Phonon dispersion curves of transition metals using modified general tensor force model

The general tensor force model has been modified by incorporating separately the electron-ion interactions. The model satisfies the translational symmetry requirements of the lattice and is used to obtain the phonon dispersion curves of chromium, molybdenum and tungsten. The agreement between the theoretical and experimental frequencies is very good.     

Phonon dispersion curves of VI B group metals

The phonon dispersion curves of chromium, molybdenum and tungsten have been computed along the principal symmetry directions using an angular force model with volume forces. The results obtained are in very good agreement with the experimental observations.The author is grateful to Dr. V. Ramamurthy of I.I.T. Delhi for many valuable and stimulating discussions.     

Dynamical correlations in the one-dimensional electron gas with short-range interactions

We study the dynamical correlation effects in a one-dimensional Fermion gas with repulsive delta-function interaction within the quantum version of the self-consistent field approximation of Singwi, Tosi, Land, and Sj?lander [Phys. Rev. 176, 589 (1968)]. The dynamic correlation effects are described by a frequency dependent local-field correction . There is a corresponding local-field factor for the spin-density correlations. We investigate the structure factors, spin-dependent pair-correlation functions, the frequency dependences of and , and the plasmon dispersion relation within this formalism. We compare our results with other theoretical approaches, in particular the static version of the self-consistent field approximation to highlight the importance of dynamical correlations. Received 11 December 1998 and Received in final form 25 April 1999     

Correlation effects in the one-dimensional Bose gas

We discuss interaction effects for the one-dimensional Bose gas with a repulsive delta-function interaction potential. We use the random-phase approximation and a finite local-field correction. Analytical results are given for the local-field correction, the pair-correlation function and the ground-state energy. The groundstate energy is found to be in much better agreement with the exact result than the ground-state energy calculated within the Bogoliubov approximation, where local-field corrections are neglected.     

Spin correlations in electron liquids: Analytical results for the local-field correction in two and three dimensions

We study the spin correlations in two- and three-dimensional electron liquids within the sum-rule version of the self-consistent field approach of Singwi, Tosi, Land, and Sj?lander. Analytic expressions for the spin-antisymmetric static structure factor and the corresponding local-field correction are obtained with density dependent coefficients. We calculate the spin-dependent pair-correlation functions, paramagnon dispersion, and static spin-response function within the present model, and discuss the spin-density wave instabilities in double-layer electron systems. Received: 22 September 1997 / Revised: 1 December 1997 / Accepted: 4 December 1997     

Instability of layered quantum liquids: 5. Collective modes at low density

We calculate the plasmon dispersion of electron superlattices by taking into account many-body effects via the local-field correction. For small electron density we find a weak roton structure in the dispersion of optical plasmons. Landau damping is strongly enhanced by many-body effects, especially for acoustical plasmons. We compare with experimental results of high-T _c superconductors. Some experiments are suggested.     

Theory of surface force-constant changes in body-centered cubic lattices

A calculation has been made of force-constant changes at the (100) and (110) surfaces of body-centered cubic crystals. A model consisting of first, second and third-neighbor Lennard-Jones interactions together with harmonic angle-bending interactions is employed. The parameters characterizing the interactions are chosen to fit the equilibrium lattice spacing, the elastic constants, and certain phonon frequencies at high symmetry points in the Brillouin zone. The procedure consists of first calculating the static displacements produced by the creation of the surface and then calculating the change in force constants produced by the cubic and quartic anharmonic terms in the potential energy. Specific results have been obtained for chromium, iron, tungsten, and molybdenum.     

First-principles study on field evaporation of surface atoms from W(0 1 1) and Mo(0 1 1) surfaces

The simulations of field-evaporation processes for surface atoms on W(0 1 1) and Mo(0 1 1) surfaces are implemented using first-principles calculations based on the real-space finite-difference method. The threshold values of the external electric field for evaporation of the surface atoms, which are ∼6 V/Å for tungsten and ∼5 V/Å for molybdenum, are in agreement with the experimental results. While the threshold value of the electric field and the local-field enhancement around the evaporating atoms agree with those expected from the conclusion of the previous study using structureless jellium, the induced charge around the surface atom has a significant difference from that obtained by the jellium model.     

Local-field enhancement of spontaneous decay in nanosystems: some estimations for dielectric particles

Enhancement of the spontaneous decay rate due to local-field effects is estimated for a luminous dipole inside a small dielectric particle. To do this, we make use of an idea on multiplicativity of the local-field factor. Elongated particles are shown to be promising in respect of the local-field enhancement of the spontaneous emission. Computations are carried out within the framework of modern theory of the spontaneous decay of excited atoms in absorbing media for both the virtual-cavity and real-cavity models. Particular emphasis is placed on dielectric/semiconductor particles in the vicinity of the optical phonon frequencies. PACS 32.80.-t; 33.50.-j; 78.55.-m; 78.67.-nThis revised version was published in October 2004 with a new Figure 5.     

强耦合简并电子气中离子阻止本领和能量离散的数值计算

利用局域场修正的介电函数,研究了注入离子在强耦合简并电子气中的阻止本领和能量离散。数值结果表明在低速和高r_s值情况下,局域场修正使得阻止本领和能量离散明显地增加。     

A method to calculate the thermal conductivity of HMX under high pressure

A method based on Debye theory is developed to calculate the thermal conductivity of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The phonon–phonon interaction model is built up for solid HMX. The phonon lifetime formula is derived by the phonon–phonon scattering mechanism, and the thermal conductivity tensor is derived by the phonon dispersion model. The thermal conductivities of α/β/δ-HMX are calculated in the temperature range 0–700?K and pressure range of 0–10?GPa. The phonon softening process of HMX is investigated. We have proven that the Debye frequency and thermal conductivity tend to 0 at the phonon softening point. A physical picture of the phonon–phonon interaction, phonon lifetime and phonon softening is built up.     

