Bose-Fermi Kondo model with Ising anisotropy: cluster-Monte Carlo approach

The Bose-Fermi Kondo model captures the physics of the destruction of Kondo screening, which is of extensive current interest to the understanding of quantum critical heavy fermion metals. There are presently limited theoretical methods to study the finite temperature properties of the Bose-Fermi Kondo model. Here we provide some of the consistency checks on the cluster-Monte Carlo method, which we have recently applied to the Ising-anisotropic Bose-Fermi Kondo model. We show that the method correctly captures the scaling properties of the Kondo phase, as well as those on approach to the Kondo-destroying quantum critical point. We establish that comparable results are obtained when the Kondo couplings are placed at or away from a Toulouse point.Comment: 2 pages, 2 figures, to appear in the proceedings of SCES 07 (the international conference on strongly correlated electron systems 2007

Low-Energy Spin Dynamics of CuO Chains in YBa(2)Cu(3)O(6+x)

We study the spin fluctuation dynamics of Cu-O chains in the oxygen deficient planes of YBa(2)Cu(3)O(6+x). The chains are described by a model including antiferromagnetic interactions between the spins and Kondo-like scattering of the oxygen holes by the copper spins. There are incommensurate spin fluctuations along the direction of the chains. The dynamic structure factor of this system is qualitatively different from that of a quasi one-dimensional localized antiferromagnet due to the presence of itinerant holes. We compute the dynamic structure factor that could be measured in neutron scattering experiments.Comment: 2 pages, 2 eps figures, LT22 proceedings, phbauth style file include

A large-N analysis of the local quantum critical point and the spin-liquid phase

We study analytically the Kondo lattice model with an additional nearest-neighbor antiferromagnetic interaction in the framework of large-N theory. We find that there is a local quantum critical point between two phases, a normal Fermi-liquid and a spin-liquid in which the spins are decoupled from the conduction electrons. The local spin susceptibility displays a power-law divergence throughout the spin liquid phase. We check the reliability of the large-N results by solving by quantum Monte Carlo simulation the N=2 spin-liquid problem with no conduction electrons and find qualitative agreement. We show that the spin-liquid phase is unstable at low temperatures, suggestive of a first-order transition to an ordered phase.Comment: 4 pages and 1 figur

Magnetoconductance through a vibrating molecule in the Kondo regime

The effect of a magnetic field on the equilibrium spectral and transport properties of a single-molecule junction is studied using the numerical renormalization group method. The molecule is described by the Anderson-Holstein model in which a single vibrational mode is coupled to the electron density. The effect of an applied magnetic field on the conductance in the Kondo regime is qualitatively different in the weak and strong electron-phonon coupling regimes. In the former case, the Kondo resonance is split and the conductance is strongly suppressed by a magnetic field $g mu_B B \gtrsim k_BT_K$, with $T_K$ the Kondo temperature. In the strong electron-phonon coupling regime a charge analog of the Kondo effect develops. In this case the Kondo resonance is not split by the field and the conductance in the Kondo regime is enhanced in a broad range of values of $B$.Comment: 6 pages, 4 figure

Locally critical point in an anisotropic Kondo lattice

We report the first numerical identification of a locally quantum critical point, at which the criticality of the local Kondo physics is embedded in that associated with a magnetic ordering. We are able to numerically access the quantum critical behavior by focusing on a Kondo-lattice model with Ising anisotropy. We also establish that the critical exponent for the q-dependent dynamical spin susceptibility is fractional and compares well with the experimental value for heavy fermions.Comment: 4 pages, 3 figures; published versio

Continuous quantum phase transition in a Kondo lattice model

We study the magnetic quantum phase transition in an anisotropic Kondo lattice model. The dynamical competition between the RKKY and Kondo interactions is treated using an extended dynamic mean field theory (EDMFT) appropriate for both the antiferromagnetic and paramagnetic phases. A quantum Monte Carlo approach is used, which is able to reach very low temperatures, of the order of 1% of the bare Kondo scale. We find that the finite-temperature magnetic transition, which occurs for sufficiently large RKKY interactions, is first order. The extrapolated zero-temperature magnetic transition, on the other hand, is continuous and locally critical.Comment: 4 pages, 4 figures; updated, to appear in PR

Disorder effects in the quantum Heisenberg model: An Extended Dynamical mean-field theory analysis

We investigate a quantum Heisenberg model with both antiferromagnetic and disordered nearest-neighbor couplings. We use an extended dynamical mean-field approach, which reduces the lattice problem to a self-consistent local impurity problem that we solve by using a quantum Monte Carlo algorithm. We consider both two- and three-dimensional antiferromagnetic spin fluctuations and systematically analyze the effect of disorder. We find that in three dimensions for any small amount of disorder a spin-glass phase is realized. In two dimensions, while clean systems display the properties of a highly correlated spin-liquid (where the local spin susceptibility has a non-integer power-low frequency and/or temperature dependence), in the present case this behavior is more elusive unless disorder is very small. This is because the spin-glass transition temperature leaves only an intermediate temperature regime where the system can display the spin-liquid behavior, which turns out to be more apparent in the static than in the dynamical susceptibility.Comment: 15 pages, 7 figure