Abstract
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 [Sakurai et al 1968 Phys. Rev. 167 510]. Despite identifying two magnon modes at the zone boundary, the team was unable to parameterise the low energy magnetic excitation spectrum below TN using conventional pseudo-bosonic approaches, instead achieving only qualitative agreement. It would not be for another 40 years that Roger, now at Oxford and motivated by the discovery of the high-Tc cuprate superconductors [Bednorz and Muller 1986 Z. Phys. B 64 189], would make another attempt at the parameterisation of the magnetic excitation spectrum that had previously alluded him at the start of his career. 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 Mg0.97Co0.03O. These experiments would prove instrumental in the determination of both single-ion [Cowley et al 2013 Phys. Rev. B 88 205117] and cooperative magnetic parameters [Sarte et al 2018 Phys. Rev. B 98 024415] for CoO. Both these sets of parameters would eventually be used in a spin–orbit exciton model [Sarte et al 2019 Phys. Rev. B 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.
Highlights
In the years following Brockhouse’s invention of the neutron triple axis spectrometer [1,2,3], Chalk River was considered a premier neutron scattering facility, attracting some of the most brilliant scientific minds of the 20th century [4,5,6,7]
We have described how Roger, through his extraction of the single-ion parameters of Co2+ via chemical dilution [88, 111], and Bill, through the establishment of the spin-orbit exciton model [131], laid for the foundation for the future success of Sarte et al [119] in parameterising the low energy magnetic fluctuations in CoO that were first measured with neutrons by these two at Chalk River more than 50 years ago [22]
While the failure of the model at the zone boundaries can be attributed to a magnetovibrational contribution to the neutron cross section [250,251,252], the physical origin underlying the rapid decay of the column of magnetic fluctuations at high energy transfers remains an open question
Summary
In the years following Brockhouse’s invention of the neutron triple axis spectrometer [1,2,3], Chalk River was considered a premier neutron scattering facility, attracting some of the most brilliant scientific minds of the 20th century [4,5,6,7]. In the face of a seemingly Herculean task, little progress was seen for almost another 20 years [27] It wouldn’t be until 1986 with the discovery of high temperature superconductivity in the cuprates by Bednorz and Mueller [89] that there would be rejuvenated interest from both Roger and the community as a whole in their parent compounds, the Mott insulators [45,56,62,90,91,92]. From Roger’s perspective, the use of such a Hamiltonian would provide a much clearer understanding of the complex low temperature magnetism in systems such as CoO by permitting a direct input of spin-orbit, anisotropy, and magnetic exchange parameters, instead of relying on indirect contributions based on expressions derived from higher order perturbation theory [120,121,122,123]
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