4,120 research outputs found
Wave function Monte Carlo method for polariton condensates
We present a quantum jump approach to describe coupled quantum and classical
systems in the context of Bose-Einstein condensation in the solid state. In our
formalism, the excitonic gain medium is described by classical rate equations,
while the polariton modes are described fully quantum mechanically. We show the
equivalence of our method with a master equation approach. As an application,
we compute the linewidth of a single mode polariton condensate. Both the line
broadening due to the interactions between polaritons and the interactions with
the reservoir excitons is taken into account.Comment: 6 pages, 2 figure
Design and experimental validation of a compact collimated Knudsen source
In this paper we discuss the design and performance of a collimated Knudsen
source which has the benefit of a simple design over recirculating sources.
Measurements of the flux, transverse velocity distribution and brightness at
different temperatures were conducted to evaluate the performance. The scaling
of the flux and brightness with the source temperature follow the theoretical
predictions. The transverse velocity distribution in the transparent operation
regime also agrees with the simulated data. The source was found able to
produce a flux of s at a temperature of 433 K. Furthermore the
transverse reduced brightness of an ion beam with equal properties as the
atomic beam reads A/(m sr eV) which is sufficient for
our goal: the creation of an ultra-cold ion beam by ionization of a
laser-cooled and compressed atomic rubidium beam
Lagekostenbedrijf behaalt laag vervangingspercentage
In de praktijk blijkt de veevervanging op het Lagekostenbedrijf de nodige knelpunten op te leveren
Cavity-enhanced photoionization of an ultracold rubidium beam for application in focused ion beams
A two-step photoionization strategy of an ultracold rubidium beam for
application in a focused ion beam instrument is analyzed and implemented. In
this strategy the atomic beam is partly selected with an aperture after which
the transmitted atoms are ionized in the overlap of a tightly cylindrically
focused excitation laser beam and an ionization laser beam whose power is
enhanced in a build-up cavity. The advantage of this strategy, as compared to
without the use of a build-up cavity, is that higher ionization degrees can be
reached at higher currents. Optical Bloch equations including the
photoionization process are used to calculate what ionization degree and
ionization position distribution can be reached. Furthermore, the ionization
strategy is tested on an ultracold beam of Rb atoms. The beam current is
measured as a function of the excitation and ionization laser beam intensity
and the selection aperture size. Although details are different, the global
trends of the measurements agree well with the calculation. With a selection
aperture diameter of 52 m, a current of pA is
measured, which according to calculations is 63% of the current equivalent of
the transmitted atomic flux. Taking into account the ionization degree the ion
beam peak reduced brightness is estimated at A/(msreV).Comment: 13 pages, 9 figure
Longitudinal static optical properties of hydrogen chains: finite field extrapolations of matrix product state calculations
We have implemented the sweep algorithm for the variational optimization of
SU(2) x U(1) (spin and particle number) invariant matrix product states (MPS)
for general spin and particle number invariant fermionic Hamiltonians. This
class includes non-relativistic quantum chemical systems within the
Born-Oppenheimer approximation. High-accuracy ab-initio finite field results of
the longitudinal static polarizabilities and second hyperpolarizabilities of
one-dimensional hydrogen chains are presented. This allows to assess the
performance of other quantum chemical methods. For small basis sets, MPS
calculations in the saturation regime of the optical response properties can be
performed. These results are extrapolated to the thermodynamic limit.Comment: Submitted to J. Chem. Phy
Energy-weighted density matrix embedding of open correlated chemical fragments
We present a multi-scale approach to efficiently embed an ab initio
correlated chemical fragment described by its energy-weighted density matrices,
and entangled with a wider mean-field many-electron system. This approach,
first presented in Phys. Rev. B, 98, 235132 (2018), is here extended to account
for realistic long-range interactions and broken symmetry states. The scheme
allows for a systematically improvable description in the range of correlated
fluctuations out of the fragment into the system, via a self-consistent
optimization of a coupled auxiliary mean-field system. It is discussed that the
method has rigorous limits equivalent to existing quantum embedding approaches
of both dynamical mean-field theory, as well as density matrix embedding
theory, to which this method is compared, and the importance of these
correlated fluctuations is demonstrated. We derive a self-consistent local
energy functional within the scheme, and demonstrate the approach for Hydrogen
rings, where quantitative accuracy is achieved despite only a single atom being
explicitly treated.Comment: 14 pages, 8 figure
Controlling the pair momentum of the FFLO state in a 3D Fermi gas through a 1D periodic potential
The question whether a spin-imbalanced Fermi gas can accommodate the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been the subject of intense
study. This state, in which Cooper pairs obtain a nonzero momentum, has
hitherto eluded experimental observation. Recently, we demonstrated that the
FFLO state can be stabilized in a 3D Fermi gas, by adding a 1D periodic
potential. Until now it was assumed that the FFLO wave vector always lies
parallel to this periodic potential (FFLO-P). In this contribution we show
that, surprisingly, the FFLO wave vector can also lie skewed with respect to
the potential (FFLO-S). Starting from the partition sum, the saddle-point free
energy of the system is derived within the path-integral formalism. Minimizing
this free energy allows us to study the different competing ground states of
the system. To qualitatively understand the underlying pairing mechanism, we
visualize the Fermi surfaces of the spin up and spin down particles. From this
visualization, we find that tilting the FFLO wave vector with respect to the
direction of the periodic potential, can result in a larger overlap between the
pairing bands of both spin species. This skewed FFLO state can provide an
additional experimental signature for observing FFLO superfluidity in a 3D
Fermi gas.Comment: 19 pages, 3 figure
The diurnal evolution of the urban heat island of Paris: a model-based case study during Summer 2006
The urban heat island (UHI) over Paris during summer 2006 was simulated using the Advanced Regional Prediction System (ARPS) updated with a simple urban parametrization at a horizontal resolution of 1 km. Two integrations were performed, one with the urban land cover of Paris and another in which Paris was replaced by cropland. The focus is on a five-day clear-sky period, for which the UHI intensity reaches its maximum. The diurnal evolution of the UHI intensity was found to be adequately simulated for this five day period. The maximum difference at night in 2 m temperature between urban and rural areas stemming from the urban heating is reproduced with a relative error of less than 10%. The UHI has an ellipsoidal shape and stretches along the prevailing wind direction. The maximum UHI intensity of 6.1 K occurs at 23:00 UTC located 6 km downstream of the city centre and this largely remains during the whole night. An idealized one-column model study demonstrates that the nocturnal differential sensible heat flux, even though much smaller than its daytime value, is mainly responsible for the maximum UHI intensity. The reason for this nighttime maximum is that additional heat is only affecting a shallow layer of 150 m. An air uplift is explained by the synoptic east wind and a ramp upwind of the city centre, which leads to a considerable nocturnal adiabatic cooling over cropland. The idealized study demonstrates that the reduced vertical adiabatic cooling over the city compared to cropland induces an additional UHI build-up of 25%. The UHI and its vertical extent is affected by the boundary-layer stability, nocturnal low-level jet as well as radiative cooling. Therefore, improvements of representing these boundary-layer features in atmospheric models are important for UHI studies
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