7,532 research outputs found
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 meson decays and - mixing: a QCD sum rule analysis
The radiative transitions and are analysed using QCD sum-rules. The results we obtain for and are in very good agreement with
existing experimental data. The related issue of mixing is
discussed and we give predictions for the and 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
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