T. D. Black

Deposition of diamond films at low pressures and their characterization by positron annihilation, Raman, scanning electron microscopy, and x-ray photoelectron spectroscopy

We have deposited diamond films with micron‐size crystals on Si〈111〉 using low‐pressure hot‐filament‐assisted chemical vapor deposition. These films have been characterized by positron annihilation, Raman spectroscopy, scanning electron microscopy, and x‐ray photoelectron spectroscopy. In addition to the results for the electronic structure and morphology, we also present new results for the lattice defects present in these films.

Field-ball milling induced anisotropy in magnetic particles

Nd2Fe14B and Sm2Co17 particles of submicrometre sizes have been prepared by ball milling in a magnetic field. Structural and magnetic characterization reveal that these submicrometre particles milled in a magnetic field, consisting of nanosize grains, exhibit strong magnetic anisotropy compared with the particles milled without a magnetic field. Based on in situ observations of the field-ball milling in a transparent container, the mechanism of field-induced anisotropy in the nanostructured hard magnetic particles is discussed.

Optical modulation by a traveling surface acoustic wave and a holographic reference grating

The interaction of an optical beam with a traveling surface wave and a stationary holographic reference grating is studied theoretically and experimentally. The spatial phase modulation produced by the traveling wave results in an intensity modulation in the diffracted orders of the optical beam. The intensity modulation is at the fundamental frequency of the traveling wave, and its amplitude depends periodically on the separation distance between the plane of the traveling wave and the plane of the stationary holographic reference grating. Experiments were carried out with 87-MHz surface acoustic waves traveling on a LiNbO3 substrate separated from a holographic reference grating in film. The amplitudes of the detected modulation in the m = 0 and m = ±1 diffraction orders of a probing beam are in close agreement with a plane-wave analysis in the Raman–Nath diffraction regime.

Observation of the cubic‐field splitting of an excited S=2 manifold in a cubic copper tetramer

EPR measurements on single crystals of Cu4OCl6(triphenylphosphine oxide)4 at liquid helium temperatures in the frequency ranges 14–17 and 34–35 GHz were fitted to a simple cubic S=2 spin Hamiltonian with g=2.10±0.01 and a zero‐field splitting of (0.53±0.01) cm−1. From the decrease in intensity of the S=2 spectrum on cooling below 4.2 K and the absence of an S=1 spectrum, the S=2 manifold was deduced to lie (14±1) cm−1 above a nonmagnetic ground state. The EPR results are used as a test of the various theories developed to explain the magnetic susceptibility of copper tetramers.

Visualization of aerodynamic flow fields using photorefractive crystals

Interferometry based on double exposure Fourier transform holography in photorefractive crystals is applied for visualization of aerodynamic flow fields. The interferograms obtained are of similar quality as those produced using holographic film but with greatly simplified procedures. The results presented are obtained using a high-power cw argon laser and iron doped lithium niobate crystals. The angular characteristics of the Fourier transform data holograms are studied.

Jahn‐Teller EPR spectra of Cu2+ in MgSiF6⋅6H2O

The 34 GHz EPR spectrum of Cu2+ in MgSiF6⋅6H2O showed a ‘‘static’’ Jahn–Teller effect at 4.2 K with two inequivalent Jahn–Teller sites per unit cell. The six axially symmetric sets of Cu2+ lines had their z axes parallel to the three tetragonal axes of two cubes, which were rotated by approximately 40° with respect to each other about a common [111] axis, which is the crystal c axis. The measured spin‐Hamiltonian parameters at 4.2 K for each set of lines were g∥=2.47±0.01, g⊥=2.10±0.01, and ‖A∥‖=(110±3)×10−4 cm−1. There was a gradual decrease in the anisotropy of the spectrum on warming the crystal, with a single, nearly isotropic line being observed above 220 K. At 270 K the spectrum had axial symmetry about the c axis with g′∥ =2.23±0.01 and g⊥ =2.25±0.01. The temperature evolution of the spectrum was interpreted in terms of a Boltzmann distribution over inequivalent distorted Jahn–Teller configurations, with one potential well lowered by an amount Δ≊105 cm−1 below the other two.

Holographic interferometry using iron‐doped lithium niobate

Holographic interferometry using crystals of iron‐doped lithium niobate is reported. The interferograms presented are produced via double‐exposure Fourier transform holography in heavily doped crystals without applied electric fields. Flow‐field visualization is used as an example application area.

Infrared detection using acousto‐optic interaction with thermally induced grating in optical waveguides

A new method of detecting infrared radiation which allows direct coupling into integrated optics is presented. The method utilized the coherent interaction of surface acoustic waves and a thermally induced grating in an optical waveguide. The sensitivity was found experimentally to be approximately 1.5×10−4 J/cm2 for 1‐msec CO2 laser pulses. Analysis indicated that the sensitivity could be improved by orders of magnitude, since the waveguide film absorbed less than 0.2% of the radiation and that rise times of the order of several nanoseconds should be possible. The method is not limited to infrared radiation; any mechanism which can change the index of refraction could, in principle, be used.

Zeeman, hyperfine, and magnetic dipolar effects on the EPR spectrum of a cubic copper tetramer

The Zeeman interaction for an isolated multiplet of a cubic Cu2+ tetramer, characterized by the total spin quantum number S, is shown to be essentially isotropic, with g=(g∥+2g⊥)/3, where g∥ and g⊥ are single Cu2+ ion components. Matrix elements are given for the off‐diagonal part of the Zeeman interaction within the three S=1 multiplets. The hyperfine structure for each isotope in a [100] direction is shown to be a 13 line structure. The intracluster magnetic dipole–dipole (MDD) interaction has nonzero matrix elements within the three S=1 multiplets, which are degenerate for an isotropic exchange interaction with equal coupling constants. With the MDD interaction treated as a perturbation of the Zeeman interaction, it is found that the single line at g=2.10, expected on the basis of the Zeeman interaction alone, is split into four lines separated from the former line by ±h0 and ±2h0, where h0≂480 G. The MDD coupling constant, calculated assuming point dipoles, is an order of magnitude too small to explai...

Electron paramagnetic resonance in CuTiF6⋅4H2O

Electron paramagnetic resonance in single crystals of CuTiF6⋅4H2O was studied between 77 and 485 K in the frequency ranges 23–25 and 34–36 GHz. The monoclinic lattice has two magnetically distinct Cu2+ sites, but (except at 4.2 K) only one resonance line was observed at all orientations. Assuming tetragonal symmetry at each Cu2+ site, the observed g values were used to determine the single ion parameters g∥ = 2.403±0.005 and g⊥ = 2.082±0.002 at 77 K. Linewidth data in the ac plane were explained in terms of the exchange narrowing of the line broadened by the dipolar and unresolved hyperfine interactions.