619 research outputs found
Different Clusters of Text from Ancient China, Different Mathematical Ontologies
Sources attesting to mathematical activities in ancient China form at least four distinct clusters of texts, bespeaking at least four different—though overlapping—ways of practicing mathematics. I will focus on two such sets of documents: the canons that in the seventh century constituted one of the two curricula taught in the Imperial “School of Mathematics,” and manuscripts recently excavated from tombs sealed in the last centuries BCE. I will argue that these two sets of documents testify to two different ways of practicing mathematics, which related to different material practices. Accordingly, we can perceive that mathematical objects were shaped and explored in different ways, with significant consequences for the knowledge produced
Ultrafast Enhancement of Ferromagnetism via Photoexcited Holes in GaMnAs
We report on the observation of ultrafast photo-enhanced ferromagnetism in
GaMnAs. It is manifested as a transient magnetization increase on a 100-ps time
scale, after an initial sub-ps demagnetization. The dynamic magnetization
enhancement exhibits a maximum below the Curie temperature Tc and dominates the
demagnetization component when approaching Tc. We attribute the observed
ultrafast collective ordering to the p-d exchange interaction between
photoexcited holes and Mn spins, leading to a correlation-induced peak around
20K and a transient increase in Tc.Comment: 4 page
Observation of inter-Landau-level quantum coherence in semiconductor quantum wells
Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a
non-radiative coherence between the discrete Landau levels of an undoped
quantum well and study its dynamics. We observe quantum beats that reflect the
time evolution of the coherence between the two lowest Landau level
magnetoexcitons. We interpret our observations using a many-body theory and
find that the inter Landau level coherence decays with a new time constant,
substantially longer than the corresponding interband magnetoexciton dephasing
times. Our results indicate a new intraband excitation dynamics that cannot be
described in terms of uncorrelated interband excitations.Comment: 5 pages, 5 figures, to appear in Phys. Rev. B Rapid Communication
Ultrafast dynamics of coherences in the quantum Hall system
Using three-pulse four-wave-mixing optical spectroscopy, we study the
ultrafast dynamics of the quantum Hall system. We observe striking differences
as compared to an undoped system, where the 2D electron gas is absent. In
particular, we observe a large off-resonant signal with strong oscillations.
Using a microscopic theory, we show that these are due to many-particle
coherences created by interactions between photoexcited carriers and collective
excitations of the 2D electron gas. We extract quantitative information about
the dephasing and interference of these coherences.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let
Photonic crystals of coated metallic spheres
It is shown that simple face-centered-cubic (fcc) structures of both metallic
and coated metallic spheres are ideal candidates to achieve a tunable complete
photonic bandgap (CPBG) for optical wavelengths using currently available
experimental techniques. For coated microspheres with the coating width to
plasma wavelength ratio and the coating and host
refractive indices and , respectively, between 1 and 1.47, one can
always find a sphere radius such that the relative gap width (gap
width to the midgap frequency ratio) is larger than 5% and, in some cases,
can exceed 9%. Using different coatings and supporting liquids, the width
and midgap frequency of a CPBG can be tuned considerably.Comment: 14 pages, plain latex, 3 ps figures, to appear in Europhys. Lett. For
more info on this subject see
http://www.amolf.nl/research/photonic_materials_theory/moroz/moroz.htm
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Photocarrier relaxation in the quantum kinetics regime
We present a study of carrier relaxation in GaAs in the quantum- kinetics regime. The results cannot be explained by Bolzmann kinetics. They are in satisfactory agreement with quantum-kinetic theory calculation of the occupation number in k-space
Proportional Relations Between Systolic, Diastolic and Mean Pulmonary Artery Pressure are Explained by Vascular Properties
Recently, it was shown that proportional relationships exist between systolic, diastolic and mean pulmonary artery pressure (Psys, Pdia and Pmean) and that they are maintained under various conditions in both health and disease. An arterial-ventricular interaction model was used to study the contribution of model parameters to the ratios Psys/Pmean, and Pdia/Pmean. The heart was modeled by a time-varying elastance function, and the arterial system by a three-element windkessel model consisting of peripheral resistance, Rp, arterial compliance Ca, and pulmonary artery characteristic impedance Z0. Baseline model parameters were estimated in control subjects and compared to values estimated in patients with pulmonary hypertension. Results indicate that experimentally derived ratios Psys/Pmean and Pdia/Pmean could be accurately reproduced using our model (1.59 and 0.61 vs. 1.55 and 0.64, respectively). Sensitivity analysis showed that the (empirical) constancy of Psys/Pmean and Pdia/Pmean was primarily based on the inverse hyperbolic relation between total vascular resistance (RT; calculated as Rp + Z0) and Ca, (i.e. constant RTCa product). Of the cardiac parameters, only heart rate affected the pressure ratios, but the contribution was small. Therefore, we conclude that proportional relations between systolic, diastolic and mean pulmonary artery pressure result from the constancy of RTCa thus from pulmonary arterial properties, with only little influence of heart rate
Theory of exciton-exciton correlation in nonlinear optical response
We present a systematic theory of Coulomb interaction effects in the
nonlinear optical processes in semiconductors using a perturbation series in
the exciting laser field. The third-order dynamical response consists of
phase-space filling correction, mean-field exciton-exciton interaction, and
two-exciton correlation effects expressed as a force-force correlation
function. The theory provides a unified description of effects of bound and
unbound biexcitons, including memory-effects beyond the Markovian
approximation. Approximations for the correlation function are presented.Comment: RevTex, 35 pages, 10 PostScript figs, shorter version submitted to
Physical Review
Giant Superfluorescent Bursts from a Semiconductor Magnetoplasma
Currently, considerable resurgent interest exists in the concept of
superradiance (SR), i.e., accelerated relaxation of excited dipoles due to
cooperative spontaneous emission, first proposed by Dicke in 1954. Recent
authors have discussed SR in diverse contexts, including cavity quantum
electrodynamics, quantum phase transitions, and plasmonics. At the heart of
these various experiments lies the coherent coupling of constituent particles
to each other via their radiation field that cooperatively governs the dynamics
of the whole system. In the most exciting form of SR, called superfluorescence
(SF), macroscopic coherence spontaneously builds up out of an initially
incoherent ensemble of excited dipoles and then decays abruptly. Here, we
demonstrate the emergence of this photon-mediated, cooperative, many-body state
in a very unlikely system: an ultradense electron-hole plasma in a
semiconductor. We observe intense, delayed pulses, or bursts, of coherent
radiation from highly photo-excited semiconductor quantum wells with a
concomitant sudden decrease in population from total inversion to zero. Unlike
previously reported SF in atomic and molecular systems that occur on nanosecond
time scales, these intense SF bursts have picosecond pulse-widths and are
delayed in time by tens of picoseconds with respect to the excitation pulse.
They appear only at sufficiently high excitation powers and magnetic fields and
sufficiently low temperatures - where various interactions causing decoherence
are suppressed. We present theoretical simulations based on the relaxation and
recombination dynamics of ultrahigh-density electron-hole pairs in a quantizing
magnetic field, which successfully capture the salient features of the
experimental observations.Comment: 21 pages, 4 figure
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