Effect of pressure on the tiny antiferro-magnetic moment in URu 2Si 2

Abstract

We propose a scenario to explain the anomalous pressure dependence of the antiferro-magnetic moment observed in URu 2Si 2, the steep change from tiny to large moments at lower pressures (P<P c =15 GPa) and the gradual change of large moment at higher pressures ( P> P c).We speculate that at ambient pressure the ionic ground state of U is non-magnetic quadrupolar doublet Γ 5 with (5f) 2, which is hybridized with the excited state of magnetic dipolar doublet Γ 7 with (5f) 1 via conduction electrons due to the Kondo effect with respect to quadrupolar degrees of freedom. By decreasing temperature T, quadrupolar order sets in at a certain temperature T o and then octupolar order sets in additionally at a certain lower temperature T t< T o leading to a tiny magnetic moment induced through the weak hybridization of the magnetic excited state to the non-magnetic ground state. By increasing P, the hybridization energy increases steeply and so does the induced magnetic moment at P< P c. On the other hand, at P> P c, the character of the ground state of uranium ions changes to be magnetic somehow. Then the magnetic order with large magnetic moment manifests itself at T< T o. This scenario is confirmed by using a two-channel Anderson lattice model.