B. Büchner

Technische Universität Dresden, Dresden, Saxony, Germany

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Publications (842)2559.2 Total impact

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    ABSTRACT: A series of new mononuclear and heteronuclear complexes of carboxylate phosphonium betaines with biologically important metals (Zn(II), Cd(II), Hg(II) and Cu(II)) were obtained in aqueous media at ambient temperature. X-ray single crystal diffraction showed that carboxylate phosphonium betaines exhibit versatile complexation abilities, producing monomeric, dimeric and tetrameric homonuclear and mixed metal complexes. The binding mode was found to change in the series of Zn(II), Cd(II) and Hg(II) complexes with an increase of the cation radius. Water and chlorine atoms, bound to the metal, play an important role in crystal formation. In the case of α- and β-substituted phosphabetaines, the crystallization of various stereoisomers was observed including a rare case of co-crystallization of two diastereomers in one crystal.
    CrystEngComm 08/2014; · 3.88 Impact Factor
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    ABSTRACT: A very fundamental and unconventional characteristic of superconductivity in iron-based materials is that it occurs in the vicinity of {\it two} other instabilities. Apart from a tendency towards magnetic order, these Fe-based systems have a propensity for nematic ordering: a lowering of the rotational symmetry while time-reversal invariance is preserved. Setting the stage for superconductivity, it is heavily debated whether the nematic symmetry breaking is driven by lattice, orbital or spin degrees of freedom. Here we report a very clear splitting of NMR resonance lines in FeSe at $T_{nem}$ = 91K, far above superconducting $T_c$ of 9.3 K. The splitting occurs for magnetic fields perpendicular to the Fe-planes and has the temperature dependence of a Landau-type order-parameter. Spin-lattice relaxation rates are not affected at $T_{nem}$, which unequivocally establishes orbital degrees of freedom as driving the nematic order. We demonstrate that superconductivity competes with the emerging nematicity.
    08/2014;
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    ABSTRACT: We report an experimental and theoretical investigation of the electron-boson interaction in KFe2As2 by point-contact (PC) spectroscopy, model, and ab-initio LDA-based calculations for the standard electron-phonon Eliashberg function. The PC spectrum viz. the second derivative of the I - V characteristic of representative PC exhibits a pronounced maximum at about 20meV and surprisingly a featureless behavior at lower and higher energies. We discuss phonon and non-phonon (excitonic) mechanisms for the origin of this peak. Analysis of the underlying source of this peak may be important for the understanding of serious puzzles of superconductivity in this type of compounds.
    08/2014;
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    ABSTRACT: Monolayer graphene grown by chemical vapor deposition and transferred to SiO_2 is used to introduce vacancies by Ar^+ ion bombardment at a kinetic energy of 50 eV. The density of defects visible in scanning tunneling microscopy (STM) is considerably lower than the ion fluence implying that most of the defects are single vacancies. The vacancies are characterized by scanning tunneling spectroscopy (STS) on graphene and HOPG exhibiting a peak close to the Fermi level. The peak persists after air exposure up to 180 min, albeit getting broader. After air exposure for less than 60 min, electron spin resonance (ESR) at 9.6 GHz is performed. For an ion flux of 10/nm^2, we find a signal corresponding to a $g$-factor of 2.001-2.003 and a spin density of 1-2 spins/nm^2. The ESR signal consists of a mixture of a Gaussian and a Lorentzian of equal weight exhibiting a width down to 0.17 mT, which, however, depends on details of the sample preparation. The $g$-factor anisotropy is about 0.02 \%. Temperature dependent measurements reveal antiferromagnetic correlations with a Curie-Weiss temperature of -10 K. Albeit the electrical conductivity of graphene is significantly reduced by ion bombardment, the spin resonance induced change in conductivity is below 10^{-5}.
    07/2014;
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    ABSTRACT: We present an inelastic neutron scattering study on single-crystalline LiFeAs devoted to the characterization of the incommensurate antiferromagnetic fluctuations at $\mathbf{Q}=(0.5\pm\delta, 0.5\mp\delta, q_l)$. Time-of-flight measurements show the presence of these magnetic fluctuations up to an energy transfer of 60 meV, while polarized neutrons in combination with longitudinal polarization analysis on a triple-axis spectrometer prove the pure magnetic origin of this signal. The normalization of the magnetic scattering to an absolute scale yields that magnetic fluctuations in LiFeAs are by a factor eight weaker than the resonance signal in nearly optimally Co-doped BaFe$_2$As$_2$, although a factor two is recovered due to the split peaks owing to the incommensurability. The longitudinal polarization analysis indicates weak spin space anisotropy with slightly stronger out-of-plane component between 6 and 12 meV. Furthermore, our data suggest a fine structure of the magnetic signal most likely arising from superposing nesting vectors.
