Screening and interaction of magnetic impurities in Luttinger liquids

The spin and charge correlations induced in the conduction electron sea by the presence of a spin-1=2 magnetic impurity are investigated for one-dimensional electrons. For correlated conduction electrons, the RKKY interaction between magnetic impurities exhibits only a slow algebraic decay with distance. Increasing the exchange coupling between conduction electrons and magnetic impurity leads to a competition between the RKKY interaction and the Kondo effect. For a two-impurity model, we study the influence of the electronic correlations on this competition. Furthermore, the Kondo screening cloud and the local spin susceptibility far away from a magnetic impurity are discussed.     

Kondo stripes in an Anderson-Heisenberg model of heavy fermion systems

We study the interplay between the spin-liquid and Kondo physics, as related to the nonmagnetic part of the phase diagram of heavy fermion materials. Within the unrestricted mean-field treatment of the infinite-U 2D Anderson-Heisenberg model, we find that there are two topologically distinct nondegenerate uniform heavy Fermi liquid states that may form as a consequence of the Kondo coupling between spinons and conduction electrons. For certain carrier concentrations, the uniform Fermi liquid becomes unstable with respect to the formation of a new kind of anharmonic "Kondo stripe" state with inhomogeneous Kondo screening strength and the charge density modulation. These features are experimentally measurable and thus may help to establish the relevance of the spin-liquid correlations to heavy fermion materials.     

SU(4) Fermi liquid state and spin filtering in a double quantum dot system

We study a symmetrical double quantum dot (DD) system with strong capacitive interdot coupling using renormalization group methods. The dots are attached to separate leads, and there can be a weak tunneling between them. In the regime where there is a single electron on the DD the low-energy behavior is characterized by an SU(4)-symmetric Fermi liquid theory with entangled spin and charge Kondo correlations and a phase shift pi/4. Application of an external magnetic field gives rise to a large magnetoconductance and a crossover to a purely charge Kondo state in the charge sector with SU(2) symmetry. In a four-lead setup we find perfectly spin-polarized transmission.     

Quantum dot in the Kondo regime coupled to superconductors

The Kondo effect and superconductivity are both prime examples of many-body phenomena. Here we report transport measurements on a carbon nanotube quantum dot coupled to superconducting leads that show a delicate interplay between both effects. We demonstrate that the superconductivity of the leads does not destroy the Kondo correlations on the quantum dot when the Kondo temperature, which varies for different single-electron states, exceeds the superconducting gap energy.     

Kondo resonance assisted thermoelectric transport through strongly correlated quantum dots

We theoretically studied the thermoelectric transport properties of a strongly correlated quantum dot system in the presence of the Kondo effect based on accurate numerical evaluations using the hierarchical equations of motion approach. The thermocurrent versus gate voltage shows a distinct sawtooth line-shape at high temperatures. In particular, the current changes from positive(hole charge) to negative(particle charge) in the electron number N = 1 region due to the Coulomb blockade effect. However,at low temperatures, where the Kondo effect occurs, the thermocurrent's charge polarity reverses, along with a significantly enhanced magnitude. As anticipated, the current sign can be analyzed by the occupation difference between particle and hole.Moreover, the characteristic turnover temperature can be further defined at which the influences of the Coulomb blockade and Kondo resonance are in an effective balance. Remarkably, the identified characteristic turnover temperature, as a function of the Coulomb interaction and dot-lead coupling, possessed a much higher value than the Kondo temperature. When a magnetic field is applied, a spin-polarized thermocurrent can be obtained, which could be tested in future experiments.     

Auto- and cross-correlations in the spinful topological Kondo model

The correlations in charge transport in multiple Majorana fermions systems contain much richer physics and carry more information than the conventional transport coefficients. We calculate the auto- and cross-correlations in the spinful topological Kondo model (SKTM) and point out the frequencies dependencies of the correlations that may serve as a proof of the existence of the Majorana fermions in our system, particularly the change of the cross-correlation from positive to negative at higher frequency.     

