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Recently, UME has acquired an argon triple point system in order to extend its temperature scale down to 83.8058 K. The detailed description of the UME argon triple point apparatus which was developed to obtain the argon triple point plateau by means of adiabatic working principles will be presented below. A standard platinum resistance thermometer which was calibrated at argon triple point at PTB low temperature laboratory has also been calibrated in our equipment. Results related to these measurements are presented in the paper.

UME radiance temperature scale is derived from the UME Silver fixed point black body and is maintained and disseminated by the UME standard radiation thermometer. According to ITS-90, temperature scale is derived from the Planckian ratio equation by comparing the thermometer signal at an arbitrary temperature with the thermometer reference signal at the fixed point. Therefore, the uncertainty associated with the reference signal is transferred to radiance temperatures calculated. Using the Wien's approximation, the temperature uncertainty of the reference signal must be lower than 30 mK in order to keep the uncertainty component due to reference signal within 100 mK at 2000 degreesC. Uncertainty in the reference signal obtained at the silver point originates from two sources. These are the uncertainties associated to the fixed point itself and, the uncertainties associated to the radiance thermometer. For the first part of the uncertainty, metal impurity, emissivity, cavity uniformity, aperture uniformity of the fixed point are considered. For the second part, spectral responsivity function, size of source effect, residual response out of spectral responsivity function, time drift of the standard radiation thermometer are considered. Finally, to assess the total uncertainty associated with the reference signal, we have also considered the standard uncertainty of the freezing plateaus obtained during the melting and freezing cycles.

UME Temperature Laboratory has realized the International Temperature Scale of 1990 (ITS-90) between the argon triple point and the freezing point of silver using commercially obtained sealed cells.

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Photoinduced absorption (PA) of hydrogenated amorphous silicon (a-Si:H) was studied by excitation spectra analysis. The nonlinear PA response taken at different excitation photon energies was found to follow the corresponding absorption coefficient for intrinsic and p-doped samples in the 80-300 K temperature range. Significant contribution of the electronic PA in the observed response was demonstrated. Thermal effects are discussed. A simple model of trapping during the pump with consequent recombination from the localized state was found to be in good agreement with experimental observations. (C) 1995 American Institute of Physics.

We have investigated the ground state and electronic properties of structure using a generalised gradient approximation of the density functional theory and the ab initio pseudopotential method. We find that the calculated electronic properties of exhibit three-dimensional rather than two-dimensional characteristics in spite of the apparent two dimensionality in its atomic structure. An interesting feature of the phonon dispersion curves is the phonon anomaly in the lowest transverse acoustic branch as well as the longitudinal acoustic branch. We have shown that these phonon anomalies give rise to large electron-phonon interaction, as is evident from the calculated Eliashberg spectral function. By integrating this spectral function, the value of average electron-phonon coupling parameter is calculated to be 0.85, from which the superconducting critical temperature is estimated to be 3.74 K, in gratifying accord with its experimental value of 4.0 K.

Ab initio pseudopotential calculations have been performed to investigate the structural, electronic, and vibrational properties of SrPd2Ge2, SrPd2As2, and CaPd2As2 crystallizing in the ThCr2Si2-type body-centered tetragonal structure. Our electronic results show that the density of states at the Fermi level is mainly dominated by the strong hybridization of Pd d states and Ge (or As) p states. The linear response method and the Migdal-Eliashberg approach have been used to calculate the Eliashberg spectral function for all these compounds. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter (lambda) is found to be 0.74 for SrPd2Ge2, 0.66 for SrPd2As2, and 0.72 for CaPd2As2. Using the calculated values of lambda and the logarithmically averaged phonon frequency omega(ln) the superconducting critical temperature (T-c) values for SrPd2Ge2, SrPd2As2, and CaPd2As2 are found to be 3.20, 2.05, and 2.48 K, respectively, which are in acceptable agreement with the corresponding experimental values. The relative differences in the T-c values between the Ge and As compounds have been explained in terms of some key physical parameters.

Ab initio pseudopotential calculations have been made to calculate the structural, electronic, vibrational, and superconducting properties of the borocarbide superconductors RM2B2C (R = Lu, La, Y; M = Ni,Pd,Pt). Using the electronic and vibrational spectra, the average electron-phonon coupling parameter is calculated to be 0.78, 0.83, 0.96, and 1.48 for LaPt2B2C, LuNi2B2C, YPt2B2C, and YPd2B2C, respectively. From a detailed spectral analysis throughout the irreducible Brillouin zone (IBZ), we found that the highest contribution to the electron-phonon coupling parameter for these materials comes from the acoustic and low-frequency optical modes characterized with anomalous dispersion, instead of the A(1g) optical-phonon mode claimed in previous studies. Using the calculated electron-phonon coupling parameter values, the superconducting critical temperatures for LaPt2B2C, LuNi2B2C, YPt2B2C, and YPd2B2C are found to be 10.40, 15.94, 11.30, and 20.60 K, respectively, which are in good accordance with their experimental values of 10.50, 16.10, 10.00, and 21.00 K.