Jahn-Teller effect and Electron correlation in manganites

Jahn-Teller (JT) effect both in the absence and presence of the strong Coulomb correlation is studied theoretically focusing on the reduction $\Delta K$ of the kinetic energy gain which is directly related to the spin wave stiffness. Without the Coulomb interaction, the perturbative analysis gives $\Delta K / (g^2/M\Omega^2) \cong 0.05-0.13$ depending on the electron number [$g$: electron-phonon(el-ph) coupling constant, $M$: mass of the oxygen atom, $\Omega$: frequency of the phonon]. Although there occurs many channels of the JT el-ph interaction in the multiband system, the final results of $\Delta K$ roughly scales with the density of states at the Fermi energy. In the limit of strong electron correlation, the magnitude of the orbital polarization saturate and the relevant degrees of freedom are the direction (phase) of it. An effective action is derived for the phase variable including the effect of the JT interaction. In this limit, JT interaction is {\it{enhanced}} compared with the non-interacting case, and $\Delta K$ is given by the lattice relaxation energy $E_{L}$ for the localized electrons, although the electrons remains itinerant. Discussion on experiments are given based on these theoretical results.Comment: 24 pages, 7 figure

Charge order and phase segregation in overdoped bilayer manganites

There have been recent reports of charge ordering around $x=0.5$ in the bilayer manganites. At $x=0.5$, there appears to be a coexistence region of layered A-type antiferromagnetc and charge order. There are also reports of orbital order in this region without any Jahn-Teller effect. Based on physical grounds, this region is investigated from a model that incorporates the two $e_g$ orbitals at each Mn site and a near-neighbour Coulomb repulsion. It is shown that there indeed is both charge and orbital order close to the half-doped region coincident with a layered magnetic structure. Although the orbital order is known to drive the magnetic order, the layered magnetic structure is also favoured in this system by the lack of coherent transport across the planes and the reduced dimensionality of the lattice. The anisotropic hopping across the $e_g$ orbitals and the underlying layered structure largely determine the orbital arrangements in this region, while the charge order is primarily due to the long range interactions.Comment: 6 pages, 6 figure

A computational scheme to evaluate Hamaker constants of molecules with practical size and anisotropy

We propose a computational scheme to evaluate Hamaker constants, $A$, of molecules with practical sizes and anisotropies. Upon the increasing feasibility of diffusion Monte Carlo (DMC) methods to evaluate binding curves for such molecules to extract the constants, we discussed how to treat the averaging over anisotropy and how to correct the bias due to the non-additivity. We have developed a computational procedure for dealing with the anisotropy and reducing statistical errors and biases in DMC valuations, based on possible validations on predicted $A$. We applied the scheme to cyclohexasilane molecule, Si$_6$H$_{12}$, used in 'printed electronics' fabrications, getting $A \sim 105 \pm 2$ [zJ], being in plausible range supported even by other possible extrapolations. The scheme provided here would open a way to use handy {\it ab initio} evaluations to predict wettabilities as in the form of materials informatics over broader molecules.Comment: The manuscript was revised according to review comment

Spin and orbital ordering in double-layered manganites

We study theoretically the phase diagram of the double-layered perovskite manganites taking into account the orbital degeneracy, the strong Coulombic repulsion, and the coupling with the lattice deformation. Observed spin structural changes as the increased doping are explained in terms of the orbital ordering and the bond-length dependence of the hopping integral along $c$-axis. Temperature dependence of the neutron diffraction peak corresponding to the canting structure is also explained. Comparison with the 3D cubic system is made.Comment: 7 figure

Magnetic Phases of Electron-Doped Manganites

We study the anisotropic magnetic structures exhibited by electron-doped manganites using a model which incorporates the double-exchange between orbital ly degenerate $e_{g}$ electrons and the super-exchange between $t_{2g}$ electrons with realistic values of the Hund's coupling($J_H$), the super-exchange coupling($J_{AF}$), and the bandwidth($W$). We look at the relative stabilities of the G, C and A type antiferromagnetic ph ases. In particular we find that the G-phase is stable for low electron doping as seen in experiments. We find good agreement with the experimentally observed magnetic phase diagrams of electron-doped manganites ($x > 0.5$) such as Nd$_{1-x}$Sr$_{x}$MnO$_{3}$, Pr$_{1-x}$Sr$_{x}$MnO$_{3}$, and Sm$_{1-x}$Ca$_{x}$MnO$_{3}$. We can also explain the experimentally observed orbital structures of the C a nd A phases. We also extend our calculation for electron-doped bilayer manganites of the form R$_{2-2x}$A$_{1+2x}$Mn$_2$O$_7$ and predict that the C-phase will be absent in t hese systems due to their reduced dimensionality.Comment: 7 .ps files included. To appear in Phys. Rev. B (Feb 2001

