F. Ventriglia

Modeling of strain effects in manganite films

Thickness dependence and strain effects in films of La 1 - x A x MnO 3 perovskites are analyzed in the colossal magnetoresistance regime. The calculations are based on a generalization of a variational approach previously proposed for the study of manganite bulk. It is found that a reduction in the thickness of the film causes a decrease of critical temperature and magnetization, and an increase of resistivity at low temperatures. The strain is introduced through the modifications of in-plane and out-of-plane electron hopping amplitudes due to substrate-induced distortions of the film unit cell. The strain effects on the transition temperature and transport properties are in good agreement with experimental data only if the dependence of the hopping matrix elements on the Mn-O-Mn bond angle is properly taken into account. Finally variations of the electron-phonon coupling linked to the presence of strain turn out to be important in influencing the balance of coexisting phases in the film.

Infrared absorption of the charge-ordering phase: Lattice effects

The phase diagram of the half-filled spinless Holstein model for electrons interacting with quantum phonons is derived in three dimensions extending at finite temperature T a variational approach introduced for the one-dimensional T=0 case. Employing the variational scheme, the spectral and optical properties of the system are evaluated in the different regimes that characterize the normal and ordered states. The effects of the charge ordering (CO) induce a transfer of spectral weight from low to high energies in the conductivity spectra, as the temperature decreases or the strength of the electron-phonon interaction increases. The inclusion of effects of lattice fluctuations is able to smooth the inverse square-root singularity expected for the case of the mean-field approach and determines a subgap tail absorption. Moreover, in the weak- to intermediate-coupling regime, a two-component structure is obtained within the CO phase at low frequency: the remnant Drude-like term and the incipient absorption hand centered around the gap energy.

Conductivity steps of a thick square barrier in a transverse magnetic field

The authors calculate the tunnelling current and the different conductivity of a thick square barrier in a transverse magnetic field. This field has the same extension as the barrier. They investigate the role of the quasi-levels localized at the barrier and appearing when the barrier is sufficiently wide. They choose the parameters suitable to describe low barriers in semiconductor heterostructures. The tunnelling current and the differential conductivity can exhibit steps in the applied voltages and Fermi levels.

Infrared conductivity of a one-dimensional charge-ordered state: Quantum lattice effects

The optical properties of the charge-ordering (CO) phase of the one-dimensional (ID) half-filled spinless Holstein model are derived at zero temperature within a well-known variational approach improved including second-order lattice fluctuations. Within the CO phase the static lattice distortions give rise to the optical interband gap, that broadens as the strength of the electron-phonon (el-ph) interaction increases The lattice fluctuation effects induce a long subgap tail in the infrared conductivity and a wide band above the gap energy. The first term is due to the multiphonon emission by the charge carriers and the second to the interband transitions accompanied by the multiphonon scattering. The results show a good agreement with experimental spectra of quasi-1D conductors such as K 0 . 3 MoO 3 and (TaSe 4 ) 2 I.

Effects of magnetic field and isotopic substitution upon the infrared absorption of manganites

Employing a variational approach that takes into account electron-phonon and magnetic interactions in La 1 - x A x MnO 3 perovskites with 0<x<0.5, the effects of the magnetic field and the oxygen isotope substitution on thephase diagram, the lattice distortions, and the infrared absorption are studied at x=0.3. The lattice displacements show a strong correlation with the conductivity and the magnetic properties of the system. Then the conductivity spectra are characterized by a marked sensitivity to the external parameters near the phase boundary.

Quantum Harmonic Oscillator in a Magnetic Field: An Example of Holomorphic Representation

The propagator of an harmonic oscillator in a constant magnetic field is given in closed holomorphic form. The analytical expression of the unitary map is evaluated, which converts the holomorphic representation into the coordinate one. This is applied to obtain a closed form of the Feynman’s propagator in polar coordinates. The procedure is of general validity and can be extended to many non interacting oscillators.

S-Matrix formalism for particles interacting with phonon fields: Coherent state representation

We apply the quasiparticle picture to the interaction between a fermion and a boson field using a coherent states representation of theS matrix. Its matrix elements between single particle states are explicitly evaluated in terms of a path integral. The method is extended to include dispersion in the excitation spectrum and applied to the case of a metal with electron-hole symmetry. Its relation with perturbation theory is discussed and the second order perturbative result for polarons in insulators is recovered.