The Orientation and Molar Volume Dependence of Second Sound in HCP 4He Crystals
While second sound has been shown to exist in NaF1 and Bi,2 solid helium3–7 remains the only case where the phenomenon is well developed and free from resistive scattering. Furthermore, the elastic properties of helium are readily changed by large amounts with the application of modest pressures. Thus solid helium is ideally suited to test theoretical predictions for second sound in solids. The one major drawback is that sound velocities in solid helium are anisotropic and crystal orientation is not easily controlled.
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