System Of Free Electrons Research Articles

Diameter dependence of ferromagnetic spin moment in Au nanocrystals

Au nanoparticles exhibit ferromagnetic spin polarization and show diameter dependence in magnetization. The magnetic moment per Au atom in the particle attains its maximum value at a diameter of about 3 nm in the magnetization-diameter curve. Because Au metal is a typical diamagnetic material, its ferromagnetic polarization mechanism is thought to be quite different from the ferromagnetism observed in transition metals. The size effect strongly suggests the existence of some spin-correlation effect at the nanoscale. The so-called ``Fermi hole effect'' is the most probable one given in the free-electron gas system. Ferromagnetism in Au nanoparticles is discussed using this model.

Noise power spectrum of a long-channel current line with electron traps: Slave-boson mean field theory

We calculated the noise power spectrum of a one-dimensional free electron system corresponding to a long-channel current line when there are several electron traps near the current line. The calculation was performed starting from the Anderson Hamiltonian in the framework of the slave-boson mean field theory. When there is a single trap, the noise power spectrum shows a peak structure. When there are two traps and the distance between the two traps is less than the Fermi momentum, the peak of the noise power is enhanced because of interference between the two traps. When there are multiple traps, the noise power is analytically found to have an ω−2 dependence in the low-frequency limit and an ω−1/2 dependence in the high-frequency limit. These results are applicable to the noise analysis of nanodevices such as a single-electron transistor (SET) if the traps are regarded as the SET islands.

From phase- to amplitude-fluctuation-driven superconductivity in systems with precursor pairing

The change-over from phase- to amplitude-fluctuation driven superconductivity is examined for a composite system of free electrons (Fermions with concentration n_F) and localized electron-pairs (hard-core Bosons with concentration n_B) as a function of doping-changing n_B. The coupling together of these two subsystems via a charge exchange term induces electron pairing below a certain T^* (showing up in form of a pseudogap) and ultimately superconductivity in the Fermionic subsystem. T^* steadily decreases with decreasing n_B. Below T^* this electron pairing leads to electron-pair resonant states (Cooperons) with quasi-particle features which strongly depend on $n_B$. For high concentrations, (n_B \simeq 0.5), correlation effects between the hard-core Bosons lead to itinerant Cooperons having a heavy mass m_p, but are long-lived. Upon reducing n_B, the mass as well as the lifetime of those Cooperons is considerably reduced. For high values of n_B, a superconducting state sets in at a T_c, being controlled by the phase stiffness D_\phi=\hbar^2 n_p/m_p of those Cooperons, where n_p denotes their density. Upon reducing n_B, the phase stiffness steadily increases, and eventually exceeds the pairing energy k_B T^*. The Cooperons loose their well defined itinerant quasi-particle features and superconductivity gets controlled by amplitude fluctuations. The resulting phase diagram with doping is reminiscent of that of the phase fluctuation scenario for high T_c superconductivity, except that in our scenario the determinant factors are the mass and the lifetime of the Cooperons rather than their density.

RKKY exchange interaction within the parabolic quantum-well

Indirect magnetic exchange in a semimagnetic semiconductor heterostructure with the parabolic quantum-well barrier potential is considered. Within the analytical method, we provide the exact derivation of the spatial dependence of the RKKY exchange integral. Using the effective dimensionality approach, we show that the spectral dimensionality of the free electron (hole) system equals four. We prove, that the RKKY exchange integral shows conventional, sign reversal variation with the 2 k F period, however, the envelope function falls off in a manner characteristic to 4D systems.

Level mixing in free-electron systems for Aharonov-Bohm geometries without disorder

Level mixing in free-electron systems for Aharonov-Bohm geometries without disorder

Co-tunneling Current through a Single Quantum Dot

1253 Coulomb interaction plays an essential role in transport phenomena in small quantum structures. The main difficulty in understanding the transport phenomena through quantum structures arises from the fact that it is necessary to take into account not only the electron transfer or the interaction across substructures, but also the strong correlation in each substructure. We have recently proposed a theoretical framework for dealing with such a situation in an adequate manner. Among many transport phenomena attributable to Coulomb interactions, we discuss the co-tunneling in a quantum dot. In addition, we focus our attention on the effects or the phenomena which are predicted from our theory and are confirmed by experiments. Averin and Nazarov (AN) had found the fourth order of tunneling processes a quantum dot, called cotunneling process, in which a tunneling current is proportional to V 3 when the applied voltage V is much smaller than the offset voltage Voff . The co-tunneling process, which is one of sequential processes, is given by the Feynman diagram shown in Fig. 1 because it describes the simultaneous transfer of two electrons through different junctions, L and R. The standard formula derived by AN for co-tunneling current is reproduced from this figure if ± lines are replaced by the Green’s functions for the free electron system, and the lines r and a by those in the noninteracting system in which the charging effect is taken into account in molecular field approximation. This implies that no correlation is taken into account in AN’s theory. As a result, only the Fermi-Dirac distribution functions are contained in AN’s even though each electron in the dot feels a strong Coulomb repulsion, so that the dramatic reduction in the fluctuation of the number of electrons induced by the Coulomb blockade is not en into account appropriately. In addition, it is known that, in the small voltage region eV EC with the charging energy EC = e/2C, it is difficult to fit the wide range of experimental data by a single term proportional to V . 3) In our analysis, on the other hand, the effects of correlation resulting from a strong Coulomb repulsion are taken into account by replacing the Green’s functions, through

