Cooperative effects on Optical Forces- Dicke's Bullet

We investigate the cooperative effects on optical forces in a system of N two level atoms occupying a volume of dimensions to within $\lambda ^3$, where lambda is radiation wavelength and is driven by a coherent radiation field with a spatial profile like Laguerre-Gaussian beam or ideal Bessel beam.We show a dramatic enhancement on optical forces as well as the angular momentum imparted to the atom by a factor of $N ^2$.Comment: 10 pages + 1 figure (submitted to PRL

Spin-Charge Separation in Two Dimensions - A Numerical Study

The question of spin-charge separation in two-dimensional lattices has been addressed by numerical simulations of the motion of one hole in a half-filled band. The calculations have been performed on finite clusters with Hubbard and t-J models. By comparing the time evolution of spin and charge polarisation currents in one and two dimensions, evidence in favor of spin-charge separation in two dimensions is presented. In contrast with this, spin-charge separation is absent in a highly doped, metallic, system.Comment: RevTeX 3.0, 10 Pages, 6 PostScript Figures (on request

Effect of screening on exchange effects for an inhomogeneous electron gas at finite temperatures

Recently, Gupta and Rajagopal [Phys. Rev. A 21, 2064 (1980)] have calculated thermodynamic-exchange energy and potential in an "exchange-only" approximation and later calculated the correlation potential exactly within the random-phase approximation [Phys. Rev. A 22, 2792 (1980)]. In this paper we estimate the effect of screening in an approximate way. The formula obtained for the correlation potential is shown to be the product of two functions, one being that of density and the other being that of dimensionless temperature. The calculated values agree reasonably in the intermediate degeneracy region and are good at higher temperatures and densities. It is shown that the specific heat for the system can also be expressed in terms of the same parameters and the (T/lnT)-type divergence disappears

Specific heat of a two-dimensional electron gas in the exchange approximation

The specific heat of a two-dimensional inhomogeneous electron gas has been calculated using Bardeen's formula in the exchange approximation. High- and low-temperature limits are obtained. The specific heat vanishes inversely with temperature in the high-temperature limit and the expected T in T behaviour is obtained in the low-temperature limit. A universal curve is obtained which gives the specific heat in the exchange approximation for any density and temperature

Energy states of a hydrogenic atom placed between two metal slabs

We have derived expressions for the self-energy of a hydrogenic atom placed in the cavity formed by two semi-infinite metal slabs. The excitations of the metal surfaces are described in terms of the surface plasmon modes and the atom is assumed to interact with these modes. The self-energies are obtained for the 10S and 14S excited states of the atom. The earlier calculations of the self-energy are based on the dipolar approximation but we show in this paper that when the gap size is sufficiently small, the multipolar interactions can exceed the dipolar contribution. We consider the case when the atom is placed at the center of the gap and obtain the variations in the self-energy when the gap size is varied

Exchange and correlation potential for a two-dimensional electron gas at finite temperatures

Calculations for an inhomogeneous electron gas using density-functional theory require suitable exchange and correlation potentials. We present here the local exchange and correlation potential (Vxc) for the two-dimensional electron gas over a wide range of temperatures and densities based upon summation of ring graphs. A suitable parametric fit for Vxc is presented which is good to within 2% of the exact values. Various limits and graphs of the polarizability are presented

Compton profiles for neon and argon from Xα wavefunctions

The dependence of Compton profiles calculated from X alpha wavefunctions on the choice of exchange parameter alpha is discussed in the light of the non-locality correction of Lam and Platzman (1974) and Tong and Lam (1978). The Compton profiles for neon and argon, using the alpha values suggested by Gopinathan (1977), are reported. The Compton profile for neon with this alpha value agrees very well with the Hartree-Fock profile if the non-locality corrections is included. The agreement for the case of argon is not so good. The possible origins of the discrepancies for argon are discussed

Cohesive, electronic and magnetic properties of the transition metal aluminides FeAl CoAl and NiAl

Electronic structure calculations using the tight-binding linear muffin tin orbital (TB-LMTO) method have been performed for three transition metal aluminides, viz. FeAl, CoAl and NiAl. The band structures and density of states (DOS), valence electron charge density contours and Fermi surfaces have been obtained and compared with the available experimental results as well as with existing theoretical calculations. The lattice constants, cohesive energies and heat of formation at equilibrium lattice constants and bulk moduli agree with the experimental values. The calculations show varying degrees of charge transfer from Al site to the transition metal (TM) sites as one goes from FeAl to CoAl to NiAl. The magnetism of pure elements Fe, Co, Ni is mostly quenched in the stoichiometric phases, with only FeAl retaining a magnetic moment of about 0.7 mu B/atom within the framework of the LMTO

Wavefunction renormalization of one hole in a strongly correlated system

The wavefunction renormalization factor Z<SUB>k</SUB> of a hole injected in a half-filled band in a strongly correlated system in two dimensions is evaluated analytically using the extended quasiparticle operator introduced by Dagotto and Schrieffer. A model Hamiltonian for such an extended hole is proposed. We analytically calculate Z<SUB>k</SUB> for different parameter values and observe that the Z<SUB>k</SUB> value increases as compared with what it would be if a bare hole operator is used. This is in conformity with the numerical observation of Dagotto and Schrieffer