    07/2014;
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    ABSTRACT: We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at Tsm ~ 160K. The mode frequencies of two first-order Raman modes B1g and Eg, both involving displacement of Fe atoms, show sharp increase below Tsm. Concomitantly, the linewidths of all the first-order Raman modes show anomalous broadening below Tsm, attributed to strong spin-phonon coupling. The high frequency modes observed between 400-1200 cm-1 are attributed to the electronic Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The splitting between xz and yz d-orbital levels is shown to be ~ 25 meV which increases as temperature decreases below Tsm. A broad Raman band observed at ~ 3200 cm-1 is assigned to two-magnon excitation of the itinerant Fe 3d antiferromagnet.
    Journal of Physics Condensed Matter 06/2014; 26(30). · 2.22 Impact Factor
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    ABSTRACT: We discuss the magnetic properties of a Sm$_{2}$Mo$_{2}$O$_{7}$ single crystal as investigated by means of different experimental techniques. In the literature, a conventional itinerant ferromagnetic state is reported for the Mo$^{4+}$ sublattice below $\sim 78$ K. However, our results of dc magnetometry, muon spin spectroscopy ($\mu^{+}$SR) and high-harmonics magnetic ac susceptibility unambiguously evidence highly disordered conditions in this phase, in spite of the crystalline and chemical order. This disordered magnetic state shares several common features with amorphous ferromagnetic alloys. This scenario for Sm$_{2}$Mo$_{2}$O$_{7}$ is supported by the anomalously high values of the critical exponents, as mainly deduced by a scaling analysis of our dc magnetization data and confirmed by the other techniques. Moreover, $\mu^{+}$SR detects a significant static magnetic disorder at the microscopic scale. At the same time, the critical divergence of the third-harmonic component of the ac magnetic susceptibility around $\sim 78$ K leads to additional evidence towards the glassy nature of this magnetic phase. Finally, the longitudinal relaxation of $\mu^{+}$ spin polarization (also supported by results of ac susceptibility) evidences re-entrant glassy features similar to amorphous ferromagnets.
    05/2014;
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    ABSTRACT: We present an ARPES study of the surface states of Ru$_2$Sn$_3$, a new type of a strong 3D topological insulator (TI). In contrast to currently known 3D TIs, which display two-dimensional Dirac cones with linear isotropic dispersions crossing through one point in the surface Brillouin Zone (SBZ), the surface states on Ru$_2$Sn$_3$ are highly anisotropic, displaying an almost flat dispersion along certain high-symmetry directions. This results in quasi-one dimensional (1D) Dirac electronic states throughout the SBZ that we argue are inherited from features in the bulk electronic structure of Ru$_2$Sn$_3$, where the bulk conduction bands are highly anisotropic. Unlike previous experimentally characterized TIs, the topological surface states of Ru$_2$Sn$_3$ are the result of a d-p band inversion rather than an s-p band inversion. The observed surface states are the topological equivalent to a single 2D Dirac cone at the surface Brillouin zone
    Scientific reports. 05/2014; 4.
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    ABSTRACT: We have investigated CeTIn5 (T = Co, Rh, Ir) by temperature and angle-dependent X-ray photoemission spectroscopy. The Ce 3d core level has a very similar shape for all three materials and is indicative of weak f-hybridization. The spectra have been analyzed using a simplified version of the Anderson Impurity model, which yields a Ce 4f occupancy that is larger than 0.9. The temperature dependence shows a continuous, irreversible and exclusive broadening of the Ce 3d peaks, due to oxidation of Ce at the surface.