Feedback control of cavity detuning in short-pulse free-electron lasers

Feedback control of cavity detuning length in a short-pulse free-electron laser oscillator system is suggested and numerical simulation results are presented. By changing the detuning length with reference to the position of the optical pulse at the end of every pass, the system operation is very stable, and very high power and comparably good spectral characteristics have been achieved. Feasibility of this scheme is also discussed.

ELECTRONIC TOPOLOGICAL TRANSITIONS IN 2D ELECTRON SYSTEM ON A SQUARE LATTICE AS A MOTOR FOR THE `STRANGE-METAL' BEHAVIOUR IN HIGH- T c CUPRATES

We show that a 2D system of free electrons on a square lattice with hoping between more than nearest neighbours is characterised by two quantum critical points associated with a change in topology of Fermi surface as a function of electron concentration. This simple model (when taking into account a positive interaction in a triplet channel) allows us to consistently explain some crucial experiments in the underdoped regime of hole-doped high- T c cuprates (ARPES, neutron scattering).

New Acceleration Principle of Charged Particles for Electronic Applications. The General Hierarchic Description

A new principle of acceleration of changed particles and quasineutral plasma beams is proposed and theoretically substantiated. The essence of this new acceleration principle is die utilization of EH-undulated fields, which resemble those of EH free-electron laser pumping systems. Here the charged particles are moving in a superposition of crossed magnetic and electric vortex undulated fields (EH-accelerator). The advantage of such systems is that both negative (electrons) and positive (ions) charged particles are accelerated simultaneously in the same longitudinal direction. In addition to the concept of the EH-accelerators, a new theoretical approach (the theory of hierarchic oscillations and waves) is given further development here. This approach has been used as a basis for the general nonlinear theory of the EH-accelerators and some other similar isochronous electronic devices with long-time interactions. In addition, several new calculation methods are presented, including the method for nonlinear current density calculations (called the averaged current-density equation hierarchic method) and two versions of hierarchic asymptotic algorithms for the integration of Maxwell's equations.

Transport Coefficients for Many Electron System – Comparative Study of Mori Formula and Kubo Formula –

By using the closed-form solution of the Mori formula for transport coefficients obtained very recently, the Mori formula and the Kubo formula have been proved to be identical to each other formally. However, once any approximation procedure is introduced, there may appear some discrepancy between these formula. Then, as for typical examples, we calculate the Mori formula and the Kubo formula for the optical conductivity in a free electron system with impurity scattering and for that in a U -infinite d - p model in some approximate ways and show that both formula coincide with each other practically and the memory function approximation which has been often employed as one of the most important applications of the Mori formula is justified only in the high frequency limit. These results suggest that the Mori formula is no more convenient to be used for calculating transport coefficients than the Kubo formula.

Short wavelength free electron lasers in 1996

Feedback control of cavity detuning length in a short-pulse free-electron laser oscillator system is suggested and numerical simulation results are presented. By changing the detuning length with reference to the position of the optical pulse at the end of every pass, the system operation is very stable, and very high power and comparably good spectral characteristics have been achieved. Feasibility of this scheme is also discussed.

Generation of squeezed states on reflection of light from a system of free electrons

The statistical characteristics of an electromagnetic wave reflected from a system of successive free-electron mirrors are investigated. Such a configuration of the electron stream is characteristic in particular for free-electron lasers. An expresson is obtained for the squeeze factor of the field of the reflected wave, and the optimal geometric configuration of the system of electrons is found. Numerical estimates show that the squeezing can be substantial when the number of electronic mirrors is large.

Comments on the Mori Formula for Transport Coefficients

Three decades ago, the quantum Langevin equation (Mori formula) was used to reformulate the linear response theory (Kubo formula). Since then, the Mori formula has often been used to evaluate transport coefficients, but only within the Born approximation. In the present study, we obtain a rigorous expression for the transport coefficients as a series expansion in terms of the projection operator. We then evaluate the optical conductivity for a free electron system with impurity scattering in order to discuss the mutual consistency between the Mori formula and the Kubo formula.

