Filling dependence of correlation exponents and metal-Mott insulator transition in strongly correlated electron systems

Using a universal relation between electron filling factor and ground state energy,this paper studies the dependence of correlation exponents on the electron filling factor of one-dimensional extended Hubbard model in a strong coupling regime,and demonstrates that in contrast to the usual Hubbard model(gc = 1/2),the dimensionless coupling strength parameter g c heavily depends on the electron filling,and it has a "particle-hole" symmetry about electron quarter filling point.As increasing the nearest neighbouring repulsive interaction,the single particle spectral weight is transferred from low energy to high energy regimes.Moreover,at electron quarter filling,there is a metal-Mott insulator transition at the strong coupling point gc = 1/4,and this transition is a continuous phase transition.     

Perturbative analysis of the probability of the nonthermal transfer of an electron

The probability of the nonthermal transfer of an electron of an excited donor-acceptor complexes was calculated from the electron coupling parameter within the framework of time-dependent perturbation theory. Using the analysis of the two first corrections of the perturbation theory series for the standard spinboson model with the Debye spectral density of thermostat oscillators as a basis, we revealed the structure of higher corrections. This allowed us to sum up the series and obtain a new analytical expression distinct from the widely used Padé-approximation for the probability of the nonthermal transfer of an electron. The stochastic approach was shown to allow overestimation of the probability of nonthermal electron transitions by 40.0% in the solvent-controlled regime.     

Improved equation of state for finite‐temperature spin‐polarized electron liquids on the basis of Singwi–Tosi–Land–Sjölander approximation

An accurate expression for the exchange‐correlation free energy f _xc of homogeneous electron fluids at finite temperatures is presented on the basis of Singwi–Tosi–Land–Sjölander (STLS ) approximation. In addition to the derivation for the paramagnetic state, that for the ferromagnetic state is carried out afresh in which a correction in the strong Coulomb coupling regime is incorporated into the construction of the analytic expression as a function of the coupling constant and Fermi degeneracy. The fitting formula for f _xc is then extended over any degree of spin polarization with the aid of available interpolation scheme. The proposed equation of state, called the iSTLS formula, shows reasonable agreement with the existing quantum Monte Carlo evaluations at finite temperatures in the paramagnetic state, thus giving a consensus for the thermodynamic functions between many‐body theories and computer simulations. On the other hand, the current status for the agreement between various evaluations of f _xc is relatively unsatisfactory in the ferromagnetic state, suggesting the necessity for further investigations.     

Theoretical investigation of superconductivity in ternary silicide NaAlSi with layered diamond-like structure

We have investigated the electronic structure, phonon modes and electron–phonon coupling to understand superconductivity in the ternary silicide NaAlSi with a layered diamond-like structure. Our electronic results, using the density functional theory within a generalized gradient approximation, indicate that the density of states at the Fermi level is mainly governed by Si p states. The largest contributions to the electron–phonon coupling parameter involve Si-related vibrations both in the x–y plane as well as along the z-axis in the x–z plane. Our results indicate that this material is an s-p electron superconductor with a medium level electron–phonon coupling parameter of 0.68. Using the Allen–Dynes modification of the McMillan formula we obtain the superconducting critical temperature of 6.98 K, in excellent agreement with experimentally determined value of 7 K.     

A nanometric electron multiplexer

A simple nanostructure resonantly coupling two semiconductor quantum wires is proposed as an electron multiplexer, which selectively transfers electrons of a given energy E₀ from one wire to the other, while letting all the neighbouring states within a certain energy range around E₀ propagate virtually unaffected along the input wire. Closed-form expressions are derived for characteristic wire lengths within the coupling structure facilitating such a directional transfer, which enables one to determine optimal parameters for the device fabrication.     

Quantum Transition of Two-Level System in a Parabolic Quantum Dot

The time evolution of the quantum mechanical state of an electron is calculated by using variational method of Pekar type on the condition of electric-LO phonon strong coupling in a parabolic quantum dot. We obtained the eigen energies of the ground state and the first-excited state, the eigen functions of the ground state and the first-excited state this system in a quantum dot may be employed as a two-level quantum system-qubit. The supposition electron is in system’s ground state in the initial time, the electron transit from the ground state to the excited state in presence of an electric field F along the x axis. The results indicate that the electron transition probability and the oscillation period increase with decreasing the electron-LO-phonon coupling constant, increasing the electric field and the confinement length.     