    Journal of Physics Condensed Matter 05/2014; 26(20):205601. · 2.22 Impact Factor
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    ABSTRACT: We report on a detailed investigation of the itinerant ferromagnets LaCoAsO, PrCoAsO and SmCoAsO performed by means of muon spin spectroscopy upon the application of external hydrostatic pressures $p$ up to $2.4$ GPa. These materials are shown to be magnetically hard in view of the weak dependence of both critical temperatures $T_{C}$ and internal fields at the muon site on $p$. In the cases $R$ = La and Sm, the behaviour of the internal field is substantially unaltered up to $p = 2.4$ GPa. A much richer phenomenology is detected in PrCoAsO instead, possibly associated with a strong $p$ dependence of the statistical population of the two different crystallographic sites for the muon. Surprisingly, results are notably different from what is observed in the case of the isostructural compounds $R$CoPO, where the full As/P substitution is already inducing a strong chemical pressure within the lattice but $p$ is still very effective in further affecting the magnetic properties.
    04/2014;
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    ABSTRACT: Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic electronic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them, and determine their respective time scales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are, to a very good approximation, independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence.
    Physical Review Letters 04/2014; 112(14):147401. · 7.73 Impact Factor
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    ABSTRACT: Fe-based superconductors bridge a gap between MgB2 and the cuprate high temperature superconductors as they exhibit multiband character and transition temperatures up to around 55 K. Investigating Fe-based superconductors thus promises answers to fundamental questions concerning the Cooper pairing mechanism, competition between magnetic and superconducting phases, and a wide variety of electronic correlation effects. The question addressed in this review is, however, is this new class of superconductors also a promising candidate for technical applications? Superconducting film-based technologies range from high-current and high-field applications for energy production and storage to sensor development for communication and security issues and have to meet relevant needs of today's society and that of the future. In this review we will highlight and discuss selected key issues for Fe-based superconducting thin film applications. We initially focus our discussion on the understanding of physical properties and actual problems in film fabrication based on a comparison of different observations made in the last few years. Subsequently we address the potential for technological applications according to the current situation.
    Reports on Progress in Physics 04/2014; 77(4):046502. · 13.23 Impact Factor
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    ABSTRACT: We report P31 and Li7 nuclear magnetic resonance (NMR) studies in new non-olivine LiZnPO4-type LiCoPO4tetra microcrystals, where the Co2+ ions are tetrahedrally coordinated. Olivine LiCoPO4, which was directly transformed from LiCoPO4tetra by an annealing process, was also studied and compared. The uniform bulk magnetic susceptibility and the P31 Knight shift obey the Curie-Weiss law for both materials with a high spin Co2+ (3d7, S =3/2), but the Weiss temperature Θ and the effective magnetic moment μeff are considerably smaller in LiCoPO4tetra. The spin-lattice relaxation rate T1-1 reveals a quite different nature of the spin dynamics in the paramagnetic state of both materials. Our NMR results imply that strong geometrical spin frustration occurs in tetrahedrally coordinated LiCoPO4, which may lead to the incommensurate magnetic ordering.
    03/2014; 89(13).
  • DPG-Frühjahrstagung (DPG Spring Meeting) Dresden, 30 March - 04 April 2014; 03/2014
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    ABSTRACT: We report $^{63}$Cu nuclear magnetic resonance and muon spin rotation measurements on the S=1/2 antiferromagnetic Heisenberg spin chain compound Sr$_{1.9}$Ca$_{0.1}$CuO$_3$. An exponentially decreasing spin-lattice relaxation rate 1/T$_1$ indicates the opening of a spin gap. This behavior is very similar to what has been observed for the cognate zigzag spin chain compound Sr$_{0.9}$Ca$_{0.1}$CuO$_2$, and confirms that the occurrence of a spin gap upon Ca doping is independent of the interchain exchange coupling $J'$. Our results therefore generally prove the appearance of a spin gap in an antiferromagnetic Heisenberg spin chain induced by a local bond disorder of the intrachain exchange coupling $J$. A low temperature upturn of 1/T$_1$ evidences growing magnetic correlations. However, zero field muon spin rotation measurements down to 1.5 K confirm the absence of magnetic order in this compound which is most likely suppressed by the opening of the spin gap.
