Spin and charge dynamics of the ferromagnetic and antiferromagnetic two-dimensional half-filled Kondo lattice model - art. no. 155114

We present a detailed numerical study of ground state and finite temperatur e spin and charge dynamics of the two-dimensional Kondo lattice model with hopping t and exchange J. Our numerical results stem from auxiliary field q uantum Monte Carlo simulations formulated in such a way that the sign probl em is absent at half-band filling thus allowing us to reach lattice sizes u p to 12 x 12. At T = 0 and antiferromagnetic couplings J>0 the competition between the Ruderman-Kittel-Kasuya-Yosida interaction and the Kondo effect triggers a quantum phase transition between antiferromagnetically ordered a nd magnetically disordered insulators: J(c)/t = 1.45+/-0.05. At J<0 the sys tem remains an antiferromagnetically ordered insulator and irrespective of the sign of J, the quasiparticle gap scales as \J\. The dynamical spin stru cture factor S(<(q)over right arrow>, omega) evolves smoothly from its stro ng-coupling form with spin gap at (q) over right arrow = (pi, pi) to a spin -wave form. For J>0, the single-particle spectral function A((k) over right arrow, omega) shows a dispersion relation following that of hybridized ban ds as obtained in the noninteracting periodic Anderson model. In the ordere d phase this feature is supplemented by shadows, thus allowing an interpret ation in terms of the coexistence of Kondo screening and magnetic ordering. In contrast, at J<0 the single-particle dispersion relation follows that o f noninteracting electrons in a staggered external magnetic field. At finit e temperatures spin T-S and charge T-C scales are defined by locating the m aximum in the charge and spin uniform susceptibilities. For weak to interme diate couplings, T-S marks the onset of antiferromagnetic fluctuations-as o bserved by a growth of the staggered spin susceptibility-and follows a J(2) law. At strong couplings T-S scales as J. On the other hand T-C scales as J both in the weak- and strong-coupling regime. At and slightly below T-C w e observe (i) the onset of screening of the magnetic impurities, (ii) a ris e in the resistivity as a function of decreasing temperature, (iii) a dip i n the integrated density of states at the Fermi energy, and finally (iv) th e occurrence of hybridized bands in A(<(k)over right arrow>, omega). It is shown that in the weak-coupling limit, the charge gap of order J is formed only at T-S and is hence of magnetic origin. The specific heat shows a two- peak structure. The low-temperature peak follows T-S and is hence of magnet ic origin. Our results are compared to various mean-field theories.