Ultrasonic Amplification in Semimetals

When a semimetal is placed in crossed electric (E) and magnetic fields (H), both electrons and holes drift in the E\ifmmode\times\else\texttimes\fi{}H direction with velocity $\frac{\mathrm{Ec}}{H}$. A sound wave propagating in this direction with velocity $s$ may be amplified if $\frac{\mathrm{Ec}}{H}>s$. The effect is similar to that described by Hutson, McFee, and White, except that the present arrangement for semimetals allows one to obtain conditions of amplification at much lower current densities, since the magnetic field limits the current flowing in the electric field direction. The theory of sound amplification in a semimetal has been worked out for the above conditions, assuming the field produced by the sound wave can be described by a deformation potential. Because of the relatively large carrier densities in semimetals such as bismuth, it is found that large amplification factors can be obtained.

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