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Register a new userCooperative effects of Jahn-Teller distortion, magnetism and Hunds coupling in the insulating phase of BaCrO$_3$
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We employ a combination of density functional theory and dynamical mean-field theory to investigate the electronic structure of the recently synthesized insulator BaCrO$_3$. Our calculations show that Hunds coupling is responsible for strong correlation effects, which are however not sufficient to turn the system insulating. A finite Jahn-Teller distortion lifting the orbital degeneracy is necessary to stabilize an insulating state with orbital ordering and consequent magnetic ordering.
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The charge order of CE phase in half-doped manganites is studied, based on an argument that the charge-ordering is caused by the Jahn-Teller distortions of MnO6 octahedra rather than Coulomb repulsion between electrons. The uantitative calculation on the ferromagnetic zigzag chain as the basic structure unit of CE phase within the framework of two-orbital double exchange model including Jahn-Teller effect is performed, and it is shown that the charge-disproportionation of Mn cations in the charge-ordered CE phase is less than 13%. In addition, we predict the negative charge-disproportionation once the Jahn-Teller effect is weak enough.
The origin of the cooperative Jahn-Teller distortion and orbital-order in LaMnO3 is central to the physics of the manganites. The question is complicated by the simultaneous presence of tetragonal and GdFeO3-type distortions and the strong Hunds rule coupling between e_g and t_2g electrons. To clarify the situation we calculate the transition temperature for the Kugel-Khomskii superexchange mechanism by using the local density approximation+dynamical mean-field method, and disentangle the effects of super-exchange from those of lattice distortions. We find that super-exchange alone would yield T_KK=650 K. The tetragonal and GdFeO3-type distortions, however, reduce T_KK to 550 K. Thus electron-phonon coupling is essential to explain the persistence of local Jahn-Teller distortions to at least 1150 K and to reproduce the occupied orbital deduced from neutron scattering.
The first known magnetic mineral, magnetite (Fe$_3$O$_4$), has unusual properties which have fascinated mankind for centuries; it undergoes the Verwey transition at $T_{rm V}$ $sim$120 K with an abrupt change in structure and electrical conductivity. The mechanism of the Verwey transition however remains contentious. Here we use resonant inelastic X-ray scattering (RIXS) over a wide temperature range across the Verwey transition to identify and separate out the magnetic excitations derived from nominal Fe$^{2+}$ and Fe$^{3+}$ states. Comparison of the RIXS results with crystal-field multiplet calculations shows that the spin-orbital $dd$ excitons of the Fe$^{2+}$ sites arise from a tetragonal Jahn-Teller active polaronic distortion of the Fe$^{2+}$O$_6$ octahedra. These low-energy excitations, which get weakened for temperatures above 350 K but persist at least up to 550 K, are distinct from optical excitations and best explained as magnetic polarons.
Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling in double-exchange models are examined for Mn perovskite oxides. We study the undoped Mott insulator as well as metal-insulator transitions by hole doping, and especially strong incoherence of ferromagnetic metal. We derive models where all the spins are fully polarized in two-dimensional planes as in the experimental indications, and investigate their ground-state properties by quantum Monte Carlo method. At half filling where the number of $e_{g}$ electron is one per site on average, the Coulomb interaction opens a Mott gap and induces a staggered orbital ordering. The opening of the Mott gap is, however, substantially slower than the mean-field results if the Jahn-Teller coupling is absent. The synergy between the strong correlation and the Jahn-Teller coupling largely enhances the Mott gap amplitude and reproduces realistic amplitudes and stabilization energy of the Jahn-Teller distortion. Upon doping, the orbital ordering stabilized by the Coulomb interaction is destroyed immediately. Toward the metal-insulator transition, the short-ranged orbital correlation is critically enhanced in metals, which should be related to strong incoherence of charge dynamics observed in experiments. Our model, moreover, exhibits a uniform ordering of $d_{x^{2}-y^{2}}$ orbital in a wide region of doping in agreement with experimental indications.
The emergence of a ferromagnetic component in $LaMnO_{3}$ with low Cr-for-Mn substitution has been studied by x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Cr K edges. The local magnetic moment strength for the Mn and Cr are proportional to each other and follows the macroscopic magnetization. The net ferromagnetic components of $Cr^{3+}$ and $Mn^{3+}$ are found antiferromagnetically coupled. Unlike hole doping by La site substitution, the inclusion of $Cr^{3+}$ ions up to x = 0.15 does not decrease the Jahn-Teller (JT) distortion and consequently does not significantly affect the orbital ordering. This demonstrates that the emergence of the ferromagnetism is not related to JT weakening and likely arises from a complex orbital mixing.
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