}$ related charge carriers are identified. One is located at t… ▽ More We investigated \LAO\ - \STO\ heterointerfaces grown either in oxygen rich or poor atmosphere by soft x-ray spectroscopy. Resonant photoemission across the Ti L$_{2,3}$ absorption edge of the valence band and Ti 2p core level spectroscopy directly monitor the impact of oxygen treatment upon the electronic structure. Two types of Ti$^{3+}$ related charge carriers are identified. One is located at the Fermi energy and related to the filling of the \STO\ conduction band. It appears for low oxygen pressure only. The other one is centered at E$_{B}$ $\approx$ 1 eV and independent of the oxygen pressure during growth. It is probably due to defects. The magnitude of both excitations is comparable. It is shown that low oxygen pressure is detrimental for the Ti - O bonding. Our results shed light on the nature of the charge carriers in the vicinity of the \LAO\ - \STO\ interface. △ Less

Submitted 20 December, 2011; v1 submitted 29 March, 2011; originally announced March 2011.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. B 84, 245121 (2011)

Exciton character in picene molecular solids

Authors: Friedrich Roth, Benjamin Mahns, Bernd Büchner, Martin Knupfer

Abstract: We have studied the low-energy electronic excitations of solid picene at 20 K using momentum dependent electron energy-loss spectroscopy. Our results demonstrate the presence of five excitonic features below the transport energy gap of picene, which all are characterized by a negligible dispersion. One of these excitons has not been observed in the optical absorption spectrum of picene molecules i… ▽ More We have studied the low-energy electronic excitations of solid picene at 20 K using momentum dependent electron energy-loss spectroscopy. Our results demonstrate the presence of five excitonic features below the transport energy gap of picene, which all are characterized by a negligible dispersion. One of these excitons has not been observed in the optical absorption spectrum of picene molecules in solution and thus is assigned to a (solid-state induced) charge transfer exciton. This conclusion is supported by the momentum dependent intensity variation of this exciton which clearly signals a significant dipole forbidden contribution, in contrast to the other low energy excitations. △ Less

Submitted 21 March, 2011; originally announced March 2011.

Comments: 11 pages, 3 figures

Journal ref: Phys. Rev. B 83, 165436 (2011)

Signatures of electronic polarons in La$_{1-x}$Sr$_{1+x}$MnO$_4$ observed by electron energy-loss spectroscopy

Authors: R. Kraus, M. Schrade, R. Schuster, M. Knupfer, A. Revcolevschi, B. Büchner, J. Geck

Abstract: The dielectric properties of La$_{1-x}$Sr$_{1+x}$MnO$_4$ single crystals with x = 0, 0.125, 0.25, and 0.5 were studied by means of electron energy-loss spectroscopy as a function of temperature and momentum transfer. A clear signature of the doped holes is observed around 1.65 eV energy loss, where spectral weight emerges with increasing x. For all $x \neq 0$, this doping-induced excitation can pr… ▽ More The dielectric properties of La$_{1-x}$Sr$_{1+x}$MnO$_4$ single crystals with x = 0, 0.125, 0.25, and 0.5 were studied by means of electron energy-loss spectroscopy as a function of temperature and momentum transfer. A clear signature of the doped holes is observed around 1.65 eV energy loss, where spectral weight emerges with increasing x. For all $x \neq 0$, this doping-induced excitation can propagate within the ab-plane, as revealed by a clear upward dispersion of the corresponding loss peak with increasing momentum transfer. The hole-induced excitation also shifts to higher energies with the onset of magnetic correlations for x = 0.5, implying a strong coupling of charge and spin dynamics. We conclude that (i) the loss feature at 1.65 eV is a signature of electronic polarons, which are created around doped holes, and that (ii) this low-energy excitation involves the charge transfer between manganese and oxygen. The finite dispersion of these excitations further indicates significant polaron-polaron interactions. △ Less

Submitted 2 March, 2011; originally announced March 2011.

