International Journal of Modern Physics B

We study the exact solution for a two-mode model describing coherent coupling between atomic and molecular Bose–Einstein condensates (BEC), in the context of the Bethe ansatz. By combining an asymptotic and numerical analysis, we identify the scaling behaviour of the model and determine the zero temperature expectation value for the coherence and average atomic occupation. The threshold coupling for production of the molecular BEC is identified as the point at which the energy gap is minimum. Our numerical results indicate a parity effect for the energy gap between ground and first excited state depending on whether the total atomic number is odd or even. The numerical calculations for the quantum dynamics reveals a smooth transition from the atomic to the molecular BEC.

We discuss several examples of three-dimensional critical phenomena that can be described by Landau–Ginzburg–Wilson ϕ⁴ theories. We present an overview of field-theoretical results obtained from the analysis of high-order perturbative series in the frameworks of the ∊ and of the fixed-dimension d=3 expansions. In particular, we discuss the stability of the O(N)-symmetric fixed point in a generic N-component theory, the critical behaviors of randomly dilute Ising-like systems and frustrated spin systems with noncollinear order, and the multicritical behavior arising from the competition of two distinct types of ordering with symmetry O(n₁) and O(n₂) respectively.

We present calculations of the electronic and magnetic structure of thin films of Ni, using the recursion technique based on the tight-binding linearized muffin-tin orbitals basis. We find that the surface layers of free standing Ni thin films do carry enhanced magnetic moment.

Various compositions of xCaO·(70-x)P₂O₅·30Na₂O glasses (0≤x≤30% mol%) are investigated in order to estimate glass formation, glass stability and activation energy of crystallization as well as dissolution properties of samples. Transparent vitreous samples were obtained in the entire composition range from 40 to 70 mol% P₂O₅. The glass transition temperatures and crystallization temperatures increased with CaO content. Dissolution properties in distilled water and simulated biological fluids were measured. The dissolution rate increases with the extent of substitution of P₂O₅ by CaO in samples.

Some thermodynamic features of an assembly of a finite number of bosons trapped in a bounded harmonic potential are investigated. P–V isotherms are drawn for both the degenerate and the non-degenerate phases. At any temperature, the pressure is a decreasing function of volume, unlike the free Bose gas for which the pressure becomes independent of volume in the degenerate phase. At the absolute zero of temperature the quantum pressure for a spherical enclosure of radius r₀ equal to a_ho, a_ho being the characteristic harmonic oscillator length, is found to be of order 10^-10 Torr while for it is of order 10^-12 Torr. The isothermal compressibility has a sharp drop near the critical point and becomes negligibly small for temperatures above the critical temperature, irrespective of the size of the trap. The coefficient of thermal expansion also shows a sudden drop at the critical temperature. The specific heat at constant pressure shows a peak and is well-defined in the degenerate phase, in contradistinction to the ideal Bose gas. Results recently derived by Singh on the basis of local equilibrium theory are found to be in good agreement with our numerical computations.

A unified description of elementary and collective excitations in quantum Hall systems is presented within the single-mode approximation (SMA) framework, with emphasis on revealing an intimate link with Chern–Simons theories. It is shown that for a wide class of quantum Hall systems the SMA in general yields, as an effective theory, a variant of the bosonic Chern–Simons theory. For single-layer systems the effective theory agrees with the standard Chern–Simons theory at long wavelengths whereas substantial deviations arise for collective excitations in bilayer systems. It is suggested, in particular, that Hall-drag experiments would be a good place to detect out-of-phase collective excitations inherent to bilayer systems. It is also shown that the intra-Landau-level modes bear a similarity in structure (though not in scale) to the inter-Landau-level modes, and its implications on the composite-fermion and composite-boson theories are discussed.

The basic kinematic behaviour of a threshold switch in a system with a sparse population is investigated. We determine the basic quantities such as the number probability density function, the survival probability, the characteristic switching time, and the response to external triggering of the switch. The modelling approach is then extended to systems with response retardation, which, it is argued, may improve the stability of the switch.

