Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userMagnetic and orbital correlations in a two-site molecule
120
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We analyze the role of orbital degeneracy in possible magnetic and orbital instabilities by solving exactly a two-site molecule with two orbitals of either $e_g$ or $t_{2g}$ symmetry at quarter-filling. As a generic feature of both models one finds that the spin and orbital correlations have opposite signs in the low temperature regime when the orbitals are degenerate, in agreement with the Goodenough-Kanamori rules. While Hunds exchange coupling $J_H$ induces ferromagnetic spin correlations in both models, it is more efficient for $t_{2g}$ orbitals where the orbital quantum number is conserved along the hopping processes. We show that the ground state and finite temperature properties may change even qualitatively with increasing Coulomb interaction when the crystal field splitting of the two orbitals is finite, and the Goodenough-Kanamori rules may not be followed.
rate research
Read More
We investigate the electronic structure of the highly anisotropic $beta$ phase of metallic plutonium, within the combination of density functional theory (DFT) and dynamical mean field theory (DMFT). Its crystal structure gives rise to site and orbital selective electronic correlations, with coherent Pu-5$f_{5/2}$ states and very incoherent Pu-5$f_{7/2}$ states. The Hunds coupling is essential for determining the level of correlations of electrons in Pu-5$f$ states, and for the quasiparticle multiplets features in the Pu-5$f$ spectral function.
$Fe_3O_4$ is a mixed-valence strongly correlated transition metal oxide which displays the intriguing metal to insulator Verwey transition. Here we investigate the electronic and magnetic structure of $Fe_3O_4$ by a unique combination of high-resolution Fe 2p3d resonant inelastic scattering magnetic circular (RIXS-MCD) and magnetic linear (RIXS-MLD) dichroism. We show that by coupling the site selectivity of RIXS with the magnetic selectivity imposed by the incident polarization handedness, we can unambiguously identify spin-flip excitations and quantify the exchange interaction of the different sublattices. Furthermore, our RIXS-MLD measurements show spin-orbital excitations that exhibit strong polarization and magnetic field dependence. Guided by theoretical simulations, we reveal that the angular dependence arises from a strong interplay between trigonal crystal-field, magnetic exchange and spin-orbit interaction at the nominal $Fe^{2+}$ sites. Our results highlight the capabilities of RIXS magnetic dichroism studies to investigate the ground state of complex systems where in-equivalent sites and bonds are simultaneously present.
The inversion and volume effects on magnetism in a spinel-type magnetically frustrated compound, CoAl2O4, and its gallium-substituted system, CoAl2-xGaxO4, were investigated. Magnetically frustrated Co2+ with spin S = 3/2 on the tetrahedral site formed a diamond lattice in CoAl2O4 located in the vicinity of the magnetic phase boundary between Neel and spin-spiral states. In the Ga-substituted system, the number of Co ions, the so-called inversion h dominating the octahedral site, increased with increasing x. From comprehensive crystallographic, magnetic, and thermal measurements, increments of both volume and inversion strongly reduced the Neel point, while the latter also induced a spin-glass state above the critical value of hc = 0.09. In the spin glass state, h > hc, the orbital degree of freedom of Co2+ ions in the octahedral site appeared in the magnetic entropy, which couples strongly with that of spin, even above the magnetic transitions. Above h ~ hc, the field-induced quenched magnetic moment appeared above the transitions. Therefore, a short range ordered state emerged among the paramagnetic, antiferromganetic, and spin-glass states in the magnetic phase diagram.
We report an angular resolved photoemission study of Na0.73CoO2 where it is found that the renormalization of the quasiparticle (QP) dispersion changes dramatically upon a rotation from GM to GK. The comparison of the experimental data to the calculated band structure reveals that the QP-renormalization is most pronounced along the GK-direction, while it is significantly weaker along the GM-direction. We discuss the observed anisotropy in terms of multiorbital effects and point out the relevance of magnetic correlations for the band structure of Na0.73CoO2.
We review our recent x-ray scattering studies of charge and orbital order in doped manganites, with specific emphasis on the role of orbital correlations in Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable from the undoped structure with long range orbital order at low temperatures. For dopings 0.3<x<0.5, we find scattering consistent with a charge and orbitally ordered CE-type structure. While in each case the charge order peaks are resolution limited, the orbital order exhibits only short range correlations. We report the doping dependence of the correlation length and discuss the connection between the orbital correlations and the finite magnetic correlation length observed on the Mn^3+ sublattice with neutron scattering techniques. The physical origin of these domains, which appear to be isotropic, remains unclear. We find that weak orbital correlations persist well above the phase transitions, with a correlation length of 1-2 lattice constants at high temperatures. Significantly, we observe similar correlations at high temperatures in La_0.7Ca_0.3MnO_3, which does not have an orbitally ordered ground state, and we conclude that such correlations are robust to variations in the relative strength of the electron-phonon coupling.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
