Linear electro-optic effect and temperature coefficient of birefringence in 4-nitro-4script-methylbenzylidene aniline single crystals

A study of the linear electro?optic effect in single crystals of the organic compound, 4?nitro�4??methylbenzylidene aniline is reported. The reduced half?wave voltages have been found to have values 2.8, 1.3, and 1.1 kV at 632.8, 514.5, and 488.0 nm, respectively and the corresponding values of the largest linear electro?optic coefficient have been calculated. The thermal variation of the birefringence has also been investigated and the temperature variation of the refractive index difference is found to have the value, d?n/dT = 15.8 × 10?5 K?1.

The concentration dependent cooperative friction coefficient of dilute polymer solutions at the theta point

We combine multiple scattering and renormalization group methods to calculate the leading order dimensionless virial coefficient k(s) for the friction coefficient of dilute polymer solutions under conditions where the osmotic second virial coefficient vanishes (i.e., at the theta point T-theta). Our calculations are formulated in terms of coupled kinetic equations for the polymer and solvent, in which the polymers are modeled as continuous chains whose configurations evolve under the action of random forces in, the velocity field of the solvent. To lowest order in epsilon=4-d, we find that k(s) = 1.06. This result compares satisfactorily with existing experimental estimates of k(s), which are in the range 0.7-0.8. It is also in good agreement with other theoretical results on chains and suspensions at T-theta. Our calculated k(s) is also found to be identical to the leading order virial coefficient of the tracer friction coefficient at the theta point. We discuss possible reasons for the difficulties encountered when attempting to evaluate k(s) by extrapolating prior renormalization group calculations from semidilute concentrations to the infinitely dilute limit. (C) 1996 American Institute of Physics.

Multifunctional cryogenic sensors from n-BaTiO3 ceramics having strong negative temperature coefficient of resistance

Donor-doped n-BaTiO3 polycrystalline ceramics show a strong negative temperature coefficient of resistivity below the orthorhombic-rhombohedral phase transition point, from 10(2-3) Omega cm af 190 K to 10(10-13) Omega cm at less than or similar to 50 K, with thermal coefficient of resistance alpha = 20-23% K-1. Stable thermal sensors for low-temperature applications are realized therefrom. The negative temperature coefficient of resistivity region can be modified by substituting isovalent ions in the lattice. Highly nonlinear current-voltage (I-V) curves are observed at low temperatures, with a voltage maximum followed by the negative differential resistance. The I-V curves are sensitive to dissipation so that cryogenic sensors can be fabricated for liquid level control, flow rate monitoring, radiation detection or in-rush voltage limitation.

Subclinical autonomic neuropathy in alcoholics: Decreased heart rate variability

Cardiac autonomic neuropathy is known to occur in alcoholics but the extent of its subclinical form is not usually recognized, Heart Rate Variability (HRV) analysis can detect subclinical autonomic neuropathy. In this study the HRV parameters were compared in 20 neurologically asymptomatic alcoholics, 20 age-matched normals and 16 depressives. All were males, ECG was recorded in a quiet room for four minutes in supine position. Time and Frequency domain parameters of HRV were computed by a researcher blind to clinical details. Alcoholics had significantly smaller Coefficient of Variation of R-R intervals (CVR-R) on time domain analysis and smaller HF band (0.15-0.5 Hz) power on spectral analysis. The decreased Heart Rate Variability indicates cardiac autonomic dysfunction.

Temperature-dependent valence-band photoemission spectra of La1-xSrxMnO3

We have studied the temperature dependence of the photoemission spectra of La1-xSrxMnO3 (x=0.0, 0.2, and 0.4) and found that the spectral line shape dramatically changes in the entire valence-band region, particularly for x=0.2 and 0.4. By contrast, the spectra of La0.6Sr0.4CoO3 show no significant temperature dependence. From comparison between the temperature-and composition-(x) dependent spectral changes and the temperature-composition phase diagram of La1-xSrxMnO3, we suggest that the changes are related to the degree of hole localization on oxygen p orbitals, which is influenced by electron-lattice coupling and magnetic correlations.

