Exceptional Ising magnetic behavior of itinerant spin-polarized carriers inURu2Si2

Abstract

We show on the basis of electronic structure calculations that the uranium 5f magnetic moment in URu2Si2 exhibits a unique Ising behavior, which surprisingly, arises from itinerant electronic states. The origin of the unusual Ising behavior is analyzed as due to the peculiar near-Fermi edge nested electronic structure of URu2Si2 involving its strong spin-orbit interaction. The Ising anisotropy has pertinent implications for theories applicable to explaining the Hidden Order phase in URu2Si2. PACS numbers: 71.28.+d,75.30.Gw The origin of the "Hidden Order" (HO) phase emerging below T 0 = 17.5 K in the uranium-based heavy-fermion compound URu 2 Si 2 has remained a mystery even after more than a quarter century of intensive investigations (see, e.g., Ref.[1] for a recent review). This second-order phase transition appears unmistakably in the thermodynamic and transport properties [2][3][4], yet local solidstate probes such as x rays, neutron scattering, NMR or µSR fail to give a clue for the emerging order parameter. Long-range ordered (dipolar) magnetism has been excluded as a cause for the "hidden order" (HO), but in close proximity to the HO phase a long-range ordered antiferromagnetic phase exists, which is stabilized through only a small pressure of ∼ 0.5 GPa [5].Multifarious theories have been proposed to explain the intriguing appearance of the HO phase, see [1] and [6] for an overview. Since the actinide 5f electrons can, in general, assume localized or itinerant character, correspondingly theories adopting localized 5f behavior have been proposed (e.g., [6][7][8][9][10][11]) as well as competing theories based on the assumption of itinerant 5f behavior (e.g., [12][13][14][15][16][17][18][19][20]). In several of the latter models the existence of a Fermi surface instability is typically connected to appearance of an unconventional density wave [17,18], a spin resonance mode [13] or hybridization wave [14] that triggers formation of a Fermi surface gap. Theories based on localized 5f states often elaborate from a mainly localized 5f 2 configuration possibly with some hybridization with conduction electrons [6,11].Experimental evidence in favor of either localized or itinerant 5f behavior is unquestionably crucial. Recent quantum oscillation measurements [21,22] have drawn attention to a previously unrecognized aspect of the HO quasiparticles in URu 2 Si 2 , namely, their extreme Ising character. From the angular dependence of the de Haasvan Alphen amplitudes [23] a g-factor anisotropy g c /g a , along the c and a crystallographic axes, was estimated to exceed 30, implying that HO emerges out of quasiparticles with giant Ising anisotropy [21,22]. This feature has become salient in the quest for understanding the exotic HO and its concomitant superconductivity [6,24]. The Ising behavior of the near Fermi-energy quasiparticles nicely supports the picture of localized 5f states in URu 2 Si 2 , possibly having a small hybridization with conduction electrons [21,22]. This extreme magnetic anisotropy is a cent...

First-principles Theory of Magnetic Multipoles in Condensed Matter Systems

“…To predict the most likely multipoles, ab initio electronic structure calculations of URu 2 Si 2 in the nonmagnetic and AFM phases have been performed. 160,318,361,[371][372][373] These calculations notably correctly reproduce many of the known properties of URu 2 Si 2 , as e.g.…”

Fermi-surface selective determination of the g -factor anisotropy in URu2Si2

“…A relativistic DFT calculation predicted an Ising behavior for the band-like 5f electrons in URu 2 Si 2 with magnetic moments along the c axis and no anisotropy in the basal plane. 22,91 Here, the 5f electrons are treated as fully itinerant and the calculation is performed for the antiferromagnetic phase which has practically the same Fermi surface than the hidden order state. 29,[43][44][45] This is justified as the Fermi surfaces for the localized 5f 2 or for the localized 5f 3 uranium configuration are not in correspondence to the experimentally observed ones.…”

“…21 The strong Ising character of the 5f electrons in URu 2 Si 2 has been also confirmed on the basis of density functional theory (DFT) electronic structure calculations. 22 The Ising anisotropy arises from a combination of the peculiar Fermi surface nesting and strong spin-orbit interaction. While in this electronic structure calculations the 5f electrons are treated fully itinerant, other models supposing a localized 5f 2 non-Kramers doublet ground state could explain the large magnetic uniaxial anisotropy as well.…”

Hidden order behaviour in URu2Si2(A critical review of the status of hidden order in 2014)

“…This scenario of a localised CEF doublet explains the Ising behaviour, but for other CEF schemes, e.g., the two CEF singlets invoking hexadecapolar order [63,67], Ising anisotropy is unproven. Surprisingly recent relativistic DFT calculations [97] showed that the bandlike 5f electrons in URu 2 Si 2 exhibit a colossal Ising behaviour, a property which is truly exceptional for itinerant electrons. In Fig.…”