Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userElectrons doped in cubic perovskite SrMnO3: isotropic metal versus chainlike ordering of Jahn-Teller polarons
223
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Single crystals of electron-doped SrMnO3 with a cubic perovskite structure have been systematically investigated as the most canonical (orbital-degenerate) double-exchange system, whose ground states have been still theoretically controversial. With only 1-2% electron doping by Ce substitution for Sr, a G-type antiferromagnetic metal with a tiny spin canting in a cubic lattice shows up as the ground state, where the Jahn-Teller polarons with heavy mass are likely to form. Further electron doping above 4%, however, replaces this isotropic metal with an insulator with tetragonal lattice distortion, accompanied by a quasi-one-dimensional 3z^2-r^2 orbital ordering with the C-type antiferromagnetism. The self-organization of such dilute polarons may reflect the critical role of the cooperative Jahn-Teller effect that is most effective in the originally cubic system.
rate research
Read More
We consider the superexchange in `frustrated Jahn-Teller systems, such as the transition metal oxides NaNiO_2, LiNiO_2, and ZnMn_2O_4, in which transition metal ions with doubly degenerate orbitals form a triangular or pyrochlore lattice and are connected by the 90-degree metal-oxygen-metal bonds. We show that this interaction is much different from a more familiar exchange in systems with the 180-degree bonds, e.g. perovskites. In contrast to the strong interplay between the orbital and spin degrees of freedom in perovskites, in the 90-degree exchange systems spins and orbitals are decoupled: the spin exchange is much weaker than the orbital one and it is ferromagnetic for all orbital states. Due to frustration, the mean-field orbital ground state is strongly degenerate. Quantum orbital fluctuations select particular ferro-orbital states, such as the one observed in NaNiO_2. We also discuss why LiNiO_2 may still behave as an orbital liquid.
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 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.
We study an effective one-dimensional (1D) orbital t-J model derived for strongly correlated e_g electrons in doped manganites. The ferromagnetic spin order at half filling is supported by orbital superexchange prop. to J which stabilizes orbital order with alternating x^2-y^2 and 3z^2-r^2 orbitals. In a doped system it competes with the kinetic energy prop. to t. When a single hole is doped to a half-filled chain, its motion is hindered and a localized orbital polaron is formed. An increasing doping generates either separated polarons or phase separation into hole-rich and hole-poor regions, and eventually polarizes the orbitals and gives a it metallic phase with occupied 3z^2-r^2 orbitals. This crossover, investigated by exact diagonalization at zero temperature, is demonstrated both by the behavior of correlation functions and by spectral properties, showing that the orbital chain with Ising superexchange is more classical and thus radically different from the 1D spin t-J model. At finite temperature we derive and investigate an effective 1D orbital model using a combination of exact diagonalization with classical Monte-Carlo for spin correlations. A competition between the antiferromagnetic and ferromagnetic spin order was established at half filling, and localized polarons were found for antiferromagnetic interactions at low hole doping. Finally, we clarify that the Jahn-Teller alternating potential stabilizes the orbital order with staggered orbitals, inducing the ferromagnetic spin order and enhancing the localized features in the excitation spectra. Implications of these findings for colossal magnetoresistance manganites are discussed.
The quadratic Jahn-Teller effect of C$_{60}^{n-}$ ($n=$ 1-5) is investigated from the first principles. Employing the density functional theory calculations with hybrid functional, the quadratic vibronic coupling constants of C$_{60}^-$ were derived. The warping of the adiabatic potential energy surface of C$_{60}^-$ by the quadratic vibronic coupling is estimated about 2 meV, which is much smaller than the Jahn-Teller stabilization energy ($approx$ 50 meV). Because of the selection rule and the vibronic reduction, the quadratic coupling slightly modifies the vibronic states of C$_{60}$ anions. Particularly, in the case of C$_{60}^{3-}$, parity and symmetry selection rule significantly reduces the effect of quadratic coupling on vibronic states. The present results confirm that the low-energy vibronic dynamics of C$_{60}^{n-}$ is of pseudorotational type.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
