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Register a new userMagnetic and transport properties of the one-dimensional ferromagnetic Kondo lattice model with an impurity
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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.
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We have studied the energy spectrum of a one-dimensional Kondo lattice, where the localized magnetic moments have SU(N) symmetry and two channels of conduction electrons are present. At half filling, the system is shown to exist in two phases: one dominated by RKKY-exchange interaction effects, and the other by Kondo screening. A quantum phase transition point separates these two regimes at temperature $T = 0$. The Kondo-dominated phase is shown to possess soft modes, with spectral gaps much smaller than the Kondo temperature.
We investigate the many-body effects of a magnetic adatom in ferromagnetic graphene by using the numerical renormalization group method. The nontrivial band dispersion of ferromagnetic graphene gives rise to interesting Kondo physics different from that in conventional ferromagnetic materials. For a half-filled impurity in undoped graphene, the presence of ferromagnetism can bring forth Kondo correlations, yielding two kink structures in the local spectral function near the Fermi energy. When the spin splitting of local occupations is compensated by an external magnetic field, the two Kondo kinks merge into a full Kondo resonance characterizing the fully screened ground state. Strikingly, we find the resulting Kondo temperature monotonically increases with the spin polarization of Dirac electrons, which violates the common sense that ferromagnetic bands are usually detrimental to Kondo correlations. Doped ferromagnetic graphene can behave as half metals, where its density of states at the Fermi energy linearly vanishes for one spin direction but keeps finite for the opposite direction. In this regime, we demonstrate an abnormal Kondo resonance that occurs in the first spin direction, while completely absent in the other one.
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 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.
Magnetization, heat capacity, electrical resistivity, thermoelectric power, and Hall effect have been investigated on single-crystalline Ce_2PdSi_3. This compound is shown to order antiferromagnetically below Neel temperature (T_N) ~3 K. The Sommerfeld coefficient far below T_N is found to be about 110 mJ/K^2 mol Ce, which indicates the heavy-fermion character of this compound. The transport and magnetic properties exhibit large anisotropy with an interplay between crystalline-electric-field (CEF) and Kondo effects. The sign of thermoelectric power is opposite for different directions at high temperatures and the ordinary Hall coefficient is anisotropic with opposite sign for different geometries, indicating the anisotropic Fermi surface. The CEF analysis from the temperature dependence of magnetic susceptibility suggests that the ground state is |+/-1/2>. The first and the second excited CEF doublet levels are found to be located at about 30 and 130 K, respectively. The Kondo temperature is estimated to be the same order as T_N, indicating the presence of a delicate competition between the Kondo effect and magnetic order.
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