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تسجيل مستخدم جديدNeutron scattering measurements of $dd$ and spin-orbit excitations below the Mott-Hubbard gap in CoO
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Neutron scattering is used to investigate the single-ion spin and orbital excitations below the Mott-Hubbard gap in CoO. Three excitations are reported at 0.870 $pm$ 0.009 eV, 1.84 $pm$ 0.03, and 2.30 $pm$ 0.15 eV. These were parameterized within a weak crystal field scheme with an intra-orbital exchange of $J(dd)$=1.3 $pm$ 0.2 eV and a crystal field splitting 10Dq=0.94 $pm$ 0.10 eV. A reduced spin-orbit coupling of lambda=-0.016 $pm$ 0.003 eV is derived from dilute samples of Mg$_{0.97}$Co$_{0.03}$O, measured to remove complications due to spin exchange and structural distortion parameters which split the cubic phase degeneracy of the orbital excitations complicating the inelastic spectrum. The 1.84 eV, while reported using resonant x-ray and optical techniques, was absent or weak for non resonant x-ray experiments and overlaps with the expected position of a $^{4}A_{2}$ level. This transition is absent in the dipolar approximation but expected to have a finite quadrupolar matrix element that can be observed with neutron scattering techniques at larger momentum transfers. Our results agree with a crystal field analysis (in terms of Racah parameters and Tanabe-Sugano diagrams) and with previous calculations performed using local-density band theory for Mott insulating transition metal oxides. The results also demonstrate the use of neutron scattering for measuring dipole forbidden transitions in transition metal oxide systems.
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Motivated by the presence of an unquenched orbital angular momentum in CoO, a team at Chalk River, including a recently hired research officer Roger Cowley, performed the first inelastic neutron scattering experiments on the classic Mott insulator [S
akurai $textit{et al.}$ 1968 Phys. Rev. $mathbf{167}$ 510]. Despite identifying magnon modes at the zone boundary, the team was unable to parameterise the low energy magnetic excitation spectrum below $Trm{_{N}}$ using conventional pseudo-bosonic approaches. It would not be for another 40 years that Roger, now at Oxford and motivated by the discovery of the high-$T_{c}$ cuprate superconductors [Bednorz & Muller 1986 Z. Phys. B $mathbf{64}$ 189], would make another attempt at the parameterisation of the magnetic excitation spectrum that had previously alluded him. Upon his return to CoO, Roger found a system embroiled in controversy, with some of its most fundamental parameters still remaining undetermined. Faced with such a formidable task, Roger performed a series of inelastic neutron scattering experiments in the early 2010s on both CoO and a magnetically dilute structural analogue MgO. These experiments would prove instrumental in the determination of both single-ion [Cowley $textit{et al.}$ 2013 Phys. Rev. B $mathbf{88}$ 205117] and cooperative magnetic parameters [Sarte $textit{et al.}$ 2018 Phys. Rev. B $mathbf{98}$ 024415] for CoO. Both these sets of parameters would eventually be used in a spin-orbit exciton model [Sarte $textit{et al.}$ 2019 Phys. Rev. B $mathbf{100}$ 075143], developed by his longtime friend and collaborator Bill Buyers, to successfully parameterise the complex spectrum that both measured at Chalk River almost 50 years prior. The story of CoO is of one that has come full circle, one filled with both spectacular failures and intermittent, yet profound, little victories.
CoO has an odd number of electrons in its unit cell, and therefore is expected to be metallic. Yet, CoO is strongly insulating owing to significant electronic correlations, thus classifying it as a Mott insulator. We investigate the magnetic fluctuat
ions in CoO using neutron spectroscopy. The strong and spatially far-reaching exchange constants reported in [Sarte et al. Phys. Rev. B 98 024415 (2018)], combined with the single-ion spin-orbit coupling of similar magnitude [Cowley et al. Phys. Rev. B 88, 205117 (2013)] results in significant mixing between $j_{eff}$ spin-orbit levels in the low temperature magnetically ordered phase. The high degree of entanglement, combined with the structural domains originating from the Jahn-Teller structural distortion at $sim$ 300 K, make the magnetic excitation spectrum highly structured in both energy and momentum. We extend previous theoretical work on PrTl$_{3}$ [Buyers et al. Phys. Rev. B 11, 266 (1975)] to construct a mean-field and multi-level spin exciton model employing the aforementioned spin exchange and spin-orbit coupling parameters for coupled Co$^{2+}$ ions on a rocksalt lattice. This parameterization, based on a tetragonally distorted type-II antiferromagnetic unit cell, captures both the sharp low energy excitations at the magnetic zone center, and the energy broadened peaks at the zone boundary. However, the model fails to describe the momentum dependence of the excitations at high energy transfers, where the neutron response decays faster with momentum than the Co$^{2+}$ form factor. We discuss such a failure in terms of a possible breakdown of localized spin-orbit excitons at high energy transfers.
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