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  • 1990-1994  (15)
  • 1994  (7)
  • 1992  (8)
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  • 1990-1994  (15)
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  • 1
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.355594
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  • 2
    Digitale Medien
    Digitale Medien
    Magnetostrictive and quadrupolar anisotropy in nuclear magnetic fcc systems (1992)
    Springer
    The European physical journal 89 (1992), S. 305-311 
    Details
    ISSN: 1434-6036
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract The spin hamiltonian for nuclear magnetic order in copper is investigated with respect to magnetoelastic couplings to the lattice. These arise due to the dipolar, Ruderman Kittel and the quadrupolar interaction. While the latter is quenched for the ideal fcc-lattice, it is found that for copper it will dominate the magnetoelastic terms of the nuclear spin hamiltonian. The absolute size of the quadrupole contribution is determined by the effective charge and (anti-) shielding effects. This interaction can give rise to an effective anisotropy in the fcc-system which can be quite large compared to the small stabilisation energies for nuclear order in copper. Consequences for the nuclear ordering in copper are briefly discussed and compared to the available experimental and theoretical data.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01318161
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  • 3
    Digitale Medien
    Digitale Medien
    Reflection-scanning near-field optical microscopy and spectroscopy of opaque samples (1994)
    Springer
    Applied physics 59 (1994), S. 103-108 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 07.60.Pb ; 07.65.Eh ; 78.65-s
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00332201
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  • 4
    Digitale Medien
    Digitale Medien
    Influence of growth conditions on the structural, optical, and electrical properties of ZnSe films (1992)
    Springer
    Applied physics 54 (1992), S. 455-459 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 61.10.-i ; 78.55.-m ; 72.80.Ey
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract ZnSe films were grown by chemical vapour deposition on GaAs substrates. The influence of the source temperature (between 820 and 900° C) and the substrate temperature (between 620 and 790° C) on the film properties were investigated by Hall measurements, X-ray diffraction, and photoluminescence. With respect to blue luminescent devices the ratio of excitonic to deep level transitions was found to be optimum at low growth rates when the source temperatures were kept below 840° C. P-type conduction up to a net carrier concentration of 8×1018 cm−3 could be obtained by substrate temperatures above 700° C. Lattice contraction versus substrate temperature pointed to a reduced incorporation of donors at higher growth temperatures.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00324171
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  • 5
    Digitale Medien
    Digitale Medien
    High—Pressure phase transition in67ZnO (1992)
    Springer
    Hyperfine interactions 70 (1992), S. 1067-1070 
    Details
    ISSN: 1572-9540
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Using a high-pressure Mössbauer spectrometer for the 93.3-keV resonance in67Zn and an X-ray diffractometer of the Guinier type67ZnO powder was investigated at pressures up to ≈19% at the phase transition. Surprisingly, the center shift (CS) of the cubic phase is by ≈20μm/s more negative than CS of the wurtzite structure. The Mössbauer data as well as theoretical cluster calculations show that the volume collapse is mainly due to the increase in coordination number of nearest-neighbour (nn) atoms. Covalency of the Zn−O bond, however, is reduced due to a≈7% increase of nn distance.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02397513
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  • 6
    Digitale Medien
    Digitale Medien
    High-pressure phase transition in ZnSe (1994)
    Springer
    Hyperfine interactions 93 (1994), S. 1505-1510 
    Details
    ISSN: 1572-9540
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02072900
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  • 7
    Digitale Medien
    Digitale Medien
    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
    Schlagwort(e): 75.10.Dg ; 75.25.+z ; 75.30.Et
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01316838
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  • 8
    Digitale Medien
    Digitale Medien
    Temperature and magnetic field dependent correlations in the singlet ground state system CsFeBr3 (1992)
    Springer
    The European physical journal 86 (1992), S. 257-262 
    Details
    ISSN: 1434-6036
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract The magnetic excitations in CsFeBr3 have been measured with inelastic scattering of cold neutrons to high precision at 80 mK. The fact that the lowest frequency mode softens with decreasing temperature but stabilizes at 0.11 THz below 2.5 K is the indication that CsFeBr3 remains a SGS system forT→0. From dispersion curves measured earlier in a magnetic field along the chain axis experimental intensities were derived and in turn correlation lengths. Correlation lengths were also calculated using the new parameters for exchange and anisotropy. The experimental results and the calculations both show that the correlation lengths increase for increasing magnetic field, flatten off around the phase transformation at 4.1 Tesla with a maximum of about 35 Å and then decrease again for higher fields. No critical phenomena connected with this phase transformation could be detected. The “Mirror Mode” published earlier was found to be of spurious origin due to double scattering. It does not exist.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01313834
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  • 9
    Digitale Medien
    Digitale Medien
    Charge-changing cross sections of the neutron-rich isotopes8,9,11Li (1992)
    Springer
    The European physical journal 343 (1992), S. 375-379 
    Details
    ISSN: 1434-601X
    Schlagwort(e): 21.10.−k ; 27.20.+n ; 25.70.Np ; 21.10.Re
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Total charge-changing cross sections have been measured for8Li on C and Pb targets, for9Li on C, Al, Cu, Sn and Pb targets, as well as for11Li on C, Sn and Pb targets at about 80 MeV/nucleon. These data are compared to measured total reaction cross sections and Glauber-type calculations using Hartree-Fock density distributions. These comparisons allow to draw conclusions on the proton density distribution of the neutronrich lithium isotopes. The results show that even for the most exotic nucleus11Li the proton distribution is only very weakly influenced by the long tail in the neutron density distribution already established in several experiments.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01289813
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  • 10
    Digitale Medien
    Digitale Medien
    High-resolution67Zn-Mössbauer study of oxide spinels (1992)
    Springer
    Hyperfine interactions 68 (1992), S. 161-164 
    Details
    ISSN: 1572-9540
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract We report on first experiments to investigate the electronic structure in the normal spinels ZnAl2O4 and ZnFe2O4 and in the inverse spinels Zn2SnO4 and Zn2TiO4 using the high-resolution67Zn-Mössbauer spectroscopy. The electric field gradient for67Zn at the B (octahedral) site in ZnAl2O4 is negative, whereas the A (tetrahedral) site remains essentially cubic, however, with a more positive center shift. ZnFe2O4 orders antiferromagnetically at ≈10K. Due to superexchange a magnetic field is observed at67Zn. In the inverse spinels short range order leads to only few (Zn,Sn) and (Zn,Ti) configurations at the octahedral sites. The s-electron densities at the67Zn sites are distinct and cover a surprisingly broad range for Zn2TiO4. This strongly suggests that d-electrons of Ti play an essential role in the chemical bond of this compound.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02396461
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