Interplay of magnetism, Fermi surface reconstructions, and hidden order in the heavy-fermion material URu ₂Si ₂

URu ₂Si ₂ is surely one of the most mysterious of the heavy-fermion compounds. Despite more than 20 years of experimental and theoretical works, the order parameter of the transition at T ₀=17.5 K is still unknown. The state below T ₀ is still called the "hidden-order phase," and the stakes are still to identify the energy scales driving the system to this phase. We present new magnetoresistivity and magnetization measurements performed on very-high-quality single crystals in pulsed magnetic fields up to 60 T. We show that the transition to the hidden-order state in URu ₂Si ₂ is initially driven by a high-temperature crossover at around 40-50 K, which is a fingerprint of intersite electronic correlations. In a magnetic field H applied along the easy-axis c, the vanishing of this high-temperature scale precedes the polarization of the magnetic moments, as well as driving the destabilization of the hidden-order phase. Strongly impurity-dependent magnetoresistivity confirms that the Fermi surface is reconstructed below T ₀ and is strongly modified in a high magnetic field applied along c, i.e., at a sufficiently high magnetic polarization. The possibility of a sharp crossover in the hidden-order state controlled by a field-induced change of the Fermi surface is pointed out.