No abstract

No abstract

No abstract

The intermetallic compound Ni, Al orders in the Cu, Au structure, and it is generally believed to be a weak itinerant ferromagnet. However, the presence of any magnetic inhomogeneities, as suggested by specific-heat measurements, cannot be easily understood within the framework of an itinerant electron theory. In the present work inelastic neutron scattering techniques have been used to study the room-temperature phonon dispersion curves of Ni, Al.The force constants obtained by fitting the experimental data to a three-nearest-neighbor Born-von Karman model were used to evaluate the phonon density of states and the lattice specific heat. We find that the electronic specific heat of Ni, Al {obtained by subtracting the lattice contribution from the measured total specific heat) does not exhibit any anoaiialous features at low temperature. Thus, within experimental precision, we do not find any evidence for the presence of magnetic inhomogeneities in Ni, Al.

Inelastic neutron scattering techniques have been used to study the lattice dynamics of hcp Hf. The phonon dispersion curves along the [001], [100], and [110]symmetry directions were determined at 295 and 1300 K, and a selected number of phonon frequencies were measured also at 800 K. As the temperature decreases we observe a rather large increase in the frequencies of all but the [001] LO branch. The zone-center mode of the [001] LO branch, on the other hand, softens appreciably, and at room temperature this branch exhibits a dip at the zone center. These features of the phonon dispersion curves of Hf are similar to those of hcp Zr and Ti. The data were used to evaluate the lattice specific heat at constant pressure as a function of temperature. The calculated total specific heat, obtained by taking into account the electronic contribution, was found to agree, to within experimental uncertainties, with the results of specific-heat measurements. We find that the phonon anomalies (and their temperature dependence) in the dispersion curves of the superconducting elements of the IV column of the Periodic Table can be understood qualitatively as originating from the splitting about the Fermi level of doubly degenerate bands by the lattice distortion corresponding to the [001] LO mode. We argue that this mechanism may also be responsible for the phonon anomalies observed in other superconducting elements (Tc) and compounds (LaSn3).

We report on the optical transmission properties of narrow-band-gap (Eg<0.1 eV) InAs/GaSb superlattices grown by molecular-beam epitaxy. Energy band gaps of 0.15 and 0.085 eV at 4.8 K are determined for a 102-Å-period and a 124-Å-period superlattice, respectively. The absorption edge is extremely soft due to the spatial mismatch of hole and electron wave functions. In addition we show the first reported x-ray diffraction measurements on this materials system.

We present an experimental and theoretical study of n-type Hg1−xCdxTe photoconductors in which a large band-gap alloy was grown on top of a smaller band-gap active region and contacts were made to the larger gap material. The larger band-gap material causes an energy barrier to holes which decreases the rate at which they reach the high recombination region of the metal-semiconductor interface. As a result, this heterojunction contact greatly reduces the effects of carrier sweepout on device performance and leads to much higher detector responsivities. Experimental results in a symmetric device with a cutoff wavelength of 7.8 μm at 77 K show responsivities in excess of 106 V/W and detectivities close to the background limited value and nonsaturation of responsivity with bias voltage. In an asymmetric device, in which only one heterojunction contact was used, an order of magnitude increase in responsivity was observed when the heterojunction contact was biased to attract minority carriers, compared with the opposite bias polarity. A theoretical model of the heterojunction contact photoconductor is presented. Calculated results are in good agreement with experimental results. The results of the calculation suggest that the optimum compositional difference Δx of the two layers should be Δx∼0.04, and that the thickness of the large band-gap region should be 2–3 μm.

No abstract