Kondo effect for electron transport through an artificial quantum dot

We have calculated the transport properties of electron through an artificial quantum dot by using the numerical renormalization group technique in this paper. We obtain the conductance for the system of a quantum dot which is embedded in a one-dimensional chain in zero and finite temperature cases. The external magnetic field gives rise to a negative magnetoconductance in the zero temperature case. It increases as the external magnetic field increases, We obtain the relation between the coupling coefficient and conductance. If the interaction is big enough to prevent conduction electrons from tunnelling through the dot, the dispersion effect is dominant in this case. In the Kondo temperature regime, we obtain the conductivity of a quantum dot system with Kondo correlation.

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.

Transition from the Kondo effect to a Coulomb blockade in an electron shuttle

We investigate the effect of the mechanical motion of a quantum dot on the transport properties of a quantum dot shuttle, Employing the equation of motion method for the nonequilibrium Green＇s function, we show that the oscillation of the dot, i.e., the time-dependent coupling between the dot＇s electron and the reservoirs, can destroy the Kondo effect. With the increase in the oscillation frequency of the dot, the density of states of the quantum dot shuttle changes from the Kondo-like to a Coulomb-blockade pattern. Increasing the coupling between the dot and the electrodes may partly recover the Kondo peak in the spectrum of the density of states. Understanding of the effect of mechanical motion on the transport properties of an electron shuttle is important for the future application of nanoelectromechanical devices.

Fano-Kondo effect in the T-shaped double quantum dots coupled to ferromagnetic leads

Using an equation-of-motion technique, we theoretically study the Fano-Kondo effect in the T-shaped double quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian. We calculate the density of states in this system with both parallel and antiparaIlel lead-polarization alignments, and our results reveal that the interdot coupling, the spin-polarized strength and the energy level of the side coupled quantum dot greatly influence the density of states of the central quantum dot. This system is a possible candidate for spin valve transistors and may have potential applications in the spintronics.

Transport through artificial single-molecule magnets:Spin-pair state sequential tunneling and Kondo effects

The transport properties of an artificial single-molecule magnet based on a CdTe quantum dot doped with a single Mn＋2 ion（S=5/2） are investigated by the non-equilibrium Green function method.We consider a minimal model where the Mn-hole exchange coupling is strongly anisotropic so that spin-flip is suppressed and the impurity spin S and a hole spin s entering the quantum dot are coupled into spin pair states with（2S＋1） sublevels.In the sequential tunneling regime,the differential conductance exhibits（2S＋1） possible peaks,corresponding to resonance tunneling via（2S＋1） sublevels.At low temperature,Kondo physics dominates transport and（2S＋1） Kondo peaks occur in the local density of states and conductance.These peaks originate from the spin-singlet state formed by the holes in the leads and on the dot via higher-order processes and are related to the parallel and antiparallel spin pair states.

Kondo effect on electrical properties of TmIn_3

We presented results of electrical resistivity, magnetoresistivity and Seebek effect measurements done in the paramagnetic state of an antiferromagnetically ordering intermetalfic compound Trains. It was found that the magnetic part of the electrical resistivity showed maximum at 18 K and the lgT dependence in temperature range 20〈T〈100 K. The magnetoresistivity was negative. The txansversal magnetoresistivity fulfilled the single-ion Kondo scaling for temperatures 2〈（T＋0.5 K）〈28 K. Thermcelectricpower displayed a sharp minimum at 10 K. Electrical properties indicated that Tmln3 is likely a dense Kondo system.

Kondo Effect Versus Magnetic Coupling in Indirectly Coupled Double Quantum Dots

We theoretically investigate a device consisting of two quantum dots (QDs) side-coupled to a quantum wirewhich has many physical ingredients of an artificial heavy fermion system.An extra parameter, the distance L betweenthe two QDs, is introduced and it plays an important role on the competition of the Kondo temperature and magneticcoupling.Three different phases are found: antiferromagnetic phase, Kondo phase with spin S = 1/2, and Kondo phasewith S=1, depending on the distance L, the magnetic coupling, and the Kondo temperature.Quantum transportproperties are qualitatively different for different phases: for the S=1 Kondo and the antiferromagnetic phases, theconductance tends to the unitary value 2e^2/h; for the S=1/2 Kondo phase the conductance is strongly dependent onthe distance.

Spin current Kondo effect in frustrated Kondo systems

Magnetic frustrations can enhance quantum fluctuations in spin systems and lead to exotic topological insulating states.When coupled to mobile electrons,they may give rise to unusual non-Fermi liquid or metallic spin liquid states whose nature has not been well explored.Here,we propose a spin current Kondo mechanism underlying a series of non-Fermi liquid phases on the border of Kondo and magnetic phases in a frustrated three-impurity Kondo model.This mechanism is confirmed by renormalization group analysis and describes movable Kondo singlets called"holons"induced by an effective coupling between the spin current of conduction electrons and the vector chirality of localized spins.Similar mechanisms may widely exist in all frustrated Kondo systems and be detected through spin current noise measurements.