Size effects and plasmon absorption in three-dimensional and two-dimensional metallic systems

The dependence of the plasmon position and plasmon width on the radius of the metallic sphere is examined with the aid of the quantum-mechanical polarizability of a non-interacting free-electron system having a definite size and spherical shape. In a further step, the polarizability of the two-dimensional gas is applied for the calculation of the peak positions of the surface plasmons as a function of the size of the wave-vector component parallel to the metal surface. These data are rather widely compared with the experimental results. For a normal incidence of electrons on the gas plate the dependence of positions of the plasmon maxima on the thickness of the electron-gas layer is also examined.

Optical sum rules for inhomogeneous electron systems.

Optical properties can be determined by either the transverse dielectric function ${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\perp}}}$(q,\ensuremath{\omega}) or the longitudinal dielectric function ${\mathrm{\ensuremath{\epsilon}}}_{\mathcal{n}}$(q,\ensuremath{\omega}) in the long wavelength limit, i.e., q\ensuremath{\rightarrow}0. By expressing ${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\perp}}}$(q\ensuremath{\rightarrow}0,\ensuremath{\omega}) in terms of density response functions, we calculate the third, fifth, and seventh frequency moments of Im${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\perp}}}$(q\ensuremath{\rightarrow}0,\ensuremath{\omega}) in addition to the first frequency moment, the well-known conductivity sum rule. While the third and fifth moments reflect the inhomogeneity of the system, correlation effects contribute explicitly to the seventh moment. In addition, the frequency moments of Im[-1/${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\parallel}}}$(q\ensuremath{\rightarrow}0,\ensuremath{\omega})] can be constructed from these moments. The dielectric functions evaluated in a self-consistent field (SCF) approximation, such as the random phase approximation (RPA) or the adiabatic local density approximation (ALDA), satisfy the first three odd frequency moments provided the single particle states on which the SCF calculation is based are calculated in the corresponding SCF approximation. The seventh frequency moment constitutes a severe requirement that ${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{\ensuremath{\perp}}}$(q\ensuremath{\rightarrow}0,\ensuremath{\omega}) evaluated in the RPA or the ALDA cannot satisfy. This is demonstrated for nearly free electron systems.

Optical properties of polyvalent metals in the solid and liquid state: lead

Recently we proposed a semiphenomenological theory for the calculation of the optical properties of polyvalent metals. In this paper, we apply it to the heavy metal lead and show that, in contrast to the preceding paper, no fitting parameter is basically necessary. The general agreement between experimental results and theoretical curves is satisfactory for all properties in both the solid and the liquid state. The optical properties in the liquid state are more similar to those of the solid state than to those of a free-electron system and therefore cannot be described by a simple Drude model.

Detailed Analysis of de Haas-van Alphen Effect in Free Electron System.

The detailed analysis of de Haas-van Alphen effect, one of magneticoscillation phenomena in metallic materials, is given for the free electron system, and the mechanism for its occurrence is reviewed. This effect may be veryinteresting for applications to the engineering field. However, unfortunately, since the observations are limited to only the liquid Helium temperature for thepresent available external magnetic field, it may be difficult at the present stageto apply the effect to the engineering. In future, if we can make the four timeslarger magnetic field, the effect can easily happen at the room temperature.Good applications of this effect in future are hoped.

Quantum Mechanical Derivation of the Classical Drude-Like Optical Conductivity Tensor of the Free Electron System in an Uniform Magnetic Field.

Quantum Mechanical Derivation of the Classical Drude-Like Optical Conductivity Tensor of the Free Electron System in an Uniform Magnetic Field.

On the edge of tetravalent Ce in CePd7

Data on the magnetic susceptibility, low-temperature specific heat and X-ray absorption spectroscopy (Ce LIII and MIV.V edges) for CePd7 and its reference compound YPd7 are presented. The respective Pauli susceptibilities chi 0 of 0.11*10-3 emu mol-1 and 0.50*10-3 emu mol-1 and the respective Sommerfeld coefficients gamma of 9.8 mJ K-2 mol-1 and 35.8 mJ K-2 mol-1 are much smaller (per atom) than those of the corresponding components (Ce, Y and Pd). The valence extracted from the Ce LIII edge is 3.5. The chi 0/ gamma -ratios are found to be 0.012 emu K-2 J-1 and 0.014 emu K2 J-1 for CePd7 and YPd7, respectively. Such a value for CePd7 corresponds to that of a free-electron system instead of an intermediate-valence system (0.035 emu K2 J-1).

Collective modes in the U=

We investigate with the aid of the 1/N expansion the dispersion relation of the collective mode in the infinite-U Hubbard model in the presence of a long-range Coulomb term. We calculate this numerically in three dimensions and for a parabolic dispersion for the full range of fillings. For moderate fillings the results are consistent with Landau theory and weak-coupling approaches, but for fillings close to one-half the nature of the collective mode is completely changed. In this limit it becomes equivalent to a movement of the electron system as a whole, thereby avoiding the U=\ensuremath{\infty} restriction. This mode never decays into single-particle excitations, and the mass scale on which it appears is that of the free-electron system.