Valence-bond charge-transfer solvation model for nonlinear optical properties of organic molecules in polar solvents

A simple model is developed for predicting solvation effects on the nonlinear optical properties of charge transfer organic materials such as 1,1 dicyano,6-(di-butyl amine) hexatriene. This model is based on the valence-bond charge-transfer (VB-CT) framework, using a continuum description of the solvent. The resulting VB-CT solvation model leads to analytic formulas for the absorption frequency (Eg), the polarizability (α), the hyperpolarizabilities (β,γ,δ), and the bond length alternation with only one solvent dependent parameter (ε, the dielectric constant of the solution). The theory involves just four solvent-independent parameters, V0, t, SF, and Q which are related to the band gap, bandwidth, geometry, and dipole moment of the CT molecule [plus a length (R DA) and force constant (k) derivable from standard force fields]. The results are in good agreement with experiment. © 1994 American Institute of Physics.published_or_final_versio

The valence-bond charge-transfer-exciton model for predicting nonlinear optical properties (hyperpolarizabililies and saturation length) of polymeric materials

A simple theory is developed and applied to the polarizability (α), second hyperpolarizability (γ), and the saturation lengths for nine polymeric materials. The theory is based on a valence bond view of the ground and excited states and considers the excited states as charge transfer excitons. It involves just two parameters which can be extracted from simple molecular orbital calculations and/or from experimental values of band gap and bandwidth. For the one system (oligothiophenes) with experimental data on saturation length, the predicted behavior of α and γ with polymer length is in good agreement with experiment. © 1994 American Institute of Physics.published_or_final_versio

The valence-bond charge-transfer-exciton model for predicting nonlinear optical properties (hyperpolarizabililies and saturation length) of polymeric materials

A simple theory is developed and applied to the polarizability (α), second hyperpolarizability (γ), and the saturation lengths for nine polymeric materials. The theory is based on a valence bond view of the ground and excited states and considers the excited states as charge transfer excitons. It involves just two parameters which can be extracted from simple molecular orbital calculations and/or from experimental values of band gap and bandwidth. For the one system (oligothiophenes) with experimental data on saturation length, the predicted behavior of α and γ with polymer length is in good agreement with experiment. © 1994 American Institute of Physics.published_or_final_versio

The magnon pairing mechanism of superconductivity in cuprate ceramics

The magnon pairing mechanism is derived to explain the high-temperature superconductivity of both the LA2-xSrxCu1O 4 and Y1Ba2Cu3O7 systems. Critical features include (i) a one- or two-dimensional lattice of linear Cu-O-Cu bonds that contribute to large antiferromagnetic (superexchange) coupling of the CuII (d9) orbitals; (ii) holes in the oxygen pπ bands [rather than CuIII (d8)] leading to high mobility hole conduction; and (iii) strong ferromagnetic coupling between oxygen pπ holes and adjacent CuII (d9) electrons. The ferromagnetic coupling of the conduction electrons with copper d spins induces the attractive interaction responsible for the superconductivity, leading to triplet-coupled pairs called "tripgems." The disordered Heisenberg lattice of antiferromagnetically coupled copper d spins serves a role analogous to the phonons in a conventional system. This leads to a maximum transition temperature of about 200 K. For La1.85Sr0.15Cu 1O4, the energy gap is in excellent agreement with experiment. For Y1Ba2Cu3O7, we find that both the CuO sheets and the CuO chains can contribute to the supercurrent.link_to_subscribed_fulltex

Elementary excitations for the two-dimensional quantum Heisenberg antiferromagnet

The excitation spectrum of the antiferromagnetic spin-1/2 Heisenberg Hamiltonian H=tsumJSj on L×L lattices is evaluated by a projector quantum Monte Carlo method. These results suggest that the exact spectrum for the finite lattice is ωZ(L)ωkSW, for all k, where ωkSW=4JS 1-γk2 with γk=(coskx+cosky)/2 is from linear-spin-wave theory. We find Z(L)=ZE+B/L3, leading to ZE=1.21±0.03 for L=. A comparison with experiments on La2CuO4 is discussed. © 1992 The American Physical Society.link_to_subscribed_fulltex

Electron-phonon interactions and superconductivity in K3C60

Using electronic states and phonon states from first-principles calculations on K3C60 to evaluate the quantities in the McMillan equation, we examine the effects of both electron-phonon dynamic charge coupling (Q) and Jahn-Teller coupling (JT) on various superconducting properties, including Tc, ΔTc (the shift of Tc for 1 GPa pressure), αC (the isotope exponent for →1312C), and αK (the isotope exponent for →4139K). All quantities including electron-phonon coupling are evaluated (without modification) directly from theory, except for the screening length between conducting electrons and ions, Rsc. With Rsc=0.8-1.0, we find that the calculated properties, Tc, ΔTc, and αC, are all in good agreement with experimental measurements. We find that the superconducting properties depend critically upon the synergy between Q and JT coupling. © 1993 The American Physical Society.link_to_subscribed_fulltex

Valence-bond charge-transfer model for nonlinear optical properties of charge-transfer organic molecules

The nonlinear optical properties of charge-transfer organic materials are discussed in the framework of a simple valence-bond charge-transfer model. This model leads to analytic formulas for the absorption frequency, hyperpolarizabilities, and bond length alternation, all of which are described in terms of three parameters, V, t, and Q related to the band gap, bandwidth, and dipole moment of the charge-transfer state. These parameters are derivable from experiment or from theory. The valence-bond charge-transfer model provides a clear physical picture for the dependence of the hyperpolarizabilities on the structure of charge-transfer molecules and leads to good agreement with the trends predicted by the AMI calculations.link_to_subscribed_fulltex

Resonant tunneling through quantum-dot arrays

We apply the Hubbard Hamiltonian to describe quantum-dot arrays weakly coupled to two contacts. Exact diagonalization is used to calculate the eigenstates of the arrays containing up to six dots and the linear-response conductance is then calculated as a function of the Fermi energy. In the atomic limit the conductance peaks form two distinct groups separated by the intradot Coulomb repulsion, while in the band limit the peaks occur in pairs. The crossover is studied. A finite interdot repulsion is found to cause interesting rearrangements in the conductance spectrum. © 1994 The American Physical Society.link_to_subscribed_fulltex