Two-impurity Kondo effect in potassium-doped single-layer p-sexiphenyl films

Herein, we show that a self-assembled phase of potassium(K)-doped single-layer para-sexiphenyl(PSP) film on a gold substrate is an excellent platform for studying the two-impurity Kondo model. On K-doped PSP molecules well separated from others, we observe a Kondo resonance peak close to EFwith a Kondo temperature of 30 K. The Kondo resonance peak splits when another K-doped PSP molecule is present in the vicinity, and the splitting gradually increases with the decrease in intermolecular distance without signs of phase transition. Our data demonstrate how a Kondo singlet state gradually evolves into an antiferromagnetic singlet state due to the competition between Kondo screening and antiferromagnetic Ruderman-Kittel-Kasuya-Yosida coupling,as described in the two-impurity Kondo model. Intriguingly, the antiferromagnetic singlet is quickly destroyed on increasing temperature and transforms back to a Kondo singlet below the Kondo temperature. Our data provide a comprehensive picture and quantitative constraints on related theories and calculations of the two-impurity Kondo model.

Charge density wave and weak Kondo effect in a Dirac semimetal CeSbTe

Using angle-resolved photoemission spectroscopy(ARPES) and low-energy electron diffraction(LEED), together with densityfunctional theory(DFT) calculation, we report the formation of charge density wave(CDW) and its interplay with the Kondo effect and topological states in CeSbTe. The observed Fermi surface(FS) exhibits parallel segments that can be well connected by the observed CDWordering vector, indicating that the CDWorder is driven by the electron-phonon coupling(EPC) as a result of the nested FS. The CDW gap is large(~0.3 eV) and momentum-dependent, which naturally explains the robust CDWorder up to high temperatures. The gap opening leads to a reduced density of states(DOS) near the Fermi level(EF), which correspondingly suppresses the many-body Kondo effect, leading to very localized 4 f electrons at 20 K and above. The topological Dirac cone at the X point is found to remain gapless inside the CDW phase. Our results provide evidence for the competition between CDWand the Kondo effect in a Kondo lattice system. The robust CDWorder in CeSbTe and related compounds provide an opportunity to search for the long-sought-after axionic insulator.

Composite electronic orders induced by orbital Kondo effect

In a large number of rare-earth and actinide systems,Kondo effect tends to suppress magnetic orders by making the spin singlet between localized and conduction electron spins.In the presence of orbital degrees of freedom,however,there emerge exotic electronic orders induced by Kondo effect.The orbital Kondo effect can collectively make diagonal and off-diagonal(superconducting) orders.With the particle-hole symmetry in conduction bands,these orders are all degenerate,forming a macroscopic SO(5) multiplet.This paper discusses recent theoretical development on these electronic orders which are relevant to Pr^(3+) and U^(4+) systems with even number of f electrons per site.In the superconducting order,each conduction–electron pair is coupled with local degrees of freedom,forming a composite entity with a staggered spatial pattern.The quasi-particle spectrum is best interpreted as virtual hybridization with resonant states at the Fermi level.Possible order parameter for URu_2Si_2 in the hidden order state is discussed in the context of composite orders.Briefly discussed are related issues such as homogeneous odd-frequency pairing and SO(5) theory for hightemperature superconductors.

Uniaxial stress effect on quasi-one-dimensional Kondo lattice CeCo_(2)Ga_(8)

Quantum critical phenomena in the quasi-one-dimensional limit remain an open issue.We report the uniaxial stress effect on the quasi-one-dimensional Kondo lattice CeCo_(2)Ga_(8) by electric transport and AC heat capacity measurements.CeCo_(2)Ga_(8) is speculated to sit in close vicinity but on the quantum-disordered side of a quantum critical point.Upon compressing the c axis,parallel to the Ce-Ce chain,the onset of coherent Kondo effect is enhanced.In contrast,the electronic specific heat diverges more rapidly at low temperature when the intra-chain distance is elongated by compressions along a or b axis.These results suggest that a tensile intra-chain strain(ε_(c)>0)pushes CeCo_(2)Ga_(8) closer to the quantum critical point,while a compressive intra-chain strain(ε_(c)<0)likely causes departure.Our work provides a rare paradigm of manipulation near a quantum critical point in a quasi-1D Kondo lattice by uniaxial stress,and paves the way for further investigations on the unique feature of quantum criticality in the quasi-1D limit.

Asymmetric conductivity of the Kondo effect in cold atomic systems

Motivated by recent theoretical and experimental advances in quantum simulations using alkaline earth(AE)atoms,we put forward a proposal to detect the Kondo physics in a cold atomic system.It has been demonstrated that the intrinsic spin-exchange interaction in AE atoms can be significantly enhanced near a confinement-induced resonance(CIR),which facilitates the simulation of Kondo physics.Since the Kondo effect appears only for antiferromagnetic coupling,we find that the conductivity of such system exhibits an asymmetry across a resonance of spin-exchange interaction.The asymmetric conductivity can serve as the smoking gun evidence for Kondo physics in the cold atom context.When an extra magnetic field ramps up,the spin-exchange process near Fermi surface is suppressed by Zee-man energy and the conductivity becomes more and more symmetric.Our results can be verified in the current experimental setup.

