= Switch-on drift in a circularly driven parabolic band
{title2=$\mathbf k=(eE_0/\hbar\omega)(\cos\omega t-1,-\sin\omega t)$}
A charge $-e$ starting at a band minimum when the field $E_0(\sin\omega t,\cos\omega t)$ is applied acquires the displayed <crystal momentum>. Its orbit is a shifted <circle>. For masses $m_x,m_y$, the real-space <coordinates> are proportional to $(\sin\omega t-\omega t)/m_x$ and $(\cos\omega t-1)/m_y$: an elliptic oscillation plus a uniform drift. Omitting the integration constant changes the physical initial condition and reduces the maximum light-axis kinetic energy by a factor of four.
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