We consider the polarization of unstable type-IIB D0-branes in the presence of a background five-form field strength. This phenomenon is studied from the point of view of the leading terms in the non-abelian Born Infeld action of the unstable D0-branes. The equations have SO(4) invariant solutions describing a non-commutative 3-sphere, which becomes a classical 3-sphere in the large-N limit. We discuss the interpretation of these solutions as spherical D3-branes. The tachyon plays a tantalizingly geometrical role in relating the fuzzy S 3 geometry to that of a fuzzy S 4 .

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We discuss impact ionisation of polarised atoms (Li, Na, K and metastable excited He) by polarised electrons. The consequences of the 'maximum interference approximation' in theoretical 'distorted wave' calculations for the total cross sections and the spin asymmetry are investigated. Numerical results obtained with the DWBA method are compared with recent experimental data. The agreement between theory and experiment is only marginal. This indicates that more sophisticated methods are needed to enable correct theoretical predictions of the spin dependent ionisation asymmetry. 0!953-4075/90/060981+07~03SO

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