Showing papers by "Mark Hillery published in 1989"

Formation of a large singlet bipolaron: Application to high-temperature bipolaronic superconductivity

Abstract: We variationally determine the conditions for the formation of large singlet bipolarons within the adiabatic approximation We find that, in two- and three-dimensional electronic systems with only a short-range electron-lattice interaction, the only bipolarons are small bipolarons Furthermore, with only the long-range (Fr\"ohlich) electron-lattice interaction characteristic of an ionic solid, we find the classical result that bipolarons will not form However, with the presence of both short- and long-range components of the electron-lattice interaction, we find a novel domain within which large bipolarons can be formed In particular, an exceptionally large ratio of the static to high-frequency dielectric constants, ${\ensuremath{\epsilon}}_{0}$ and ${\ensuremath{\epsilon}}_{\ensuremath{\infty}}$, respectively, is critical to the formation of large singlet bipolarons The remaining conditions for the formation of large singlet bipolarons are much less stringent for electronic systems of two dimensions than for those of three dimensions Therefore, with the high-temperature superconductors having ${\ensuremath{\epsilon}}_{0}$\ensuremath{\gg}${\ensuremath{\epsilon}}_{\ensuremath{\infty}}$, the notion that their charge carriers are singlet large bipolarons is a real possibility Estimating the transition temperature for bipolaronic superconductivity as the temperature of the Bose-Einstein condensation, transition temperatures of the order of those found in the high-temperature superconductors are reasonably obtained Furthermore, if, as envisioned in the ${\mathrm{CuO}}_{2}$-based materials, the mass of the large bipolaron is dominated by the short-range interaction of the carrier occupying oxygen sites with the surrounding (relatively heavy) cations, there is only a slight dependence of the transition temperature on isotopic substitutions for the solid's oxygen atoms Finally, the transition temperature increases linearly with the thickness of the disk-shaped bipolaron (the number of contiguous ${\mathrm{CuO}}_{2}$ sheets) until a limiting value is achieved when the bipolaron's shape approaches three dimensionality These findings are consistent with the general features of the high-temperature superconducting materials

Total noise and nonclassical states

Abstract: The total noise of a field state is a measure of the fluctuations of the field amplitude. It is a minimum for coherent states. As the behavior of a state becomes more nonclassical, its total noise increases. This is shown first for several specific types of nonclassical states, among them squeezed and sub-Poissonian states. These results are generalized by using nonclassical distance to measure how nonclassical a field state is. A lower bound for the total noise is derived that is an increasing function of nonclassical distance. From it one can conclude that highly nonclassical states have large amplitude fluctuations.

Role of pumping statistics in maser and laser dynamics: density-matrix approach

Abstract: We discuss in detail the influence that the statistical properties of the pump source have on maser and laser dynamics. We derive a general master equation for the radiation field that is valid for a wide range of different pump mechanisms. If the pump noise is eliminated, we find that the photon number noise in a micromaser and a laser can be significantly reduced below the shot-noise level. In contrast, the phase fluctuations for both maser and laser are unaffected by the noise contribution of the pump.

Squeezing and photon number in the Jaynes-Cummings model.

Abstract: It was recently shown that large amounts of squeezing can be obtained in the Jaynes-Cummings model by choosing the initial photon number to be large enough. A general property of squeezed states can be used to derive the complementary result that for small initial photon numbers the squeezing that is produced will be relatively weak for all times. It also illustrates the fact that for some quantum effects to be substantial, large photon numbers are required.

Time-averaged properties of the Jaynes-Cummings model.

Abstract: Examen du modele de Jaynes Cummings avec le champ initialement dans un etat coherent. On obtient en particulier de nouvelles conditions sous lesquelles le champ est fortement compresse, ainsi que des informations sur le comportement a long terme de l'inversion atomique