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Twdimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in lowdensity helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values, The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously d e t a i n e d in lifetime measurements; the widths of the Ps component in helium. neon and argon are consistent with a Ps-atom scattering cross Section below 6.8 eV in all three gases of approximately (9-0.5E) nu:, where E is the Ps energy in eV.

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Materials that exhibit colossal magnetoresistance (CMR) are currently the focus of an intense research effort, driven by the technological applications that their sensitivity lends them to. Using the angular correlation of photons from electronpositron annihilation, we present a first glimpse of the Fermi surface of a material that exhibits CMR, supported by "virtual crystal" electronic structure calculations. The Fermi surface is shown to be sufficiently cubic in nature that it is likely to support nesting.Since the recent discovery of the phenomenon of colossal magnetoresistance (CMR), research efforts have been intense [1][2][3]. The reason is the number of potentially important applications for CMR materials in magnetic memory systems, magnetic read heads and in other magnetic sensors [4]. Experimental studies of CMR materials have concentrated on the manganite perovskites [3], T 1−x D x MnO 3 , where T is a trivalent lanthanide cation, and D is a divalent (e.g. alkaline earth) cation. As implied by the Jahn-Teller distortions manifest in the undoped parent compounds (e.g. LaMnO 3 ), these are systems where there is strong coupling between the electronic and lattice degrees of freedom [5]. That, and the multiplicity of crystallographic and magnetic phases in the doped crystals, suggests that the ground states of these systems depend upon a subtle interplay between their microscopic electrical, magnetic and lattice properties and excitations [6,7]. Calculations of electronic band structures have been reported [7,8], but experimental evidence concerning those band structures and their associated electronic spectra remains scarce.In particular, a knowledge of the Fermi surface (FS) is vital for an understanding of transport properties. The half-metallic character (and therefore the existence of a FS in only one spin) of these materials is clearly of importance, owing to the absence of a spin-flip scattering contribution to the resistivity. Moreover, there has been speculation that one of these sheets has nesting properties [9], implying additional consequences for the transport. In this Letter, we present the first glimpse of the FS of La 0.7 Sr 0.3 MnO 3 , presented in conjunction with calculations of the momentum density and band structure.In such complicated systems, the traditional tools of Fermiology are precluded, since the disordered nature of the alloy means that the mean-free-paths are too short. However, the occupied momentum states, and hence the FS, can be accessed via the momentum distribution using the 2-Dimensional Angular Correlation of electronpositron Annihilation Radiation (2D-ACAR) technique [10]. A 2D-ACAR measurement yields a 2D projection (integration over one dimension) of the underlying twophoton momentum density, ρ(p) i.e. ρ(p) = occ.j,k | dr γ(r)ψ k,j (r)ψ + (r) exp(−ip.r)| 2