... ¹

Physics and Chemistry of Solids Group, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, UK. CB3 0HE

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... ²

Girton College, University of Cambridge, Huntingdon Road, Cambridge, UK. CB3 0JG

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... ³

vi5u0-website@yahoo.co.uk

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... ⁴

Physics and Chemistry of Solids Group, Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge, UK. CB3 0HE

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...$^{\textrm{,}}$⁵

Selwyn College, University of Cambridge, Grange Road, Cambridge, UK. CB3 9DQ

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...$^{\textrm{,}}$⁶

jacb1@phy.cam.ac.uk

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