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  • 1990-1994  (7)
  • 1994  (7)
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  • 1990-1994  (7)
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  • 1
    Electronic Resource
    Electronic Resource
    Optical transitions in (ZnCo)Se and (ZnFe)Se: Role of an effective p–d exchange (invited) : 38th Annual Confrerence on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 5719-5724 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this paper we discuss photoluminescence (PL) and Raman excitations in the diluted magnetic semiconductors Zn1−xCoxSe and Zn1−xFexSe. The PL spectra associated with isolated crystal field d levels are found to rapidly weaken and/or broaden as the transition metal ion concentration approaches and increases beyond a critical value. In addition a Raman continuum, that overlaps with the optical phonon, is observed to develop when the PL features are modified. In the corresponding Zn1−xCoxSe samples the optical phonons acquire a characteristic Fano line shape suggestive of its coupling to the Raman continuum. It is proposed that these changes to the optical transitions arise from the enhancement of an effective p–d hybridization between low-lying d levels and band electrons with increasing x—an effect largely due to the proximity of the Co2+ and Fe2+ crystal field ground levels to the valence band maximum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355594
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Magnetic excitations of CsFeCl3 in an external magnetic field parallel to the hexagonal plane (1994)
    Springer
    The European physical journal 95 (1994), S. 13-21 
    Details
    ISSN: 1434-6036
    Keywords: 75.10.Dg ; 75.25.+z ; 75.30.Et
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract In CsFeCl3 the Fe2+-ion with effective spin one has locally a singlet ground state (m s =0). The ferromagnetic interactions along thec-direction and the anti-ferromagnetic interactions perpendicular to it are too week as compared to the anisotropy to introduce longrange order in the absence of an external field. By inelastic neutron scattering we have investigated the excitation spectrum in an external magnetic field up to 6 T applied perpendicular to thec-axis. Preliminary measurements in zero field show that dipolar effects lead to a shift of the minimum of the dispersion curve away from theK-point. We show that with increasing magnetic field the whole dispersion sheet is shifted towards higher frequencies and a splitting appears. With the help of a local Hamiltonian which neglects exchange interactions between all neighbours we give a qualitative explanation of the intensity of the magnetic excitations in dependence of the magnetic field. A more sophisticated theory (RPA-diagonalisation of the Heisenberg-Hamiltonian that is used to describe the magnetic behaviour of CsFeCl3) yields — apart from a mismatch for the modes at high energy transfers — a satisfactory description of the magnetic excitations of CsFeCl3 in dependence of the magnetic field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01316838
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Reflection-scanning near-field optical microscopy and spectroscopy of opaque samples (1994)
    Springer
    Applied physics 59 (1994), S. 103-108 
    Details
    ISSN: 1432-0630
    Keywords: 07.60.Pb ; 07.65.Eh ; 78.65-s
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Opaque samples are imaged by Scanning Nearfield Optical Microscopy (SNOM) in reflection mode: A quartz glass fiber tip is used both to illuminate the sample and to collect light locally reflected from or emitted by the surface. The collected light is coupled out by a 2×2 fiber coupler and fed into a grating spectrometer for spectral analysis at each sampled point. The tip-sample distance is controlled by a shear-force feedback system. The simultaneous measurement of topography and optical signals allows an assessment of imaging artifacts, notably topography-induced intensity changes. It is demonstrated that an optical reflectance contrast not induced by topographic interference can be found on suitable samples. Local spectral analysis is shown in images of a photoluminescent layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00332201
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    High-pressure phase transition in ZnSe (1994)
    Springer
    Hyperfine interactions 93 (1994), S. 1505-1510 
    Details