    03/2014;
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    ABSTRACT: We investigate the effect of disorder on the heat transport properties of the $S=\tfrac{1}{2}$ Heisenberg chain compound Sr$_2$CuO$_3$ upon chemically substituting Sr by increasing concentrations of Ca. As Ca occupies sites outside but near the Cu-O-Cu spin chains, bond disorder, i.e. a spatial variation of the exchange interaction $J$, is expected to be realized in these chains. We observe that the magnetic heat conductivity ($\kappa_{\mathrm{mag}}$) due to spinons propagating in the chains is gradually but strongly suppressed with increasing amount of Ca, where the doping dependence can be understood in terms of increased scattering of spinons due to Ca-induced disorder. This is also reflected in the spinon mean free path which can be separated in a doping independent but temperature dependent scattering length due to spinon-phonon scattering, and a temperature independent but doping dependent spinon-defect scattering length. The latter spans from very large ($>$ 1300 lattice spacings) to very short ($\sim$ 12 lattice spacings) and scales with the average distance between two neighboring Ca atoms. Thus, the Ca-induced disorder acts as an effective defect within the spin chain, and the doping scheme allows to cover the whole doping regime between the clean and the dirty limits. Interestingly, at maximum impurity level we observe, in Ca-doped Sr$_2$CuO$_3$, an almost linear increase of $\kappa_{\mathrm{mag}}$ at temperatures above 100 K which reflects the intrinsic low temperature behavior of heat transport in a Heisenberg spin chain. These findings are quite different from that observed for the Ca-doped double spin chain compound, SrCuO$_2$, where the effect of Ca seems to saturate already at intermediate doping levels.
    03/2014; 89(10).
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    ABSTRACT: Single crystals of the charge-transfer salt picene/2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane have been grown using physical vapor transport. The crystal structure was determined using single-crystal X-ray diffraction. It was found that the crystals grow in a 1:1 molecular ratio and adopt a monoclinic structure with alternate stacking. Both X-ray data and Raman measurements show that the grown crystals are of good quality. From structure and infrared data, the charge transfer between acceptor and donor molecules was estimated to be approximately 0.14–0.19 electron. Transport measurements indicate a nonmetallic ground state with an activation energy of 0.6 eV. The supporting density functional theory calculations on molecular model systems as well as on crystalline structures confirm the amount of charge transfer and provide first insights into the electronic structure of the new material.
    Crystal Growth & Design 02/2014; 14(3):1338. · 4.69 Impact Factor
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    ABSTRACT: Randomness is an important characteristic of a spin-glass. An example are magnetic ions diluted in a metallic host such as Mn in Cu, the spins of which are randomly oriented below a characteristic temperature. While a spin-glass behavior is mostly caused by time-independent quenched disorder, it is believed that it could also be driven by competing interactions. Here we demonstrate, by means of $^{139}$La nuclear magnetic resonance (NMR) measurements on La$_{2-x}$Sr$_x$CuO$_4$ (LSCO:x, $0.07\leq x \leq 0.15$) and 1/8-doped La$_{2-x}$Ba$_x$CuO$_4$ (LBCO:1/8) single crystals, that the glassy behavior emerging in 1/8-doped lanthanum cuprates is self-generated by charge stripe order, which is distinct from that for light dopings. Taking the glassiness as the fingerprint, we conclude that bulk charge stripe order in LSCO:1/8 sets in at $\sim 70$ K and competes with superconductivity.
    02/2014;
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    ABSTRACT: We analyze the structure of the pairing interaction and superconducting gap in LiFeAs by decomposing the pairing interaction for various kz cuts into s- and d-wave components and by studying the leading superconducting instabilities. We use the ten orbital tight-binding model, derived from ab-initio LDA calculations with hopping parameters extracted from the fit to ARPES experiments. We find that the pairing interaction almost decouples between two subsets, one consists of the outer hole pocket and two electron pockets, which are quasi-2D and are made largely out of dxy orbital, and the other consists of the two inner hole pockets, which are quasi-3D and are made mostly out of dxz and dyz orbitals. Furthermore, the bare inter-pocket and intra-pocket interactions within each subset are nearly equal. In this situation, small changes in the intra-pocket and inter-pocket interactions due to renormalizations by high-energy fermions give rise to a variety of different gap structures. We find four different configurations of the s-wave gap immediately below Tc: the one in which superconducting gap changes sign between two inner hole pockets and between the outer hole pocket and two electron pockets, the one in which the gap changes sign between two electron pockets and three hole pockets, the one in which the gap on the outer hole pocket differs in sign from the gaps on the other four pockets, and the one in which the gaps on two inner hole pockets have one sign, and the gaps on the outer hole pockets and on electron pockets have different sign. Different s-wave gap configurations emerge depending on whether the renormalized interactions increase attraction within each subset or increase the coupling between particular components of the two subsets. We argue that the state with opposite sign of the gaps on the two inner hole pockets has the best overlap with ARPES data.
