Shear relaxation in iron under the conditions of earth's inner core.

Large scale molecular dynamics simulations of iron at high pressure and temperature are performed to investigate the physics of shear softening. A solid 16×10(6) atoms sample of iron is grown out of the liquid with a small solid immersed in it at the start of simulation. We observe that diffusion in the sheared solid is similar to that in liquid, even though at different time scales. This allows us to describe the time dependence of shear stress in terms of elastic and hydrodynamic relaxation. The elastic response of the sample is close to the elastic response of Earth's inner core. This explains the abnormally low shear modulus in the core. The reason for the low shear modulus is the presence of defects of the crystal structure.