Search

A variational procedure is developed to study the properties of the bipolaron in the Fröhlich model including explicitly the electron band structure and taking into account the long-range repulsive electron-electron interaction. Adopting two different ansatze for the longitudinal optical phonon distribution function, the large-bipolaron and small-bipolaron limits are obtained. The evolution of the bipolaron ground state as a function of the electron-phonon coupling constant and of the bare-electronic bandwidth is discussed and the bipolaron phase diagram is presented. A clear crossover from the large-bipolaron to the small-bipolaron regime takes place.

The critical behaviour of the Fully Frustrated XY model in presence of weak positional disorder is studied in a square lattice by Monte Carlo methods. The critical exponent associated to the divergence of the chiral correlation length is found to be equal to 1.7 already at very small values of disorder. Furthermore the helicity modulus jump is found larger than the universal value expected in the XY model., Comment: 8 pages, 4 figures (revtex)

A single-particle Hamiltonian approach is used for describing dynamical screening of excitons in optically excited semiconductors. Upon considering a particular electron-hole pair, the excitation spectrum of all the other electrons and holes is replaced by a single plasmon mode omega (q). From a Frohlich-type Hamiltonian, 1s exciton binding energy and wavefunctions are calculated variationally with a generalization of the Lee-Low-Pines transformation. The advantage of the method, beside the simplicity, is that bandgap renormalization is accounted for within the same Hamiltonian so that a consistent comparison with experimental data is possible. In particular, our model reproduces both the remarkable persistence of the 1s GaAs exciton line under strong optical excitation and the measured transition density from bound to unbound states.

Single-parameter charge pumping, induced by a nearby radio-frequency antenna, is achieved in suspended carbon nanotubes close to the mechanical resonance. The charge pumping is due to an important dynamic adjustment of the oscillating motion to the antenna signal and it is different from the mechanism active in the two-parameter pumping. Finally, the second harmonic oscillator response shows an interesting relationship with the first harmonic that should be experimentally observed.

We investigate the effect of a phenomenological relaxation time on the magnetoplasma excitations of a 2D strip of electrons by using the scheme proposed in a previous paper. Particular attention is put on the lowest frequency mode which exhibits an "anomalous" dependence on the static magnetic field (perpendicular to the strip). We show that also the life time of the first mode has an anomalous dependence on the wavevector parallel to the strip and on the magnetic field.