Kondo screening cloud in the single-impurity Anderson model: A density matrix renormalization group study (original) (raw)
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Spin Correlations and Finite-Size Effects in the One-Dimensional Kondo Box
Physical Review Letters, 2006
We analyze the Kondo effect of a magnetic impurity attached to an ultrasmall metallic wire using the density matrix renormalization group. The spatial spin correlation function and the impurity spectral density are computed for system sizes of up to L = 511 sites, covering the crossover from L < ℓK to L > ℓK , with ℓK the spin screening length. We establish a proportionality between the weight of the Kondo resonance and ℓK as function of L. This suggests a spectroscopic way of detecting the Kondo cloud.
Physical Review Letters, 2004
We investigate the effects of spin-polarized leads on the Kondo physics of a quantum dot using the numerical renormalization group method. Our study demonstrates in an unambiguous way that the Kondo effect is not necessarily suppressed by the lead polarization: While the Kondo effect is quenched for the asymmetric Anderson model, it survives even for finite polarizations in the regime where charge fluctuations are negligible. We propose the linear tunneling magnetoresistance as an experimental signature of these behaviors. We also report on the influence of spin-flip processes. PACS numbers: 72.15.Qm, 72.25.Mk, 73.63.Kv Introduction.-Magnetic impurities embedded in metallic hosts cause anomalous resonant scattering of conduction band electrons. At the same time, the localized magnetic moments are screened at low temperature by the itinerant electron spins. This is the celebrated Kondo effect [1], which has been recently revived in mesoscopic physics . Ever since the theoretical predictions and the experimental demonstrations [5], the Kondo effect in phase-coherent systems such as quantum dots (QD's) has stimulated great interest in this field. The remarkable success behind this is the fine tunability of the parameter space (impurity level and hybridization couplings). The controlled manipulation in mesoscopic systems has not only allowed to test various aspects of the Kondo effect, which is a hard task in bulk solids, but also has posed further exciting questions. For example, when the spin-degeneracy of the impurity level is lifted by an external magnetic field, the Kondo peak in the density of states (DOS) of the dot is expected to split [6]. However, new experiments [7] and theoretical studies suggest that the situation is more subtle.
Kondo screening regimes of a quantum dot with a single Mn ion
Physical Review B, 2011
We study the Kondo and transport properties of a quantum dot with a single magnetic Mn ion connected to metallic leads. By employing a numerical renormalization group technique we show that depending on the value of ferromagnetic coupling strength between the local electronic spin and the magnetic moment of the Mn, two distinct Kondo regimes exist. In the weak coupling limit, the system can be found in a completely screened Kondo state describing a local magnetic moment decoupled from the rest of the system. In contrast, in the strong coupling regime the quantum dot spin and the local magnetic moment form a single large-spin entity partially Kondo screened. A crossover between these two regimes can be suitably tuned by varying the tunnel coupling between the quantum dot electron and the leads. The model investigated here is also suitable to study magnetic molecules adsorbed on a metallic surface. The rich phenomenology of these systems is reflected in the conductance across the system.
Kondo screening suppression by spin-orbit interaction in quantum dots
Physical Review B, 2009
We study the transport properties of a quantum dot embedded in an Aharonov-Bohm ring in the presence of spin-orbit interactions. Using a numerical renormalization group analysis of the system in the Kondo regime, we find that the competition of Aharonov-Bohm and spin-orbit dynamical phases induces a strong suppression of the Kondo state singlet, somewhat akin to an effective intrinsic magnetic field in the system. This effective field breaks the spin degeneracy of the localized state and produces a finite magnetic moment in the dot. By introducing an in-plane Zeeman field we show that the Kondo resonance can be fully restored, reestablishing the spin singlet and a desired spin filtering behavior in the Kondo regime, which may result in full spin polarization of the current through the ring.
Physical Review Letters, 2003
The Kondo effect in quantum dots (QDs)-artificial magnetic impurities-attached to ferromagnetic leads is studied with the numerical renormalization group method. It is shown that the QD level is spin split due to the presence of ferromagnetic electrodes, leading to a suppression of the Kondo effect. We find that the Kondo effect can be restored by compensating this splitting with a magnetic field. Although the resulting Kondo resonance then has an unusual spin asymmetry with a reduced Kondo temperature, the ground state is still a locally screened state, describable by Fermi liquid theory and a generalized Friedel sum rule, and transport at zero temperature is spin independent.
