74 research outputs found
Dynamics of matter solitons in weakly modulated optical lattices
It is shown that matter solitons can be effectively managed by means of
smooth variations of parameters of optical lattices in which the condensate is
loaded. The phenomenon is based on the effect of lattice modulations on the
carrier wave transporting the soliton and that is why is well understood in
terms of the effective mass approach, where a particular spatial configuration
of the band structure is of primary importance. Linear, parabolic, and
spatially localized modulations are considered as the case examples. It is
shown that these defects can originate accelerating and oscillating motion of
matter solitons as well as simulate soliton interaction with attractive and
repulsive defects.Comment: 6 pages, 7 figures (text with major revision
Scattering properties of a PT dipole
Electromagnetic response of a PT dipole is studied both analytically and numerically. In the analytical approach, the dipole is represented by two point scatterers. Within the first Born approximation, the asymmetry of the scattering field with respect to the orientation of the dipole is proven. In numerical simulations, the dipole is represented by two infinitely long, parallel cylinders with opposite sign of the imaginary part of a refractive index. Numerical data confirm the validity of the Born approximation in the weak scattering limit, while significant deviations from the Born approximation were observed for stronger scatterers and in the near-field range.Peer ReviewedPostprint (published version
Nonlinear tunneling in two-dimensional lattices
We present thorough analysis of the nonlinear tunneling of Bose-Einstein
condensates in static and accelerating two-dimensional lattices within the
framework of the mean-field approximation. We deal with nonseparable lattices
considering different initial atomic distributions in the highly symmetric
states. For analytical description of the condensate before instabilities are
developed, we derive several few-mode models, analyzing both essentially
nonlinear and quasi-linear regimes of tunneling. By direct numerical
simulations, we show that two-mode models provide accurate description of the
tunneling when either initially two states are populated or tunneling occurs
between two stable states. Otherwise a two-mode model may give only useful
qualitative hints for understanding tunneling but does not reproduce many
features of the phenomenon. This reflects crucial role of the instabilities
developed due to two-body interactions resulting in non-negligible population
of the higher bands. This effect becomes even more pronounced in the case of
accelerating lattices. In the latter case we show that the direction of the
acceleration is a relevant physical parameter which affects the tunneling by
changing the atomic rates at different symmetric states and by changing the
numbers of bands involved in the atomic transfer
Suppressed superconductivity in ultrathin Mo2N films due to pair-breaking at the interface
A strong disorder characterized by a small product of the Fermi vector kF and
the electron mean free l drives superconductors towards insulating state. Such
disorder can be introduced by making the films very thin. Here, we present 3-nm
Mo2N film with k_F*l ~ 2 with a resistive superconducting transition
temperature Tc = 2 K heavily suppressed in comparison with the bulk Tc.
Superconducting density of states (DOS) with smeared gap-like peaks and in-gap
states, so called Dynes DOS, is observed by the low temperature tunneling
spectroscopy despite a sharp resistive transition. By scanning tunneling
microscope the spectral maps are obtained and related to the surface
topography. The maps show a spatial variation of the superconducting energy gap
on the order of 20 % which is not accidental but well correlates with the
surface corrugation: protrusions reveal larger gap, smaller spectral smearing
and smaller in-gap states. In agreement with our previous measurements on
ultrathin MoC films we suggest that the film-substrate interface introducing
the local pair-breaking is responsible for the observed effects and generally
for the suppression of the superconductivity in these ultrathin films.Comment: Manuscript with 3 Figure
Superfluidity of Bose-Einstein condensates in toroidal traps with nonlinear lattices
Superfluid properties of Bose-Einstein condensates (BEC) in toroidal
quasi-one-dimensional traps are investigated in the presence of periodic
scattering length modulations along the ring. The existence of several types of
stable periodic waves, ranging from almost uniform to very fragmented chains of
weakly interacting and equally spaced solitons, is demonstrated. We show that
these waves may support persistent atomic currents and sound waves with spectra
of Bogoliubov type. Fragmented condensates can be viewed as arrays of Josephson
junctions and the current as a BEC manifestation of the dc-Josephson effect.
The influence of linear defects on BEC superfluidity has been also
investigated. We found that for subcritical velocities, linear defects that are
static with respect to the lattice (while the condensate moves in respect to
both the optical lattice and the defect) preserve the BEC superfluidity
Optical control of photon tunneling through an array of nanometer scale cylindrical channels
We report first observation of photon tunneling gated by light at a different
wavelength in an artificially created array of nanometer scale cylindrical
channels in a thick gold film. Polarization properties of gated light provide
strong proof of the enhanced nonlinear optical mixing in nanometric channels
involved in the process. This suggests the possibility of building a new class
of "gated" photon tunneling devices for massive parallel all-optical signal and
image processing.Comment: 4 pages, 4 figure
Superfluidity breakdown of periodic matter waves in quasi one-dimensional annular traps via resonant scattering with moving defects
We investigate, both analytically and numerically, the quasisuperfluidity properties of periodic Bose-Einstein condensates (BECs) in a quasi-one-dimensional (1D) ring with optical lattices (OL) of different kinds (linear and nonlinear) and with a moving defect of an infinite mass inside. To study the dynamics of the condensate we used a mean-field approximation describing the condensate by use of the Gross-Pitaevskii equation for the order parameter. We show that the resonant scattering of sound Bloch waves with the defect profoundly affect BEC superfluidity. In particular, a moving defect always leads to the breakdown of superfluidity independently of the value of its velocity. For weak periodic potentials the superfluidity breakdown may occur on a very long time scale (quasisuperfluidity) but the breakdown process can be accelerated by increasing the strength of the OL. Quite remarkably, we find that when the length of the ring is small enough to imply the discreteness of the reciprocal space, it becomes possible to avoid the resonant scattering and to restore quasi-superfluidity.Fundação para a Ciência e a Tecnologia (FCT)Programma di Ricerca Scientifica di Rilevante Interesse Nazionale (Italy
Disorder- and magnetic field-tuned fermionic superconductor-insulator transition in MoN thin films. Transport and STM studies
Superconductor-insulator transition (SIT) driven by disorder and transverse
magnetic field has been investigated in ultrathin MoN films by means of
transport measurements and scanning tunneling microscopy and spectroscopy. Upon
decreasing thickness, the homogeneously disordered films show increasing sheet
resistance Rs, shift of the superconducting transition Tc to lower temperatures
with the 3 nm MoN being the last superconducting film and thinner films already
insulating. Fermionic scenario of SIT is evidenced by applicability of the
Finkelsteins model, by the fact that Tc and the superconducting gap are coupled
with a constant ratio, and by the spatial homogeneity of the superconducting
and electronic characteristics. The logarithmic anomaly found in the tunneling
spectra of the non-superconducting films is further enhanced in increased
magnetic field due to the Zeeman spin effects driving the system deeper into
the insulating state and pointing also to fermionic SIT.Comment: Manuscript (6 Figures) including Supplemental Materials (7 Figures
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