925 research outputs found
Modification of the Bloch law in ferromagnetic nanostructures
The temperature dependence of magnetization in ferromagnetic nanostructures
(e.g., nanoparticles or nanoclusters) is usually analyzed by means of an
empirical extension of the Bloch law sufficiently flexible for a good fitting
to the observed data and indicates a strong softening of magnetic coupling
compared to the bulk material. We analytically derive a microscopic
generalization of the Bloch law for the Heisenberg spin model which takes into
account the effects of size, shape and various surface boundary conditions. The
result establishes explicit connection to the microscopic parameters and
differs significantly from the existing description. In particular, we show
with a specific example that the latter may be misleading and grossly
overestimates magnetic softening in nanoparticles. It becomes clear why the
usual dependence appears to be valid in some nanostructures, while
large deviations are a general rule. We demonstrate that combination of
geometrical characteristics and coupling to environment can be used to
efficiently control magnetization and, in particular, to reach a magnetization
higher than in the bulk material.Comment: 7 pages, 4 figure
Impurity effects on Fabry-Perot physics of ballistic carbon nanotubes
We present a theoretical model accounting for the anomalous Fabry-Perot
pattern observed in the ballistic conductance of a single-wall carbon
nanotubes. Using the scattering field theory, it is shown that the presence of
a limited number of impurities along the nanotube can be identified by a
measurement of the conductance and their position determined. Impurities can be
made active or silent depending on the interaction with the substrate via the
back-gate. The conceptual steps for designing a bio-molecules detector are
briefly discussed.Comment: 4 pages, 4 figure
Quantum pumping in deformable quantum dots
The charge current pumped adiabatically through a deformable quantum dot is
studied within the Green's function approach. Differently from the
non-deformable case, the current shows an undefined parity with respect to the
pumping phase \phi. The unconventional current-phase relation, analyzed in the
weak pumping regime, is due to a dynamical phase shift \phi_D caused by the
elastic deformations of the central region (classical phonons). The role of the
quality factor Q of the oscillator, the effects induced by a mechanical
resonance and the implications for current experiments on molecular systems are
also discussed
Quantum Bose Josephson Junction with binary mixtures of BECs
We study the quantum behaviour of a binary mixture of Bose-Einstein
condensates (BEC) in a double-well potential starting from a two-mode
Bose-Hubbard Hamiltonian. We focus on the small tunneling amplitude regime and
apply perturbation theory up to second order. Analytical expressions for the
energy eigenvalues and eigenstates are obtained. Then the quantum evolution of
the number difference of bosons between the two potential wells is fully
investigated for two different initial conditions: completely localized states
and coherent spin states. In the first case both the short and the long time
dynamics is studied and a rich behaviour is found, ranging from small amplitude
oscillations and collapses and revivals to coherent tunneling. In the second
case the short-time scale evolution of number difference is determined and a
more irregular dynamics is evidenced. Finally, the formation of Schroedinger
cat states is considered and shown to affect the momentum distribution.Comment: 14 pages, 4 figure
Bosonization and entanglement spectrum for one-dimensional polar bosons on disordered lattices
The extended Bose-Hubbard model subjected to a disordered potential is
predicted to display a rich phase diagram. In the case of uniform random
disorder one finds two insulating quantum phases -- the Mott-insulator and the
Haldane insulator -- in addition to a superfluid and a Bose glass phase. In the
case of a quasiperiodic potential further phases are found, eg the
incommensurate density wave, adiabatically connected to the Haldane insulator.
For the case of weak random disorder we determine the phase boundaries using a
perturbative bosonization approach. We then calculate the entanglement spectrum
for both types of disorder, showing that it provides a good indication of the
various phases.Comment: Submitted to NJ
Noise-assisted Thouless pump in elastically deformable molecular junctions
We study a Thouless pump realized with an elastically \textit{deformable
quantum dot} whose center of mass follows a non-linear stochastic dynamics. The
interplay of noise, non-linear effects, dissipation and interaction with an
external time-dependent driving on the pumped charge is fully analyzed. The
results show that the quantum pumping mechanism not only is not destroyed by
the force fluctuations, but it becomes stronger when the forcing signal
frequency is tuned close to the resonance of the vibrational mode. The
robustness of the quantum pump with temperature is also investigated and an
exponential decay of the pumped charge is found when the coupling to the
vibrational mode is present. Implications of our results for
nano-electromechanical systems are also discussed.Comment: 2 Appendices and figures adde
Phase rigidity breaking in open Aharonov-Bohm ring coupled to a cantilever
The conductance and the transmittance phase shifts of a two-terminal
Aharonov-Bohm (AB) ring are analyzed in the presence of mechanical
displacements due to coupling to an external can- tilever. We show that phase
rigidity is broken, even in the linear response regime, by means of inelastic
scattering due to phonons. Our device provides a way of observing continuous
variation of the transmission phase through a two-terminal
nano-electro-mechanical system (NEMS). We also propose measurements of phase
shifts as a way to determine the strength of the electron-phonon coupling in
NEMS.Comment: 7 pages, 8 figure
- …