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A theoretical crystal-field model for the X-(X = 0, S) centres with U,-symmetry leads to three equivalent operators for the np' 'P ground state, from which the g tensor of the defects can be calculated analytically. Three equivalent operator coefficients p , q , r a r e fitted to four a priori independent experimental quantities g,,, g",, gzz and q . This leads to an excellent agreement for NaCl: S-, RbCl : 0-and KCI : 0 -, and to a fair agreement for KCI : S -.

An ultrahigh vacuum scanning tunneling microscope (STM) with a new piezo-driven inchworm for coarse positioning has been designed and constructed. The inchworm consists of five piezoelectric stacks and moves, being pressed from the side by a spring. This structure makes it durable to bakeout of the vacuum chamber. The tip and the sample set in the tunneling unit are exchangable without breaking vacuum. The STM chamber is attached to a conventional molecular beam epitaxy chamber equipped with surface analysis instruments without external vibration isolation. We have ascertained that the STM has an atomic-order resolution even under poor conditions, where much acoustic noise and mechanical vibration are produced by vacuum pumps and cooling water in the main chamber.

Formation of titanium hydrides in (Al,Ga)As doubleheterostructure laser contacts and their effect on degradation rate