Is there anyone familiar with berry phase?(adsbygoogle = window.adsbygoogle || []).push({});

Wilczek and Zee have a classic paper PRL 52_2111

I can not understand their equation (6)

I can not see why the first equality should hold

\eta_a and \eta_b should be orthogonal to each other, but why should \eta_b be orthogonal to the time derivative of \eta_a ?

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# Berry phase in degenerare case

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