The log delta log t method for the determination of the coefficient of consolidation

The behaviour of saturated soils undergoing consolidation is very complex, It may not follow Terzaghi's theory over the entire consolidation process, Different soils may behave in such a way as to fit into Terzaghi's theory over some specific stages of the consolidation process (percentage of consolidation), This may be one of the reasons for the difficulties faced by the existing curve-fitting procedures in obtaining the coefficient of consolidation, c(v). It has been shown that the slope of the initial linear portion of the theoretical log U-log T curve is constant over a wider range of degree of consolidation, U, when compared with the other methods in use, This initial well-defined straight line in the log U-log T plot intersects the U = 100% line at T = pi/4, which corresponds to U = 88.3%, The proposed log delta-log t method is based on this observation, which gives the value of c(v) through simple graphical construction, In the proposed method, which is more versatile, identification of the characteristic straight lines is very clear; the intersection of these lines is more precise and the method does not depend upon the initial compression for the determination of c(v).

Vibronic model for band A emission in diamond

In this paper, recent results on band A emission in chemical vapor-deposited diamond films have been analyzed within a vibronic model. The blue-band A (2.8 eV) spectra from undoped diamond films grown by two different techniques have been simulated using the same phonon density distribution g(Omega) and Huang-Rhys factor (S). The same g(Omega) at higher S gives a good fit with the green band A (2.32 eV) as well. This model provides a reasonable alternative approach to the long standing donor-acceptor pair recombination model.

Zn incorporation and band gap shrinkage in p-type GaAs

Dimethylzine (DMZn) was used as a p-type dopant in GaAs grown by low pressure metalorganic vapor phase epitaxy using trimethylgallium and arsine (AsH3) as source materials, The hole carrier concentrations and zinc (Zn) incorporation efficiency are studied by using the Hall effect, electrochemical capacitance voltage profiler and photoluminescence (PL) spectroscopy, The influence of growth parameters such as DMZn mole fraction, growth temperature, and AsH, mole fraction on the Zn incorporation have been studied. The hole concentration increases with increasing DMZn and AsH3 mole fraction and decreases with increasing growth temperature. This can be explained by vacancy control model. The PL experiments were carried out as a function of hole concentration (10(17)-1.5 x 10(20) cm(-3)). The main peak shifted to lower energy and the full width at half maximum (FWHM) increases with increasing hole concentrations. We have obtained an empirical relation for FWHM of PL, Delta E(p)(eV) = 1.15 x 10(-8)p(1/3). We also obtained an empirical relation for the band gap shrinkage, Delta E-g in Zn doped GaAs as a function of hole concentration. The value of Delta E-g(eV) = -2.75 x 10(-8)p(1/3), indicates a significant band gap shrinkage at high doping levels, These relations are considered to provide a useful tool to determine the hole concentration in Zn doped GaAs by low temperature PL measurement. The hole concentration increases with increasing AsH3 mole fraction and the main peak is shifted to a lower energy side. This can be explained also by the vacancy control model. As the hole concentration is increased above 3.8 x 10(18) cm(-3), a shoulder peak separated from the main peak was observed in the PL spectra and disappears at higher concentrations. (C) 1997 American Institute of Physics.

Nonlinear electrical conduction and broad band noise in the charge-ordered rare earth manganate Nd0.5Ca0.5MnO3

Measurements of the dc transport properties and the low-frequency conductivity noise in films of charge-ordered Nd0.5Ca0.5MnO3 grown on Si substrate reveal the existence of a threshold field in the charge-ordered regime beyond which strong nonlinear conduction sets in along with a large broad band conductivity noise. Threshold-dependent conduction disappears as T --> T-CO, the charge-ordering temperature. This observation suggests that the charge-ordered state gets depinned at the onset of the nonlinear conduction. (C) 1999 American Institute of Physics. [S0003-6951(99)05247-X].

