Phonons in random alloys: the itinerant coherent-potential approximation

We present the itinerant coherent-potential approximation(ICPA), an analytic, translationally invariant and tractable form of augmented-space-based, multiple-scattering theory in a single-site approximation for harmonic phonons in realistic random binary alloys with mass and force-constant disorder. We provide expressions for quantities needed for comparison with experimental structure factors such as partial and average spectral functions and derive the sum rules associated with them. Numerical results are presented for Ni_{55} Pd_{45} and Ni_{50} Pt_{50} alloys which serve as test cases, the former for weak force-constant disorder and the latter for strong. We present results on dispersion curves and disorder-induced widths. Direct comparisons with the single-site coherent potential approximation(CPA) and experiment are made which provide insight into the physics of force-constant changes in random alloys. The CPA accounts well for the weak force-constant disorder case but fails for strong force-constant disorder where the ICPA succeeds.Comment: 19 pages, 12 eps figures, uses RevTex

SCUBA - The first year on the telescope [Abstract]

SCUBA is a revolutionary new bolometer camera operating at submillimetre wavelengths on the James Clerk Maxwell Telescope (JCMT). With two arrays of detectors operating simultaneously (131 pixels in total), SCUBA can take images up to a thousand times faster than the JCMT's previous (single-channel) continuum instrument to the same noise level. With each pixel having a factor of 10 improvement in sensitivity, the performance of SCUBA is limited purely by the sky background level on the telescope. After a lengthy commissioning period beginning in July 1996, SCUBA began carrying out observations for the astronomical community in the early summer of 1997. We describe the current performance of the instrument, and illustrate this, using some spectacular examples of the type of science SCUBA is now producing