Critical Kondo destruction and the violation of the quantum-to-classical mapping of quantum criticality

Antiferromagnetic heavy fermion metals close to their quantum critical points display a richness in their physical properties unanticipated by the traditional approach to quantum criticality, which describes the critical properties solely in terms of fluctuations of the order parameter. This has led to the question as to how the Kondo effect gets destroyed as the system undergoes a phase change. In one approach to the problem, Kondo lattice systems are studied through a self-consistent Bose-Fermi Kondo model within the extended dynamical mean field theory. The quantum phase transition of the Kondo lattice is thus mapped onto that of a sub-Ohmic Bose-Fermi Kondo model. In the present article we address some aspects of the failure of the standard order-parameter functional for the Kondo-destroying quantum critical point of the Bose-Fermi Kondo model.     

Interplay between the mesoscopic Stoner and Kondo effects in quantum dots

We consider electrons confined to a quantum dot interacting antiferromagnetically with a spin-1 / 2 Kondo impurity. The electrons also interact among themselves ferromagnetically with a dimensionless coupling J , where J =1 denotes the bulk Stoner transition. We show that as J approaches 1 there is a regime with enhanced Kondo correlations, followed by one where the Kondo effect is destroyed and impurity is spin polarized opposite to the dot electrons. The most striking signature of the first, Stoner-enhanced Kondo regime is that a Zeeman field increases the Kondo scale, in contrast to the case for noninteracting dot electrons. Implications for experiments are discussed.     

由杂质自旋散射引起的纠缠

研究了自由电子与固定杂质自旋的散射过程中的纠缠行为.研究发现,固定自旋的杂质能起到纠缠过滤器的作用,实现自由电子间的纠缠浓缩.当两无相互作用的电子先后经过同一Kondo杂质并与之发生散射时,可以使得原本不纠缠的电子发生纠缠.类似地,当运动的电子先后经过两无相互作用的Kondo杂质并与它们发生散射,可以使得原本不纠缠的K...     

Universal scaling in nonequilibrium transport through a single channel Kondo dot

Scaling laws and universality play an important role in our understanding of critical phenomena and the Kondo effect. We present measurements of nonequilibrium transport through a single-channel Kondo quantum dot at low temperature and bias. We find that the low-energy Kondo conductance is consistent with universality between temperature and bias and is characterized by a quadratic scaling exponent, as expected for the spin-1/2 Kondo effect. We show that the nonequilibrium Kondo transport measurements are well described by a universal scaling function with two scaling parameters.     

Interacting quantum dot coupled to a kondo spin: a universal Hamiltonian study

We study a Kondo spin coupled to a mesoscopic interacting quantum dot that is described by the "universal Hamiltonian." The problem is solved numerically by diagonalizing the system Hamiltonian in a good-spin basis and analytically in the weak and strong Kondo coupling limits. The ferromagnetic exchange interaction within the dot leads to a stepwise increase of the ground-state spin (Stoner staircase), which is modified nontrivially by the Kondo interaction. We find that the spin-transition steps move to lower values of the exchange coupling for weak Kondo interaction, but shift back up for sufficiently strong Kondo coupling. The interplay between Kondo and ferromagnetic exchange correlations can be probed with experimentally tunable parameters.     

Kondo effect in a deformed molecule coupled asymmetrically to ferromagnetic electrodes

The nonequilibrium Kondo effect is studied in a molecule quantum dot coupled asymmetrically to two ferromagnetic electrodes by employing the nonequilibrium Green function technique. The current-induced deformation of the molecule is taken into account, modeled as interactions with a phonon system, and phonon-assisted Kondo satellites arise on both sides of the usual main Kondo peak. In the antiparallel electrode configuration, the Kondo satellites can be split only for the asymmetric dot-lead couplings, distinguished from the parallel configuration where splitting also exists, even though it is for symmetric case. We also analyze how to compensate the splitting and restore the suppressed zero-bias Kondo resonance. It is shown that one can change the TMR ratio significantly from a negative dip to a positive peak only by slightly modulating a local external magnetic field, whose value is greatly dependent on the electron--phonon coupling strength.     

Junction of three off-critical quantum Ising chains and two-channel Kondo effect in a superconductor

We show that a junction of three off-critical quantum Ising chains can be regarded as a quantum spin chain realization of the two-channel spin-1/2 overscreened Kondo effect with two superconducting leads. We prove that, as long as the Kondo temperature is larger than the superconducting gap, the equivalent Kondo model flows towards the two channel Kondo fixed point. We argue that our system provides the first controlled realization of two channel Kondo effect with superconducting leads. Besides its theoretical interest, this result is of importance for potential applications to a number of contexts, including the analysis of the quantum entanglement properties of a Kondo system.     

Splitting and Restoration of Kondo Peak in a Deformed Molecule Quantum Dot Coupled to Ferromagnetic Electrodes

We adopt the nonequilibrium Green＇s function method to theoretically study the Kondo effect in a deformed molecule, which is treated as an electron-phonon interaction （EPI） system. The self-energy for phonon part is calculated in the standard many-body diagrammatic expansion up to the second order in EPI strength. We find that the multiple phonon-assisted Kondo satellites arise besides the usual Kondo resonance. In the antiparallel magnetic configuration the splitting of main Kondo peak and phonon-assisted satellites only happen for asymmetrical dot-lead couplings, but it is free from the symmetry for the parallel magnetic configuration. The EPI strength and vibrational frequency can enhance the spin splitting of both main Kondo and satellites. It is shown that the suppressed zero-bias Kondo resonance can be restored by applying an external magnetic field, whose magnitude is dependent on the phononic effect remarkably. Although the asymmetry in tunnel coupling has no contribution to the restoration of spin splitting of Kondo peak, it can shrink the external field needed to switch tunneling magnetoresistance ratio between large negative dip and large positive peak.     

