Elasticity of dense helium

Brillouin scattering measurements have been conducted on crystals of ⁴He at high pressure using diamond anvil cells. Complete sets of elastic moduli, aggregate sound velocities, adiabatic and isothermal equations of state, and other thermodynamic properties were obtained up to 320.3em0exGPa at room temperature. Although He adopts a hexagonal close-packed structure over the measured P1000-0ptT range, its elasticity is close to that of a cubic material. The Cauchy condition is violated to an increasing degree with pressure and indicates an increasing role of noncentral forces such as many-body exchange interactions. The results provide detailed constraints on the density dependence of interatomic interactions and the associated lattice dynamics of this quantum solid.