B. Buchner

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

Are you B. Buchner?

Claim your profile

Publications (69)112.17 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have studied the magnetic ordering in Na doped BaFe$_2$As$_2$ by unpolarized and polarized neutron diffraction using single crystals. Unlike previously studied FeAs-based compounds that magnetically order, Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ exhibits two successive magnetic transitions: For x=0.35 upon cooling magnetic order occurs at $\sim$70\ K with in-plane magnetic moments being arranged as in pure or Ni, Co and K-doped BaFe$_2$As$_2$ samples. At a temperature of $\sim$46\ K a second phase transition occurs, which the single-crystal neutron diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ point along the $c$ direction. Magnetic correlations in these materials cannot be considered as Ising like, and spin-orbit coupling must be included in a quantitative theory.
    07/2014;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report a detailed investigation of RCoPO (R = La, Pr) and LaCoAsO materials performed by means of muon spin spectroscopy. Zero-field measurements show that the electrons localized on the Pr3+ ions do not play any role in the static magnetic properties of the compounds. Magnetism at the local level is indeed fully dominated by the weakly itinerant ferromagnetism from the Co sublattice only. The increase of the chemical pressure triggered by the different ionic radii of La3+ and Pr3+, on the other hand, plays a crucial role in enhancing the value of the magnetic critical temperature and can be mimicked by the application of external hydrostatic pressure up to 24 kbar. A sharp discontinuity in the local magnetic field at the muon site in LaCoPO at around 5 kbar suggests a sizable modification in the band structure of the material upon increasing pressure. This scenario is qualitatively supported by ab initio density-functional-theory calculations.
    Physical Review B 02/2013; 87:064401. · 3.66 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165 K using femtosecond time-resolved pump-probe spectroscopy. The contributions in transient differential reflectivity arising from exponentially decaying QP relaxation and oscillatory coherent acoustic phonon mode show large variations in the vicinity of T_SDW. From the temperature-dependence of the QP recombination dynamics in the SDW phase, we evaluate a BCS-like temperature dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW, whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has been estimated from the linear temperature-dependence of the QP relaxation time. The long-wavelength coherent acoustic phonons with typical time-period of ~100 ps have been analyzed in the light of propagating strain pulse model providing important results for the optical constants, sounds velocity and the elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.
    Journal of the Physical Society of Japan 01/2013; 82(4):044715. · 2.09 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Co2FeAl0.5Si0.5 films with a surface roughness of 0.12 nm have been grown epitaxially on lattice-matched MgAl2O4 (001) substrates by off-axis sputtering. X-ray diffraction shows pronounced Laue oscillations, rocking curves as narrow as 0.0043°, and clear Co2FeAl0.5Si0.5 (111) peaks indicating L21 ordering. Magnetic characterizations show a clear magnetocrystalline anisotropy comprising cubic and epitaxy-induced uniaxial terms. Nuclear magnetic resonance measurements reveal L21 order of 81% in the Co2FeAl0.5Si0.5 films. Magnetotransport measurements show a distinct separation of anisotropic magnetoresistance and ordinary magnetoresistance. These results demonstrate the state-of-the-art crystalline quality and magnetic uniformity of the Co2FeAl0.5Si0.5 films.
    Applied Physics Letters 01/2013; 103(16):162404-162404-5. · 3.79 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Nonequilibrium quasiparticle relaxation dynamics is reported in superconducting Ca(Fe0.944Co0.056)2As2 single crystals by measuring transient reflectivity changes using femtosecond time-resolved pump-probe spectroscopy. Large changes in the temperature-dependent differential reflectivity values in the vicinity of the spin density wave (TSDW) and superconducting (TSC) transition temperatures of the sample have been inferred to have charge gap opening at those temperatures. We have estimated the zero-temperature charge gap value in the superconducting state to be ∼1.8kBTSC and an electron–phonon coupling constant λ of ∼0.1 in the normal state that signifies the weak coupling in iron pnictides. From the peculiar temperature-dependence of the quasiparticle dynamics in the intermediate temperature region between TSC and TSDW we infer a temperature scale where the charge gap associated with the spin ordered phase is maximum and closes on either side while approaching the two phase transition temperatures.
    Solid State Communications 01/2013; 160:8–12. · 1.53 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report temperature evolution of coherently excited acoustic and optical phonon dynamics in superconducting iron pnictide single crystal Ca(Fe_0.944Co_0.056)_2As_2 across the spin density wave transition at T_SDW ~ 85 K and superconducting transition at T_SC ~20 K. Strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range of 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below T_SC indicating a coupling of these low energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T ~ 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before T_SDW.
