Many-particle effects in adsorbed magnetic atoms with easy-axis anisotropy: the case of Fe on CuN/Cu(100) surface

We study the effects of the exchange interaction between an adsorbed magnetic atom with easy-axis magnetic anisotropy and the conduction-band electrons from the substrate. We model the system using an anisotropic Kondo model and we compute the impurity spectral function which is related to the differential conductance (dI/dV) spectra measured using a scanning tunneling microscope. To make contact with the known experimental results for iron atoms on the CuN/Cu(100) surface [Hirjibehedin et al., Science {\bf 317}, 1199 (2007)], we calculated the spectral functions in the presence of an external magnetic field of varying strength applied along all three spatial directions. It is possible to establish an upper bound on the coupling constant J: in the range of the magnetic fields for which the experimental results are currently known (up to 7T), the low-energy features in the calculated spectra agree well with the measured dI/dV spectra if the exchange coupling constant J is at most half as large as that for cobalt atoms on the same surface. We show that for even higher magnetic field (between 8 and 9T) applied along the ``hollow direction'', the impurity energy states cross, giving rise to a Kondo effect which takes the form of a zero-bias resonance. The paper introduces an approach for calculating the expectation values of global spin operators and all components of the impurity magnetic susceptibility tensor in numerical renormalization group (NRG) calculations with no spin symmetry. An appendix contains a density-functional-theory (DFT) study of the Cu and Fe adsorbates on CuN/Cu(100) surface: we compare magnetic moments, as well as orbital energies, occupancies, centers, and spreads by calculating the maximally localized Wannier orbitals of the adsorbates.Comment: 18 pages, 7 figure

Kondo effect in triple quantum dots

Numerical analysis of the simplest odd-numbered system of coupled quantum dots reveals an interplay between magnetic ordering, charge fluctuations and the tendency of itinerant electrons in the leads to screen magnetic moments. The transition from local-moment to molecular-orbital behavior is visible in the evolution of correlation functions as the inter-dot coupling is increased. Resulting novel Kondo phases are presented in a phase diagram which can be sampled by measuring the zero-bias conductance. We discuss the origin of the even-odd effects by comparing with the double quantum dot.Comment: 4 pages, 4 figure

Superconductivity in the Kondo lattice model

We study the Kondo lattice model with additional attractive interaction between the conduction electrons within the dynamical mean-field theory using the numerical renormalization group to solve the effective quantum impurity problem. In addition to normal-state and magnetic phases we also allow for the occurrence of a superconducting phase. In the normal phase we observe a very sensitive dependence of the low-energy scale on the conduction-electron interaction. We discuss the dependence of the superconducting transition on the interplay between attractive interaction and Kondo exchange.Comment: Submitted to ICM 2009 Conference Proceeding

Spin qubits in double quantum dots - entanglement versus the Kondo effect

We investigate the competition between pair entanglement of two spin qubits in double quantum dots attached to leads with various topologies and the separate entanglement of each spin with nearby electrodes. Universal behavior of entanglement is demonstrated in dependence on the mutual interactions between the spin qubits, the coupling to their environment, temperature and magnetic field. As a consequence of quantum phase transition an abrupt switch between fully entangled and unentangled states takes place when the dots are coupled in parallel.Comment: 3 figure

Convergence acceleration and stabilization for dynamical-mean-field-theory calculations

The convergence to the self-consistency in the dynamical-mean-field-theory (DMFT) calculations for models of correlated electron systems can be significantly accelerated by using an appropriate mixing of hybridization functions which are used as the input to the impurity solver. It is shown that the techniques and the past experience with the mixing of input charge densities in the density-functional-theory (DFT) calculations are also effective in DMFT. As an example, the increase of the computational requirements near the Mott metal-insulator transition in the Hubbard model due to critical slowing down can be strongly reduced by using the modified Broyden's method to numerically solve the non-linear self-consistency equation. Speed-up factors as high as 3 were observed in practical calculations even for this relatively well behaved problem. Furthermore, the convergence can be achieved in difficult cases where simple linear mixing is either not effective or even leads to divergence. Unstable and metastable solutions can also be obtained. We also determine the linear response of the system with respect to the variations of the hybridization function, which is related to the propagation of the information between the different energy scales during the iteration.Comment: 9 pages, 8 figure

Enhanced Conductance Through Side-Coupled Double Quantum Dots

Conductance, on-site and inter-site charge fluctuations and spin correlations in the system of two side-coupled quantum dots are calculated using the Wilson's numerical renormalization group (NRG) technique. We also show spectral density calculated using the density-matrix NRG, which for some parameter ranges remedies inconsistencies of the conventional approach. By changing the gate voltage and the inter-dot tunneling rate, the system can be tuned to a non-conducting spin-singlet state, the usual Kondo regime with odd number of electrons occupying the dots, the two-stage Kondo regime with two electrons, or a valence-fluctuating state associated with a Fano resonance. Analytical expressions for the width of the Kondo regime and the Kondo temperature are given. We also study the effect of unequal gate voltages and the stability of the two-stage Kondo effect with respect to such perturbations.Comment: 11 pages, 12 figure

The Kondo effect in the presence of the Rashba spin-orbit interaction

We study the temperature scale of the Kondo screening of a magnetic impurity which hybridizes with a two-dimensional electron gas in the presence of the Rashba spin-orbit interaction. The problem is mapped to an effective single-band impurity model with a hybridization function having an inverse-square-root divergence at the bottom of the band. We study the effect of this divergence on the Kondo screening. The problem is solved numerically without further approximations using the numerical renormalization group technique. We find that the Rashba interaction leads to a small variation of the Kondo temperature (increase or decrease) which depends on the values of the impurity parameters