M. Acquarone

ELECTRONIC AND PHONONIC STATES OF THE HOLSTEIN-HUBBARD DIMER OF VARIABLE LENGTH

We consider a model Hamiltonian for a dimer of length a including all the electronic one- and two-body terms consistent with a single orbital per site, a free Einstein phonon term for a frequency V, and an electronphonon coupling g0 of the Holstein type. The bare electronic interaction parameters were evaluated in terms of Wannier functions built from Gaussian atomic orbitals. An effective polaronic Hamiltonian was obtained by an unrestricted displaced-oscillator transformation, followed by evaluation of the phononic terms over a squeezedphonon variational wave function. For the cases of quarter-filled and half-filled orbitals, and over a range of dimer length values, the ground state for given g0 and V was identified by simultaneously and independently optimizing the orbital shape, the phonon displacement, and the squeezing effect strength. As a varies, we generally find discontinuous changes of both electronic and phononic states, accompanied by an appreciable renormalization of the effective electronic interactions across the transitions, due to the equilibrium shape of the wave functions strongly depending on the phononic regime and on the type of ground state. @S0163-1829~98!06035-4#

Diamagnetic impurity effects on the Néel temperature in La2CuO4 and related materials

Abstract To investigate why the sensitivity of the Neel temperature TN of the antiferromagnetic (AF) layered copper perovskites (typically La2CuO4) to diamagnetic impurities such as Zn is reportedly much larger than in the AF members of the K2NiF4 family, we first treat the effect of a concentration c of impurities on the uncorrelated electronic states in the coherent potential approximation (CPA). Then we consider the Heisenberg hamiltonian as the large correlation limit of the Hubbard hamiltonian for a single band of impurity-modified electronic states. The correlation effects are treated variationally. The model is solved explicity by using a rectangular density of states, and we obtain the c-dependent exchange J, staggered moment Sq, spin wave velocity and transverse susceptibility at zero temperature. We take into consideration several recently proposed formulae for TN in the clean limit, and include the impurity effects by exploiting the results obtained, in order to test their predictions against the experimental TN(c) data for La2Cu1−cZncO4. Our results suggest that, to explain the difference between the K2NiF4 and the La2 CuO4 families, one should consider both the sign and the magnitude of the difference I≡ϵB−ϵA between impurity (B) and host (A) ionic potentials. The slowly decreasing trend of TN(c) in the K2NiF4 family is reproduced if I is negative and sizeable, or positive but very small, while the quick decrease typical of the copper perovskites requires a positive and rather large I. For reasonable values of the interaction parameters, among the several models we compare, only the model of Chakravarty, Halperin and Nelson is able to semi-quantitatively reproduce the non-linear behaviour of TN(c) reported for La2Cu1−cZncO4, provided the spin stiffness is assumed to scale with c as appropriate to Fermi liquids.

Model calculation of the interaction terms and ground states of the extended Hubbard model on a dimer

Abstract We consider the complete extended Hubbard Hamiltonian on a dimer, including all one- and two-body terms. The energy eigenvalues and eigenvectors and various correlation functions have been obtained in explicit form. Introducing model Wannier orbitals we have evaluated the interaction parameters as functions of the dimer length and orbital shape. Minimization of the lowest-energy eigenvalue yields the equilibrium shape for different fillings.

Mixed valence effects in itinerant magnetism

Abstract We study the electronic states of a system with a degenerate band of e g type. Removing the off-diagonal hopping terms in the Hamiltonian results in a renormalized band structure exhibiting similarities with mixed valence systems. Inclusion of Coulomb correlation and exchange produces a variety of magnetic configurations.

Variational study of the extended Hubbard-Holstein model on clusters of variable site spacing

We study the complete extended Hubbard-Holstein Hamiltonian on a four-site chain with equally spaced sites, with spacing-dependent electronic interaction parameters evaluated in terms of Wannier functions built from Gaussian atomic orbitals. By successive application of generalized displacement and squeezing transformations, an effective polaronic Hamiltonian is obtained, containing a purely phonon-induced, long-range intersite charge interaction. The phase diagrams for

The Holstein-Hubbard dimer of variable length as the building block of the CuO plane: Electronic and phononic transitions

N=1,

Can electronic interactions alone explain the diamagnetic impurity effects on antiferromagnetism in La2CuO4 and related materials

2, 3, and 4 electrons are determined by variational minimization of the sum of the electronic and the phononic energy, paying special attention to the effects of the above-mentioned phonon-induced long-range interaction. To characterize the physics of each ground state, we evaluate how variations of the site spacing affect the behavior of several correlation functions of experimental interest, with the aim of representing lattice deformation effects, either spontaneous or induced by external pressure.

A study of the Hubbard–Holstein model on a four-site chain

A polaronic Hamiltonian is obtained for a dimer of lengtha, including an Einstein phonon of frequency Ω, a Holstein coupling g0, and all the electronic one- and two-body terms consistent with a single orbital per site (the tatter evaluated in terms of Wannier functions built from Gaussian atomic orbitals). Its ground state for givena,g0, and Ω is identified by independent and simultaneous optimization of the electronic and phononic parameters. Asa varies, we find discontinuous changes of both electronic and phononic states, with interesting physical implications.

Doping dependence of the delocalization energy of spin polarons

Abstract We discuss a model combining diamagnetic impurity and electron correlation effects to explain, in La 2 Cu 1− c Zn c O 4 and other materials related to HTCSC, the vanishing of T N for small Z n concentration.

Electronic and phononic transitions in the two-site Holstein model

Abstract Using the displacement and the squeezing transformations, we obtain the polaronic Hamiltonian for a chain of four sites including all the electronic one- and two-body terms for a single orbital, the free phonon term for a single frequency Ω , and an electron–phonon coupling g0 of the Holstein type. The ground state was studied for up to four electrons by exact diagonalization and variational optimization of the displacement and squeezing parameters. For any filling we find an abrupt polaron localization transition, with strong renormalization of all the electronic interactions.