Solutions of Eliashberg equations for an electron-phonon coupling with a cutoff

In this paper we discuss the Eliashberg equations for the case of an electron-phonon coupling with an energy cutoff. This cutoff is imposed either for the energy difference by means of a strip function, or for both energies, with a Cooper-like expression. The strip function cutoff requires explicit calculation of not only the frequency renormalization functionZ but also the energy renormalizationX. The physical origin of such cutoffs might lie in the very strong electron-electron interaction which seems typical for highT _c superconductivity. If such cutoffs are admitted, the hypothesis thatT _c is caused at least in part by a strong electron-phonon interaction can be reconsidered. We find that the combination of strong coupling and low-energy cutoff could produce highT _c with only small isotope effect and with little damping or pulling of the phonon modes. Correlation with other physical properties, such as specific heat, is reexamined in view to estimate the coupling constant . Some objections to the model using strong electron phonon interaction are removed and better agreement with observed properties is obtained     

Effect of electron-phonon interaction and temperature dependence of resistivity in some heavy fermion systems

Here we have considered the electron-phonon interaction in the Periodic Anderson Model (PAM) to describe the temperature dependence of resistivity in some heavy fermion (HF) systems. Since the resistivity is related to the imaginary part of the electron self energy, the expression of the same is evaluated from electron Green function by the double time temperature dependant Green function technique of the Zubarev type. In order to understand the effect of electron-phonon interaction in these systems, we couple the phonons to both the f-electrons as well as to the electrons of the hybridization band of both conduction and f-electrons. The influence of various model parameters, namely, the position of 4f level E ₀, the electron-phonon coupling strengths f ₁(q) and f ₂(q), the effecting coupling strength g = N(0)γ ₀²/ω ₀ have been studied on the temperature dependence of resistivity in HF systems. The numerical analysis is performed for q = 0 and for finite temperature in the static limit. The analysis of the results gives satisfactorily in comparison to the experimental observations.     

Numerical analyses of the nonequilibrium electron transport through the Kondo impurity beside the Toulouse point

Nonequilibrium electron transport through the Kondo impurity is investigated numerically for the system with 20 conduction-electron levels. The electron current under finite voltage drop is calculated in terms of the ‘conductance viewed as transmission’ picture proposed by Landauer. Here, we take into account the full transmission processes of both the many-body correlation and the hybridization amplitude up to infinite order. Our results demonstrate, for instance, how the exact solution of the differential conductance by Schiller and Hershfield obtained at the Toulouse point becomes deformed by more realistic interactions. The differential-conductance-peak height is suppressed below e²/h with the width hardly changed through reducing the Kondo coupling from the Toulouse point, whereas it is kept unchanged by further increase of the coupling. We calculated the nonequilibrium local Green function as well. This clarifies the spectral property of the Kondo impurity driven far from equilibrium.     

New polaron effect in magnetooptic phenomena in a quantum well

It is predicted that resonance coupling between two discrete electron energy levels corresponding to different size-quantization quantum numbers and different Landau quantum numbers can occur in a quantum well in a quantizing magnetic field. The resonance coupling is due to the interaction of an electron with LO phonons and results in the formation of polaron states of a new type. It is shown that for a certain value of the magnetic field, which depends on the splitting of the electron size-quantization levels, the absorption peak and the two-phonon resonance Raman scattering peak split into two components, the separation between which is determined by the electron-phonon coupling constant. The resonance coupling between size-quantization levels with the same Landau quantum numbers is also studied. The splitting of the peaks in this case is virtually independent of the magnetic field and can be observed in much weaker fields. The experimental observation of the effect will make it possible to determine the relative position of the electronic levels and the electron-phonon coupling constant. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 511–515 (10 April 1997)     

Kondo effect and mesoscopic fluctuations

Two important themes in nanoscale physics in the last two decades are correlations between electrons and mesoscopic fluctuations. Here we review our recent work on the intersection of these two themes. The setting is the Kondo effect, a paradigmatic example of correlated electron physics, in a nanoscale system with mesoscopic fluctuations; in particular, we consider a small quantum dot coupled to a finite reservoir (which itself may be a large quantum dot). We discuss three aspects of this problem. First, in the high-temperature regime, we argue that a Kondo temperature T _k which takes into account the mesoscopic fluctuations is a relevant concept: for instance, physical properties are universal functions of T/T _k. Secondly, when the temperature is much less than the mean level spacing due to confinement, we characterize a natural cross-over from weak to strong coupling. This strong coupling regime is itself characterized by well-defined single-particle levels, as one can see from a Nozières Fermi-liquid theory argument. Finally, using a mean-field technique, we connect the mesoscopic fluctuations of the quasiparticles in the weak coupling regime to those at strong coupling.     

