24,032 research outputs found
Accurate control of a Bose-Einstein condensate by managing the atomic interaction
We exploit the variation of the atomic interaction in order to move
ultra-cold atoms across an AC-driven periodic lattice. By breaking relevant
symmetries, a gathering of atoms is achieved. Accurate control of the gathered
atoms positions can be demonstrated via the control of the atomic localization
process. The localization process is analyzed with the help of the nonlinear
Floquet states where the Landau-Zener tunneling between states is observed and
controlled. Transport effects in the presence of disorder are discussed.Comment: 14 pages, 5 Figures, PACS numbers: 03.75.Lm, 05.60.-k, 63.20.P
Conductance of nano-systems with interactions coupled via conduction electrons: Effect of indirect exchange interactions
A nano-system in which electrons interact and in contact with Fermi leads
gives rise to an effective one-body scattering which depends on the presence of
other scatterers in the attached leads. This non local effect is a pure
many-body effect that one neglects when one takes non interacting models for
describing quantum transport. This enhances the non-local character of the
quantum conductance by exchange interactions of a type similar to the
RKKY-interaction between local magnetic moments. A theoretical study of this
effect is given assuming the Hartree-Fock approximation for spinless fermions
in an infinite chain embedding two scatterers separated by a segment of length
L\_c. The fermions interact only inside the two scatterers. The dependence of
one scatterer onto the other exhibits oscillations which decay as 1/L\_c and
which are suppressed when L\_c exceeds the thermal length L\_T. The
Hartree-Fock results are compared with exact numerical results obtained with
the embedding method and the DMRG algorithm
The innermost regions of the jet in NRAO 150. Wobbling or internal rotation?
NRAO 150 is a very bright millimeter to radio quasar at redshift =1.52 for
which ultra-high-resolution VLBI monitoring has revealed a counter-clockwise
jet-position-angle wobbling at an angular speed /yr in the
innermost regions of the jet. In this paper we present new total and linearly
polarized VLBA images at 43 GHz extending previous studies to cover the
evolution of the jet in NRAO 150 between 2006 and early 2009. We propose a new
scenario to explain the counter-clockwise rotation of the jet position angle
based on a helical motion of the components in a jet viewed faced-on. This
alternative scenario is compatible with the interpretation suggested in
previous works once the indetermination of the absolute position of the
self-calibrated VLBI images is taken into account. Fitting of the jet
components motion to a simple internal rotation kinematical model shows that
this scenario is a likely alternative explanation for the behavior of the
innermost regions in the jet of NRAO 150.Comment: 5 pages, 4 figures, Presented in 'The Innermost Regions of
Relativistic Jets and Their Magnetic Fields' conference. Granada, Spain, 201
Effect of flux-dependent Friedel oscillations upon the effective transmission of an interacting nano-system
We consider a nano-system connected to measurement probes via non interacting
leads. When the electrons interact inside the nano-system, the coefficient
|ts(E_F)|^2 describing its effective transmission at the Fermi energy E_F
ceases to be local. This effect of electron-electron interactions upon
|ts(E_F)|^2 is studied using a one dimensional model of spinless fermions and
the Hartree-Fock approximation. The non locality of |ts(E_F)|^2 is due to the
coupling between the Hartree and Fock corrections inside the nano-system and
the scatterers outside the nano-system via long range Friedel oscillations.
Using this phenomenon, one can vary |ts(E_F)|^2 by an Aharonov-Bohm flux
threading a ring which is attached to one lead at a distance Lc from the
nano-system. For small distances Lc, the variation of the quantum conductance
induced by this non local effect can exceed 0.1 (e^2/h)
Area Littlewood-Paley functions associated with Hermite and Laguerre operators
In this paper we study Lp-boundedness properties for area Littlewood-Paley
functions associated with heat semigroups for Hermite and Laguerre operator
Length-dependent oscillations of the conductance through atomic chains: The importance of electronic correlations
We calculate the conductance of atomic chains as a function of their length.
Using the Density Matrix Renormalization Group algorithm for a many-body model
which takes into account electron-electron interactions and the shape of the
contacts between the chain and the leads, we show that length-dependent
oscillations of the conductance whose period depends on the electron density in
the chain can result from electron-electron scattering alone. The amplitude of
these oscillations can increase with the length of the chain, in contrast to
the result from approaches which neglect the interactions.Comment: 7 pages, 4 figure
Inhomogeneous soliton ratchets under two ac forces
We extend our previous work on soliton ratchet devices [L. Morales-Molina et
al., Eur. Phys. J. B 37, 79 (2004)] to consider the joint effect of two ac
forces including non-harmonic drivings, as proposed for particle ratchets by
Savele'v et al. [Europhys. Lett. 67}, 179 (2004); Phys. Rev. E {\bf 70} 066109
(2004)]. Current reversals due to the interplay between the phases, frequencies
and amplitudes of the harmonics are obtained. An analysis of the effect of the
damping coefficient on the dynamics is presented. We show that solitons give
rise to non-trivial differences in the phenomenology reported for particle
systems that arise from their extended character. A comparison with soliton
ratchets in homogeneous systems with biharmonic forces is also presented. This
ratchet device may be an ideal candidate for Josephson junction ratchets with
intrinsic large damping
Ratchet behavior in nonlinear Klein-Gordon systems with point-like inhomogeneities
We investigate the ratchet dynamics of nonlinear Klein-Gordon kinks in a
periodic, asymmetric lattice of point-like inhomogeneities. We explain the
underlying rectification mechanism within a collective coordinate framework,
which shows that such system behaves as a rocking ratchet for point particles.
Careful attention is given to the kink width dynamics and its role in the
transport. We also analyze the robustness of our kink rocking ratchet in the
presence of noise. We show that the noise activates unidirectional motion in a
parameter range where such motion is not observed in the noiseless case. This
is subsequently corroborated by the collective variable theory. An explanation
for this new phenomenom is given
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