107 research outputs found
Dicke Superradiance in Solids
Recent advances in optical studies of condensed matter have led to the
emergence of phenomena that have conventionally been studied in the realm of
quantum optics. These studies have not only deepened our understanding of
light-matter interactions but also introduced aspects of many-body correlations
inherent in optical processes in condensed matter systems. This article is
concerned with superradiance (SR), a profound quantum optical process predicted
by Dicke in 1954. The basic concept of SR applies to a general -body system
where constituent oscillating dipoles couple together through interaction with
a common light field and accelerate the radiative decay of the system. In the
most fascinating manifestation of SR, known as superfluorescence (SF), an
incoherently prepared system of inverted atoms spontaneously develops
macroscopic coherence from vacuum fluctuations and produces a delayed pulse of
coherent light whose peak intensity . Such SF pulses have been
observed in atomic and molecular gases, and their intriguing quantum nature has
been unambiguously demonstrated. Here, we focus on the rapidly developing field
of research on SR in solids, where not only photon-mediated coupling but also
strong Coulomb interactions and ultrafast scattering exist. We describe SR and
SF in molecular centers in solids, molecular aggregates and crystals, quantum
dots, and quantum wells. In particular, we will summarize a series of studies
we have recently performed on quantum wells in strong magnetic fields. These
studies show that cooperative effects in solid-state systems are not merely
small corrections that require exotic conditions to be observed; rather, they
can dominate the nonequilibrium dynamics and light emission processes of the
entire system of interacting electrons.Comment: 23 pages, 26 figure
Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma
We investigate photoluminescence from a high-density electron-hole plasma in
semiconductor quantum wells created via intense femtosecond excitation in a
strong perpendicular magnetic field, a fully-quantized and tunable system. At a
critical magnetic field strength and excitation fluence, we observe a clear
transition in the band-edge photoluminescence from omnidirectional output to a
randomly directed but highly collimated beam. In addition, changes in the
linewidth, carrier density, and magnetic field scaling of the PL spectral
features correlate precisely with the onset of random directionality,
indicative of cooperative recombination from a high density population of free
carriers in a semiconductor environment
Fermi-Edge Superfluorescence from a Quantum-Degenerate Electron-Hole Gas
We report on the observation of spontaneous bursts of coherent radiation from
a quantum-degenerate gas of nonequilibrium electron-hole pairs in semiconductor
quantum wells. Unlike typical spontaneous emission from semiconductors, which
occurs at the band edge, the observed emission occurs at the quasi-Fermi edge
of the carrier distribution. As the carriers are consumed by recombination, the
quasi-Fermi energy goes down toward the band edge, and we observe a
continuously red-shifting streak. We interpret this emission as cooperative
spontaneous recombination of electron-hole pairs, or superfluorescence, which
is enhanced by Coulomb interactions near the Fermi edge. This novel many-body
enhancement allows the magnitude of the spontaneously developed macroscopic
polarization to exceed the maximum value for ordinary superfluorescence, making
electron-hole superfluorescence even more "super" than atomic
superfluorescence.Comment: 10 pages, 5 figure
Unusual manganese enrichment in the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa
An unusual sediment-hosted manganese deposit is described from the Mesoarchean Mozaan Group, Pongola Supergroup, South Africa. MnO contents up to 15 wt.% were observed in marine clastic and chemical sedimentary rocks. Mn enrichment is interpreted to have resulted from the hydrothermal alteration of manganiferous shale and BIF parent rocks, the primary MnO contents of which are as high as 8.5 wt.%. A detailed mineralogical and petrographic study shows that these parent rocks are characterized by manganoan siderite, ferroan rhodochrosite and other Mn-Fe-rich mineral phases, such as kutnohorite and Fe-Mn-chlorite. Their hypogene alteration gave rise to a diversification of mineral assemblages where ferroan tephroite, calcian rhodochrosite, rhodochrosite, pyrochroite, pyrophanite, cronstedtite, manganoan Fe-rich chlorite and manganoan phlogopite partially or totally replaced the previous mineral assemblage. Thermodynamic modeling performed on chlorite phases associated with the described mineral assemblages illustrates a decrease of average crystallization temperatures from ca. 310 °C during early metamorphic stages to ca. 250 °C during a hydrothermal stage. Mineral transformation processes were thus related to retrograde metamorphism and/or hydrothermal alteration post-dating metamorphism and gave rise to progressive Mn enrichment from unaltered parent to altered rocks. The timing of hypogene alteration was constrained by 40Ar/39Ar dating to between about 1500 and 1100 Ma ago, reflecting tectonic processes associated with the Namaqua-Natal orogeny along the southern Kaapvaal Craton margin. Manganiferous shale and BIF of the Mozaan Group may represent the oldest known examples of primary sedimentary Mn deposition, related to oxidation of dissolved Mn(II) by free oxygen in a shallow marine environment. Oxygenic photosynthesis would have acted as a first-order control during Mn precipitation. This hypothesis opens a new perspective for better constraining secular evolution of sediment-hosted mineral deposits linked to oxygen levels in the atmosphere-hydrosphere system during the Archean Eon
Новые образовательные технологии в информационно-образовательном пространстве НГТУ им. Р. Е. Алексеева
Report on new developments and information technology used in the classroom. Interactive browser e-Learning Course. Webinars OpenMeetings. Software for creating electronic books FlippingBook Publisher.Доклад о новых разработках и информационных технологиях используемых в учебном процессе. Интерактивный броузер e-Learning Course. Вебинары OpenMeetings. ПО для создания электронных книг FlippingBook Publisher
Harmonic Generation from Relativistic Plasma Surfaces in Ultra-Steep Plasma Density Gradients
Harmonic generation in the limit of ultra-steep density gradients is studied
experimentally. Observations demonstrate that while the efficient generation of
high order harmonics from relativistic surfaces requires steep plasma density
scale-lengths () the absolute efficiency of the harmonics
declines for the steepest plasma density scale-length , thus
demonstrating that near-steplike density gradients can be achieved for
interactions using high-contrast high-intensity laser pulses. Absolute photon
yields are obtained using a calibrated detection system. The efficiency of
harmonics reflected from the laser driven plasma surface via the Relativistic
Oscillating Mirror (ROM) was estimated to be in the range of 10^{-4} - 10^{-6}
of the laser pulse energy for photon energies ranging from 20-40 eV, with the
best results being obtained for an intermediate density scale-length
Do Evaporating Black Holes Form Photospheres?