Time-Dependent Variational Approach to Ground-State Phase Transition and Phonon Dispersion Relation of the Quantum Double-Well Model

The ground-state phase transition and the phonon dispersion relation of the quantum double-well model are studied by means of the time-dependent variational approach combined with a Hartree-type many-body trial wavefunction. The single-particle state is taken to be a frozen Jackiw-Kerman wavefunction. Under the condition of minimum uncertainty relation, we obtain an effective classical Hamiltonian for the system and equations of motion for the particle＇s expectation values. It is shown that the effective substrate potential transits from a symmetric double-well potential to a symmetric single-well potential, and the ground state exhibits a transition from a broken symmetry phase to a restored symmetry phase as increasing the strength of quantum fluctuations. We also obtain the phonon dispersion relations and the phonon gaps at the two phases.     

Dynamical exchange corrections to the dielectric function in a two-dimensional electron gas

The effect of dynamical exchange interactions on the dielectric function of a two-dimensional electron gas is studied using a variational approach. Exchange effects are introduced via the local-field correction. The variationally obtained local-field factor is compared to the earlier perturbative result to first order in the electron-electron interaction.     

Phonon-induced polariton superlattices

We show that the coherent interaction between microcavity polaritons and externally stimulated acoustic phonons forms a tunable polariton superlattice with a folded energy dispersion determined by the phonon population and wavelength. Under high phonon concentration, the strong confinement of the optical and excitonic polariton components in the phonon potential creates weakly coupled polariton wires with a virtually flat energy dispersion.     

Phonon anomalies due to collective stripe modes in high T(c) cuprates

Phonon anomalies observed in various high T(c) cuprates by neutron experiments are analyzed theoretically in terms of the stripe concept. The phonon self-energy correction is evaluated by taking into account the charge collective modes of stripes, giving rise to dispersion gap, or kink and shadow phonon modes at twice the wave number of spin stripe. These features coincide precisely with observations. The gapped branches of the phonon are found to be in-phase and out-of-phase oscillations relative to the charge collective mode.     

Dynamical exchange corrections to the dielectric function in a two-dimensional electron gas

The effect of dynamical exchange interactions on the dielectric function of a two-dimensional electron gas is studied using a variational approach. Exchange effects are introduced via the local-field correction. The variationally obtained local-field factor is compared to the earlier perturbative result to first order in the electron-electron interaction.     

Time-Dependent Variational Approach to the Phonon Dispersion Relation of the Commensurate Quantum Frenkel-Kontorova Model

The phonon dispersion relation of the commensurate quantum Frenkel-Kontorova model is studied by means of the time-dependent variational approach combined with a Hartree-type many-body trial wavefunction for the particles. The single-particle state is taken to be a frozen Jackiw-Kerman wavefunction. Under the condition of minimum uncertainty, equations of motion for the particle expectation values are derived to obtain the phonon dispersion relation. It is shown that the strength of the substrate potential and the phonon excitation gap are reduced due to the quantum fluctuations in comparison with those of the classical model. We also compare our results with those previously obtained by using the path-integral molecular dynamics.     

Three‐Phonon and Four‐Phonon Interaction Processes in a Pair‐Condensed Fermi Gas

We study the interactions among phonons and the phonon lifetime in a pair‐condensed Fermi gas in the BEC‐BCS crossover in the collisionless regime. To compute the phonon‐phonon coupling amplitudes we use a microscopic model based on a generalized BCS Ansatz including moving pairs, which allows for a systematic expansion around the mean field BCS approximation of the ground state. We show that the quantum hydrodynamic expression of the amplitudes obtained by Landau and Khalatnikov apply only on the energy shell, that is for resonant processes that conserve energy. The microscopic model yields the same excitation spectrum as the Random Phase Approximation, with a linear (phononic) start and a concavity at low wave number that changes from upwards to downwards in the BEC‐BCS crossover. When the concavity of the dispersion relation is upwards at low wave number, the leading damping mechanism at low temperature is the Beliaev‐Landau process 2 phonons ? 1 phonon while, when the concavity is downwards, it is the Landau‐Khalatnikov process 2 phonons ? 2 phonons. In both cases, by rescaling the wave vectors to absorb the dependence on the interaction strength, we obtain a universal formula for the damping rate. This universal formula corrects and extends the original analytic results of Landau and Khalatnikov [ZhETF 19 , 637 (1949)] for the 2?2 processes in the downward concavity case. In the upward concavity case, for the Beliaev 1? 2 process for the unitary gas at zero temperature, we calculate the damping rate of an excitation with wave number q including the first correction proportional to q ⁷ to the q ⁵ hydrodynamic prediction, which was never done before in a systematic way.     

Local-field correction of the charged Bose condensate for two and three dimensions

The local-field correction for the dielectric function of the two-dimensional and of the three-dimensional Bose condensate is calculated within a sum-rule version of the Singwi et al. (Phys. Rev.176, 589 (1968)) approach. We derive analytical expressions for small and large wave numbers and give analytical expressions for the density dependence. We compare the results of the groundstate energy for the three-dimensional system with Monte-Carlo computations. In two dimensions a roton structure in the plasmon dispersion is found at low boson density. The plasmon density of states is calculated. A correlation induced charge-density-wave instability in layered structures of two-dimensional Bose gases is discussed.