Elementary excitations for the Anderson model at finite temperatures

The elementary excitation spectrums for the Anderson model at finite temperatures are calculated by using the Bethe-ansatz solution. The formulation is based on the method of Yang and Yang, which was developed for the one-dimensional boson systems with the -function type interaction. We obtain the temperature dependence of the spin and the charge excitation spectrums. When the impurity level lies deeply from the Fermi level and the Coulomb interaction is suitably large, the resonant peak structure develops in the low energy region of the spin excitation spectrum and the hump structure grows around the impurity level of the charge excitation spectrum with decreasing temperature. Received: 21 January 1998 / Accepted: 17 March 1998     

Electron correlations in alloys of simple and transition metals

The effect of dynamical electron-electron correlations on the electronic structure of the paramagnetic disordered binary alloy of the transition and simple metals is studied for the Anderson model extended to arbitrary concentration of the transition metal impurities. We use the terminal-point approximation for the many-body quantities, which allows us to solve the random part of the problem within the single-site approximation. The many-body part of the problem is treated within the selfconsistent T-matrix approximation, valid for low, but finite concentration of particles ( ? 0.3/atom/spin). For low concentration of the transition metal impurities we obtain results identical with those for the Anderson model. The theory is illustrated numerically for a simple bandstructure model.     

On the spin fluctuation approach to the anderson and Kondo hamiltonians

The free energy expression of the full Anderson model is derived in a similar way as has been done before for the Kondo model. Use is made of the “asymptotic time approximation” first invented to study the x-ray threshold singularity. Again the procedure leads to a classical Coulomb gas on a ring. The magnetic field is included and plays the role of an electric field for the Coulomb gas. Further it turns out that the “symmetric” Anderson model (2ε _d =?U) is identical to the antiferromagnetic Kondo model. The method and the results suggest the construction of a “polaron” model which in the approximation used is equivalent to the Kondo model as well as the Anderson model. From this a new picture of the “Kondo effect” in terms of spin fluctuations is developed.     

Correlations and conductivity of interacting fermions in a disordered chain

A one dimensional system of interacting fermions in a random potential is investigated within a Hartree-Fock approximation. The model in consideration combines the onsite interaction of the Hubbard model and the stochastic site energies of the Anderson model. For a system of 120 sites and several values of the model parameters the equal time correlation functions for the charge and the spin density as well as the low frequency conductivity are calculated numerically. The gross features of the conductivity thus obtained agree with published results obtained in a Monte Carlo simulation, but there are also interesting differences.     

Electron-spin polarization in a non-magnetic heterostructure

The spin dependent electron transmission through a non-magnetic III-V semiconductor symmetric well is studied theoretically so as to investigate the output transmission current polarization at zero magnetic field. Transparency of electron transmission is calculated as a function of electron energy as well as the well width, within the one electron band approximation along with the spin-orbit interaction. Enhanced spin-polarized resonant tunneling in the heterostructure due to Dresselhaus and Rashba spin-orbit coupling induced splitting of the resonant level is observed. We predict that a spin-polarized current spontaneously emerges in this heterostructure. This effect could be employed in the fabrication of spin filters, spin injectors and detectors based on non-magnetic semiconductors.     

Decay of multispin multiquantum coherent states in the NMR of a solid

A model based on the Anderson adiabatic approximation, which is widely used for describing various aspects of dynamic phenomena in conventional radiospectroscopy, is proposed for describing the decay of multispin multiquantum coherent states in a solid. The coherent state relaxation function is represented by the product of two functions corresponding to spin precession in a two-component local field with a correlated and an uncorrelated component. Theoretical results of this study explain the experimental data reported in a number of publications and are in good agreement with these data.     

Current bistability in resonant tunneling through a semiconductor quantum dot

We discuss resonant tunneling through quantum dot energy levels considering the charging energy of the dot. The hamiltonian of the system is reduced to a form of the Anderson hamiltonian of resonant tunneling. The mean-field approximation is applied and current–voltage characteristics are evaluated. The self-consistent solution is investigated for the low tunneling rate case in the low-temperature condition. The current bistability and the related current hysteresis are pointed out. The Coulomb staircase is shown in the current–voltage characteristics. These features are all due to Coulomb repulsion within the dot.     

