Heterogeneous Metallic State in URu2Si2

A review of recent attempts to understand the pressure-temperature phase diagram of URu2 Si-2 is given. At ambient (and weak) pressures, the low-T properties of this system are dominated by an unidentified ordered state (hidden order: HO) and non-trivial superconductivity, while by applying hydrostatic (or uniaxial or chemical) pressures, a change to a regime dominated by antiferromagnetic order (AF) is observed. In between these two regimes, an unusual heterogeneous metallic state is found. This change in regime is clearly demonstrated by neutron scattering, NMR and mu SR measurements. The experimental results and strain analyses show that the low-T state of this system is extremely sensitive to the shape effects, and a slight increase in the axial strain delta(c/a)/( c/a ) (equivalent to(eta) over cap) can easily change the ground state from HO to AF. The critical value is estimated to be very small ((eta) over capc similar to 10(-3) ), suggesting that a strain distribution in a crystal is responsible for the observed heterogeneity. We will demonstrate this by showing our recent neutron-scattering experimental data using a high-quality single crystal.