Abstract
The strongly correlated insulator Ca$_{2}$RuO$_4$ is considered as a paradigmatic realization of both spin-orbital physics and a band-Mott insulating phase, characterized by orbitally selective coexistence of a band and a Mott gap. We present a high-resolution oxygen $K$-edge resonant inelastic X-ray scattering study of the antiferromagnetic Mott insulating state of Ca$_{2}$RuO$_4$. A set of low-energy ($\sim$80 and 400 meV) and high-energy ($\sim$1.3 and 2.2 eV) excitations are reported that show strong incident light polarization dependence. Our results strongly support a spin-orbit coupled band-Mott scenario and explore in detail the nature of its exotic excitations. Guided by theoretical modelling, we interpret the low-energy excitations as a result of composite spin-orbital excitations. Their nature unveil the intricate interplay of crystal-field splitting and spin-orbit coupling in the band-Mott scenario. The high-energy excitations correspond to intra-atomic singlet-triplet transitions at an energy scale set by the Hund's coupling. Our findings give a unifying picture of the spin and orbital excitations in the band-Mott insulator Ca$_{2}$RuO$_4$.
Highlights
Spin-orbit coupling (SOC) is a central thread in the search for novel quantum material physics [1]
Good agreement with previous published XAS experiments [26,27,28] is found when overlap in temperature, light polarization, and incident angle allows for a comparison
As previously reported [26,27,31], the apical and planar oxygen K-edge peaks are found at about 529.1 eV and 529.8 eV [see Fig. 1(b)]. These resonances stem from hybridization of the oxygen p-bands with the ruthenium t2g states, whereas the resonances at higher photon energies are related to hybridization with unoccupied eg states
Summary
Spin-orbit coupling (SOC) is a central thread in the search for novel quantum material physics [1]. Such as Sr2IrO4 are prime examples of systems where SOC plays a defining role in shaping the Mott insulating ground state [2]. Measurements of the paramagnetic insulating band structure [9] were interpreted in favor of an orbitally differentiated band-Mott insulating ground state [10,11]. This rich phenomenology of Ca2RuO4 is a manifestation of the interplay between multiple energy scales— the Coulomb interaction U, Hund’s coupling JH, the crystal-field splitting δ, and SOC λ.
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