A model of resonant Raman scattering via magnetic exciton in ferromagnetic phase

A semi-localized magneticd exciton model is proposed to explain the strong magnetic field dependence of the resonant Raman scattering observed in Eu-chalcogenides in ferro-magnetic phase. Choosing, in comparison with magneto-optical experiment, a single intermediate energy level in which all the spin and orbital angular momenta couple parallel, the observed frequency-, polarization- and magnetic field dependences are explained quantitatively in molecular field approximation. The discrepancy in the temperature dependence is discussed in terms of the short range order and the neglected intermediate energy levels.     

Soliton antiferromagnetic lattice in carbon nanotubes

Based on the Hubbard model for electrons of carbon nanotubes and the Anderson model in the theory of indirect interactions, an effective Hamiltonian describing the interaction of spin degrees of freedom in the longwave approximation and having the form of the Hamiltonian of the nonlinear sigma-model was proposed. The developed model was reduced to the classical nonlinear sigma-model. Periodic solutions to the equations of evolution of spin moments were obtained, which were interpreted as lattices composed of solitons of domain walls.     

Kondo resonance for orbitally degenerate systems

Formation of the Kondo state in the general two-band Anderson model has been investigated within the numerical renormalization group calculations. The Abrikosov-Suhl resonance is essentially asymmetric for the model with one electron per impurity (quarter filling case) in contrast with the one-band case. An external magnetic (pseudomagnetic) field breaking spin (orbital) degeneracy leads to asymmetric splitting and essential broadening of the many-body resonance. Unlike the standard Anderson model, the "spin-up" Kondo peak is pinned against the Fermi level, but not suppressed by the magnetic field.     

嵌入T型耦合双量子点介观A-B环系统的显著Fano 效应

利用平均场近似理论,研究了一个嵌入T型弱耦合双量子点的介观环系统的基态性质. 结果表明,体系中复杂的基态性质源于Kondo效应与Fano效应相互竞争. 当介观环的尺寸达到足以产生完全Kondo共振时,随双量子点间耦合强度的增强,尖锐的持续电流峰出现了,且越发显著,这说明体系中存在着显著的Fano 效应. 但介观环的Kondo共振持续电流峰值却几乎不发生变化,这为测定Kondo 屏蔽云提供了一个新的可能模型. 关键词： 耦合量子点 持续电流 Kondo效应 Fano 效应     

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.     

A self-consistent approach to localization transition

The Anderson transition is investigated using a self-consistent approximation in analogy with mean-field approximations in classical spin systems. Mobility edge trajectories in a three-dimensional disordered system with various distributions of the site energies are obtained. The present results are qualitatively in good agreement with the results obtained by using the finite-size scaling method.     

Quantum-mechanical interference in charge exchange between hydrogen and graphene-like surfaces

The neutral to negative charge fluctuation of a hydrogen atom in front of a graphene surface is calculated by using the Anderson model within an infinite intra atomic Coulomb repulsion approximation. We perform an ab initio calculation of the Anderson hybridization function that allows investigation of the effect of quantum-mechanical interference related to the Berry phase inherent to the graphene band structure. We find that consideration of the interaction of hydrogen on top of many C atoms leads to a marked asymmetry of the imaginary part of the hybridization function with respect to the Fermi level. Consequently, Fano factors larger than one and strongly dependent on the energy around the Fermi level are predicted. Moreover, the suppression of the hybridization for energies above the Fermi level can explain the unexpected large negative ion formation measured in the scattering of protons by graphite-like surfaces.     