    EPL (Europhysics Letters) 12/2012; 100(5):57007. · 2.26 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In our tunneling investigation using Andreev superconductor - normal metal - superconductor contacts on LiFeAs single crystals we observed two reproducible independent subharmonic gap structures at dynamic conductance characteristics. From these results, we can derive the energy of the large superconducting gap $\Delta_L=(2.5 \div 3.4)$ meV and the small gap $\Delta_L=(0.9 \div 1)$ meV at $T = 4.2$ K for the $T_C^{local} \approx (10.5 \div 14)$ K (the contact area critical temperature which deviation causes the variation of $\Delta_L$). The BCS-ratio is found to be $2\Delta_L/k_BT_C = (4.6 \div 5.6)$, whereas $2\Delta_S/k_BT_C \ll 3.52$ results from induced superconductivity in the bands with the small gap.
    JETP Letters 05/2012; 95(10). · 1.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present an angle resolved photoemission study of the surface and bulk electronic structure of the single layer ruthenate Sr$_2$RuO$_4$. As the early studies of its electronic structure by photoemission and scanning tunneling microscopy were confronted with a problem of surface reconstruction, surface aging was previously proposed as a possible remedy to access the bulk states. Here we suggest an alternative way by demonstrating that, in the case of Sr$_2$RuO$_4$, circularly polarised light can be used to disentangle the signals from the bulk and surface layers, thus opening the possibility of investigating many-body interactions both in bulk and surface bands. The proposed procedure results in improved momentum resolution, which enabled us to detect an unexpected splitting of the surface $\beta$ band. We propose that spin--orbit splitting might be responsible for this, and discuss possible relations of the newly observed surface feature to topological matter.
    03/2011;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A local magnetization measurement was performed by magnetic force microscopy (MFM) to determine magnetization in domains of an exchange coupled [Co/Pt]/Co/Ru multilayer with predominant perpendicular anisotropy. The quantitative MFM measurements were conducted with an iron-filled carbon nanotube tip, which is shown to behave like a monopole. As a result we determined an additional in-plane magnetization component of the multilayer, which is explained by estimating the effective permeability of the sample within the μ<sup>*</sup> -method.
    Applied Physics Letters 01/2011; · 3.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: ISI Document Delivery No.: 833GD
    Phys. Rev. B. 01/2011; 84:10.
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report magnetization measurements of As-deficient LaO(0.9)F(0.1)FeAs(1-delta) (delta approximate to 0.06) samples with improved superconducting properties as compared with As-stoichiometric optimally doped La-1111 samples [i.e., a slightly higher T(c) (enhanced by about 2 K) and nearly a twice as large slope of -dH(c2)/dT at T(c)]. In this As-deficient system with almost homogeneously distributed As-vacancies (AV), as suggested by the (75)As-nuclear quadrupole resonance measurements, we observe a strong enhancement of the spin susceptibility by a factor of 3-7. This observation is attributed to the presence of an electronically localized state around each AV, carrying a magnetic moment of about 3.2 mu(B) per AV or 0.8 mu(B)/Fe atom. From theoretical considerations, we find that the formation of a local moment on neighboring iron sites of an AV sets in when the local Coulomb interaction exceeds a critical value of similar to 1 eV in the dilute limit. Its estimated value amounts to similar to 2.5 eV and implies an upper bound of similar to 2 eV for the Coulomb repulsion at Fe sites beyond the first neighbor shell of an AV. Electronic correlations are thus moderate or weak in doped La-1111. The strongly enhanced spin susceptibility is responsible for the Pauli limiting behavior of the superconductivity that we observe in As-deficient LaO(0.9)F(0.1)FeAs(1-delta). In contrast, no Pauli limiting behavior is found for the optimally doped, As-stoichiometric LaO(0.9)F(0.1)FeAs superconductor in accord with its low spin susceptibility.
    Physical Review B. 01/2011; 84(13):10.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Accurate temperature control with high spatial resolution is a challenging research topic in nano- and biotechnology. Due to their extraordinary physical and chemical properties carbon nanotubes reveal a promising potential as biomedical agents for temperature sensoring on the cellular level. Filling carbon nanotubes with tailored materials realises nanoscaled containers in which the active content is encapsulated by a protecting carbon shell. In particular, the filling with NMR active substances allows the usage as markers and temperature sensors. Their potential for biomedical applications is highlighted by temperature dependent NMR studies giving a proof-of-concept for a non-invasive temperature control on the cellular level.
    Int. J. of Biomedical Nanoscience and Nanotechnology. 01/2011; 2(2):99 - 111.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We performed soft x-ray resonant scattering at the MnL{sub 2,3}- and OK edges of YMnO. While the resonant intensity at the MnL{sub 2,3} edges represent the magnetic order parameter, the resonant scattering at the OK edge is found to be directly related to the macroscopic ferroelectric polarization. The latter observation reveals the important role of the spin-dependent Mn-O hybridization for the multiferroicity of YMnO. We present details about how to obtain correct energy dependent lineshapes and discuss the origin of the resonant intensity at the OK edge.
