1,720,990 research outputs found
Polaronic Effects on Exciton States with Different Angular Momenta
No full textIt is shown that exciton states with various total angular momenta (L = 0, 1, 2, 3) are differently affected by the interaction with LO phonons in polar materials. In particular, a dependence is found of the binding energy on the value of L not expected on the basis of the statically screened electron-hole attraction. Calculations for CuCl and other ionic compounds show that the lowest L = 1 state (n = 2) is less bound than the corresponding exciton state with static screening. This effect, which is opposite to the polaronic effect on the ground state, is still present for L ≧ 2 but it decreases with increasing L. The computed binding energies of 2p excitons compare favorably with the available experimental data. The method is also applied to the calculation of bipolaron binding energies
On the boson-fermion model of superconductivity
No full textThe existence of pair excitations in a Fermi gas interacting via a short-range attractive potential is investigated. Within the ladder approximation to the Bethe-Salpeter equation for the effective two-particle interaction, evidence of pair excitation is found at energies slightly larger than the chemical potential. The link between those excitations and a boson-fermion model of superconductivity (Phys. Lett. A, 196 (1995) 359) is discussed. In particular it is shown that the charge carrier density dependence of the pair excitation (boson) energy, assumed phenomenologically in the boson-fermion model, is consistent with the properties of the interacting Fermi gas studied. These results give support to the microscopic origin of the phenomenological boson-fermion model of superconductivity
PHONON-PLASMON COOPERATIVE EFFECTS IN THE DILUTE LARGE-BIPOLARON GAS - A POSSIBLE MECHANISM FOR HIGH-TC SUPERCONDUCTIVITY
No full textA Hamiltonian for two interacting electrons coupled with longitudinal phonons and plasmons is wt up. The phonons are treated in the Frohlich scheme and the plasmons in the single-pole approximation. We are interested in systems with low electron density (n less-than-or-equal-to 10(20)-10(21)) such as high-T(c) superconductors. It is shown that, in this density range, the electron dynamics do not simply screen the electrostatic interaction but, also, cooperates to the bipolaron formation. Furthermore we find that the binding energies and the effective mass depend on the electronic density. The features of the effective electron-electron potential are discussed, mainly for what concerns the self-energy terms and the long-range tail; the former depends on the pair state and the second shows that the plasmon field tends to screen all the electrostatic interactions, even the electron-phonon one. It is also shown that the Hamiltonian formulation is equivalent to a dielectric formulation where the total dielectric function of the system is the sum of the dielectric function appropriate for an ionic of the system is the sum of the dielectric function appropriate for an ionic material and that appropriate for the electron gas. Within this model we calculate the Bose-condensation critical temperature of a system of correlated pairs and free carriers in a two-fluid model at thermodynamical equilibrium
POLARON AND BIPOLARON COEXISTENCE IN HIGH T-C SUPERCONDUCTIVITY
No full textA model for high T-c superconductivity in which polarons and bipolarons coexist is studied. The bipolarons are assumed to be in a bound or resonant state depending on the total charge carrier density and are allowed to decay into a pair of independent polarons. The critical temperature as a function of the total charge carrier density and of the ratio of the superconducting density to the superconducting charge carrier mass is calculated and compared with the experimental data. It is shown that the dependence on the total charge carrier density of the bipolaron resonant energy and of the bipolaron-polaron decay probability is important in order to obtain a satisfactory agreement with the experimental data
Conduction-band anisotropy effects in spherical semiconductor nanocrystals: a theoretical study
No full textThe quantum confinement for a conduction-band electron within a spherical quantum dot is investigated in the framework of the effective-mass approximation. The conduction-band effective-mass anisotropy is fully taken into account. The exact solution for the electronic spectrum is provided, showing how the 'isotropic' mass levels map into the new spectrum which accounts for the anisotropy. The removal of degeneracies typical of the spherical dot, as well as its effects on the infrared transition energies, are discussed
Introduction to solid state physics and crystalline nanostructures
No full textThis textbook provides conceptual, procedural, and factual knowledge on solid state and nanostructure physics. It is designed to acquaint readers with key concepts and their connections, to stimulate intuition and curiosity, and to enable the acquisition of competences in general strategies and specific procedures for problem solving and their use in specific applications. To these ends, a multidisciplinary approach is adopted, integrating physics, chemistry, and engineering and reflecting how these disciplines are converging towards common tools and languages in the field. Each chapter discusses essential ideas before the introduction of formalisms and the stepwise addition of complications. Questions on everyday manifestations of the concepts are included, with reasoned linking of ideas from different chapters and sections and further detail in the appendices. The final section of each chapter describes experimental methods and strategies that can be used to probe the phenomena under discussion. Solid state and nanostructure physics is constantly growing as a field of study where the fascinating quantum world emerges and otherwise imaginary things can become real, engineered with increasing creativity and control: from tinier and faster technologies realizing quantum information concepts, to understanding of the fundamental laws of Physics. Elements of Solid State Physics and of Crystalline Nanostructures will offer the reader an enjoyable insight into the complex concepts of solid state physics
- …