Recent progress in theoretical understanding of heavy fermion systems

A key to understand the physics of heavy Fermion systems is the competition between the RKKY interaction and the Kondo screening effect of magnetic moments. The Kondo lattice model is a theoretical model for which one can study the interplay. Recently the ground state phase diagram of the one‐dimensional Kondo lattice model has been completed. After reviewing the properties of the three phases in the phase diagram we discuss the Kondo insulator in one‐dimension with particular emphasis on the difference between the spin excitation gap and the charge excitation gap. We argue that the Kondo insulators may be distinguished from the ordinary band insulators by the difference between the two gaps. Another topic which is discussed in this article is the complicated magnetic phase diagram of the low‐carrier‐density system of cerium monopnictides. We propose that the new mechanism which exists for Kondo semimetals may be responsible for the complicated magnetic structures in these compounds. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlated disorder in a Kondo lattice

Motivated by recent experiments on Yb-doped CeCoIn5, we study the effect of correlated disorder in a Kondo lattice. Correlations between the impurities are considered at the two-particle level. We use a mean-field theory approximation for the Anderson lattice model to calculate how the emergence of coherence in the Kondo lattice is impacted by correlations between impurities. We show that the rate at which disorder suppresses coherence temperature depends on the length of the impurity correlations. As the impurity concentration increases, we generally find that the suppression of coherence temperature is significantly reduced. The results are discussed in the context of available experimental data.     

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.     

Universal scaling and a novel quantum critical behavior of CeRhSb1-xSnx

We observe and explain a universal scaling rhochi = const for the electrical resistivity rho with the inverse magnetic susceptibility chi(-1) for the Kondo insulator CeRhSb(1-x)Snx. In the regime where the Kondo gap disappears (x > 0.12), the system forms a non-Fermi liquid (NFL), which transforms into a Fermi liquid at higher temperature. The NFL behavior is associated with the presence of a novel quantum critical point (QCP) at the Kondo insulator-correlated metal boundary. The divergent behavior of the resistivity, the susceptibility, and the specific heat has been observed when approaching the QCP from the metallic side and is interpreted as due to the competition between the Kondo and the intersite magnetic correlations.     

Competition between Kondo and RKKY correlations in the presence of strong randomness

We propose that competition between Kondo and magnetic correlations results in a novel universality class for heavy fermion quantum criticality in the presence of strong randomness. Starting from an Anderson lattice model with disorder, we derive an effective local field theory in the dynamical mean-field theory approximation, where randomness is introduced into both hybridization and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Performing the saddle-point analysis in the U(1) slave-boson representation, we reveal its phase diagram which shows a quantum phase transition from a spin liquid state to a local Fermi liquid phase. In contrast with the clean limit case of the Anderson lattice model, the effective hybridization given by holon condensation turns out to vanish, resulting from the zero mean value of the hybridization coupling constant. However, we show that the holon density becomes finite when the variance of the hybridization is sufficiently larger than that of the RKKY coupling, giving rise to the Kondo effect. On the other hand, when the variance of the hybridization becomes smaller than that of the RKKY coupling, the Kondo effect disappears, resulting in a fully symmetric paramagnetic state, adiabatically connected to the spin liquid state of the disordered Heisenberg model. We investigate the quantum critical point beyond the mean-field approximation. Introducing quantum corrections fully self-consistently in the non-crossing approximation, we prove that the local charge susceptibility has exactly the same critical exponent as the local spin susceptibility, suggesting an enhanced symmetry at the local quantum critical point. This leads us to propose novel duality between the Kondo singlet phase and the critical local moment state beyond the Landau-Ginzburg-Wilson paradigm. The Landau-Ginzburg-Wilson forbidden duality serves the mechanism of electron fractionalization in critical impurity dynamics, where such fractionalized excitations are identified with topological excitations.     

Charge distribution in a Kondo-correlated quantum dot

We report on direct measurement of charge and its distribution in a Kondo correlated quantum dot (QD). A noninvasive potential-sensitive detector, in proximity with a QD, reveals that, although the conductance of the QD is significantly enhanced as it enters the Kondo regime, the average charge remains unaffected. This demonstrates the separation between spin and charge degrees of freedom. We find, however, under certain conditions, an abrupt redistribution of charge in the QD, taking place with an onset of Kondo correlation. This suggests a correlation between the spin and charge degrees of freedom.     

Double-dot charge qubit and transport via dissipative cotunneling

We investigate transport through an exotic charge qubit composed of two strongly capacitively coupled quantum dots, each being independently connected to a side gate which in general exhibits a fluctuating electrostatic field (i.e., Johnson-Nyquist noise). Two quantum phases are found: the "Kondo" phase where an orbital-Kondo entanglement emerges and a "local moment" phase in which the noise destroys the Kondo effect leaving the orbital spin unscreened and resulting in a clear suppression of the conductance. In the Kondo realm, the transfer of charge across the setting is accompanied by zero-point charge fluctuations in the two dissipative environments and then the I-V characteristics are governed by what we call "dissipative cotunneling."     

