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Register a new userCoupled spin-orbital fluctuations in a three orbital model for $4d$ and $5d$ oxides with electron fillings $n=3,4,5$ -- Application to $rm NaOsO_3$, $rm Ca_2RuO_4$, and $rm Sr_2IrO_4$
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A unified approach is presented for investigating coupled spin-orbital fluctuations within a realistic three-orbital model for strongly spin-orbit coupled systems with electron fillings $n=3,4,5$ in the $t_{2g}$ sector of $d_{yz},d_{xz},d_{xy}$ orbitals. A generalized fluctuation propagator is constructed which is consistent with the generalized self-consistent Hartree-Fock approximation where all Coulomb interaction contributions involving orbital diagonal and off-diagonal spin and charge condensates are included. Besides the low-energy magnon, intermediate-energy orbiton and spin-orbiton, and high-energy spin-orbit exciton modes, the generalized spectral function also shows other high-energy excitations such as the Hunds coupling induced gapped magnon modes. We relate the characteristic features of the coupled spin-orbital excitations to the complex magnetic behavior resulting from the interplay between electronic bands, spin-orbit coupling, Coulomb interactions, and structural distortion effects, as realized in the compounds $rm NaOsO_3$, $rm Ca_2RuO_4$, and $rm Sr_2IrO_4$.
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Strongly anisotropic spin-orbit coupling (SOC) renormalization and strongly enhanced orbital magnetic moments are obtained in the fully self consistent approach including the orbital off-diagonal spin and charge condensates. For moderate tetragonal distortion as in $rm Ca_2 RuO_4$, dominantly planar antiferromagnetic (AFM) order with small canting of moments in and about the crystal $c$ axis are obtained. For reduced tetragonal distortion, we find a tunable regime wherein the magnetic order can be tuned (AFM or FM) by the bare SOC strength and octahedral tilting magnitude. In this regime, with decreasing tetragonal distortion, AFM order is maintained by progressively decreasing octahedral tilting, as observed in $rm Ca_{2-x}Sr_x RuO_4$. For purely planar order, the only self consistent solution is FM order along crystal $b$ axis, which is relevant for the bilayer ruthenate compound $rm Ca_3 Ru_2 O_7$.
Including the orbital off-diagonal spin and charge condensates in the self consistent determination of magnetic order within a realistic three-orbital model for the $4d^4$ compound $rm Ca_2 Ru O_4$, reveals a host of novel features including strong and anisotropic spin-orbit coupling (SOC) renormalization, coupling of strong orbital magnetic moments to orbital fields, and a magnetic reorientation transition. Highlighting the rich interplay between orbital geometry and overlap, spin-orbit coupling, Coulomb interactions, tetragonal distortion, and staggered octahedral tilting and rotation, our investigation yields a planar antiferromagnetic (AFM) order for moderate tetragonal distortion, with easy $a-b$ plane and easy $b$ axis anisotropies, along with small canting of the dominantly $yz,xz$ orbital moments. With decreasing tetragonal distortion, we find a magnetic reorientation transition from the dominantly planar AFM order to a dominantly $c$ axis ferromagnetic (FM) order with significant $xy$ orbital moment.
We have measured the change in the resistivity of thin films of ${rm SrRuO_3}$ and ${rm CaRuO_3}$ upon introducing point defects by electron irradiation at low temperatures, and we find significant deviations from Matthiessens rule. For a fixed irradiation dose, the induced change in resistivity {it decreases} with increasing temperature. Moreover, for a fixed temperature, the increase in resistivity with irradiation is found to be {it sublinear}. We suggest that the observed behavior is due to the marked anisotropic scattering of the electrons together with their relatively short mean free path (both characteristic of many metallic oxides including cuprates) which amplify effects related to the Pippard ineffectiveness condition.
With octahedrally coordinated $t_{rm 2g}$ orbitals which are active at filling $n=2$, the $rm Sr_2CrO_4$ compound exhibits rich interplay of spin-orbital physics with tetragonal distortion induced crystal field tuning by external agent such as pressure. Considering both reversed and restored crystal field regimes, collective spin-orbital excitations are investigated in the antiferromagnetic (AFM) state of a realistic three-orbital model using the generalized self consistent and fluctuation approach including spin-orbit coupling (SOC). Important effects of SOC and Coulomb interaction induced orbital mixing terms are highlighted. The behavior of the calculated energy scales of collective excitations with crystal field is in striking similarity to that of the transition temperatures with pressure as obtained from susceptibility and resistivity anomalies in high-pressure studies.
We report the crystal field levels of several newly-discovered rare-earth kagome compounds: $rm{Nd_3Sb_3Mg_2O_{14}}$, $rm{Nd_3Sb_3Zn_2O_{14}}$, and $rm{Pr_3Sb_3Mg_2O_{14}}$. We determine the CEF Hamiltonian by fitting to neutron scattering data using a point-charge Hamiltonian as an intermediate fitting step. The fitted Hamiltonians accurately reproduce bulk susceptibility measurements, and the results indicate easy-axis ground state doublets for $rm{Nd_3Sb_3Mg_2O_{14}}$ and $rm{Nd_3Sb_3Zn_2O_{14}}$, and a singlet ground state for $rm{Pr_3Sb_3Mg_2O_{14}}$. These results provide the groundwork for future investigations of these compounds and a template for CEF analysis of other low-symmetry materials.
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