    The European Physical Journal Special Topics 01/2011; 208(1). · 1.80 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on the magnetic properties of an individual, high-quality single-crystalline iron nanowire with diameter d=26 nm . The nanowire is embedded in a carbon nanotube which provides complete shielding against oxidation. Magnetization reversal is associated with domain wall formation where domain nucleation is initiated by curling. The observed nucleation fields of up to 900 mT are much higher than reported previously and nearly reach the shape anisotropy field of iron nanowires.
    Applied Physics Letters 12/2010; · 3.79 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We performed magnetic force microscopy (MFM) measurements in external magnetic fields parallel to the sample plane to qualitatively study their effect on the magnetization of different kinds of MFM probes. As a test structure we used an array of rectangular ferromagnetic thin film elements aligned with the external magnetic field direction. MFM images were taken while the field was increased stepwise to monitor the onset of a tilt in the MFM probe magnetization. Three different probes were investigated: a conventional pyramidal probe coated with 40 nm of CoCrTa, a coated high aspect ratio MFM probe, and a MFM probe based on an iron filled carbon nanotube (FeCNT). The results show that the magnetization of the pyramidal probe is only stable in in-plane fields of up to ∼60 mT , in larger fields a considerable in-plane component of the tip magnetization is observed. This makes it difficult to distinguish the effect of the external field on the sample from those on the MFM probe. The coated high aspect ratio probe is at first stable up to ∼200 mT and then shows a history dependent behavior, in the second sweep the onset of the magnetization tilt is already visible at ∼100 mT . With the FeCNT probe, no tilt of the probe magnetization could be observed in in-plane fields up to the experimental limit of 230 mT. Due to the large shape anisotropy of the enclosed iron nanowire its direction of magnetization stays mainly oriented along the long axis even in moderate fields that deviate from this easy axis. Consequently, this probe is best suited for MFM measurements in external fields.
    Journal of Applied Physics 08/2010; · 2.21 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Superconductor/ferromagnet (S/F) proximity effect theory predicts that the superconducting critical temperature of the F1/F2/S or F1/S/F2 trilayers for the parallel orientation of the F1 and F2 magnetizations is smaller than for the antiparallel one. This suggests a possibility of a controlled switching between the superconducting and normal states in the S layer. Here, using the spin switch design F1/F2/S theoretically proposed by Oh et al. [Appl. Phys. Lett. 71, 2376 (1997)], that comprises a ferromagnetic bilayer separated by a non-magnetic metallic spacer layer as a ferromagnetic component, and an ordinary superconductor as the second interface component, we have successfully realized a full spin switch effect for the superconducting current. Comment: 5 pages, 4 figures
    Applied Physics Letters 07/2010; · 3.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Lankau, Alexander Koepernik, Klaus Borisenko, Sergey Zabolotnyy, Volodymyr Buechner, Bernd van den Brink, Jeroen Eschrig, Helmut
    Physical Review B. 01/2010; 82.
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this work, we present systematic electron spin resonance (ESR) experiments on SiGe quantum-dot structures. A series of samples with different sizes of quantum dots is prepared by varying growth temperature and spacing between quantum-dot layers. At a frequency of about 9.5 GHz, two ESR signals with g factors around 1.9992 and 1.9994 are observed with magnetic field in growth direction. The signals shift and broaden with magnetic field in the in-plane direction. The estimated dephasing time T-2(*) amounts up to 500 ns. The saturation behavior yields relaxation time T-1 of about 10 mu s. The relative intensity between the two peaks can be changed with illumination with subband-gap light. The two peaks are interpreted as s- and p-like states of electrons confined in the strained Si around the SiGe nanostructures.
    Physical Review B 01/2010; 81. · 3.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the cleaving behavior of LiFeAs and determine its surface electronic structure by detailed density functional calculations. We show that due to the neutral surface of LiFeAs after cleaving, barely any influence of the surface on the electronic states is present. Therefore the data of surface sensitive probes such as angle-resolved photoemission spectroscopy (ARPES) represent to a high degree the bulk electronic structure. This we highlighted by a direct comparison of the calculations to ARPES spectra.
    Physical Review B. 01/2010; 82(18).
  • [Show abstract] [Hide abstract]
    ABSTRACT: A wide variety of properties such as half-metallicity is found among Heusler compounds. In order to separate intrinsic and extrinsic properties, high quality single crystals are required. Here, we report on differently grown crystals of the half-metallic ferromagnet Co <sub>2</sub> FeSi . All crystals show excellent ordering, resulting in outstanding electrical behavior with low residual resistivity and high residual-resistivity-ratio. All Co <sub>2</sub> FeSi crystals show a plateau in the resistivity below 50 K, which might point to half-metallic ferromagnetism. The cross-over from this unusual to more conventional transport ( T<sup>2</sup> dependence) around 50 K indicates the onset of spin flip scattering and thus is indispensable for understanding the strong temperature dependence of Co <sub>2</sub> FeSi tunneling magnetoresistance-devices.
    Applied Physics Letters 11/2009; · 3.79 Impact Factor