URu2Si2 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 T0 = 17.5 K is still unknown. The state below T0 is called the “hidden-order phase,” and the stakes are still to identify the energy scales driving the system to this phase.
A new generation of magnetoresistivity and magnetization measurements on very-high-quality single crystals of URu2Si2 has been performed up to 60 T at the LNCMI-Toulouse. It permitted to show that the transition to the hidden-order state 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 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 T0 and is strongly modified in a high field applied along c, i.e., at a sufficiently high magnetic polarization. These new results will help for a future understanding of the hidden-order in URu2Si2.
More information about heavy fermions can be found here
G. W. Scheerer, W. Knafo, D. Aoki, G. Ballon, A. Mari, D. Vignolles, and J. Flouquet, Interplay of magnetism, Fermi surface reconstructions, and hidden order in the heavy-fermion material URu2Si2, Phys. Rev. B 85, 094402 (2012).
G. W. Scheerer, W. Knafo, D. Aoki, and J. Flouquet, Angular Dependence of the High-Magnetic-Field Phase Diagram of URu2Si2, Accepted in J. Phys. Soc. Jpn. Suppl.