The pomeron in closed bosonic string theory

We review the features of the pomeron in the S-matrix theory and in quantum field theory. We extend those general properties to the pomeron of closed bosonic string theory in a Minkowskian background. We compute the couplings of the pomeron to the lowest mass levels of closed bosonic string states in flat space. We recognize the deviation from the linearity of the Regge trajectories in a five dimensional anti De Sitter background.Comment: 13 page

Dynamics and Current Fluctuations in AC driven Charge Shuttle

The behavior of a charge shuttle under a pure AC field has been recently considered theoretically and experimentally. If the system presents an asymmetry in the tunneling amplitudes the device acts as a nano-electromechanical rectifier, transforming a pure AC voltage field into a direct curren. In this paper we first review the model and the appearance of the rectifying effect for bias voltages below the threshold of self-oscillation. We discuss in some details the dynamics of the central island that, like the current, presents strong dependence on the forcing AC field frequency. In presence of both a constant and a small oscillating bias voltage we analyze the transition from the static to self-oscillating solution. We then consider current fluctuations (full counting statistics) for periodic motion of the grain. We explicitly evaluate the current noise numerically and we find that it shows clear signatures of correlated transport at certain locking frequencies. In the adiabatic limit we obtain a simple expression for the full-counting statistics and calculate explicitly the first four moments.Comment: 16 pages, 7 Figures. Invited paper for the special issue on NEMS on New Journal of Physic

Bound entanglement in the XY model

We study the multi-spin entanglement for the 1D anisotropic XY model concentrating on the simplest case of three-spin entanglement. As compared to the pairwise entanglement, three-party quantum correlations have a longer range and they are more robust on increasing the temperature. We find regions of the phase diagram of the system where bound entanglement occurs, both at zero and finite temperature. Bound entanglement in the ground state can be obtained by tuning the magnetic field. Thermal bound entanglement emerges naturally due to the effect of temperature on the free ground state entanglement.Comment: 7 pages, 3 figures; some typos corrected, references adde

Properties of Mesoscopic Hybrid Superconducting Systems

In this paper we review several aspects of mesoscopic hybrid superconducting systems. In particular we consider charge and heat transport properties in hybrid superconducting-metal structures and the effect of charging energy in superconducting nanostructures.Comment: 27 pages, 8 figure

Radiative ϕ\phi−-meson decays and η\eta-η′\eta^\prime mixing: a QCD sum rule analysis

The radiative transitions $\phi \to \eta \gamma$ and $\phi \to \eta^\prime \gamma$ are analysed using QCD sum-rules. The results we obtain for ${\cal B}(\phi \to \eta \gamma)=(1.15 \pm 0.2) 10^{-2}$ and ${\cal B}(\phi \to \eta^\prime \gamma)=(1.18 \pm 0.4) 10^{-4}$ are in very good agreement with existing experimental data. The related issue of $\eta-\eta^\prime$ mixing is discussed and we give predictions for the $\eta$ and $\eta^\prime$ decay constants in the framework of a mixing scheme in the quark-flavour basis.Comment: Latex, 15 pages, 3figures. Some typos correcte

Critical behavior of dissipative two-dimensional spin lattices

We explore critical properties of two-dimensional lattices of spins interacting via an anisotropic Heisenberg Hamiltonian and subject to incoherent spin flips. We determine the steady-state solution of the master equation for the density matrix via the corner-space renormalization method. We investigate the finite-size scaling and critical exponent of the magnetic linear susceptibility associated to a dissipative ferromagnetic transition. We show that the Von Neumann entropy increases across the critical point, revealing a strongly mixed character of the ferromagnetic phase. Entanglement is witnessed by the quantum Fisher information which exhibits a critical behavior at the transition point, showing that quantum correlations play a crucial role in the transition even though the system is in a mixed state.Comment: Accepted for publication on Phys. Rev. B (6 pages, 5 figures