Electronic structure theory of the hidden-order materialURu2Si2

Abstract

We report a comprehensive electronic structure investigation of the paramagnetic ͑PM͒, the large moment antiferromagnetic ͑LMAF͒, and the hidden order ͑HO͒ phases of URu 2 Si 2 . We have performed relativistic full-potential calculations on the basis of the density-functional theory, employing different exchangecorrelation functionals to treat electron correlations within the open 5f shell of uranium. Specifically, we investigate-through a comparison between calculated and low-temperature experimental properties-whether the 5f electrons are localized or delocalized in URu 2 Si 2 . The local spin-density approximation ͑LSDA͒ and generalized gradient approximation ͑GGA͒ are adopted to explore itinerant 5f behavior, the GGA plus additional strong Coulomb interaction ͑GGA+ U approach͒ is used to approximate moderately localized 5f states, and the 5f-core approximation is applied to probe potential properties of completely localized uranium 5f states. We also performed local-density approximation plus dynamical mean-field theory calculations ͑DMFT͒ to investigate the temperature evolution of the quasiparticle states at 100 K and above, unveiling a progressive opening of a quasiparticle gap at the chemical potential when temperature is reduced. A detailed comparison of calculated properties with known experimental data demonstrates that the LSDA and GGA approaches, in which the uranium 5f electrons are treated as itinerant, provide an excellent explanation of the available low-temperature experimental data of the PM and LMAF phases. We show furthermore that due to a materialspecific Fermi-surface instability a large, but partial, Fermi-surface gapping of up to 750 K occurs upon antiferromagnetic symmetry breaking. The occurrence of the HO phase is explained through dynamical symmetry breaking induced by a mode of long-lived antiferromagnetic spin fluctuations. This dynamical symmetry breaking model explains why the Fermi-surface gapping in the HO phase is similar but smaller than that in the LMAF phase and it also explains why the HO and LMAF phases have the same Fermi surfaces yet different order parameters. A suitable order parameter for the HO is proposed to be the Fermi-surface gap, and the dynamic spin-spin correlation function is further suggested as a secondary order parameter.

Unusual Magnetic Field-Dependence of the Electronic Dispersion Relations in a Hidden Ordered Phase.

“…The transition results in a folding of the Brillouin zone through a commensurate vector Q which is consistent with Angle Resolved Photoemission [12], de Haasvan Alphen [13] and neutron scattering measurements [14]. Similar nesting conditions have been found to be satisfied in LDA calculations [15,16] of the electronic structure of URu 2 Si 2 . We have shown [9] that the transition in the underscreened Anderson Lattice Model can be considered as resulting in a coupled spin and orbital density wave such that the orbital density wave for the spin-up is exactly out of phase with the orbital density wave for the spin-down electrons.…”

Lattice dynamics of the heavy-fermion compoundURu2Si2

“…We performed two types of calculations, with and without spin-orbit coupling (SOC), assuming in both cases the non-magnetic ground state. The optimized lattice parameters show good agreement with the experimental values and the previous relativistic full-potential calculations 55 .…”

Collective spin resonance excitation in the gapped itinerant multipole hidden order phase ofURu2Si2

“…Their enhanced effective masses (m * = 8 − 25m 0 ) 7,8 cannot adequately be described by band structure calculations (m b ≤ 8m 0 ) 7 although the latter give a Fermi surface sheet topology in agreement with dHvA and SdH experiments 1,6 . Conceptually mass enhancement can be obtained within the Anderson lattice model in constrained mean field approximation 38 which also leads to a narrow indirect hybridization gap close to the Fermi level, equivalent to the Kondo temperature (T K 11.1 meV or 129 K) that can be seen in STM experiments 20,39 .…”

“…We rather model directly the heavy quasiparticle bands and their Fermi surface by a simple effective tight binding model. Ab-initio LDA calculations 6,17 have shown that states at the Fermi surface consist mostly of jj-coupled total angular mometum 5f states |j = . Therefore the heavy bands, which have twofold Kramers-pseudo spin degeneracy may be directly described within an effective two-orbital tight binding model due to Rau and Kee…”

“…1. In the early theoretical work on thermodynamical properties the HO phase was interpreted in terms of localized and crystalline-electric-field (CEF) split 5f states 2 and their localized multipolar degrees of freedom 3 . However, it has subsequently become evident from comparison of ARPES experiments 4,5 and local density electronic structure calculations 6 that the itinerant 5f electron band picture is a more appropriate starting point. The LDA calculations show that the heavy Fermi surface in URu 2 Si 2 consists mainly of electron sheets around the Γ-point and hole sheets around the body centered tetragonal (bct) Z-point (0,0, 2π c ) consisting mostly of j = 5/2 5f orbitals.…”