The ZnP2-Ge system and growth of single crystals of ZnGeP2

The phase relations in the ZnP₂-Ge join in the Zn-Ge-P system were determined. ZnGeP₂ melts congruently at 1027 ± 3°C and undergoes a solid state transformation at 950 ± 5°C, in agreement with previous work. The low temperature phase, designated as αZnGeP₂, has the tetragonal chalcopyrite structure, while the structure of the high temperature phase, designated as βZnGeP₂, is unknown. Two eutectics are present: one between αZnGeP₂ and Ge at 865 ± 4°C and 95 mol % Ge, and the other, between βZnGeP₂ and ZnP₂ at 970°C ± 4°C and 15 mol % Ge. Large crack- and bubble-free single crystals of ZnGeP₂ can be grown by directional solidification without seeding, and the phase transition appears to be no impediment to the growth of large crystals. The extra absorption of a few cm^-1 which exists between the band edge and 2μ has been shown to depend on crystal growth conditions. The source of the absorption, however, has not been identified. The absorption spectrum of a crystal grown from a liquid Bi-Pb alloy is compared with the spectra of crystals grown directly from ZnGeP₂ melts.     

A fast approach for accurate content-adaptive mesh generation

Mesh modeling is an important problem with many applications in image processing. A key issue in mesh modeling is how to generate a mesh structure that well represents an image by adapting to its content. We propose a new approach to mesh generation, which is based on a theoretical result derived on the error bound of a mesh representation. In the proposed method, the classical Floyd-Steinberg error-diffusion algorithm is employed to place mesh nodes in the image domain so that their spatial density varies according to the local image content. Delaunay triangulation is next applied to connect the mesh nodes. The result of this approach is that fine mesh elements are placed automatically in regions of the image containing high-frequency features while coarse mesh elements are used to represent smooth areas. The proposed algorithm is noniterative, fast, and easy to implement. Numerical results demonstrate that, at very low computational cost, the proposed approach can produce mesh representations that are more accurate than those produced by several existing methods. Moreover, it is demonstrated that the proposed algorithm performs well with images of various kinds, even in the presence of noise.     

Helical spin structure in MnSi-NMR studies

The zero field NMR signals of MnSi have been observed at 1.6 K. From the distribution of the resonance frequency and the external field dependence it is concluded that MnSi has a helical spin structure at zero field. The magnetization process is interpreted as due to the helical→ conical→ ferromagnetic transitions.     

Bayesian kernel methods for analysis of functional neuroimages

We propose an approach to analyzing functional neuroimages in which 1) regions of neuronal activation are described by a superposition of spatial kernel functions, the parameters of which are estimated from the data and 2) the presence of activation is detected by means of a generalized likelihood ratio test (GLRT). Kernel methods have become a staple of modern machine learning. Herein, we show that these techniques show promise for neuroimage analysis. In an on-off design, we model the spatial activation pattern as a sum of an unknown number of kernel functions of unknown location, amplitude, and/or size. We employ two Bayesian methods of estimating the kernel functions. The first is a maximum a posteriori (MAP) estimation method based on a Reversible-Jump Markov-chain Monte-Carlo (RJMCMC) algorithm that searches for both the appropriate model complexity and parameter values. The second is a relevance vector machine (RVM), a kernel machine that is known to be effective in controlling model complexity (and thus discouraging overfitting). In each method, after estimating the activation pattern, we test for local activation using a GLRT. We evaluate the results using receiver operating characteristic (ROC) curves for simulated neuroimaging data and example results for real fMRI data. We find that, while RVM and RJMCMC both produce good results, RVM requires far less computation time, and thus appears to be the more promising of the two approaches.     

The melt growth and doping of CdGeP2

Crystals of the ternary chalcopyrite CdGeP2 have been grown from doped and undoped melts by directional solidification. We find that the stoichiometry and temperature gradient in the melt are critical factors in determining crystal quality. The effect of excess phosphorous is particularly dramatic, causing a rapid deterioration of crystal quality when the excess is greater than 3 at.% in the charge. This empirical observation led us to perform measurements of the total vapor pressure over CdGeP₂ as a function of temperature. The vapor species was found to consist almost entirely of phosphorous and the measured pressure at 790°C was 1.92±.15 atm. An extensive search for dopants suitable for the production of low-resistivity CdGeP₂ was unsuccessful. It was found that doping heavily with In and Ga produced high-resistivity n- and p-type crystals, respectively, but no shallow donors or acceptors were found.     