We performed ab initio, local density functional calculations for the electronic structure of MgB₂. The Fermi level of MgB₂ cuts through relatively narrow electron bands which have a dominant contribution from B(2p) states. The above metallic behavior of boron layers can also be seen from the calculated partial density of states. The metallic nature, strong covalent bonds, and the light atomic mass in the boron layers could lead to a strong electron-phonon (e–ph) interaction. The strong e–ph interaction is particularly important for the high energy phonons that are associated with boron layers in MgB₂.

In this paper we study an impact of viscosity on DNA behaviour. We investigate a so-called big viscosity using extended Peyrard-Bishop model to describe nonlinear DNA dynamics.

In the present communication we consider a time dependent ion-field interaction. Here we discuss the interaction between a single trapped ion and two fields taking into account the coupling parameter to be time dependent and allowing for amplitude modulation of the laser field radiating the trapped ion. At exact resonances the analytic solution for the Heisenberg equations of motion is obtained. We examine the effect of the velocity and the acceleration on the Rabi oscillations by studying the second order correlation function. The phenomenon of squeezing for single and two fields cases is considered. The cross correlation between the fields is discussed.

Using a quantum field theoretical canonical generator for the scale transformation of the second quantized Schrödinger fields describing a mixture of Fermion and Boson systems, the equation of state is derived. The derivation is based on the equal-time canonical commutation relations of the field operators and no approximation is employed. The result can be applied to liquid ³He–⁴He mixture.

We investigate the variant regularly diluted situations of in S=1 isotropic antiferromagnetic chains by the quantum Monte Carlo loop cluster algorithm. Our results manifest significant different magnetic properties in the ground state with respect to the odd (even) host S=1 spins in one unit cell with an impurity S=1/2, and the doped system gradually transits to the pure Haldane chain in two different tendencies with the decreasing of the impurity concentrations.

Effects of variational parameters on different dielectric functions in the study of semiconductor–metal transition are investigated for a shallow donor in an isolated well of GaAs/Ga_1-xAl_sAs superlattice system within the effective mass approximation. Vanishing of the donor ionization energy as a function of well width and donor concentration suggests that no transition is possible below a well width of 30 Å supporting the scaling theory of localization. The effects of Anderson localization, exchange and correlation in the Hubbard model are included in a simple way. The relationship between the present model and the Mott criterion in terms of Hubbard model is also brought out. The critical concentration is enhanced when the Hartree–Fock dielectric function is used. Results are compared with the existing data available and discussed in the light of existing literature.

We measured the angular momentum density distribution of YNi₂B₂C to acquire information about its electronic structure. The measurements were performed using the full-scale utility of the two-dimensional angular correlation of annihilation radiation (2D-ACAR). The measured spectra clarified that Ni (3d) like state, predominantly, affected the Fermi surface of YNi₂B₂C. Further, s- and p-like-states enhanced its superconducting properties. The Fermi surface of YNi₂B₂C. was reconstructed using Fourier transformation followed by the LCW (Loucks, Crisp and West) folding procedure. It showed a large and complex surface similar to that of the high temperature superconductors HTS, with anisotropic properties. It also disclosed the effect of d-like state. Nevertheless, the current Fermi surface could deliver the needed topological information to isolate its features. The general layouts of this Fermi surface are; two large electron surfaces running along Γ–Z direction; as well as an additional large electron surface centered on X point; beside one hole surface centered on 100 point. This Fermi surface was interpreted in view of the earlier results.

We show within the framework of Variational Path Integration that the density matrix and the ground state energy of the trapped Bose gas can be obtained in a simple way and it is in agreement with the result obtained by the variational Gross–Pitaevskii equation. The advantage of this method is the analytical result can be found for various forms of interaction between particles.

We present a simple and pedagogical derivation of the spin current as the linear response to an external electric field for both Rashba and Luttinger spin–orbital coupling Hamiltonians. Except for the adiabatic approximation, our derivation is exact to the linear order of the electric field for both models. The spin current is a direct result of the difference in occupation levels between different bands. Moreover, we show a general topological spin current can be defined for a broad class of spin–orbit coupling systems.

