Hallmarks of Hunds coupling in the Mott insulator Ca2RuO4

Abstract

A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realized in Ca2RuO4. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide—using angle-resolved photoemission electron spectroscopy—the band structure of the paramagnetic insulating phase of Ca2RuO4 and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund's coupling J=0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the dxy orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund's coupling that together generate the Mott insulating state of Ca2RuO4.

In situ strain tuning of the metal-insulator-transition of Ca2RuO4 in angle-resolved photoemission experiments

“…Earlier dynamical mean field theory (DMFT) calculations of the MIT in Ca 2 RuO 4 predict that the full d xy orbital polarization with n xy  ≈ 2, n xz  ≈  n yz  ≈ 1 and p  =  n xy  − ( n xz  +  n yz )/2 ≈ 1 observed in the insulating state vanishes in the metallic L-Pbca phase 4,7,14,16,38,39 . This is qualitatively consistent with our experiments showing signatures of all three t 2 g orbitals on the Fermi surface.…”

“…3d) and consistent with ref. 16 , we identify these features with the fully occupied xy orbital and the lower Hubbard band (LHB) of xz / yz character, respectively. This confirms a basic electronic configuration in the insulating S-Pbca phase with fully occupied d xy orbital and half-filled d xz / d yz bands split into lower and upper Hubbard band, as proposed on the basis of DMFT calculations 4,14 .…”

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

“…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 . Nevertheless, as the multiband Hubbard model was solved using the single-site dynamical mean-field theory approach, it is not clear to what extent the spin polaron physics is present there.…”

Determination of Hund's coupling in 5d oxides using resonant inelastic x-ray scattering

“…On the other hand, it promotes strongly correlated bad-metal behavior by rendering Fermi liquid quasiparticles incoherent. This dichotomous role played by J H is now recognized to be important in the widely studied iron pnictides [7][8][9][10][11][12][13] , as well as in ruthenates like Ca 2 RuO 4 14, 15 . In recent years, there has been an increasing interest in complex 5d oxides.…”