Effect of porosity on the permeability tensor of polycrystalline ferrites (independent grain approximation)

The effect of porosity on the form of the permeability tensor is calculated using the independent-grain approximation; this procedure is similar to the Schlömann method. The theoretical curve for the resonant-field distribution is approximated by the Lorentz curve using the method of least squares. It is shown that with this approximation, porosity increases the width of the ferromagnetic resonance line for a non-porous material by the width of the Lorentz distribution curve; thus the resonant field shifts toward lower values. Formulas are obtained for the resonant-field shift due to porosity and the broadening in the ferromagnetic resonance line; these formulas differ somewhat from the Schlömann formulas. In order to check the working formulas and the applicability of the independent-grain approximation, measurements were performed on the tensor for magnesium-chromium-copper ferrites with variable porosity and a magnetization on the order of 1200 gauss at a frequency of 4000 mHz. Specimens having the form of longitudinally magnetized circular cylinders were used so that there was no degeneration in uniform precession of magnetization with long spin waves. The observed effect of porosity on the width of the ferromagnetic resonance line (determined by measuring the tensor) was found to be in good quantitative agreement with calculation. The shift in the resonant field due to porosity was negligibly small, which also agreed with calculation. The experimental results show that when there is no degeneration in uniform precession with spin waves, the independent-grain approximation can be used in experiments even when the magnetization and resonant field are approximately equal.Here, we must allow for the static magnetic susceptibility in the formulas for the resonant-field shift and the broadening in the ferromagnetic resonance line.     

Influence of magnetic impurities on the de Haas-van Alphen oscillations

Using a simple extension of Shiba's formula for the de Haas-van Alphen-oscillations, the influence of scattering by magnetic impurities is studied. Already for a classical spin model one obtains a minimum in the dHvA amplitudes as function of the magnetic-field, which is influenced by normal scattering. The change of this behavior for the Anderson model is studied within the Non-Crossing-Approximation, and raises questions for a comparison between experiment and theory.     

Narrow band limit of the equivalence between Anderson and Kondo Hamiltonians

It is shown that for a narrow band version of the Anderson Hamiltonian, the transformation of Schrieffer and Wolff can be carried out to all orders in the mixing parameter. The resulting Kondo-like Hamiltonian is in diagonal form. The results can be used to derive a systematic approximation scheme for the Anderson Hamiltonian, through use of the level shift operator of Primas. The resulting interaction parameters remain regular when the energy of the localized level approaches the Fermi level.     

Nonlinear behavior in magnetic transients

We examine the motion of a spin subject to a static external field, dynamic demagnetizing fields, and a relatively weak oscillatory driving field. More specifically, we study the initial transient behavior just after all these fields are turned on and present analytic calculations based on an approximation scheme for the motion of the spin. We find that two modes - a mode at the driving frequency and a mode at the natural resonant frequency - play an important role in the initial transient. Because of the nonlinear nature of the spin equations of motion, one also finds, only in the initial transient, modes with doubled frequencies of the initial two modes and modes involving the sum and difference frequencies of the initial two modes. We also examine large amplitude transients in specific cases through numerical calculations.     

Polarized nuclei from unpolarized atoms two-photon resonantly ionized via hyperfine structure energy levels

Nuclear spin polarization of the residual ion formed from an alkali atom after its resonant two-photon two-light beam ionization via hyperfine structure energy states is calculated in second-order time-dependent perturbation approach for atom-light interaction and taking into account all photoionization channels permitted in the electric-dipole approximation.     

The effect of spin on the non-linear resonant motions of a dynamical system

The motions of a two degree of freedom mechanical oscillator in a state of internal resonance due to the non-linear coupling between its modes are analyzed by the method of multiple scales. The system is connected by a motor to a vertical shaft driven at a constant spin rate relative to inertial space. It is shown that the non-linear resonance phenomenon can effectively be controlled by properly changing the spin rate of the motor. In addition, the transition curves that separate the non-linear resonant and the non-resonant motions of the system are also determined analytically by a straightforward perturbation method. The analytical expression for the transition curves is used in connection with the multiple scale analysis to yield a refined approximation for the main characteristics of the non-linear resonant motion.