E. Vallejo

Spin and charge ordering in three-leg ladders in oxyborates.

We study the spin ordering within the three-leg ladders present in the oxyborate Fe3O2BO3 consisting of localized classical spins interacting with conduction electrons (one electron per rung). We also consider the competition with antiferromagnetic superexchange interactions to determine the magnetic phase diagram. Besides a ferromagnetic phase we find (i) a phase with ferromagnetic rungs ordered antiferromagnetically and (ii) a zigzag canted spin ordering along the legs. We also determine the induced charge ordering within the different phases and the interplay with lattice instability. Our model is discussed in connection with the lattice dimerization transition observed in this system, emphasizing the role of the magnetic structure.

Magnetic polaron structures in the one-dimensional double and super-exchange model

Abstract An analytical and numerical study of the one-dimensional double and super-exchange model is presented. A phase separation between ferromagnetic and anti-ferromagnetic phases occurs at low super-exchange interaction energy. When the super-exchange interaction energy gets larger, the conduction electrons are self-trapped within separate small magnetic polarons. These magnetic polarons contain a single electron inside two or three sites depending on the conduction electron density and form a Wigner crystallization. A new phase separation is found between these small polarons and the anti-ferromagnetic phase. Our results could explain the spin-glass-like behavior observed in the nickelate one-dimensional compound Y 2−xCaxBaNiO5.

STRUCTURAL DISTORTION IN LUDWIGITES

Structural distortion in a three-leg-ladder is studied in connection with Ludwigites, in particular the Fe and Co homometallic ones. Static impurities in t2g-orbitals as infinite repulsion potentials randomly located in the three-leg-ladder and a Su–Schrieffer–Heeger like tight-binding Hamiltonian are proposed and discussed. It is found that such potentials block itinerant electrons and diminish a structural staggered order parameter, related with structural distortion, as 3-M being M the number of impurities. This diminution is in detriment of Peierls like distortion that occurs in these ladders as in the case of Fe-Ludwigite. On the other hand, this diminution could explain the lack of structural distortion as in the case of Co-Ludwigite.

Formation of spin-polarons in the ferromagnetic Kondo lattice model away from half-filling

Even though realistic one-dimensional experiments in the field of half-metallic semiconductors are not at hand yet, we are interested in the underlying fundamental physics. In this regard we study a one-dimensional ferromagnetic Kondo lattice model, a model in which a conduction band is coupled ferromagnetically to a background of localized d moments with coupling constant J(H), and investigate the T = 0 phase diagram as a function of the antiferromagnetic interaction J between the localized moments and the band-filling n, since it has been observed that doping of the compounds has led to formation of magnetic domains. We explore the spin-polaron formation by looking at the nearest-neighbour correlation functions in the spin and charge regimes for which we use the density matrix renormalization group method, which is a highly efficient method to investigate quasi-one-dimensional strongly correlated systems.

Monte Carlo study of the double and super-exchange model with lattice distortion

In this work a magneto-elastic phase transition was obtained in a linear chain due to the interplay between magnetism and lattice distortion in a double and super-exchange model. It is considered a linear chain consisting of localized classical spins interacting with itinerant electrons. Due to the double exchange interaction, localized spins tend to align ferromagnetically. This ferromagnetic tendency is expected to be frustrated by anti-ferromagnetic super-exchange interactions between neighbor localized spins. Additionally, lattice parameter is allowed to have small changes, which contributes harmonically to the energy of the system. Phase diagram is obtained as a function of the electron density and the super-exchange interaction using a Monte Carlo minimization. At low super-exchange interaction energy phase transition between electron-full ferromagnetic distorted and electron-empty anti-ferromagnetic undistorted phases occurs. In this case all electrons and lattice distortions were found within the ferromagnetic domain. For high super-exchange interaction energy, phase transition between two site distorted periodic arrangement of independent magnetic polarons ordered anti-ferromagnetically and the electron-empty anti-ferromagnetic undistorted phase was found. For this high interaction energy, Wigner crystallization, lattice distortion and charge distribution inside two-site polarons were obtained.

SPIN-POLARONS IN AN EXCHANGE MODEL

Spin-polarons are obtained using an Ising-like exchange model consisting of double and super-exchange interactions in low dimensional systems. At zero temperature, a new phase separation between small magnetic polarons, one conduction electron self-trapped in a magnetic domain of two or three sites, and the anti-ferromagnetic phase was previously reported. On the other hand the important effect of temperature was missed. Temperature diminishes Boltzmann probability allowing excited states in the system. Static magnetic susceptibility and short-range spin-spin correlations at zero magnetic field were calculated to explore the spin-polaron formation. At high temperature Curie-Weiss behavior is obtained and compared with the Curie-like behavior observed in a nickelate one-dimensional compound.