Ultrafast Photoinduced Formation of Metallic State in a Perovskite-type Manganite with Short Range Charge and Orbital Order

Femtosecond reflection spectroscopy was performed on a perovskite-type manganite, Gd0.55Sr0.45MnO3, with the short-range charge and orbital order (CO/OO). Immediately after the photoirradiation, a large increase of the reflectivity was detected in the mid-infrared region. The optical conductivity spectrum under photoirradiation obtained from the Kramers-Kronig analyses of the reflectivity changes demonstrates a formation of a metallic state. This suggests that ferromagnetic spin arrangements occur within the time resolution (ca. 200 fs) through the double exchange interaction, resulting in an ultrafast CO/OO to FM switching.Comment: 4 figure

Phase diagram of a generalized Hubbard model applied to orbital order in manganites

The magnetic phase diagram of a two-dimensional generalized Hubbard model proposed for manganites is studied within Hartree-Fock approximation. In this model the hopping matrix includes anisotropic diagonal hopping matrix elements as well as off-diagonal elements. The antiferromagnetic (AF), ferromagnetic (F), canted (C) and paramagnetic (P) states are included in the analysis as possible phases. It is found that away from half-filling only the canted and F states may exist and AF and P states which are possible for the usual Hubbard model do not appear. This is because the F order has already developed for on-site repulsion U=0 due to the hopping matrix of the generalized model. When applied for manganites the orbital degree is described by a pseudospin. Thus our ``magnetic'' phase diagram obtained physically describes how orbital order changes with $U$ and with doping for manganites. Part of our results are consistent with other numerical calculations and some experiments.Comment: 5 eps figures; a note added, to appear in Phys. Rev.

Locality Error Free Effective Core Potentials for 3d Transition Metal Elements Developed for the Diffusion Monte Carlo Method

Pseudopotential locality errors have hampered the applications of the diffusion Monte Carlo (DMC) method in materials containing transition metals, in particular oxides. We have developed locality error free effective core potentials, pseudo-Hamiltonians, for transition metals ranging from Cr to Zn. We have modified a procedure published by some of us in [M.C. Bennett et al, JCTC 18 (2022)]. We carefully optimized our pseudo-Hamiltonians and achieved transferability errors comparable to the best semilocal pseudopotentials used with DMC but without incurring in locality errors. Our pseudo-Hamiltonian set (named OPH23) bears the potential to significantly improve the accuracy of many-body-first-principles calculations in fundamental science research of complex materials involving transition metals

Orbital Structure and Magnetic Ordering in Layered Manganites: Universal Correlation and Its Mechanism

Correlation between orbital structure and magnetic ordering in bilayered manganites is examined. A level separation between the $3d_{3z^2-r^2}$ and $3d_{x^2-y^2}$ orbitals in a Mn ion is calculated in the ionic model for a large number of the compounds. It is found that the relative stability of the orbitals dominates the magnetic transition temperatures as well as the magnetic structures. A mechanism of the correlation between orbital and magnetism is investigated based on the theoretical model with the two $e_g$ orbitals under strong electron correlation.Comment: 4 pages, 4 figure

Structural, magnetic and electrical properties of single crystalline La_(1-x)Sr_xMnO_3 for 0.4 < x < 0.85

We report on structural, magnetic and electrical properties of Sr-doped LaMnO_3 single crystals for doping levels 0.4 < x < 0.85. The complex structural and magnetic phase diagram can only be explained assuming significant contributions from the orbital degrees of freedom. Close to x = 0.6 a ferromagnetic metal is followed by an antiferromagnetic metallic phase below 200 K. This antiferromagnetic metallic phase exists in a monoclinic crystallographic structure. Following theoretical predictions this metallic antiferromagnet is expected to reveal an (x^2-y^2)-type orbital order. For higher Sr concentrations an antiferromagnetic insulator is established below room temperature.Comment: 8 pages, 7 figure