Electronic double refraction due to the Rashba effect: Analytical and numerical results

By analogy with the classic effect of the double refraction of light, we investigate the relevant effect of an electron entering from the Non-Rashba region to the Rashba region in two-dimensional systems. It is shown that the effect of electronic double refraction is determined by a combined parameter γ = m ^* λ _F α/2πħ², rather than both the Rashba coefficient α and wavelength λ _F of a Fermi electron, separately. For the case of normal incidence, the analytical expressions for the wavefunction of the electron are presented; it is predicted that the Rashba spin-orbit coupling can induce a current perpendicular to the normal incident direction of the electron. Moreover, the general case of incident electron with any given momentum and spin state are studied numerically in detail, including the abrupt changes of spin direction and the two-step characters for reflection.      

Electron spin resonance spectrum for a radical in γ-irradiated N-iodosuccinimide

A radical exhibiting a very large hyperfine coupling to ¹²⁷I, together with a small coupling to ¹⁴N, formed in γ-irradiated N-iodosuccinimide at 77 K is identified as a σ* radical anion in which the added electron is accommodated primarily in the N-I antibonding σ-orbital.     

The Wigner semi-circle law in quantum electro dynamics

In the present paper, the basic ideas of thestochastic limit of quantum theory are applied to quantum electro-dynamics. This naturally leads to the study of a new type of quantum stochastic calculus on aHilbert module. Our main result is that in the weak coupling limit of a system composed of a free particle (electron, atom,...) interacting, via the minimal coupling, with the quantum electromagnetic field, a new type of quantum noise arises, living on a Hilbert module rather than a Hilbert space. Moreover we prove that the vacuum distribution of the limiting field operator is not Gaussian, as usual, but a nonlinear deformation of the Wigner semi-circle law. A third new object arising from the present theory, is the so-calledinteracting Fock space. A kind of Fock space in which then quanta, in then-particle space, are not independent, but interact. The origin of all these new features is that we do not introduce the dipole approximation, but we keep the exponential response term, coupling the electron to the quantum electromagnetic field. This produces a nonlinear interaction among all the modes of the limit master field (quantum noise) whose explicit expression, that we find, can be considered as a nonlinear generalization of theFermi golden rule.     

Damping of plasmons in electron liquid

An expression for the width of the plasmon excitations in electron liquid has been obtained using the mode coupling approximation. It is shown that the exact weak coupling (r_s → 0) results of the plasmon width in the long wavelength limit is obtained as a very special case in this approach.     

Subband states inn-inversion layers on small-gap semiconductors

Subband states inn-inversion layers on small-gap semiconductors are subject to the coupling between valence and conduction band (nonparabolicity effects). In order to account for this coupling we study different models: two of them are based on Kane's 6×6 and 8×8 bulk-Hamiltonians, the third one takes into account higher order terms of the electron momentum in a 2×2 conduction band Hamiltonian. We perform selfconsistent calculations for these models with parameters characteristic for InSb and HgCdTe and electron concentrationsN _S , for which up to two subbands are occupied. The calculated subband separations and Fermi energies are independent of the models only if the same energy band dispersion is used and depend strongly on the applied boundary conditions.Work supported in part by the Deutsche Forschungsgemeinschaft     

Electron resonance studies of the renner effect

The gas phase electron resonance spectra of NCO in its ground ²Π_3/2 vibronic state and in two excited vibronic states are described. Theoretical analysis of the spectra yields effective g values for the three states. In additon the ¹⁴N magnetic hyperfine and electric quadrupole coupling constants and the electric dipole moment are determined. The theory of the Renner coupling of electronic and vibrational motion is extended, and shown to account for anomalous contributions to the g values. The theory also shows that these contributions are closely related to the Renner coupling constant.     

Proton impact charge exchange and excitation cross sections for liquid Sn divertor studies

The quantum dynamics of charge exchange (electron capture) and excitation processes in proton collisions with Sn atoms is studied for the first time by using the two-centre atomic orbital close coupling method. The expansion basis includes the lowest nine states of singlet and triplet symmetry of Sn and all the states with n ≤ 8 of the H atom. The spin-resolved state-selective charge exchange (up to n = 5) and excitation cross sections are calculated in the energy range of 0.5 − 100 keV . Strong coupling between the electron capture and excitation channels is observed in the low-energy (E ≤ 10 keV) region associated with the energy quasi-resonance of some projectile and target states. The intensities of some strong Sn spectral lines are also calculated for a number of typical tokamak edge plasma temperatures.     

极性半导体的表面电子

有不少的极性半导休,电子与表面声学声子和体纵光学声子的耦合弱,但与表面光学声手的耦台强.本文同时考虑体纵光学声子、表面光学声子以及表面声学声予的影响,研究这类半导体的表面电予的性质,采用线性组合算符和拉格朗日乘子法,导出其有效哈密顿量和重正化质量。