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if we have an electron in the valance band and excite it to the conduction band, and if there is no input of momentum during this process, then I always thought we would be left with an electron of wavevector $\vec k$ and a hole with wavevector $\vec k$ also. You wrote it yourself : there is no input of momentum during this process. Then why do you ...


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For the relation between the first and second equation, in the book you have : $$\frac{\partial H}{\partial t} + \nabla\cdot j_E = 0$$ and it's says also that : $$ j_E = \frac{1}{2}\int d^3r\left( R\frac{\partial\mathcal{H}}{\partial t} +\frac{\partial\mathcal{H}}{\partial t}R\right) $$ but $H$ and $\mathcal{H}$ are not the same objects, the first is the ...


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I found this one: Van der Waals molecules. This answers my question to some extent.


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Most of the energy of a tsunami wave is gravitational energy. It is the rise in sea level, and the retreat of the surge, which is most devastating. Our cities are vulnerable because we build right to the edge of the sea. If we left a reasonable buffer of tidal flats, marshes, or sand berms between ourselves and the ocean, the energy of tsunami or storm ...



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