Canio Noce

Franck–Condon factors in curvilinear coordinates: the photoelectron spectrum of ammonia

An approach to the calculation of Franck–Condon factors in curvilinear coordinates is outlined. The approach is based on curvilinear normal coordinates, which allows for an easy extension of Duschinsky’s transformation to the case of curvilinear coordinates, and on the power series expansion of the kinetic energy operator. Its usefulness in the case of molecules undergoing large displacements of their equilibrium nuclear configurations upon excitation is then demonstrated by an application to the vibrational structure of the photoelectron spectrum of ammonia, using an anharmonic potential only for the symmetric stretching and bending coordinates of the radical cation.

Coexistence of ferromagnetism and singlet superconductivity via kinetic exchange.

We propose a novel mechanism for the coexistence of metallic ferromagnetism and singlet superconductivity assuming that the magnetic instability is due to kinetic exchange. Within this scenario, the unpaired electrons which contribute to the magnetization have a positive feedback on the gain of the kinetic energy in the coexisting phase by undressing the effective mass of the carriers involved in the pairing. The evolution of the magnetization and pairing amplitude and the phase diagram are first analyzed for a generic kinetic exchange model and then are determined within a specific case with spin dependent bond-charge occupation.

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#

Spin-orbital coupling in a triplet superconductor-ferromagnet junction.

We study the interplay of spin and orbital degrees of freedom in a triplet superconductor-ferromagnet junction. Using a self-consistent spatially dependent mean-field theory, we show that increasing the angle between the ferromagnetic moment and the triplet vector order parameter enhances or suppresses the p-wave gap close to the interface, according to whether the gap antinodes are parallel or perpendicular to the boundary, respectively. The associated change in condensation energy establishes an orbitally dependent preferred orientation for the magnetization. When both gap components are present, as in a chiral superconductor, first-order transitions between different moment orientations are observed as a function of the exchange field strength.

Spin-sensitive long-range proximity effect in ferromagnet/spin-triplet-superconductor bilayers

, calls for experimental methods to identify the presence of spin-triplet pairing. We here demon-strate a method which accomplishes this in an appealingly simple manner: a spin-sensitive proximity effect ina ferromagnetjtriplet superconductor bilayer. It is shown how the orientation of the field can be used to unam-biguously distinguish between different spin-triplet states. Moreover, the proximity effect becomes long-rangedin spite of the presence of an exchange field and even without any magnetic inhomogeneities, in contrast toconventional SjF junctions. Our results can be verified by STM-spectroscopy and could be useful as a tool tocharacterize the pairing state in unconventional superconducting materials.

Thermodynamical properties of the Anderson model in the atomic limit

Abstract The theoretical trend of the specific heat, the static magnetic susceptibility and the valence of the localized electrons in the atomic limit of the Anderson model has been investigated. The effect of the various parameters of the Hamiltonian on these quantities has been analyzed with a special emphasis on its relationship with the intermediate valence states of the localized electrons. A good qualitative agreement with some experimental results has been obtained.

Grain geometry effect on the magnetic properties of a granular iron-based superconductor LaFeAsO1−xFx

We have studied and compared the magnetic response of LaFeAsO0.92F0.08, in the form of granular bulk and powder, to the application of ac magnetic fields with different frequencies. The study of the third harmonics of the ac susceptibility has shown that the ac frequency plays an important role in the electromagnetic coupling of the grains in the granular sample. In particular, the characteristic harmonic response of the bulk sample at the highest frequency resembles the response of the unconnected grains of the powders, even at low frequencies. Moreover, the possibility of the occurrence of magnetic field focusing in the region between the grains, ascribed to both superconducting shielding and geometric effect of the grains, has been considered to explain the observed peak effect in the dc magnetization loops on the bulk sample. Although our investigation has be confined to the LaFeAsO0.92F0.08 granular, these results may be applied to any granular sample consisting of flat grains connected by weak links. Moreover, they also show that a peak effect in a granular superconductor can exist without necessarily invoke the existence of vortex dynamic processes.

Densities of states in the periodic Anderson model

The densities of states for the periodic Anderson model are evaluated by means of a non-standard perturbation expansion in the kinetic term of the conduction electrons. The calculations are performed at finite temperature, taking into account in an exact way the effect of the on-site Coulomb repulsion U and the hybridization coupling V. The authors reproduce most of the properties that are usually expected for the correlated electrons, especially the peaks in the density of states at energies in f and in f+U and the double peak structure with the hybridization gap in the region around the chemical potential.

Charge and spin transport through a ferromagnet/insulator/unconventional superconductor junction

We analyze the charge and spin transport through a ballistic ferromagnet/insulator/superconductor junction by means of the Bogoliubov-de Gennes equations. For the ferromagnetic side we assume that ferromagnetism may be driven by an unequal mass renormalization of oppositely polarized carriers, i.e. a spin bandwidth asymmetry, and/or by a rigid splitting of up-and down-spin electron bands, as in a standard Stoner ferromagnet, whereas the superconducting side is assumed to exhibit a d-wave symmetry of the order parameter, which can be pure or accompanied by a minority component breaking time-reversal symmetry. Several remarkable features in the charge conductance arise in this kind of junction, providing useful information about the mechanism of ferromagnetism in the ferromagnetic electrode, as well as of the order parameter symmetry in the superconducting one. In particular, we show that when a time-reversal symmetry breaking superconductor is considered, the use of the two kinds of ferromagnet mentioned above represents a valuable tool to discriminate between the different superconducting mixed states. We also explain how this junction may mimic a switch able to turn on and off a spin current, leaving the charge conductance unchanged, and we show that for a wide range of insulating barrier strengths, a spin bandwidth asymmetry ferromagnet may support a spin current larger than a standard Stoner one.

Josephson effect in S/F/S junctions: Spin bandwidth asymmetry versus Stoner exchange

We analyze the dc Josephson effect in a ballistic superconductor/ferromagnet/superconductor junction by means of the Bogoliubov-de Gennes equations in the quasiclassical Andreev approximation. We consider the possibility of ferromagnetism originating from a mass renormalization of carriers of opposite spin, i.e. a spin bandwidth asymmetry. We provide a general formula for Andreev levels which is valid for arbitrary interface transparency, exchange interaction, and bandwidth asymmetry. We analyze the current-phase relation, the critical current, and the free energy in the short junction regime. We demonstrate that a larger number of