316 research outputs found
C programs for solving the time-dependent Gross-Pitaevskii equation in a fully anisotropic trap
We present C programming language versions of earlier published Fortran
programs (Muruganandam and Adhikari, Comput. Phys. Commun. 180 (2009) 1888) for
calculating both stationary and non-stationary solutions of the time-dependent
Gross-Pitaevskii (GP) equation. The GP equation describes the properties of
dilute Bose-Einstein condensates at ultra-cold temperatures. C versions of
programs use the same algorithms as the Fortran ones, involving real- and
imaginary-time propagation based on a split-step Crank-Nicolson method. In a
one-space-variable form of the GP equation, we consider the one-dimensional,
two-dimensional, circularly-symmetric, and the three-dimensional
spherically-symmetric harmonic-oscillator traps. In the two-space-variable
form, we consider the GP equation in two-dimensional anisotropic and
three-dimensional axially-symmetric traps. The fully-anisotropic
three-dimensional GP equation is also considered. In addition to these twelve
programs, for six algorithms that involve two and three space variables, we
have also developed threaded (OpenMP parallelized) programs, which allow
numerical simulations to use all available CPU cores on a computer. All 18
programs are optimized and accompanied by makefiles for several popular C
compilers. We present typical results for scalability of threaded codes and
demonstrate almost linear speedup obtained with the new programs, allowing a
decrease in execution times by an order of magnitude on modern multi-core
computers.Comment: 8 pages, 1 figure; 18 C programs included (to download, click other
and download the source
Coherent Diffraction Imaging of Single 95nm Nanowires
Photonic or electronic confinement effects in nanostructures become
significant when one of their dimension is in the 5-300 nm range. Improving
their development requires the ability to study their structure - shape, strain
field, interdiffusion maps - using novel techniques. We have used coherent
diffraction imaging to record the 3-dimensionnal scattered intensity of single
silicon nanowires with a lateral size smaller than 100 nm. We show that this
intensity can be used to recover the hexagonal shape of the nanowire with a
28nm resolution. The article also discusses limits of the method in terms of
radiation damage.Comment: 5 pages, 5 figure
Nonresonant microwave absorption in epitaxial La-Sr-Mn-O films and its relation to colossal magnetoresistance
We study magnetic-field-dependent nonresonant microwave absorption and
dispersion in thin LaSrMnO films and show that it
originates from the colossal magnetoresistance. We develop the model for
magnetoresistance of a thin ferromagnetic film in oblique magnetic field. The
model accounts fairly well for our experimental findings, as well as for
results of other researchers. We demonstrate that nonresonant microwave
absorption is a powerful technique that allows contactless measurement of
magnetic properties of thin films, including magnetoresistance, anisotropy
field and coercive field.Comment: 20 pages, 11 figure
Large and uniform optical emission shifts in quantum dots externally strained along their growth axis
We introduce a method which enables to directly compare the impact of elastic
strain on the optical properties of distinct quantum dots (QDs). Specifically,
the QDs are integrated in a cross-section of a semiconductor core wire which is
surrounded by an amorphous straining shell. Detailed numerical simulations show
that, thanks to the mechanical isotropy of the shell, the strain field in a
core section is homogeneous. Furthermore, we use the core material as an in
situ strain gauge, yielding reliable values for the emitter energy tuning
slope. This calibration technique is applied to self-assembled InAs QDs
submitted to incremental tensile strain along their growth axis. In contrast to
recent studies conducted on similar QDs stressed perpendicularly to their
growth axis, optical spectroscopy reveals 5-10 times larger tuning slopes, with
a moderate dispersion. These results highlight the importance of the stress
direction to optimise QD response to applied strain, with implications both in
static and dynamic regimes. As such, they are in particular relevant for the
development of wavelength-tunable single photon sources or hybrid QD
opto-mechanical systems
Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)
We report on the structural properties of Ge_(1-x)Mn_x layers grown by
molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are
embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray
scattering, atomic force and transmission electron microscopy to study the
structural properties of the columns. We demonstrate how the elastic
deformation of the matrix (as calculated using atomistic simulations) around
the columns, as well as the average inter-column distance can account for the
shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure
High Field Studies of the Hidden Order Transition in URuSi
We studied in detail the low temperature/high magnetic field phases of
URuSi single crystals with specific heat, magnetocaloric effect,
and magnetoresistance in magnetic fields up to 45 T. Data obtained down to 0.5
K, and extrapolated to T = 0, show a suppression of the hidden order phase at
H(0) = 35.9 0.35 T and the appearance of a new phase for magnetic
fields in excess of H(0) = 36.1 0.35 T observed \textit{only} at
temperatures lower than 6 K. In turn, complete suppression of this high field
state is attained at a critical magnetic field H(0) = 39.7 0.35 T.
No phase transitions are observed above 40 T. We discuss our results in the
context of itinerant vs. localized \textit{f}-electron behavior and consider
the implications for the hidden order phase.Comment: 4 pages, 3 figures Submitted May 10, 2002. Revised Sep 17, 200
Fractional-Period Excitations in Continuum Periodic Systems
We investigate the generation of fractional-period states in continuum
periodic systems. As an example, we consider a Bose-Einstein condensate
confined in an optical-lattice potential. We show that when the potential is
turned on non-adiabatically, the system explores a number of transient states
whose periodicity is a fraction of that of the lattice. We illustrate the
origin of fractional-period states analytically by treating them as resonant
states of a parametrically forced Duffing oscillator and discuss their
transient nature and potential observability.Comment: 10 pages, 6 figures (some with multiple parts); revised version:
minor clarifications of a couple points, to appear in Physical Review
Loss and revival of phase coherence in a Bose-Einstein condensate moving through an optical lattice
We investigate the phase coherence of a trapped Bose-Einstein condensate that
undergoes a dynamical superfluid-insulator transition in the presence of a
one-dimensional optical lattice. We study the evolution of the condensate after
a sudden displacement of the harmonic trapping potential by solving the
Gross-Pitaevskii equation, and comparing the results with the prediction of two
effective 1D models. We show that, owing to the 3D nature of the system, the
breakdown of the superfluid current above a critical displacement is not
associated to a sharp transition, but there exists a range of displacements for
which the condensate can recover a certain degree of coherence. We also discuss
the implications on the interference pattern after the ballistic expansion as
measured in recent experiments at LENS.Comment: 7 pages, 9 figure
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