Hidden order, exotic magnetism and structure of the normal state in correlated f-electron systems: UTe2, NpO2 and PrO2

Abstract:

The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such “hidden orders” (HOs) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f - shells that may harbor high-rank multipole moments. We show how the ground-state order and magnetic excitations of a prototypical HO system NpO₂, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force theorem method. A primary non collinear order of time odd rank 5 (triakontadipolar) moments has been predicted. We show also that exotic non-chiral magnetic order in PrO₂ is a results of a strong high order multipolar interactions within whole |JM> ground state multiplet. The problem of magnetic order in PrO₂ cannot be reduced to the interactions within a ground state Kramer’s doublet.

As has been shown by Kotljar and Haule in 2007 the canonical and perhaps most investigated metallic “hidden” order materials URu₂Si₂ can also develop a “hidden” multipolar order (hexadecapolar) due to localized 5f2 configuration at low temperature. Whereas at higher temperatures a hybridization of the localized 5f-levels leads to the Kondo behavior (“Kondo arrest scenario”) and at very low temperature a hidden order phase coexists with superconductivity. We reveal on basis of correlated ab-intio calculations a close analogy between normal state behavior of URu₂Si₂ and newly discovered heavy fermion UTe₂ superconductor.

The UTe₂ compound is regarded as a heavy fermion mixed-valence system with very peculiar properties within the normal and superconducting states. It shows no signs of magnetic order but strong anisotropy of a magnetic susceptibility and a superconducting critical field. In addition to the heavy fermion-like behavior in the normal state, it exhibits also a distinctive Schottky-type anomaly at about 12 K and a characteristic excitations gap ~35-40 K. Here we show, by virtue of dynamical mean-field theory calculations with a quasi-atomic treatment of electron correlations, that ab-initio derived crystal-field splitting of the 5f2 ionic configuration yields an agreement with these experimental observations. We also analyze the symmetry of magnetic and multipolar moment fluctuations that might lead to the superconducting pairing at low temperatures. The origin of the critical fluctuations that mediate a SC in both UTe2 and URu₂Si₂ compound might be essentially the same.