    02/2014; 89(14).

Publication Stats

7k Citations
2,559.20 Total Impact Points

Institutions

  • 2007–2014
    • Technische Universität Dresden
      • • Institute of Materials Science
      • • Institute of Condensed Matter Physics
      • • Klinik und Poliklinik für Urologie
      Dresden, Saxony, Germany
    • Universität Osnabrück
      Osnabrück, Lower Saxony, Germany
    • Leibniz Institute of Polymer Research Dresden
      Dresden, Saxony, Germany
    • Universidade Federal Fluminense
      • Instituto de Física
      Vila Real da Praia Grande, Rio de Janeiro, Brazil
  • 2003–2014
    • Leibniz Institute for Solid State and Materials Research Dresden
      • Institute for Solid State Research
      Dresden, Saxony, Germany
    • Universität Augsburg
      • Institute of Physics
      Augsburg, Bavaria, Germany
  • 2000–2014
    • Los Alamos National Laboratory
      • Materials Physics and Applications Division
      Los Alamos, California, United States
  • 2013
    • Jawaharlal Nehru University
      New Dilli, NCT, India
    • The Ohio State University
      • Department of Chemistry and Biochemistry
      Columbus, Ohio, United States
  • 2011–2013
    • Lomonosov Moscow State University
      • Faculty of Physics
      Moscow, Moscow, Russia
    • A.E. Arbuzov Institute of Organic and Physical Chemistry
      Kasan, Tatarstan, Russia
    • Universität Heidelberg
      • Kirchhoff-Institute of Physics
      Heidelburg, Baden-Württemberg, Germany
    • Università della Calabria
      • Department of Pharmaco-Biology
      Rende, Calabria, Italy
  • 1996–2013
    • Russian Academy of Sciences
      • Institute of Chemistry
      Moskva, Moscow, Russia
  • 2012
    • Indian Institute of Science Education and Research, Pune
      Poona, Mahārāshtra, India
    • Università degli Studi di Salerno
      • Department of Physics "E. R. Caianiello" DF
      Fisciano, Campania, Italy
    • Bhabha Atomic Research Centre
      Mumbai, Mahārāshtra, India
  • 1990–2012
    • University of Cologne
      • II. Institute of Physics
      Köln, North Rhine-Westphalia, Germany
  • 2010–2011
    • University of Warsaw
      • Institute of Experimental Physics
      Warszawa, Masovian Voivodeship, Poland
    • National Academy of Sciences of Ukraine
      Kievo, Kyiv City, Ukraine
    • Moscow State Institute of Electronics and Mathematics
      Moskva, Moscow, Russia
    • Ludwig-Maximilian-University of Munich
      München, Bavaria, Germany
    • Sohag University
      Sawhāj, Sūhāj, Egypt
    • University of Duisburg-Essen
      Essen, North Rhine-Westphalia, Germany
  • 2009–2010
    • University of Oxford
      • Department of Materials
      Oxford, ENG, United Kingdom
    • Coventry University
      • Applied Mathematics Research Centre
      Coventry, England, United Kingdom
  • 2008–2009
    • University of Vienna
      • Faculty of Physics
      Vienna, Vienna, Austria
    • Paul Scherrer Institut
      • Laboratory for Muon Spin Spectroscopy (LMU)
      Aargau, Switzerland
    • Oak Ridge National Laboratory
      Oak Ridge, Florida, United States
    • Moscow State Textile University
      Moskva, Moscow, Russia
  • 2006
    • Catalan Institute of Nanoscience and Nanotechnology
      Barcino, Catalonia, Spain
  • 2003–2004
    • University of Geneva
      • Department of Condensed Matter Physics
      Genève, Geneva, Switzerland
  • 2002–2004
    • RWTH Aachen University
      • Institute of Physical Chemistry
      Aachen, North Rhine-Westphalia, Germany
  • 2001
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      New Brunswick, NJ, United States
    • Technische Universität Braunschweig
      Brunswyck, Lower Saxony, Germany
  • 1999
    • Université Paris-Sud 11
      • Laboratoire de Physique des Solides
      Paris, Ile-de-France, France