Polaronic aspects of the two-dimensional ferromagnetic Kondo model

Abstract

Abstract. The two-dimensional ferromagnetic Kondo model with classical corespins is studied via unbiased Monte Carlo simulations for a hole doping up to x = 12.5%. A canonical algorithm for finite temperatures is developed. We show that with realistic parameters for the manganites and at low temperatures, the double-exchange mechanism does not lead to phase separation on a twodimensional lattice but rather stabilises individual ferromagnetic polarons for this doping range. A detailed analysis of unbiased Monte Carlo results reveals that the polarons can be treated as independent particles for these hole concentrations. It is found that a simple polaron model describes the physics of the ferromagnetic Kondo model amazingly well. The ferromagnetic polaron picture provides an obvious explanation for the pseudogap in the one-particle spectral function A k (ω) observed in the ferromagnetic Kondo model.

Ferromagnetic polarons in the one-dimensional ferromagnetic Kondo model with quantum mechanicalS=3∕2core spins

“…In the latter case, it can be accounted for by the independence of the polarons at low doping, i.e., adding each polaron costs the same energy, which is balanced by the chemical potential µ * = ǫ pol . 2,7 This also corresponds to our observation that the ground state energy per site as a function of filling (not shown) lies practically on an straight line near n = 1 with dE/dn = ǫ pol .…”

“…In previous work by some of the current authors, also treating the core spins classically, similar numerical results were obtained. 2,7 However, a closer analysis of the data revealed that the features which had been interpreted by other authors to indicate phase separation (a discontinuity in the electron density versus the chemical potential, a pseudogap in the one-particle density of states) were, in fact, due to small, independent ferromagnetic polarons. Likewise, ferromagnetic polarons have been found for the almost empty lower Kondo band, for localized S = 1/2 quantum spins.…”

Two‐band ferromagnetic Kondo lattice model on a ladder with quantum S = 3/2 core spins

“…Its shape depends on the core spin superexchange: For J = 2 meV the polaron covers a 2×2 plaquette, while for J = 10 meV just a single core spin is flipped, yielding a T-shaped structure of four ferromagnetically correlated sites, similar to the shape found in a 2D study [7] with classical core spins. A simple tight-binding calculation for one hole delocalising in such a small cluster shows that the kinetic energy gain is −2t for the 2×2 plaquette, while it is only − √ 3t in the latter case.…”

Double and super-exchange model in one-dimensional systems

“…At low conduction electron density, F polarons have been found for localized S ¼ 1 2 quantum spins [8]. F polarons in an AF background were also found in 2D [9]. ''Island'' phases, periodic arrangement of F polarons coupled anti-ferromagnetically, have been clearly identified at commensurate fillings both for quantum spins in one dimension [10] and for classical spins in one [11] and two dimensions [12].…”