The Kondo Effect in Non-Equilibrium Quantum Dots: Perturbative Renormalization Group
Journal of the Physical Society of Japan, 2005
While the properties of the Kondo model in equilibrium are very well understood, much less is known for Kondo systems out of equilibrium. We study the properties of a quantum dot in the Kondo regime, when a large bias voltage V and/or a large magnetic field B is applied. Using the perturbative renormalization group generalized to stationary nonequilibrium situations, we calculate renormalized couplings, keeping their important energy dependence. We show that in a magnetic field the spin occupation of the quantum dot is non-thermal, being controlled by V and B in a complex way to be calculated by solving a quantum Boltzmann equation. We find that the well-known suppression of the Kondo effect at finite V ≫ TK (Kondo temperature) is caused by inelastic dephasing processes induced by the current through the dot. We calculate the corresponding decoherence rate, which serves to cut off the RG flow usually well inside the perturbative regime (with possible exceptions). As a consequence, the differential conductance, the local magnetization, the spin relaxation rates and the local spectral function may be calculated for large V, B ≫ TK in a controlled way.
Kondo temperature and screening extension in a double quantum dot system
Physical Review B, 2012
In this work we use the slave-boson mean-field approximation at finite U to study the effects of spin-spin correlations in the transport properties of two quantum dots coupled in series to metallic leads. Different quantum regimes of this system are studied in a wide range of the parameter space. The main aspects related to the interplay between the half-filling Kondo effect and the antiferromagnetic correlation between the quantum dots are reviewed. Slave-boson results for conductance, local density of states in the quantum dots, and the renormalized energy parameters are presented. As a different approach to the Kondo physics in a double-dot system, the Kondo cloud extension inside the metallic leads is calculated and its dependence with the interdot coupling is analyzed. In addition, the cloud extension permits the calculation of the Kondo temperature of the double quantum dot. This result is very similar to the corresponding critical temperature T c , as a function of the parameters of the system, as obtained by using the finite-temperature extension of the slave-boson mean-field approximation.
Kondo effect in systems with spin disorder
Physical Review B, 1999
We consider the role of static disorder in the spin sector of the one-and two-channel Kondo models. The distribution functions of the disorder-induced effective energy splitting between the two levels of the Kondo impurity are derived to the lowest order in the concentration of static scatterers. It is demonstrated that the distribution functions are strongly asymmetric, with the typical splitting being parametrically smaller than the average rms value. We employ the derived distribution function of splittings to study the temperature dependence of the low-temperature conductance of a sample containing an ensemble of two-channel Kondo impurities. The results are used to analyze the consistency of the two-channel Kondo interpretation of the zero-bias anomalies observed in Cu/(Si:N)/Cu nanoconstrictions. 72.10.Fk, 75.20.Hr
The existence of a length-scale xiKsim1/TK\xi_K\sim 1/T_KxiKsim1/TK (with TKT_KTK the Kondo temperature) has long been predicted in quantum impurity systems. At low temperatures TllTKT\ll T_KTllTK, the standard interpretation is that a spin-$\tfrac{1}{2}$ impurity is screened by a surrounding `Kondo cloud' of spatial extent xiK\xi_Kxi_K. We argue that renormalization group (RG) flow between any two fixed points (FPs) results in a characteristic length-scale, observed in real-space as a crossover between physical behaviour typical of each FP. In the simplest example of the Anderson impurity model, three FPs arise; and we show that `free orbital', `local moment' and `strong coupling' regions of space can be identified at zero temperature. These regions are separated by two crossover length-scales xitextLM\xi_{\text{LM}}xitextLM and xiK\xi_KxiK, with the latter diverging as the Kondo effect is destroyed on increasing temperature through TKT_KT_K. One implication is that moment formation occurs inside the `Kondo cloud', while the screening process itself occurs on flowing to the strong coupling FP at distances simxi_K\sim \xi_Ksimxi_K. Generic aspects of the real-space physics are exemplified by the two-channel Kondo model, where xi_K\xi_Kxi_K now separates `local moment' and `overscreening' clouds.
Conductance of a spin-1 quantum dot: The two-stage Kondo effect
Physical Review B, 2007
We discuss the physics of a of a spin-1 quantum dot, coupled to two metallic leads and develop a simple model for the temperature dependence of its conductance. Such quantum dots are described by a two-channel Kondo model with asymmetric coupling constants and the spin screening of the dot by the leads is expected to proceed via a two-stage process. When the Kondo temperatures of each channel are widely separated, on cooling, the dot passes through a broad cross-over regime dominated by underscreened Kondo physics. A singular, or non-fermi liquid correction to the conductance develops in this regime. At the lowest temperatures, destructive interference between resonant scattering in both channels leads to the eventual suppression of the conductance of the dot. We develop a model to describe the growth, and ultimate suppression of the conductance in the two channel Kondo model as it is screened successively by its two channels. Our model is based upon large-N approximation in which the localized spin degrees of freedom are described using the Schwinger boson formalism.