Manipulating the Kondo resonance through quantum size effects

Manipulating the Kondo effect by quantum confinement has been achieved by placing magnetic molecules on silicon-supported nanostructures. The Kondo resonance of individual manganese phthalocyanine (MnPc) molecules adsorbed on the top of Pb islands was studied by scanning tunneling spectroscopy. Oscillating Kondo temperatures as a function of film thickness were observed and attributed to the formation of the thickness-dependent quantum-well states in the host Pb islands. The present approach provides a technologically feasible way for single spin manipulation by precise thickness control of thin films.     

Shot noise through a quantum dot in the Kondo regime

The shot noise in the current through a quantum dot is calculated as a function of voltage from the high-voltage Coulomb-blockaded regime to the low-voltage Kondo regime. Using several complementary approaches, it is shown that the zero-frequency shot noise (scaled by the voltage) exhibits a nonmonotonic dependence on voltage, with a peak around the Kondo temperature. Beyond giving a good estimate of the Kondo temperature, it is shown that the shot noise yields additional information on the effects of electronic correlations on the local density of states in the Kondo regime, unaccessible in traditional transport measurements.     

Spatially dependent Kondo effect in quantum corrals

We study the Kondo screening of a single magnetic impurity inside a nonmagnetic quantum corral located on the surface of a metallic host system. We show that the spatial structure of the corral's eigenmodes leads to a spatially dependent Kondo effect whose signatures are spatial variations of the Kondo temperature T K. Moreover, we predict that the Kondo screening is accompanied by the formation of multiple Kondo resonances with characteristic spatial patterns. Our results open new possibilities to manipulate and explore the Kondo effect by using quantum corrals.     

Kondo-transport spectroscopy of single molecule magnets

We demonstrate that in a single molecule magnet strongly coupled to electrodes the Kondo effect involves all magnetic excitations. This Kondo effect is induced by the quantum tunneling of the magnetic moment. Importantly, the Kondo temperature TK can be much larger than the magnetic splittings. We find a strong modulation of the Kondo effect as a function of the transverse anisotropy parameter or a longitudinal magnetic field. Both for integer and half-integer spin this can be used for an accurate transport spectroscopy of the magnetic states in low magnetic fields on the order of the easy-axis anisotropy parameter. We set up a relationship between the Kondo effects for successive integer and half-integer spins.     

Tuning the Kondo effect with a mechanically controllable break junction

We study electron transport through C(60) molecules in the Kondo regime using a mechanically controllable break junction. By varying the electrode spacing, we are able to change both the width and the height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. The linear conductance as a function of T/T(K) agrees with the scaling function expected for the spin-1/2 Kondo problem. We are also able to tune finite-bias Kondo features which appear at the energy of the first C(60) intracage vibrational mode.     

Why could electron spin resonance be observed in a heavy fermion Kondo lattice?

We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancellation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh₂Si₂ in terms of their dependence on temperature and magnetic field.     

Spin-flip transport through an interacting quantum dot

The spin-flip associated transport based on the Anderson model is studied. It is found that the electrons are scattered due to spin-flip effect via the normal, mixed and Kondo channels. The spin-flip scattering via Kondo channel enhances the Kondo resonance peak and causes a slight blue shift. The conductance is suppressed by the spin-flip scattering. This is attributed to the reason that electrons with energy near Fermi level are scattered by Kondo channel.     

Kondo effect in the presence of itinerant-electron ferromagnetism studied with the numerical renormalization group method

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.     

Evolution of Kondo resonance from a single impurity molecule to the two-dimensional lattice

The successive spectral evolution of the Kondo resonance state was investigated from a single iron(II) phthalocyanine molecule to the two-dimensional lattice on Au(111) by interrogating the individual molecules with a scanning tunneling microscope. A sharp Kondo peak appears in the single-impurity regime, which broadens and splits as the lattice builds up. The origin of spectral evolution together with the electronic ground state of the lattice are discussed based on the competition of the Kondo effect and Rudermann-Kittel-Kasuya-Yosida coupling between the molecular spins.     

Spin-resolved splitting of Kondo resonances in the presence of RKKY-type coupling

We have performed spin-resolved measurements on a Kondo impurity in the presence of RKKY-type exchange coupling. By placing manganese phthalocyanine (MnPc) molecules on Fe-supported Pb islands, a Kondo system is devised which is exchange coupled to a magnetic substrate via conduction electrons in Pb, inducing spin splitting of the Kondo resonance. The spin-polarized nature of the split Kondo resonance and a spin filter effect induced by spin-flip inelastic electron tunneling are revealed by spin-polarized scanning tunneling microscopy and spectroscopy.     

Evolution of the Kondo effect in a quantum dot probed by shot noise

We measure the current and shot noise in a quantum dot in the Kondo regime to address the nonequilibrium properties of the Kondo effect. By systematically tuning the temperature and gate voltages to define the level positions in the quantum dot, we observe an enhancement of the shot noise as temperature decreases below the Kondo temperature, which indicates that the two-particle scattering process grows as the Kondo state evolves. Below the Kondo temperature, the Fano factor defined at finite temperature is found to exceed the expected value of unity from the noninteracting model, reaching 1.8±0.2.