Non-Kondo mechanism for resistivity minimum in spin ice conduction systems

We present a mechanism of resistivity minimum in conduction electron systems coupled with localized moments, which is distinguished from the Kondo effect. Instead of the spin-flip process in the Kondo effect, electrons are elastically scattered by local spin correlations which evolve in a particular way under geometrical frustration as decreasing temperature. This is demonstrated by the cellular dynamical mean-field theory for a spin-ice-type Kondo lattice model on a pyrochlore lattice. Peculiar temperature dependences of the resistivity, specific heat, and magnetic susceptibility in the non-Kondo mechanism are compared with the experimental data in metallic Ir pyrochlore oxides.     

Free magnetic moments in disordered metals

The screening of magnetic moments in metals, the Kondo effect, is found to be quenched with a finite probability in the presence of nonmagnetic disorder. Numerical results for a disordered electron system show that the distribution of Kondo temperatures deviates strongly from the result expected from the random matrix theory. A pronounced second peak emerges for small Kondo temperatures, showing that the probability that magnetic moments remain unscreened at low temperatures increases with disorder. Analytical calculations, taking into account the correlations between eigenfunction intensities, yield a finite width for the distribution in the thermodynamic limit. Experimental consequences for disordered mesoscopic metals are discussed. This article was submitted by the authors in English.     

Transport through a quantum dot subject to spin and charge bias

Spin and charge transport through a quantum dot coupled to external nonmagnetic leads is analyzed theoretically in terms of the non-equilibrium Green function formalism based on the equation of motion method. The dot is assumed to be subject to spin and charge bias, and the considerations are focused on the Kondo effect in spin and charge transport. It is shown that the differential spin conductance as a function of spin bias reveals a typical zero-bias Kondo anomaly which becomes split when either magnetic field or charge bias are applied. Significantly different behavior is found for mixed charge/spin conductance. The influence of electron-phonon coupling in the dot on tunneling current as well as on both spin and charge conductance is also analyzed.     

Anomalous magnetic splitting of the kondo resonance

The splitting of the Kondo resonance in the density of states of an Anderson impurity in a finite magnetic field is calculated from the exact Bethe-ansatz solution. The result gives an estimate of the electron spectral function for a nonzero magnetic field and the Kondo temperature, with consequences for transport experiments on quantum dots in the Kondo regime. The strong correlations of the Kondo ground state cause a significant low-temperature reduction of the peak splitting. Explicit formulas are found for the shift and broadening of the Kondo peaks. A likely cause of the problems of large- N approaches to spin- 1 / 2 impurities at finite magnetic field is suggested.     

Spin-flip Fano–Kondo resonant tunneling through a quantum dot interferometer responded by a rotating magnetic field

The Fano and Kondo cooperated resonant tunneling through a quantum dot interferometer under the perturbation of a rotating magnetic field is investigated theoretically. The spin-polarized current components have been derived generally by employing the Keldysh nonequilibrium Green?s function method, through which the charge and spin currents are determined directly. The numerical calculations on spin and charge currents are performed to show the compound features of mesoscopic transport associated with the Kondo, Fano, and Zeeman effects intimately. The induced spin current in the Kondo regime is much different from the one in the non-interacting regime. The spin current is tuned from resonant peak to valley by varying external parameters.     

Oscillations of the magnetic polarization in a Kondo system at finite magnetic fields

The electronic properties of a Kondo impurity are investigated in a magnetic field using linear response theory. The distribution of electrical charge and magnetic polarization are calculated in real space. The (small) magnetic field does not change the charge distribution. However, it unmasks the Kondo cloud and generates a Kondo polarization. The weight of the d-electron components with their magnetic moment up and down is shifted and the compensating moments of the s-electron clouds don’t cancel any longer (a requirement for an experimental detection of the Kondo cloud). In addition to the polarization cloud (of the conduction electrons) an oscillating polarization component with a period of half the Fermi wave length is observed. This represents an internal electronic structure of the Kondo impurity.     

Out-of-equilibrium Kondo effect in a mesoscopic device

We study the nonequilibrium regime of the Kondo effect in a quantum dot laterally coupled to a narrow wire. We observe a split Kondo resonance when a finite bias voltage is imposed across the wire. The splitting is attributed to the creation of a double-step Fermi distribution function in the wire. Kondo correlations are strongly suppressed when the voltage across the wire exceeds the Kondo temperature. A perpendicular magnetic field enables us to selectively control the coupling between the dot and the two Fermi seas in the wire. Already at fields of order 0.1 T only the Kondo resonance associated with the strongly coupled reservoir survives.