50 research outputs found
Scattering amplitudes for dark and bright excitons
Using the composite boson many-body formalism that takes single-exciton
states rather than free carrier states as a basis, we derive the integral
equation fulfilled by the exciton-exciton effective scattering from which the
role of fermion exchanges can be unraveled. For excitons made of
-spin electrons and -spin holes, as in GaAs
heterostructures, one major result is that most spin configurations lead to
brightness-conserving scatterings with equal amplitude , in spite of
the fact that they involve different carrier exchanges. A brightness-changing
channel also exists when two opposite-spin excitons scatter: dark excitons
can end either in the same dark states with an amplitude ,
or in opposite-spin bright states , with a different amplitude
, the number of carrier exchanges being even or odd respectively.
Another major result is that these amplitudes are linked by a striking
relation, , which has decisive consequence for
exciton Bose-Einstein condensation. Indeed, this relation leads to the
conclusion that the exciton condensate can be optically observed through a
bright part only when excitons have a large dipole, that is, when the electrons
and holes are well separated in two adjacent layers.Comment: 8 pages, 4 figure
Defect-Driven Superfluid Crossover for Two-Dimensional Dipolar Excitons Trapped at Thermodynamic Equilibrium
We study ultra-cold dipolar excitons confined in a 10m trap of a double
GaAs quantum well. Based on the local density approximation, we unveil for the
first time the equation of state of excitons at pure thermodynamic equilibrium.
In this regime we show that, below a critical temperature of about Kelvin,
a superfluid forms in the inner region of the trap at a local exciton density
, encircled by a more dilute and normal
component in the outer rim of the trap. Remarkably, this spatial arrangement
correlates directly with the concentration of defects in the exciton density
which exhibits a sudden decrease at the onset of superfluidity, thus pointing
towards an underlying Berezinskii-Kosterlitz-Thouless mechanism.Comment: 8 pages, 8 figure
Macroscopic coherence of a single exciton state in a polydiacetylene organic quantum wire
We show that a single exciton state in an individual ordered conjugated
polymer chain exhibits macroscopic quantum spatial coherence reaching tens of
microns, limited by the chain length. The spatial coherence of the k=0 exciton
state is demonstrated by selecting two spatially separated emitting regions of
the chain and observing their interference.Comment: 12 pages with 2 figure
Feedback cooling of a single trapped ion
Based on a real-time measurement of the motion of a single ion in a Paul
trap, we demonstrate its electro-mechanical cooling below the Doppler limit by
homodyne feedback control (cold damping). The feedback cooling results are well
described by a model based on a quantum mechanical Master Equation.Comment: 4 pages, 3 figure
Quantum to Classical Transition in a Single-Ion Laser
Stimulated emission of photons from a large number of atoms into the mode of
a strong light field is the principle mechanism for lasing in "classical"
lasers. The onset of lasing is marked by a threshold which can be characterised
by a sharp increase in photon flux as a function of external pumping strength.
The same is not necessarily true for the fundamental building block of a laser:
a single trapped atom interacting with a single optical radiation mode. It has
been shown that such a "quantum" laser can exhibit thresholdless lasing in the
regime of strong coupling between atom and radiation field. However, although
theoretically predicted, a threshold at the single-atom level could not be
experimentally observed so far. Here, we demonstrate and characterise a
single-atom laser with and without threshold behaviour by changing the strength
of atom-light field coupling. We observe the establishment of a laser threshold
through the accumulation of photons in the optical mode even for a mean photon
number substantially lower than for the classical case. Furthermore,
self-quenching occurs for very strong external pumping and constitutes an
intrinsic limitation of single-atom lasers. Moreover, we find that the
statistical properties of the emitted light can be adjusted for weak external
pumping, from the quantum to the classical domain. Our observations mark an
important step towards fundamental understanding of laser operation in the
few-atom limit including systems based on semiconductor quantum dots or
molecules.Comment: 19 pages, 4 figures, 10 pages supplement, accepted by Nature Physic
Year in review in Intensive Care Medicine 2009: II. Neurology, cardiovascular, experimental, pharmacology and sedation, communication and teaching
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Mathematical surprises and Dirac's formalism in quantum mechanics
By a series of simple examples, we illustrate how the lack of mathematical
concern can readily lead to surprising mathematical contradictions in wave
mechanics. The basic mathematical notions allowing for a precise formulation of
the theory are then summarized and it is shown how they lead to an elucidation
and deeper understanding of the aforementioned problems. After stressing the
equivalence between wave mechanics and the other formulations of quantum
mechanics, i.e. matrix mechanics and Dirac's abstract Hilbert space
formulation, we devote the second part of our paper to the latter approach: we
discuss the problems and shortcomings of this formalism as well as those of the
bra and ket notation introduced by Dirac in this context. In conclusion, we
indicate how all of these problems can be solved or at least avoided.Comment: Largely extended and reorganized version, with new title and abstract
and with 2 figures added (published version), 54 page
Exciton dans un fil quantique organique
PARIS-BIUSJ-ThĂšses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF
Les liaisons dangereuses. Mariage juif et Ătat moderne Ă Trieste au XVIIIe siĂšcle
Liaisons dangereuses : Jewish Marriage and Modernity in XVIIIth century Trieste.
This article considers Jewish emancipation, tradition and modernity through the issue of marriage law and family formation, specifically the effects upon Jews of the modern state's transfer of marriage from religious to civil law. From archival and rabbinic sources it reconstructs and analyzes the cases of two couples in late 18th- century Trieste whose unusual situations precipitated struggles between the Jewish community and the state over which marriage law - Habsburg or Jewish - would effectively prevail. It explores how individuals choices were constrained by laws custom parental authority public opinion and social class. It shows that the defense of Jewish marriage law in the face of the enlightened absolutist state also entailed rede finition of tradition and incipient secularization.Dubin Lois C., SĂ©nĂ© Jean-François. Les liaisons dangereuses. Mariage juif et Ătat moderne Ă Trieste au XVIIIe siĂšcle. In: Annales. Histoire, Sciences Sociales. 49á” annĂ©e, N. 5, 1994. pp. 1139-1170