73 research outputs found
AC-induced superfluidity
We argue that a system of ultracold bosonic atoms in a tilted optical lattice
can become superfluid in response to resonant AC forcing. Among others, this
allows one to prepare a Bose-Einstein condensate in a state associated with a
negative effective mass. Our reasoning is backed by both exact numerical
simulations for systems consisting of few particles, and by a theoretical
approach based on Floquet-Fock states.Comment: Accepted for publication in Europhysics letters, 6 pages, 4 figures,
Changes in v2: reference 7 replaced by a more recent on
Tunneling control and localization for Bose-Einstein condensates in a frequency modulated optical lattice
The similarity between matter waves in periodic potential and solid-state
physics processes has triggered the interest in quantum simulation using
Bose-Fermi ultracold gases in optical lattices. The present work evidences the
similarity between electrons moving under the application of oscillating
electromagnetic fields and matter waves experiencing an optical lattice
modulated by a frequency difference, equivalent to a spatially shaken periodic
potential. We demonstrate that the tunneling properties of a Bose-Einstein
condensate in shaken periodic potentials can be precisely controlled. We take
additional crucial steps towards future applications of this method by proving
that the strong shaking of the optical lattice preserves the coherence of the
matter wavefunction and that the shaking parameters can be changed
adiabatically, even in the presence of interactions. We induce reversibly the
quantum phase transition to the Mott insulator in a driven periodic potential.Comment: Laser Physics (in press
Quasienergy spectra of a charged particle in planar honeycomb lattices
The low energy spectrum of a particle in planar honeycomb lattices is
conical, which leads to the unusual electronic properties of graphene. In this
letter we calculate the quasienergy spectra of a charged particle in honeycomb
lattices driven by a strong AC field, which is of fundamental importance for
its time-dependent dynamics. We find that depending on the amplitude, direction
and frequency of external field, many interesting phenomena may occur,
including band collapse, renormalization of velocity of ``light'', gap opening
etc.. Under suitable conditions, with increasing the magnitude of the AC field,
a series of phase transitions from gapless phases to gapped phases appear
alternatively. At the same time, the Dirac points may disappear or change to a
line. We suggest possible realization of the system in Honeycomb optical
lattices.Comment: 4+ pages, 5 figure
Dynamics of one-dimensional tight-binding models with arbitrary time-dependent external homogeneous fields
The exact propagators of two one-dimensional systems with time-dependent
external fields are presented by following the path-integral method. It is
shown that the Bloch acceleration theorem can be generalized to the
impulse-momentum theorem in quantum version. We demonstrate that an evolved
Gaussian wave packet always keeps its shape in an arbitrary time-dependent
homogeneous driven field. Moreover, that stopping and accelerating of a wave
packet can be achieved by the pulsed field in a diabatic way.Comment: 8 pages, 6 figure
Abelian gauge potentials on cubic lattices
The study of the properties of quantum particles in a periodic potential
subject to a magnetic field is an active area of research both in physics and
mathematics; it has been and it is still deeply investigated. In this review we
discuss how to implement and describe tunable Abelian magnetic fields in a
system of ultracold atoms in optical lattices. After discussing two of the main
experimental schemes for the physical realization of synthetic gauge potentials
in ultracold set-ups, we study cubic lattice tight-binding models with
commensurate flux. We finally examine applications of gauge potentials in
one-dimensional rings.Comment: To appear on: "Advances in Quantum Mechanics: Contemporary Trends and
Open Problems", G. Dell'Antonio and A. Michelangeli eds., Springer-INdAM
series 201
Preparation and Instability of Nanocrystalline Cuprous Nitride
Low-dimensional cuprous nitride (Cu3N) was synthesized by nitridation (ammonolysis) of cuprous oxide (Cu2O) nanocrystals using either ammonia (NH3) or urea (H2NCONH2) as the nitrogen source. The resulting nanocrystalline Cu3N spontaneously decomposes to nanocrystalline CuO in the presence of both water and oxygen from air at room temperature. Ammonia was produced in 60% chemical yield during Cu3N decomposition, as measured using the colorimetric indophenol method. Because Cu3N decomposition requires H2O and produces substoichiometric amounts of NH3\u3e, we conclude that this reaction proceeds through a complex stoichiometry that involves the concomitant release of both N2 and NH3. This is a thermodynamically unfavorable outcome, strongly indicating that H2O (and thus NH3 production) facilitate the kinetics of the reaction by lowering the energy barrier for Cu3N decomposition. The three different Cu2O, Cu3N, and CuO nanocrystalline phases were characterized by a combination of optical absorption, powder X-ray diffraction, transmission electron microscopy, and electronic density of states obtained from electronic structure calculations on the bulk solids. The relative ease of interconversion between these interesting and inexpensive materials bears possible implications for catalytic and optoelectronic applications
Synthesis of Well-Defined, Surfactant-Free Co<sub>3</sub>O<sub>4</sub> Nanoparticles:The Impact of Size and Manganese Promotion on Co<sub>3</sub>O<sub>4</sub> Reduction and Water Oxidation Activity
Abstract: A surfactant-free synthetic route has been developed to produce size-controlled, cube-like cobalt oxide nanoparticles of three different sizes in high yields. It was found that by using sodium nitrite as salt-mediating agent, near-quantitative yields could be obtained. The size of the nanoparticles could be altered from 11 to 22Â nm by changing the cobalt concentration and reaction time. These surfactant-free nanoparticles form ideal substrates for facile deposition of further elements such as manganese. The effect of size of the cobalt oxide nanoparticles and the presence of manganese on the reducibility of cobalt oxide to metallic cobalt was investigated. Similarly, the effect of these parameters was investigated with a visible light promoted water oxidation system with cobalt oxide as catalyst, together with [Ru(bpy) 3] 2+ light harvester dye and an electron acceptor. Graphical Abstract: A novel surfactant-free synthetic route has been developed to produce size-controlled, cube shaped cobalt oxide nanoparticles in high yields. [Figure not available: see fulltext.]. </p
Diversity and role of cave-dwelling hematophagous insects in pathogen transmission in the Afrotropical region
XXIV.—On Old Red Sandstone Plants showing Structure, from the Rhynie Chert Bed, Aberdeenshire. Part I. Rhynia Gwynne-Vaughani, Kidston and Lang
Serious Sociological Games in the ELFE Cohort Study: Using Children’s Play to Gain Perspective on their Visions of the World
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