Tuning of the Ru4+ ground-state orbital population in the 4d4 Mott insulator C

Abstract

The ground-state orbital occupancy of the Ru 4+ ion in Ca2−xLaxRuO4 [x = 0, 0.05(1), 0.07(1) and 0.12(1)] was investigated by performing X-ray absorption spectroscopy (XAS) in the vicinity of the O K edge as a function of angle between the incident beam and the surface of the single-crystal samples. A minimal model of the hybridization between the O 2p states probed at the K edge and the Ru 4d orbitals was used to analyze the XAS data, allowing the ratio of hole occupancies nxy/nyz,zx to be determined as a function of doping and temperature. For the samples displaying a low-temperature insulating ground-state (x ≤ 0.07), nxy/nyz,zx is found to increase significantly with increasing doping, with increasing temperature acting to further enhance nxy/nyz,zx. For the x = 0.12 sample, which has a metallic ground-state, the XAS spectra are found to be independent of temperature, and not to be describable by the minimal hybridization model, while being qualitatively similar to the spectra displayed by the x ≤ 0.07 samples above their insulating to metallic transitions. To understand the origin of the evolution of the electronic structure of Ca2−xLaxRuO4 across its phase diagram, we have performed theoretical calculations based on a model Hamiltonian, comprising electron-electron correlations, crystal field (∆) and spin-orbit coupling (λ), of a Ru-O-Ru cluster, with realistic values used to parameterize the various interactions taken from the literature. Our calculations of the Ru hole occupancy as a function of ∆/λ provide an excellent description of the general trends displayed by the data. In particular they establish that the enhancement of nxy/nyz,zx is driven by significant modifications to the crystal field as the tetragonal distortion of the RuO6 octahedral changes from compressive to tensile with La doping. We have also used our model to show that the hole occupancy of the O 2p and Ru 4d orbitals display the same general trend as a function of ∆/λ, thus validating the minimal hybridization model used to analyze the data. In essence, our results suggest that the predominant mechanism driving the emergence of the low-temperature metallic phase in La doped Ca2RuO4 is the structurally induced redistribution of holes within the t2g orbitals, rather that the injection of free carriers.

Photoemission spectrum of Ca2RuO4 : Spin polaron physics in an S=1 antiferromagnet with anisotropies

“…24, 26, and 27 the spin model is highly anisotropic, with the suggested values of thê z (x) axis anisotropy being equal to = 5.6J (γ = 0.25J), respectively, with J = 5.6 meV reproducing the spin wave dispersion observed in the inelastic neutron scattering experiment 24,26,27 . We note that such a large spin anisotropy originates in the large spin-orbit coupling on the ruthenium ions which, however, is quite widely considered as not strong enough to stabilise the S = 0 ground state 21,24,26,30,32 . Although the latter result can naively be understood as a consequence of the crystal field splitting (between the xz, yz and the xy orbitals) being about twice larger than the spin-orbit coupling 30 and therefore the spin S = 0 states having considerably higher energy than the spin S = 1 states, see Fig.…”

“…To investigate this rather general problem, we take a closer look at one of the intensively investigated transition metal oxides-Ca 2 RuO 4 19-33 which is a spin S = 1 antiferromagnetically ordered 21,24,26,30,32 and Mott insulating analogue of the unconventional superconductor Sr 2 RuO 4 34 . The other reason for choosing this system is that recently its detailed photoemission spectrum was not only studied experimentally but also successfully modelled using a multiband Hubbard model 28 .…”

Competition of three-dimensional magnetic phases in Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$: A structural perspective

“…Consequently, the xy orbitals are fully populated, leading to the insulating state. Any elongation of the c-axis induced by uni-axial pressure 54 or chemical doping 19,30,31,55,56 leads a transition to a metallic state in which the xy orbitals are only partially occupied. The octahedral flattening also stabilizes magnetic order, both FM and AFM, by increasing DOS at the Fermi level for the former and shifting nesting vector through orbital polarization for the latter.…”

Resonant inelastic x-ray scattering study of Ca3Ru2O7

“…In this fashion, we probe the two unoccupied t 2g states. This indirect approach has routinely been applied to different TM oxides [21][22][23][24].…”