The study of the superconducting state parameters (SSP) viz. electron–phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T_C, isotope effect exponent α and effective interaction strength N₀V of In-based binary alloys In_1-xNa_x, In_1-xMg_x, In_1-xZn_x, In_1-xTl_x, In_1-xSn_x, In_1-xPb_x and In_1-xBi_x have been made extensively in the present work using a model potential. To explain electron–ion interaction pseudo-alloy-atom (PAA) model is applied for the first time instead of Vegard's Law. A considerable influence of various exchange and correlation functions on λ and μ* is found from the present study. It is also concluded that T_C strongly depends on the value of the effective valence Z* of binary alloys.

We report on the effects of using various doping 3d, 4d and 5d transition metal elements on the stability of the supersaturated FCC Ag–Cu solid solution prepared by mechanical alloying. We find that the addition of W does not have any influence on the stability of the Ag–Cu solid solution. Moderate effects are observed for Ru, Fe and Co, while the addition of Ni partially destabilizes the Ag–Cu solid solution. The results are discussed in the framework of kinetic and thermodynamic processes.

Changes in the physical properties such as the ground state properties, charge density wave ordering, binding energy and energy bandwidth of polaron and quantum fluctuation, and minimum uncertainty relation of phonons and nonadiabatically coupled electron–phonon systems with spin-1/2 have been investigated by our new state ansatz which can account for correlation among the phononic displacement, squeezing and polaron effects using variational method in one-dimensional Holstein model. The investigation here shows that the squeezing–antisqueezing effect (correlated) results in a decrease of the ground state energy, an increase of the binding energy of polarons, the reduction of the uncertainty and quantum fluctuation of the phonons, a decrease of polaron narrowing of electron bandwidth, an increase of tunneling effect of the polarons and an increase of CDW ordering and phonon staggered ordering when compared with the uncorrelated case. Therefore, this shows that the ground state determined by the new state ansatz is the most stable. The new ansatz which include the squeezing–antisqueezing (correlated) effect is very relevant for the coupled electron–phonon systems, especially in strongly coupled and highly squeezed cases.

The modified-weighted-density-functional approximation (MWDA) proposed by Denton and Ashcroft, is applied to study the equation of sate and freezing of the hard spheres using the generalized mean spherical approximation (GMSA) direct correlation function (DCF). Because of the attractive tail in the DCF, the perturbation method similar to that introduced by Yoon and Kim is applied. The free energy, freezing parameters and the equation of state of the hard sphere FCC crystal are obtained. The results are compared with some other previous theories and Monte Carlo simulation. Our results are in good agreement with the simulation results.

THz emission spectra from a grating-coupled two-dimensional plasmon system driven by DC current are calculated in a self-consistent manner. The emission peaks show strong resonant enhancement due to Smith–Purcell effect.

In the framework of random-phase approximation theory and applying the self-consistent field method, we study the properties of collective charge density excitations in single quantum well in consideration of dielectric mismatch. We analytically give the general solution of the image potential to the one dimensional Green's function. Our numerical results demonstrate that the dielectric mismatch between the barrier and well materials significantly changes the frequency of the intra- and inter-subband plasmon modes in contrast with dielectric match. We reasonably conclude that the image potential affects the intra- and inter-subband plasmon modes in a different way.

The interface-optical (IO) phonon modes and their interaction with electrons in a multi-heterointerfaces structure are studied by using a transfer matrix method within the framework of the dielectric continuum model. The dispersion equation of IO phonon modes and the electron–IO-phonon (e–IO-p) interaction Hamiltonian are derived by considering the pressure effect. This method facilitates systematic calculations for complex structures where the conventional method is very difficult to implement. The numerical computation has been performed for a quantum well GaN/AlN system under pressure. The e–IO-p coupling related to the IO modes with the AlN LO-frequency (phonon energy 112.46 meV) at the long-wavelength limit is strongest in the four branches of IO-phonon modes. The result indicates that the IO-phonon modes and e–IO-p interaction coupling functions increase with pressure obviously.

Negative donor ion on the hetero-interface in magnetic fields are investigated. Using a variational method, we have calculated the binding energies of D^- center for the spin-triplet state of L=-1 in this structure. Moreover, the effect of electron-interface phonon coupling on the energy of a donor located in a polar-crystal hetero-interface is found. Numerical results are produced for heterostructures of some II–VI crystals.