Competition between hidden order and antiferromagnetism in URu2Si2 under uniaxial stress studied by neutron scattering -: art. no. 214419

We have performed elastic neutron scattering experiments under uniaxial stress sigma applied along the tetragonal [100], [110], and [001] directions for the heavy-electron compound URu2Si2. We found that antiferromagnetic (AF) order with large moment is developed with sigma along the [100] and [110] directions. If the order is assumed to be homogeneous, the staggered ordered moment mu(o) continuously increases from 0.02 mu(B)/U(sigma=0) to 0.22 mu(B)/U (0.25 GPa). The rate of increase partial derivative mu(o)/partial derivative sigma is similar to 1.0 mu(B)/GPa, which is four times larger than that for the hydrostatic pressure (partial derivative mu(o)/partial derivative P similar to 0.25 mu(B)/GPa). Above 0.25 GPa, mu(o) shows a tendency to saturate, similar to the hydrostatic pressure behavior. For sigma parallel to[001], mu(o) shows only a slight increase to 0.028 mu(B)/U(sigma=0.46 GPa) with a rate of similar to 0.02 mu(B)/GPa, indicating that the development of the AF state highly depends on the direction of sigma. We have also found a clear hysteresis loop in the isothermal mu(o)(sigma) curve obtained for sigma parallel to[110] under the zero-stress-cooled condition at 1.4 K. This strongly suggests that the sigma-induced AF phase is metastable, and separated from the "hidden order" phase by a first-order phase transition. We discuss these experimental results on the basis of crystalline strain effects and elastic energy calculations, and show that the c/a ratio plays a key role in the competition between these two phases.