Comments: 7 pages, 4 figures

The catalytic potential of high-k dielectrics for graphene formation

Authors: Andrew Scott, Arezoo Dianat, Felix Börrnert, Alicja Bachmatiuk, Shasha Zhang, Jamie H. Warner, Ewa Borowiak-Palen, Martin Knupfer, Bernd Büchner, Gianaurelio Cuniberti, Mark H. Rümmeli

Abstract: The growth of single and multilayer graphene nano-flakes on MgO and ZrO2 at low temperatures is shown through transmission electron microscopy. The graphene nano-flakes are ubiquitously anchored at step edges on MgO (100) surfaces. Density functional theory investigations on MgO (100) indicate C2H2 decomposition and carbon adsorption at step-edges. Hence, both the experimental and theoretical data… ▽ More The growth of single and multilayer graphene nano-flakes on MgO and ZrO2 at low temperatures is shown through transmission electron microscopy. The graphene nano-flakes are ubiquitously anchored at step edges on MgO (100) surfaces. Density functional theory investigations on MgO (100) indicate C2H2 decomposition and carbon adsorption at step-edges. Hence, both the experimental and theoretical data highlight the importance of step sites for graphene growth on MgO. △ Less

Submitted 25 February, 2011; originally announced February 2011.

Journal ref: Appl. Phys. Lett. 98, 073110 (2011)

Dynamic response and electronic structure of potassium doped picene investigated by electron energy-loss spectroscopy

Authors: Friedrich Roth, Benjamin Mahns, Bernd Büchner, Martin Knupfer

Abstract: We performed electron energy-loss spectroscopy studies in order to get a deeper insight into the electronic properties of potassium intercalated picene, a recently discovered superconductor. A comparison of the loss function of the undoped and doped compound shows the appearance of a new peak in the optical gap, which we attribute to the charge carrier plasmon. We find a dramatic increase for the… ▽ More We performed electron energy-loss spectroscopy studies in order to get a deeper insight into the electronic properties of potassium intercalated picene, a recently discovered superconductor. A comparison of the loss function of the undoped and doped compound shows the appearance of a new peak in the optical gap, which we attribute to the charge carrier plasmon. We find a dramatic increase for the value of the background dielectric constant $ε_{\infty}$ upon doping. Our core level excitation data clearly signal filling of the conduction bands with electrons upon potassium addition. △ Less

Submitted 16 February, 2011; originally announced February 2011.

Comments: 13 pages, 5 figures

Journal ref: Phys. Rev. B 83, 144501 (2011)

Plasmons and Interband Transitions of Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$ investigated by Electron Energy-Loss Spectroscopy

Authors: Friedrich Roth, Christian Hess, Bernd Büchner, Udo Ammerahl, Alexandre Revcolevschi, Martin Knupfer

Abstract: Electron energy-loss spectroscopy studies have been performed in order to get a deeper insight into the electronic structure and elementary excitations of the two-leg ladder system Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$. We find a strong anisotropy of the loss function for momentum transfers along the a and c-crystallographic axis, and a remarkable linear plasmon dispersion for a momentum transfer paral… ▽ More Electron energy-loss spectroscopy studies have been performed in order to get a deeper insight into the electronic structure and elementary excitations of the two-leg ladder system Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$. We find a strong anisotropy of the loss function for momentum transfers along the a and c-crystallographic axis, and a remarkable linear plasmon dispersion for a momentum transfer parallel to the legs of the ladders. The investigated spectral features are attributed to localized and delocalized charge-transfer excitations and the charge carrier plasmon. The charge carrier plasmon position and dispersion in the long wave-length limit agree well with expectations based upon the band structure of the two-leg ladder, while the observed quasi-linear plasmon dispersion might be related to the peculiar properties of underdoped cuprates in general. △ Less

Submitted 19 November, 2010; originally announced November 2010.

Comments: 16 pages, 8 figures

Journal ref: Phys. Rev. B 82, 245110 (2010)

Electronic properties of molecular solids: the peculiar case of solid Picene

Authors: Friedrich Roth, Matteo Gatti, Pierluigi Cudazzo, Mandy Grobosch, Benjamin Mahns, Bernd Büchner, Angel Rubio, Martin Knupfer

Abstract: Recently, a new organic superconductor, K-intercalated Picene with high transition temperatures $T_c$ (up to 18\,K) has been discovered. We have investigated the electronic properties of the undoped relative, solid picene, using a combination of experimental and theoretical methods. Our results provide detailed insight into the occuopied and unoccupied electronic states. Recently, a new organic superconductor, K-intercalated Picene with high transition temperatures $T_c$ (up to 18\,K) has been discovered. We have investigated the electronic properties of the undoped relative, solid picene, using a combination of experimental and theoretical methods. Our results provide detailed insight into the occuopied and unoccupied electronic states. △ Less

Submitted 21 September, 2010; originally announced September 2010.