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A high-pressure Mössbauer spectrometer for the 93.3-keV resonance in67Zn and an X-ray diffractometer of the Guinier type are used to investigate67ZnSe powder at high external pressures. At ∼ 13 GPa, a phase transition from the zinc blende to the fcc (NaCl) structure is observed. The transition is accompanied by a 15.2% decrease of the volume of the unit cell. The Lamb-Mössbauer factor (LMF) increases fromf=0.53% at ambient pressure tof=1.07% at 6.1 GPa. It thendecreases tof=0.91% as the pressure is further increased to 8.2 GPa. This behavior is investigated by lattice-dynamical calculations using an 11-parameter rigid-ion model. The decrease of the LMF at high pressures is caused by the softening of the TA phonon modes, which occurs already far below the phase transition. The s-electron density ρ(0) at the67Zn nucleus increases with reduced volume. The change of ρ(0) as well as theoretical Hartree-Fock cluster calculations show that covalency of bonding 4s/4p states of Zn and Se plays an essential role and determines the isomer shift.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02072900
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  • 5
    Electronic Resource
    Electronic Resource
    Electronic structure of Zn in oxide spinels (1994)
    Springer
    Hyperfine interactions 90 (1994), S. 359-364 
    Details
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using67Zn Mössbauer absorption and emission spectroscopy, we have investigated the electronic structure at the A and B sites in the normal spinels (Zn)[Al2]O4, (Zn)[Fe2]O4 and (Zn)[Ga2]O4. Within each system, the center shift Sc at the A site is more positive. In all systems investigated, the electric field gradientV u at the B site is negative. The values for SC andV U scale with oxygen nearest-neighbour distance to Zn. In the Fe spinel, a transferred magnetic hyperfine field is observed at the Zn site below the antiferromagnetic ordering temperatureT N=10 K. For a more detailed discussion of the chemical bond, we have performed ab initio Hartree-Fock cluster calculations for the Al and Fe spinels. Our experimental and theoretical results show that all hyperfine parameters are essentially determined by covalency effects. Our data on the Ga spinel raise the question of a partially inverse structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02069139
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Investigation of electronic structure and anisotropy of the Lamb-Mössbauer factor in ZnF2 single crystals (1994)
    Springer
    Hyperfine interactions 93 (1994), S. 1453-1458 
    Details
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using the 93.3 keV resonance in67Zn the isomer shift, the quadrupole interaction, and the anisotropy of the Lamb-Mössbauer factor (LMF) in ZnF2 single crystals have been investigated. The main component of the electric field gradient (efg) tensorV zz=+(2.18+-0.22)×1017V/cm2 andη=0.29±0.03 are derived.V zz is oriented perpendicular to the crystallographicc axis. The anisotropy of the LMF is large:f x=(2.4±0.1)%,f y=(2.3±0.2)%, andf z=(1.4±0.2)%. Ab initio Hartree-Fock cluster calculations gave a detailed description of the observed charge distribution including the orientation of the efg tensor. Using a force constant model (shell model), we solved the eigenvalue problem of the dynamic matrix and quantitatively reproduced the anisotropy of the LMF.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02072892
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  • 7
    Electronic Resource
    Electronic Resource
    Theoretical calculations of electron densities in zinc chalcogenides and in zinc fluoride (1994)
    Springer
    Hyperfine interactions 90 (1994), S. 411-415 
    Details
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Self-consistent, non-relativistic Hartree-Fock calculations on a finite cluster of atoms with zinc at the center have been performed on the zinc blende compounds ZnTe, ZnSe, ZnS, on ZnO (wurtzite structure), ZnO (NaCl structure), and on ZnF2 (rutile-type structure) to obtain changes in s electron density Δρ(0) at the67Zn nucleus. We solved the eigenvalue problem of the dynamic matrix to calculate the second-order Doppler shiftS SOD using appropriate force constant models and determined the isomer shiftS from the measured center shift for each compound. Our calculations clearly show the importance of the covalency of the Zn-ligand bond for the origin ofS and fully corroborate the experimental linear correlations between decreasingS values and increasing electronegativity of the ligands. The most important contribution to Δρ(0) comes from the Zn(4s) electrons, with a smaller but significant contribution from the Zn(3s) electrons. For the change of the mean-square nuclear charge radius for the Mössbauer transition in67Zn, we obtain Δ〈r 2〉=+(13.9±1.4) × 10−3 fm2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02069148
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    Paper (German National Licenses)
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