Statistics on Graphs, Exponential Formula and Combinatorial Physics

The concern of this paper is a famous combinatorial formula known under the name "exponential formula". It occurs quite naturally in many contexts (physics, mathematics, computer science). Roughly speaking, it expresses that the exponential generating function of a whole structure is equal to the exponential of those of connected substructures. Keeping this descriptive statement as a guideline, we develop a general framework to handle many different situations in which the exponential formula can be applied

t-J model of coupled Cu2_2O5_5 ladders in Sr14−x_{14-x}Cax_xCu24_{24}O41_{41}

Starting from the proper charge transfer model for Cu$_2$O$_5$ coupled ladders in Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ we derive the low energy Hamiltonian for this system. It occurs that the widely used ladder t-J model is not sufficient and has to be supplemented by the Coulomb repulsion term between holes in the neighboring ladders. Furthermore, we show how a simple mean-field solution of the derived t-J model may explain the onset of the charge density wave with the odd period in Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$.Comment: 8 pages, 4 figures, 2 table

Exchange parameters from approximate self-interaction correction scheme

The approximate atomic self-interaction corrections (ASIC) method to density functional theory is put to the test by calculating the exchange interaction for a number of prototypical materials, critical to local exchange and correlation functionals. ASIC total energy calculations are mapped onto an Heisenberg pair-wise interaction and the exchange constants J are compared to those obtained with other methods. In general the ASIC scheme drastically improves the bandstructure, which for almost all the cases investigated resemble closely available photo-emission data. In contrast the results for the exchange parameters are less satisfactory. Although ASIC performs reasonably well for systems where the magnetism originates from half-filled bands, it suffers from similar problems than those of LDA for other situations. In particular the exchange constants are still overestimated. This reflects a subtle interplay between exchange and correlation energy, not captured by the ASIC.Comment: 10 page

Reversible strain effect on the magnetization of LaCoO3 films

The magnetization of ferromagnetic LaCoO3 films grown epitaxially on piezoelectric substrates has been found to systematically decrease with the reduction of tensile strain. The magnetization change induced by the reversible strain variation reveals an increase of the Co magnetic moment with tensile strain. The biaxial strain dependence of the Curie temperature is estimated to be below 4K/% in the as-grown tensile strain state of our films. This is in agreement with results from statically strained films on various substrates

Anharmonic effect on lattice distortion, orbital ordering and magnetic properties in Cs2AgF4

We develop the cluster self-consistent field method incorporating both electronic and lattice degrees of freedom to study the origin of ferromagnetism in Cs$_{2}$AgF$_{4}$. After self-consistently determining the harmonic and anharmonic Jahn-Teller distortions, we show that the anharmonic distortion stabilizes the staggered x$^{2}$-z$^{2}$/y$^{2}$-z$^{2}$ orbital and ferromagnetic ground state, rather than the antiferromagnetic one. The amplitudes of lattice distortions, Q$_{2}$ and Q$_{3}$, the magnetic coupling strengthes, J$_{x,y}$, and the magnetic moment, are in good agreement with the experimental observation.Comment: 13 pages, 5 figure

Structural, orbital, and magnetic order in vanadium spinels

Vanadium spinels (ZnV_2O_4, MgV_2O_4, and CdV_2O_4) exhibit a sequence of structural and magnetic phase transitions, reflecting the interplay of lattice, orbital, and spin degrees of freedom. We offer a theoretical model taking into account the relativistic spin-orbit interaction, collective Jahn-Teller effect, and spin frustration. Below the structural transition, vanadium ions exhibit ferroorbital order and the magnet is best viewed as two sets of antiferromagnetic chains with a single-ion Ising anisotropy. Magnetic order, parametrized by two Ising variables, appears at a tetracritical point.Comment: v3: streamlined introductio

Phase formation, phonon behavior, and magnetic properties of novel ferromagnetic La3BAlMnO9 (B = Co or Ni) triple perovskites

In the quest for novel magnetoelectric materials, we have grown, stabilized and explored the properties of La3BAlMnO9 (B = Co or Mn) thin films. In this paper, we report the influence of the growth parameters that promote B/Al/Mn ordering in the pseudo-cubic unit cell and their likely influence on the magnetic and multiferroic properties. The temperature dependence of the magnetization shows that La3CoAlMnO9 is ferromagnetic up to 190 K while La3NiAlMnO9 shows a TC of 130 K. The behavior of these films are compared and contrasted with related La2BMnO6 double perovskites. It is observed that the insertion of AlO6 octahedra between CoO6 and MnO6 suppresses significantly the strength of the superexchange interaction, spin-phonon and spin-polar coupling.Comment: 13 pages, 3 fig

Research for preparation of cation-conducting solids by high-pressure synthesis and other methods

It was shown that two body-centered-cubic skeleton structures, the Im3 KSbO3 phase and the defect-pyrochlore phase A(+)B2X6, do exhibit fast Na(+)-ion transport. The placement of anions at the tunnel intersection sites does not impede Na(+)-ion transport in (NaSb)3)(1/6 NaF), and may not in (Na(1+2x)Ta2 5F)(Ox). The activation energies are higher than those found in beta-alumina. There are two possible explanations for the higher activation energy: breathing of the bottleneck (site face or edge) through which the A(+) ions must pass on jumping from one site to another may be easier in a layer structure and/or A(+)-O bonding may be stronger in the cubic structures because the O(2-) ion bonds with two (instead of three) cations of the skeleton. If the former explanation is dominant, a lower activation energy may be achieved by optimizing the lattice parameter. If the latter is dominant, a new structural principle may have to be explored

Long-range Ni/Mn structural order in epitaxial double perovskite La2NiMnO6 thin films

We report and compare the structural, magnetic, and optical properties of ordered La2NiMnO6 thin films and its disordered LaNi0.5Mn0.5O3 counterpart. An x-ray diffraction study reveals that the B-site Ni/Mn ordering induces additional XRD reflections as the crystal symmetry is transformed from a pseudocubic perovskite unit cell in the disordered phase to a monoclinic form with larger lattice parameters for the ordered phase. Polarized Raman spectroscopy studies reveal that the ordered samples are characterized by additional phonon excitations that are absent in the disordered phase. The appearance of these additional phonon excitations is interpreted as the clearest signature of Brillouin zone folding as a result of the long-range Ni/Mn ordering in La2NiMnO6. Both ordered and disordered materials display a single ferromagnetic-to-paramagnetic transition. The ordered films display also a saturation magnetization close to 4.8 mB/f.u. and a transition temperature (FM-TC) around 270 K, while the disordered ones have only a 3.7 mB/f.u. saturation magnetization and a FM-TC around 138 K. The differences in their magnetic behaviours are understood based on the distinct local electronic configurations of their Ni/Mn cations.Comment: 15 pages, 5 fig