Journal ref: New J. Phys. 12 (2010) 103036

Evidence for Pauli-limiting behaviour at high fields and enhanced upper critical fields near T_c in several disordered FeAs based Superconductors

Authors: G. Fuchs, S. -L. Drechsler, N. Kozlova, M. Bartkowiak, G. Behr, K. Nenkov, H. -H. Klauss, J. Freudenberger, M. Knupfer, F. Hammerath, G. Lang, H. -J. Grafe, B. Buechner, L. Schultz

Abstract: We report resistivity and upper critical field B_c2(T) data for disordered (As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field range up to 60 T. These samples exhibit a slightly enhanced superconducting transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4 T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K above 30 T. The… ▽ More We report resistivity and upper critical field B_c2(T) data for disordered (As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field range up to 60 T. These samples exhibit a slightly enhanced superconducting transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4 T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K above 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0) approximately 63 T. We compare our results with B_c2(T)-data from the literature for clean and disordered samples. Whereas clean samples show almost no PLB for fields below 60 to 70 T, the hitherto unexplained pronounced flattening of B_c2(T) for applied fields H II ab observed for several disordered closely related systems is interpreted also as a manifestation of PLB. Consequences are discussed in terms of disorder effects within the frames of (un)conventional superconductivity, respectively. △ Less

Submitted 14 August, 2009; originally announced August 2009.

Comments: 2 pages, 3 figures, submitted to M2S Tokyo 09

Fermi surface of Ba{1-x}KxFe2As2 as probed by angle-resolved photoemission

Authors: V. B. Zabolotnyy, D. V. Evtushinsky, A. A. Kordyuk, D. S. Inosov, A. Koitzsch, A. V. Boris, G. L. Sun, C. T. Lin, M. Knupfer, B. Buechner, A. Varykhalov, R. Follath, S. V. Borisenko

Abstract: Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba1-xKxFe2As2 (Tc = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected… ▽ More Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba1-xKxFe2As2 (Tc = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected at the Brillouin zone boundary. At the X-symmetry point the Fermi surface is formed by a shallow electron-like pocket surrounded by four hole-like pockets elongated in G-X and G-Y directions. △ Less

Submitted 28 April, 2009; originally announced April 2009.

Journal ref: Physica C 469, 448-451 (2009)

New insight into the physics of iron pnictides from optical and penetration depth data

Authors: S. -L. Drechsler, H. Rosner, M. Grobosch, G. Behr, F. Roth, G. Fuchs, K. Koepernik, R. Schuster, J. Malek, S. Elgazzar, M. Rotter, D. Johrendt, H-H. Klauss, B. Büchner, M. Knupfer

Abstract: We report theoretical values for the unscreened plasma frequencies Omega_p of several Fe pnictides obtained from DFT based calculations within the LDA and compare them with experimental plasma frequencies obtained from reflectivity data. The sizable renormalization observed for all considered compounds points to the presence of many-body effects beyond the LDA. From the large empirical backgroun… ▽ More We report theoretical values for the unscreened plasma frequencies Omega_p of several Fe pnictides obtained from DFT based calculations within the LDA and compare them with experimental plasma frequencies obtained from reflectivity data. The sizable renormalization observed for all considered compounds points to the presence of many-body effects beyond the LDA. From the large empirical background dielectric constant of about 12-15, we estimate a large arsenic polarizability of about 9.5 +- 1.2 Angstroem^3 where the details depend on the polarizabilities of the remaining ions taken from the literature. This large polarizability can significantly reduce the value of the Coulomb repulsion U_d about 4 eV on iron known from iron oxides to a level of 2 eV or below. In general, this result points to rather strong polaronic effects as suggested by G.A. Sawatzky et al., in Refs. arXiv:0808.1390 and arXiv:0811.0214 (Berciu et al.). Possible consequences for the conditions of a formation of bipolarons are discussed, too. From the extrapolated muon spin rotation penetration depth data at T= 0 and the experimental Omega_p we estimate the total coupling constant lambda_tot for the el-boson interaction within the Eliashberg-theory adopting a single band approximation. For LaFeAsO_0.9F_0.1 a weak to intermediately strong coupling regime and a quasi-clean limit behaviour are found. For a pronounced multiband case we obtain a constraint for various intraband coupling constants which in principle allows for a sizable strong coupling in bands with either slow electrons or holes. △ Less

Submitted 5 April, 2009; originally announced April 2009.

Comments: 34 pages, 10 figures, submitted to New Journal of Physics (30.01.2009)