Optical,electronic and magnetic properties of the dilute magnetic semiconductor Mn:CuInte2

CuInTe₂ and CuInSe₂ are direct bandgap ternary chalcopyrite (I-III-VI₂) semiconductors which are the simplest ternary analogs of the II-VI zincblende compounds. Some of the chalcopyrites exhibit nonlinear optical properties which make them good candidates for optical device technology. We have done preliminary studies on the influence of Mn²⁺ on the optical, transport and magnetic properties of CuInTe₂ in analog to previous stud-ies of Mn:CdTe etc. Bulk polycrystalline p-type samples, with up to 12% Mn substi-tution, have been examined with polarized light microscopy, x-ray energy dispersive analysis, optical spectrometry, van der Pauw technique and SQuID magnetometer mea-surements. From the metallurgical studies, Mn is found to substitute for Cu but not In. The bandgap of Mn:CuInTe₂ is larger than 0.92 eV. Most of our samples show phonon limited mobility, with the highest mobility occurring at ≈75K. Without Mn substitu-tion, CuInTe₂(Se₂) is diamagnetic. Samples with 3% Mn substitution in Cu sites are paramagnetic. At higher Mn concentration (3% < x < 12%), the (Cu_1-xMn_x)InTe₂ sam-ples show antiferromagnetic coupling and a measured effective magnetic moment up to ≈5.4 μB/Mn atom. Samples with attempted Mn substitution in In sites (from 3% to 9%) show anomalous magnetic behavior at low temperatures.     

Permeation through a single crystal of Zeolite NaX

Steady state permeation through a 120 μm single crystal of zeolite NaX has for the first time been observed. Each crystal was imbedded in an epoxy film, polished to expose opposite faces to feed and permeate gases, and mounted in an apparatus with pmol/s permeation rate sensitivity. Permeation was constrained to occur by an intracrystalline diffusion mechanism, confirmed by temperature dependence and selectivity measurements. Rates decreased at higher temperatures due to opposing sorption and diffusion effects. Intracrystalline butane diffusivity at 25°C was 3 x 10^?4 cm²/sec, in agreement with other methods. Anomalously long time lags for approach to steady state are explained by a transition between two states of the zeolite, differing in permeability by 10²-10⁴. The state with lower permeability was selectively permeable to isobutane from a methaneísobutane mixture.     

Exafs measurements on Nb3Ge thin films

We have measured the Extended X-ray Absorption Fine Structure (EXAFS) spectra for superconducting thin films of Nb₃Ge prepared at variable deposition temperature by sputtering and electron beam coevaporation. As the deposition temperature is varied from 1050 K to 875 K, the film structure changes continuously from the A15 phase to a quasi-amorphous phase. This amorphous phase is characterized by a mean Ge-Nb distance of 2.66 A, as compared to the A15 distance, 2.87 A. The mean coordination of the Ge sites is reduced from 12 to 8±2, and the mean square spread in Ge-Nb distances (thermal + spatial) has decreased by only 0.01 A².     

Virtual bound states of Pd in Cu,Ag and Au and of Pt in Ag

The positions and widths of the virtual bound states of 5 and 10% Pd in Cu, Ag and Au and of 5 and 10% Pt in Ag have been measured by XPS. The spin—orbit splitting of the Pt state in Ag is reduced from the theoretical value, but comparable to the spectroscopic atomic value. The host lattice d-band structure is perturbed in the alloys.     

Ultra-low-frequency dust-electromagnetic modes in self-gravitating magnetized dusty plasmas

Obliquely propagating altra-low-frequency dust-electromagnetic waves in a self-gravitating, warm, magnetized, two fluid dusty plasma system have been investigated. Two special cases, namely, dust-Alfvén mode propagating parallel to the external magnetic field and dustmagnetosonic mode propagating perpendicular to the external magnetic field have also been considered. It has been shown that effects of self-gravitational field, dust fluid temperature, and obliqueness significantly modify the dispersion properties of these ultra-low-frequency dust-electromagnetic modes. It is also found that in parallel propagating dust-Alfvén mode these effects play no role, but in obliquely propagating dust-Alfvén mode or perpendicular propagating dust-magnetosonic mode the effect of self-gravitational field plays destabilizing role whereas the effect of dust/ion fluid temperature plays stabilizing role.