Biomedical subjects

LM Peng

Publications and source records attributed to LM Peng.

8 recordsLinked to original sources

PubMed2000

Automated identification of symmetry in CBED patterns: a genetic approach

The genetic algorithm has been applied for automated identification of symmetry in CBED patterns. A normalized inner product between an original and its symmetry operated CBED patterns was found to be a good measure of similarity between them, and this inner product was used as the objective function in the genetic algorithm. A real floating number implementation of this genetic approach has been applied successfully in identifying rotation axes and mirror planes in experimental CBED patterns obtained from a single crystal of silicon. In particular a three-fold rotation axis reflecting the true three-dimensional symmetry of silicon is clearly distinguished from a six-fold rotation axis as expected from a two-dimensional crystal in the experimental <1 1 1> zone axis CBED pattern.

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PubMed2000

Surface structural sensitivity of convergent-beam RHEED: Si (0 0 1) 2 x 1 models compared with dynamical simulations

The aim of this work is to evaluate the sensitivity of convergent-beam RHEED for the refinement of surface atomic structure. We have compared experimental and theoretical convergent-beam RHEED patterns from the silicon (0 0 1) reconstructed surface. The experiment was carried out in a custom designed UHV diffraction camera,using a micron sized probe. Both experimental and theoretical CB-RHEED patterns show complex details, highly sensitive to the surface structure. The multiple scattering simulations were based on two experimental structural models which make different assumptions for dimer tilt, one derived from X-ray diffraction results, and another from LEED data. The simulated CB-RHEED patterns using the X-ray model were found to be in closer agreement with our experiment than the LEED model. However, the agreement is not entirely satisfactory, suggesting that further improvement on this model is necessary.

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PubMed2000

Incommensurate valence modulation in high-Tc cuprates

An incommensurate modulation has been observed in a Cu-rich La2CuO4.003 crystal. It is shown that the modulation results from a periodically distributed holes lying on the CuO2 planes, and that the hole modulation may be regarded to be a kind of valence modulation. It is shown that appreciable valence modulation contrast may be generated by the mechanisms of hole scattering alone when the period of the modulation is of the order of 2 nm.

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PubMed2000

Quasi-dynamical electron diffraction - a kinematic type of expression for the dynamical diffracted-beam amplitudes

It is shown that to a good approximation the dynamical diffracted electron-beam amplitudes may be expressed in a form that is identical to that of the kinematic theory of electron diffraction. The validity of this approximate form of the dynamical electron diffraction is illustrated for thin films of GaAs and Au crystals, and its implications in electron crystallography for structural determination and refinement are discussed.

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PubMed2000

Debye-waller factors of compounds with the caesium chloride structure

The lattice dynamics of five compounds with the caesium chloride structure have been investigated using shell models. Debye-Waller factors for these compounds are calculated over the temperature range from 1 to 1000 K and the results are presented analytically in a polynomial form. When experimental results are available, the calculated results are compared to the experimentally measured Debye-Waller factors and typically the discrepancies between these factors are less than 10%.

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PubMed1999

Lattice dynamics and Debye-Waller factors of some compounds with the sodium chloride structure.

The lattice dynamics of 19 compounds with the sodium chloride structure have been investigated using shell models. The models are compared with existing experimentally measured phonon-dispersion curves and refined using a multidimensional downhill simplex method. Debye-Waller factors for these compounds are calculated over the temperature range from 1 to 1000 K where appropriate and the results are fitted analytically using polynomial regression. The results are compared with experimentally measured room-temperature Debye-Waller factors and for most of the compounds the agreement is found to be better than 10%. At lower temperatures, it is expected that these results would be more accurate, since the harmonic approximation, which is crucial to the calculation of the Debye-Waller factors, works better. In choosing the models for particular applications, it is recommended that the model with the smallest standard error sigma is used for fitting the experimentally measured phonon-dispersion curves, or the model that shows best agreement with reliable experimental measurements of more relevant physical quantities, such as Debye-Waller factors in crystallography.

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PubMed1999

Anisotropic dispersion of the band structure and formation of ring patterns in CBED.

An off-axis ring pattern was observed in the <1-11> zone-axis CBED pattern of an NiO single crystal. The pattern was interpreted as resulting from anisotropic dispersion of the band structure of the two most strongly excited free Bloch waves and strong absorption of the most tightly bound 1s Bloch state. Comparison has also been made with the case of MgO which has the same structure as NiO but weaker scattering power. The tightly bound 1s Bloch state in MgO was found to play an important role together with a strongly excited free Bloch state and a more conventional zone-axis ring pattern similar to that observed in the <001> zone-axis CBED pattern of silicon was observed.

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PubMed1999

Parameterization of the temperature dependence of the Debye-Waller factors.

Parameterization has been made for the temperature dependence of the Debye-Waller factors of 68 elemental crystals and 17 compounds with the zinc-blende structure. The Debye-Waller factors of 46 elemental crystals were calculated based on the available phonon density of states, and those of the remaining 22 elemental crystals were estimated using the Debye approximation for the phonon density of states. The Debye-Waller factors of the zinc-blende compounds were obtained from the 14-parameter shell-model calculations of Reid [Acta Cryst. (1983), A39, 1-13].

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