Kondo effect in quantum dots and molecular devices

Kondo effect is a very important many-body phenomenon in condensed matter physics,which explains why the resistance increases as the temperature is lowered (usually <10 K) in dilute magnetic alloy, and why the con-ductance increases as temperature is decreased in quantum dots. This paper simply introduces equilibrium and non- equilibrium Kondo effects in quantum dots together with the Kondo effect in quantum dots with even number of electrons (when the singlet and triplet states are degenerate). Fur-thermore, Kondo effect in single atom/molecular transistors is introduced, which indicates a new way to study Kondo effect.

Anomalous Kondo-Switching Effect of a Spin-Flip Quantum Dot Embedded in an Aharonov-Bohm Ring

We theoretically investigate the Kondo effect of a quantum dot embedded in a mesoscopic Aharonov-Bohm(AB)ring in the presence of the spin flip processes by means of the one-impurity Anderson Hamiltonian.Based on theslave-boson mean-field theory,we find that in this system the persistent current(PC)sensitively depends on the parityand size of the AB ring and can be tuned by the spin-flip scattering(R).In the small AB ring,the PC is suppresseddue to the enhancing R weakening the Kondo resonance.On the contrary,in the large AB ring,with R increasing,thepeak of PC firstly moves up to max-peak and then down.Especially,the PC phase shift of π appears suddenly withthe proper value of R,implying the existence of the anomalous Kondo effect in this system.Thus this system may be acandidate for quantum switch.

Effects of van Hove Singularities on Transport of Quantum Dot Systems in Kondo Regime

In the present paper, we study the effect of van Hove singularities of conduction electron on the transport of a single quantum dot system in the Kondo regime. By using both the equation-of-motion and the noncrossing approximation techniques, we show that the corrections caused by these singularities are actually minor. It can be explained by observing that the singularities in the equations, which determine the electronic DOS on the dot, are integrable. Furthermore, we find that, although each line width function is divergent at van Hove singular points, the total divergence is canceled out in the final formula to calculate the current through the system. Therefore, as far as the qualitative properties of the system is concerned, these singularities can be ignored and the wide-band approximation can be safely used in calculation.

Spin-Polarized Transport Through a Quantum Dot Coupled to Ferromagnetic Leads： Kondo Correlation Effect

We investigate the linear and nonlinear transport through a single level quantum dot connected to two ferromagnetic leads in Kondo regime, using the slave-boson mean-field approach for finite on-site Coulomb repulsion. We find that for antiparallel alignment of the spin orientations in the leads, a single zero-bias Kondo peak always appears in the voltage-dependent differential conductance with peak height going down to zero as the polarization grows to P = 1.For parallel configuration, with increasing polarization from zero, the Kondo peak descends and greatly widens with the appearance of shoulders, and finally splits into two peaks on both sides of the bias voltage around P ～ 0.7 until disappearing at even larger polarization strength. At any spin orientation angle θ, the linear conductance generally drops with growing polarization strength. For a given finite polarization, the minimum linear conductance always appears at θ = π.

Kondo-Fano Effect in T-shaped Double Quantum Dots Coupled to Ferromagnetic Leads

Using an equation-of-motion technique,we theoretically study the Kondo-Fano effect in the T-shaped double quantum dots coupled to two ferromagnetic leads by the Anderson Hamiltonian.We calculate the density of states in this system by solving Green function.Our results reveal that the density of states show some noticeable characteristics not only depending upon the interdot coupling t ab,the energy level ε d1 of the side coupled quantum dot QD b,and the relative angle θ of magnetic moment M,but also the asymmetry parameter α in ferromagnetic leads and so on.All these parameters greatly influence the density of states of the central quantum dot QD a.This system is a possible candidate for spin valve transistors and may have potential applications in the spintronics.

平行双量点中的Fano-Kondo

通过么正变换得到了平行双量子点有间库仑作用时的Fano-Anderson哈密顿量,并用格林函数运动方程的方法济南市出了与实验观测量密切相关的推迟和量子分布格林函数。借助于新得到的哈密顿量和格林函数,可以研究Kondo和Fano共振的关联效应。

AC Conductance Through a Vibrating Molecular Dot in Kondo Regime

In the present paper,by applying the Lang-Firsov canonical transformation and the so-called non-crossingapproximation technique,we investigate the joint effects of the electron-phonon interaction and an external alternatinggate voltage on the transport of a quantum dot system in the Kondo regime.We find that,while the satellite Kondoresonant peaks appear in both the averaged local density of states and the differential conductance,the main Kondopeak at the Fermi energy is greatly suppressed.These results confirm the previous ones derived by other methods,suchas the equation of motion solution.Furthermore,based on the picture of virtual transition between quasi-eigenstates inthe system,we also give a slightly different explanation on these phenomena.