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تسجيل مستخدم جديدImaging antiferromagnetic domain fluctuations and the effect of atomic-scale disorder in a doped spin-orbit Mott insulator
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Correlated oxides can exhibit complex magnetic patterns, characterized by domains with vastly different size, shape and magnetic moment spanning the material. Understanding how magnetic domains form in the presence of chemical disorder and their robustness to temperature variations has been of particular interest, but atomic-scale insight into this problem has been limited. We use spin-polarized scanning tunneling microscopy to image the evolution of spin-resolved modulations originating from antiferromagnetic (AF) ordering in a spin-orbit Mott insulator Sr3Ir2O7 as a function of chemical composition and temperature. We find that replacing only several percent of La for Sr leaves behind nanometer-scale AF puddles clustering away from La substitutions preferentially located in the middle SrO layer within the unit cell. Thermal erasure and re-entry into the low-temperature ground state leads to a spatial reorganization of the AF modulations, indicating multiple stable AF configurations at low temperature. Interestingly, regardless of this rearrangement, the AF puddles maintain scale-invariant fractal geometry in each configuration. Our experiments reveal spatial fluctuations of the AF order in electron doped Sr3Ir2O7, and shed light on its sensitivity to different types of atomic-scale disorder.
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Disentangling the relationship between the insulating state with a charge gap and the magnetic order in an antiferromagnetic (AF) Mott insulator remains difficult due to inherent phase separation as the Mott state is perturbed. Measuring magnetic and
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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.
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