Magnetic polarons due to spin-length fluctuations in ${\mathbf{d}}^4$ spin-orbit Mott systems

Abstract

Mott insulators based on $4d$ and $5d$ transition-metal ions, where spin-orbit interaction plays a key role, can exhibit various forms of unusual magnetism. A particular example is the antiferromagnet Ca${}_2$RuO${}_4$ containing $d^4$ Ru${}^{4+}$ ions. Here the spin-orbit interaction stabilizes the non-magnetic $J=0$ singlet ionic ground state, which gets dynamically mixed - via exchange interactions - with low-energy $J=1$ ionic excitations. Thanks to a sufficient strength of the exchange, these excitations condense and a long-range order emerges. The resulting ordered moments are soft and prone to fluctuations of their effective length. The corresponding amplitude mode appears as a prominent magnetic excitation and complements the conventional magnons involving rotations of the moments. Motivated by this peculiar kind of magnetic order and the specific spectrum of magnetic excitations, we study their influence on the propagation of doped carriers. To this end, we construct a microscopic model including both $d^4$ and $d^5$ degrees of freedom and address the propagation of an injected electron by employing self-consistent Born approximation. We find that the electron shows a combination of both free and a polaronic type of motion, where the mobile carrier strongly interacts with an accompanying cloud of magnetic excitations. Remarkably, in the latter case it is the exotic excitation - the amplitude mode - that is found to dominate over the contribution of magnons. Our soft-spin situation thus largely contrasts with spin polarons widely discussed in the context of doped Heisenberg-like magnets based on rigid spin moments.

22 pages, 11 figures, updated discussion of the hedgehog model

References in corpus (23)

• Mott Insulators in the Strong Spin-Orbit Coupling Limit: From Heisenberg to a Quantum Compass and Kitaev Models
• Novel Jeff = 1/2 Mott State Induced by Relativistic Spin-Orbit Coupling in Sr2IrO4
• Models and Materials for Generalized Kitaev Magnetism
• Spin-Orbital Separation in the quasi 1D Mott-insulator Sr2CuO3
• Na2IrO3 as a spin-orbit-assisted antiferromagnetic insulator with a 340 meV gap
• Anomalous high energy dispersion in photoemission spectra from insulating cuprates
• String excitations of a hole in a quantum antiferromagnet and photoelectron spectrospopy
• Spectral properties of orbital polarons in Mott insulators
• Magnetic Excitations in Spin-Orbit Coupled $d^4$ Mott Insulator on Square Lattice
• Electronic band structure changes across the antiferromagnetic phase transition of exfoliated MnPS${}_3$ probed by $\mu$-ARPES
• Spectral functions of Sr${}_2$IrO${}_4$: theory versus experiment
• Hole in the 2D Ising Antiferromagnet: Origin of the Incoherent Spectrum
• Non-linear bond-operator theory and 1/d expansion for coupled-dimer magnets II: Antiferromagnetic phase and quantum phase transition
• Collective nature of orbital excitations in layered cuprates in the absence of apical oxygens
• Spin polaron theory for the photoemission spectra of layered cobaltates
• Single hole dynamics in the Kondo Necklace and Bilayer Heisenberg models on a square lattice
• Nature of the current-induced insulator-to-metal transition in Ca${}_2$RuO${}_4$ as revealed by transport-ARPES
• On the cuprates' universal waterfall feature: evidence of a momentum-driven crossover
• Equilibrium and non-equilibrium dynamics of a hole in a bilayer antiferromagnet
• Photoemission Spectrum of Ca2RuO4: Spin Polaron Physics in an S=1 Antiferromagnet with Anisotropies
• Wave function and spatial structure of polarons in an antiferromagnetic bilayer
• Orbital-selective metal skin induced by alkali-metal-dosing Mott-insulating Ca${}_2$RuO${}_4$
• Emergent transverse-field Ising model in ${\mathbf{d}}^4$ spin-orbit Mott insulators