Quantum Oscillations of the Velocity of Sound in Gallium

Large-amplitude quantum oscillations of the ultrasonic velocity, $\left|\frac{\Delta V}{V_0}\right|\sim {10}^{-3\mathrm{}}$, were measured in single-crystal gallium at $1.2{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}<T<\mathrm{}1.6\mathrm{}{}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}$ in magnetic fields up to 150 kG. The measurements were carried out using 11- and 28-Mc/sec longitudinal sound waves with $\mathbf{q}\parallel \mathbf{H}\parallel$ ($b$ crystallographic axis). The experimental results are compared with three different theories of this phenomenon. The most satisfactory account of the experimental observations, including the amplitude of the velocity oscillations, is given by the Blank-Kaner theory, although some features of the data remain unexplained.