Measurement of axial dispersion coefficient in a packed bed using X-ray

Dispersion of the liquid in a porous media is of great importance in many areas of engineering and has been studied by several researchers so far. A new experimental method has been developed to measure the dispersion coefficient. X-ray absorption technique provides a better understanding of dispersion that characterizes the mixing phenomenon in the packed beds. This is because the method is non-invasive and also it gives tracer concentration data at every point within the bed. The axial dispersion in a cylindrical bed of non-porous and non-wetting spherical particles has been measured for the flow of water. Aqueous barium chloride solution has been used a as tracer. X-ray images, recorded on a videocassette, have been analyzed using an image processing software to extract the local interstitial velocity and concentration data in the bed. Local dispersion coefficient has been determined with the help of aforementioned data. By using these data, the overall dispersion coefficient in a packed bed can also be estimated.

Toward a Mean Field Formulation of the Babcock-Leighton Type Solar Dynamo. I. α-Coefficient versus Durney's Double-Ring Approach

We develop a model of the solar dynamo in which, on the one hand, we follow the Babcock-Leighton approach to include surface processes, such as the production of poloidal field from the decay of active regions, and, on the other hand, we attempt to develop a mean field theory that can be studied in quantitative detail. One of the main challenges in developing such models is to treat the buoyant rise of the toroidal field and the production of poloidal field from it near the surface. A previous paper by Choudhuri, Schüssler, & Dikpati in 1995 did not incorporate buoyancy. We extend this model by two contrasting methods. In one method, we incorporate the generation of the poloidal field near the solar surface by Durney's procedure of double-ring eruption. In the second method, the poloidal field generation is treated by a positive α-effect concentrated near the solar surface coupled with an algorithm for handling buoyancy. The two methods are found to give qualitatively similar results.

Structural, optical and nanomechanical properties of (1 1 1) oriented nanocrystalline ZnTe thin films

Structural, optical and nanomechanical properties of nanocrystalline Zinc Telluride (ZnTe) films of thickness upto 10 microns deposited at room temperature on borosilicate glass substrates are reported. X-ray diffraction patterns reveal that the films were preferentially oriented along the (1 1 1) direction. The maximum refractive index of the films was 2.74 at a wavelength of 2000 nm. The optical band gap showed strong thickness dependence. The average film hardness and Young's modulus obtained from load-displacement curves and analyzed by Oliver-Pharr method were 4 and 70 GPa respectively. Hardness of (1 1 1) oriented ZnTe thin films exhibited almost 5 times higher value than bulk. The studies show clearly that the hardness increases with decreasing indentation size, for indents between 30 and 300 nm in depth indicating the existence of indentation size effect. The coefficient of friction for these films as obtained from the nanoscratch test was ~0.4.

Structural, optical and nanomechanical properties of (1 1 1) oriented nanocrystalline ZnTe thin films

Structural, optical and nanomechanical properties of nanocrystalline Zinc Telluride (ZnTe) films of thickness upto 10 microns deposited at room temperature on borosilicate glass substrates are reported. X-ray diffraction patterns reveal that the films were preferentially oriented along the (1 1 1) direction. The maximum refractive index of the films was 2.74 at a wavelength of 2000 nm. The optical band gap showed strong thickness dependence. The average film hardness and Young's modulus obtained from load-displacement curves and analyzed by Oliver-Pharr method were 4 and 70 GPa respectively. Hardness of (1 1 1) oriented ZnTe thin films exhibited almost 5 times higher value than bulk. The studies show clearly that the hardness increases with decreasing indentation size, for indents between 30 and 300 nm in depth indicating the existence of indentation size effect. The coefficient of friction for these films as obtained from the nanoscratch test was ~0.4.