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Register a new userFerromagnetic Polarons, Phase Separation, Stripes and Polaron Lattice: The two- and three-dimensional ferromagnetic Kondo Model
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We investigate the two- and three-dimensional ferromagnetic Kondo lattice model by unbiased Monte Carlo simulations. A phase diagram for the two-dimensional model is presented, in which the stability of magnetic order and ferromagnetic polarons is examined with respect to the antiferromagnetic superexchange J and temperature. The Monte Carlo simulations reveal that J > 0.02 strengthens individual polarons while small J < 0.02 favors larger clusters and phase separation except for small doping. Lowering the temperature stabilizes ferromagnetic polarons for realistic J > 0.01, while phase separation is only favored for very small J < 0.01. Our Monte Carlo simulations show that low temperatures can lead to diagonal or vertical stripes depending on J. Simulations for three-dimensional systems yield ferromagnetic polarons, which form a `polaron lattice at higher doping levels 0.2 < x < 0.23, when independent polarons do no longer fit into the system. No tendency to phase separation is observed in three dimensions.
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The magnetic ground state phase diagram of the ferromagnetic Kondo-lattice model is constructed by calculating internal energies of all possible bipartite magnetic configurations of the simple cubic lattice explicitly. This is done in one dimension (1D), 2D and 3D for a local moment of S = 3/2. By assuming saturation in the local moment system we are able to treat all appearing higher local correlation functions within an equation of motion approach exactly. A simple explanation for the obtained phase diagram in terms of bandwidth reduction is given. Regions of phase separation are determined from the internal energy curves by an explicit Maxwell construction.
We study numerically the one-dimensional ferromagnetic Kondo lattice. This model is widely used to describe nickel and manganese perovskites. Due to the competition between double and super-exchange, we find a region where the formation of magnetic polarons induces a charge-ordered state. This ordering is present even in the absence of any inter-site Coulomb repulsion. There is an insulating gap associated to the charge structure formation. We also study the insulator-metal transition induced by a magnetic field which removes simultaneously both charge and spin ordering.
We have studied the ferromagnetic Kondo lattice model (FKLM) with an Anderson impurity on finite chains with numerical techniques. We are particularly interested in the metallic ferromagnetic phase of the FKLM. This model could describe either a quantum dot coupled to one-dimensional ferromagnetic leads made with manganites or a substitutional transition metal impurity in a MnO chain. We determined the region in parameter space where the impurity is empty, half-filled or doubly-occupied and hence where it is magnetic or nonmagnetic. The most important result is that we found, for a wide range of impurity parameters and electron densities where the impurity is magnetic, a singlet phase located between two saturated ferromagnetic phases which correspond approximately to the empty and double-occupied impurity states. Transport properties behave in general as expected as a function of the impurity occupancy and they provide a test for a recently developed numerical approach to compute the conductance. The results obtained could be in principle reproduced experimentally in already existent related nanoscopic devices or in impurity doped MnO nanotubes.
We present an new approach for the ferromagnetic, three-dimensional, translational-symmetric Kondo lattice model which allows us to derive both magnon energies and linewidths (lifetimes) and to study the properties of the ferromagnetic phase at finite temperatures. Both anomalous softening and anomalous damping are obtained and discussed. Our method consists of mapping the Kondo lattice model onto an effective Heisenberg model by means of the modified RKKY interaction and the interpolating self-energy approach. The Heisenberg model is approximatively solved by applying the Dyson-Maleev transformation and using the spectral density approach with a broadened magnon spectral density.
Using the Lanczos method in linear chains we study the double exchange model in the low concentration limit, including an antiferromagnetic super-exchange K. In the strong coupling limit we find that the ground state contains ferromagnetic polarons whose length is very sensitive to the value of K/t. We investigate the dispersion relation, the trapping by impurities, and the interaction between these polarons. As the overlap between polarons increases, by decreasing K/t, the effective interaction between them changes from antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit suggests an attractive interaction in the strong coupling regime (J_h > t) and no binding in the weak limit (J_h simeq t).
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