15,992 research outputs found
High-resolution imaging of ultracold fermions in microscopically tailored optical potentials
We report on the local probing and preparation of an ultracold Fermi gas on
the length scale of one micrometer, i.e. of the order of the Fermi wavelength.
The essential tool of our experimental setup is a pair of identical,
high-resolution microscope objectives. One of the microscope objectives allows
local imaging of the trapped Fermi gas of 6Li atoms with a maximum resolution
of 660 nm, while the other enables the generation of arbitrary optical dipole
potentials on the same length scale. Employing a 2D acousto-optical deflector,
we demonstrate the formation of several trapping geometries including a tightly
focussed single optical dipole trap, a 4x4-site two-dimensional optical lattice
and a 8-site ring lattice configuration. Furthermore, we show the ability to
load and detect a small number of atoms in these trapping potentials. A site
separation of down to one micrometer in combination with the low mass of 6Li
results in tunneling rates which are sufficiently large for the implementation
of Hubbard-models with the designed geometries.Comment: 15 pages, 6 figure
Anisotropic effect of field on the orthorhombic-to-tetragonal transition in the striped cuprate (La,Nd)_{2-x}Sr_xCuO_4
The Nd-doped cuprate La_{2-y-x}Nd_ySr_xCuO_4 displays a first-order phase
transition at T_d (= 74 K for x=0.10, y = 0.60) to a low-temperature tetragonal
(LTT) phase. A magnetic field H applied || the a-axis leads to an increase in
T_d, whereas T_d is decreased when H || c. These effects show that magnetic
ordering involving both Nd and Cu spins plays a key role in driving the LTO-LTT
transition. Related anisotropic effects are observed in the uniform
susceptibility and the in-plane magnetoresistance.Comment: 5 pages, 5 figure
Calibration of a single atom detector for atomic micro chips
We experimentally investigate a scheme for detecting single atoms
magnetically trapped on an atom chip. The detector is based on the
photoionization of atoms and the subsequent detection of the generated ions. We
describe the characterization of the ion detector with emphasis on its
calibration via the correlation of ions with simultaneously generated
electrons. A detection efficiency of 47.8% (+-2.6%) is measured, which is
useful for single atom detection, and close to the limit allowing atom counting
with sub-Poissonian uncertainty
Carrier-wave Rabi flopping signatures in high-order harmonic generation for alkali atoms
We present the first theoretical investigation of carrier-wave Rabi flopping
in real atoms by employing numerical simulations of high-order harmonic
generation (HHG) in alkali species. Given the short HHG cutoff, related to the
low saturation intensity, we concentrate on the features of the third harmonic
of sodium (Na) and potassium (K) atoms. For pulse areas of 2 and Na atoms,
a characteristic unique peak appears, which, after analyzing the ground state
population, we correlate with the conventional Rabi flopping. On the other
hand, for larger pulse areas, carrier-wave Rabi flopping occurs, and is
associated with a more complex structure in the third harmonic. These new
characteristics observed in K atoms indicate the breakdown of the area theorem,
as was already demonstrated under similar circumstances in narrow band gap
semiconductors
Low-cost Sensor System for Non-invasive Monitoring of Cell Growth in Disposable Bioreactors
AbstractTo ensure productivity and product quality, the parameters of biotechnological processes need to be monitored. Along temperature or pH, one important parameter is the cell density in the culture medium. In this work, we present a low-cost sensor system for online cell growth monitoring in bioreactors via permittivity measurements based on coplanar transmission lines. To evaluate the sensor, E. coli cultivations are performed. We found a good correlation between optical density of the culture medium and the effective permittivity at a frequency of 1kHz when the sensor is submerged into the culture medium. Measurements at higher frequencies additionally allow monitoring the osmolarity. Furthermore, an improved sensor was successfully used for first non-invasive measurements through the polymer wall of a disposable bioreactor
Cooperative Scattering by Cold Atoms
We have studied the interplay between disorder and cooperative scattering for
single scattering limit in the presence of a driving laser. Analytical results
have been derived and we have observed cooperative scattering effects in a
variety of experiments, ranging from thermal atoms in an optical dipole trap,
atoms released from a dark MOT and atoms in a BEC, consistent with our
theoretical predictions.Comment: submitted for special issue of PQE 201
Cold atoms near superconductors: Atomic spin coherence beyond the Johnson noise limit
We report on the measurement of atomic spin coherence near the surface of a
superconducting niobium wire. As compared to normal conducting metal surfaces,
the atomic spin coherence is maintained for time periods beyond the Johnson
noise limit. The result provides experimental evidence that magnetic near field
noise near the superconductor is strongly suppressed. Such long atomic spin
coherence times near superconductors open the way towards the development of
coherently coupled cold atom / solid state hybrid quantum systems with
potential applications in quantum information processing and precision force
sensing.Comment: Major revisions of the text for submission to New Journal of Physics
8 pages, 4 figure
Ad- and desorption of Rb atoms on a gold nanofilm measured by surface plasmon polaritons
Hybrid quantum systems made of cold atoms near nanostructured surfaces are
expected to open up new opportunities for the construction of quantum sensors
and for quantum information. For the design of such tailored quantum systems
the interaction of alkali atoms with dielectric and metallic surfaces is
crucial and required to be understood in detail. Here, we present real-time
measurements of the adsorption and desorption of Rubidium atoms on gold
nanofilms. Surface plasmon polaritons (SPP) are excited at the gold surface and
detected in a phase sensitive way. From the temporal change of the SPP phase
the Rubidium coverage of the gold film is deduced with a sensitivity of better
than 0.3 % of a monolayer. By comparing the experimental data with a Langmuir
type adsorption model we obtain the thermal desorption rate and the sticking
probability. In addition, also laser-induced desorption is observed and
quantified.Comment: 9 pages, 6 figure
Substrate-induced strain effects on Pr_{0.6}Ca_{0.4}MnO_{3} films
We report the characterization of the crystal structure, low-temperature
charge and orbital ordering, transport, and magnetization of
Pr_{0.6}Ca_{0.4}MnO_{3} films grown on LaAlO_{3}, NdGaO_{3}, and SrTiO_{3}
substrates, which provide compressive (LaAlO_{3}) and tensile (NdGaO_{3} and
SrTiO_{3}) strain. The films are observed to exhibit different crystallographic
symmetries than the bulk material, and the low-temperature ordering is found to
be more robust under compressive-- as opposed to tensile-- strain. In fact,
bulk-like charge and orbital ordering is not observed in the film grown on
NdGaO_{3}, which is the substrate that provides the least amount of nominal and
measured, but tensile, strain. This result suggests the importance of the role
played by the Mn--O--Mn bond angles in the formation of charge and orbital
ordering at low temperatures. Finally, in the film grown on LaAlO_{3}, a
connection between the lattice distortion associated with orbital ordering and
the onset of antiferromagnetism is reported.Comment: 12 pages, 7 figure
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