25,963 research outputs found
Ising metamagnets in thin film geometry: equilibrium properties
Artificial antiferromagnets and synthetic metamagnets have attracted much
attention recently due to their potential for many different applications.
Under some simplifying assumptions these systems can be modeled by thin Ising
metamagnetic films. In this paper we study, using both the Wang/Landau scheme
and importance sampling Monte Carlo simulations, the equilibrium properties of
these films. On the one hand we discuss the microcanonical density of states
and its prominent features. On the other we analyze canonically various global
and layer quantities. We obtain the phase diagram of thin Ising metamagnets as
a function of temperature and external magnetic field. Whereas the phase
diagram of the bulk system only exhibits one phase transition between the
antiferromagnetic and paramagnetic phases, the phase diagram of thin Ising
metamagnets includes an additional intermediate phase where one of the surface
layers has aligned itself with the direction of the applied magnetic field.
This additional phase transition is discontinuous and ends in a critical end
point. Consequently, it is possible to gradually go from the antiferromagnetic
phase to the intermediate phase without passing through a phase transition.Comment: 8 figures, accepted for publication in Physical Review
Towards experimental entanglement connection with atomic ensembles in the single excitation regime
We present a protocol for performing entanglement connection between pairs of
atomic ensembles in the single excitation regime. Two pairs are prepared in an
asynchronous fashion and then connected via a Bell measurement. The resulting
state of the two remaining ensembles is mapped to photonic modes and a reduced
density matrix is then reconstructed. Our observations confirm for the first
time the creation of coherence between atomic systems that never interacted, a
first step towards entanglement connection, a critical requirement for quantum
networking and long distance quantum communications
Control of decoherence in the generation of photon pairs from atomic ensembles
We report an investigation to establish the physical mechanisms responsible
for decoherence in the generation of photon pairs from atomic ensembles, via
the protocol of Duan et. al for long distance quantum communication [Nature
(London) 414, 413 (2001)] and present the experimental techniques necessary to
properly control the process. We develop a theory to model in detail the
decoherence process in experiments with magneto-optical traps. The
inhomogeneous broadening of the ground state by the trap magnetic field is
identified as the principal mechanism for decoherence. In conjunction with our
theoretical analysis, we report a series of measurements to characterize and
control the coherence time in our experimental setup. We use copropagating
stimulated Raman spectroscopy to access directly the ground state energy
distribution of the ensemble. These spectroscopic measurements allow us to
switch off the trap magnetic field in a controlled way, optimizing the
repetition rate for single-photon measurements. With the magnetic field off, we
then measure nonclassical correlations for pairs of photons generated by the
ensemble as a function of the storage time of the single collective atomic
excitation. We report coherence times longer than 10 microseconds,
corresponding to an increase of two orders of magnitude compared to previous
results in cold ensembles. The coherence time is now two orders of magnitude
longer than the duration of the excitation pulses. The comparison between these
experimental results and the theory shows good agreement. Finally, we employ
our theory to devise ways to improve the experiment by optical pumping to
specific initial states.Comment: 16 pages, 11 figures, submitted for publicatio
Phase control of La2CuO4 in thin-film synthesis
The lanthanum copper oxide, La2CuO4, which is an end member of the prototype
high-Tc superconductors (La,Sr)2CuO4 and (La,Ba)2CuO4, crystallizes in the
"K2NiF4" structure in high-temperature bulk synthesis. The crystal chemistry,
however, predicts that La2CuO4 is at the borderline of the K2NiF4 stability and
that it can crystallize in the Nd2CuO4 structure at low synthesis temperatures.
In this article we demonstrate that low-temperature thin-film synthesis
actually crystallizes La2CuO4 in the Nd2CuO4 structure. We also show that the
phase control of "K2NiF4"-type La2CuO4 versus "Nd2CuO4"-type La2CuO4 can be
achieved by varying the synthesis temperature and using different substrates.Comment: 4 pages, 5 figures, submitted to PRB, revte
Direct measurement of decoherence for entanglement between a photon and stored atomic excitation
Violations of a Bell inequality are reported for an experiment where one of
two entangled qubits is stored in a collective atomic memory for a user-defined
time delay. The atomic qubit is found to preserve the violation of a Bell
inequality for storage times up to 21 microseconds, 700 times longer than the
duration of the excitation pulse that creates the entanglement. To address the
question of the security of entanglement-based cryptography implemented with
this system, an investigation of the Bell violation as a function of the
cross-correlation between the generated nonclassical fields is reported, with
saturation of the violation close to the maximum value allowed by quantum
mechanics.Comment: 4 pages, 3 figures. Minor changes. Published versio
Superconductivity and Cobalt Oxidation State in Metastable Na(x)CoO(2-delta)*yH2O (x ~ 1/3; y ~ 4x)
We report the synthesis and superconducting properties of a metastable form
of the known superconductor NaxCoO2*yH2O (x ~ 1/3, y ~ 4x). Instead of using
the conventional bromine-acetonitrile mixture for sodium deintercalation, we
use an aqueous bromine solution. Using this method, we oxidize the sample to a
point that the sodium cobaltate becomes unstable, leading to formation of other
products if not controlled. This compound has the same structure as the
reported superconductor, yet it exhibits a systematic variation of the
superconducting transition temperature (Tc) as a function of time. Immediately
after synthesis, this compound is not a superconductor, even though it contains
appropriate amounts of sodium and water. The samples become superconducting
with low Tc values after ~ 90 h. Tc continually increases until it reaches a
maximum value (4.5 K) after about 260 h. Then Tc drops drastically, becoming
non-superconducting approximately 100 h later. Corresponding time-dependent
neutron powder diffraction data shows that the changes in superconductivity
exhibited by the metastable cobaltate correspond to slow formation of oxygen
vacancies in the CoO2 layers. In effect, the formation of these defects
continually reduces the cobalt oxidation state causing the sample to evolve
through its superconducting life cycle. Thus, the dome-shaped superconducting
phase diagram is mapped as a function of cobalt oxidation state using a single
sample. The width of this dome based on the formal oxidation state of cobalt is
very narrow - approximately 0.1 valence units wide. Interestingly, the maximum
Tc in NaxCoO2*yH2O occurs when the cobalt oxidation state is near 3.5. Thus, we
speculate that the maximum Tc occurs near the charge ordered insulating state
that correlates with the average cobalt oxidation state of 3.5.Comment: 22 pages, 9 figures, 1 tabl
Tau Polarizations in the Three-body Slepton Decays with Stau as the NLSP
In the gauge-mediated supersymmetry breaking models with scalar tau as the
next-to-lightest supersymmetric particle, a scalar lepton may decay dominantly
into its superpartner, tau lepton, and the lightest scalar tau particle. We
give detailed formulas for the three-body decay amplitudes and the polarization
asymmetry of the outgoing tau lepton . We find that the tau polarizations are
sensitive to the model parameters such as the stau mixing angle, the neutralino
to slepton mass ratio and the neutralino mixing effect.Comment: 13 pages, 5 figures, RevTe
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