Several authors, most notably Heckler, have claimed that the observable
Hawking emission from a microscopic black hole is significantly modified by the
formation of a photosphere around the black hole due to QED or QCD interactions
between the emitted particles. In this paper we analyze these claims and
identify a number of physical and geometrical effects which invalidate these
scenarios. We point out two key problems. First, the interacting particles must
be causally connected to interact, and this condition is satisfied by only a
small fraction of the emitted particles close to the black hole. Second, a
scattered particle requires a distance ~ E/m_e^2 for completing each
bremsstrahlung interaction, with the consequence that it is improbable for
there to be more than one complete bremsstrahlung interaction per particle near
the black hole. These two effects have not been included in previous analyses.
We conclude that the emitted particles do not interact sufficiently to form a
QED photosphere. Similar arguments apply in the QCD case and prevent a QCD
photosphere (chromosphere) from developing when the black hole temperature is
much greater than Lambda_QCD, the threshold for QCD particle emission.
Additional QCD phenomenological arguments rule out the development of a
chromosphere around black hole temperatures of order Lambda_QCD. In all cases,
the observational signatures of a cosmic or Galactic halo background of
primordial black holes or an individual black hole remain essentially those of
the standard Hawking model, with little change to the detection probability. We
also consider the possibility, as proposed by Belyanin et al. and D. Cline et
al., that plasma interactions between the emitted particles form a photosphere,
and we conclude that this scenario too is not supported.Comment: version published in Phys Rev D 78, 064043; 25 pages, 3 figures;
includes discussion on extending our analysis to TeV-scale,
higher-dimensional black hole
Renormalized Energies of Superfluorescent Bursts from an Electron-Hole Magneto-plasma with High Gain in InGaAs Quantum Wells
We study light emission properties of a population-inverted 2D electron-hole
plasma in a quantizing magnetic field. We observe a series of superfluorescent
bursts, discrete both in time and energy, corresponding to the cooperative
recombination of electron-hole pairs from different Landau levels. The emission
energies are strongly renormalized due to many-body interactions among the
photogenerated carriers, exhibiting red-shifts as large as 20 meV at 15 T.
However, the magnetic field dependence of the lowest Landau level emission line
remains excitonic at all magnetic fields. Interestingly, our time-resolved
measurements show that this lowest-energy burst occurs only after all upper
states become empty, suggesting that this excitonic stability is related to the
`hidden symmetry' of 2D magneto-excitons expected in the magnetic quantum
limit.Comment: 5 pages, 4 figure
On the role of continuum-driven eruptions in the evolution of very massive stars and Population III stars
We suggest that the mass lost during the evolution of very massive stars may
be dominated by optically thick, continuum-driven outbursts or explosions,
instead of by steady line-driven winds. In order for a massive star to become a
WR star, it must shed its H envelope, but new estimates of the effects of
clumping in winds indicate that line driving is vastly insufficient. We discuss
massive stars above roughly 40-50 Msun, for which the best alternative is mass
loss during brief eruptions of luminous blue variables (LBVs). Our clearest
example of this phenomenon is the 19th century outburst of eta Car, when the
star shed 12-20 Msun or more in less than a decade. Other examples are
circumstellar nebulae of LBVs, extragalactic eta Car analogs (``supernova
impostors''), and massive shells around SNe and GRBs. We do not yet fully
understand what triggers LBV outbursts, but they occur nonetheless, and present
a fundamental mystery in stellar astrophysics. Since line opacity from metals
becomes too saturated, the extreme mass loss probably arises from a
continuum-driven wind or a hydrodynamic explosion, both of which are
insensitive to metallicity. As such, eruptive mass loss could have played a
pivotal role in the evolution and fate of massive metal-poor stars in the early
universe. If they occur in these Population III stars, such eruptions would
profoundly affect the chemical yield and types of remnants from early SNe and
hypernovae.Comment: 4 pages, 1 figure, accepted by ApJ Letter
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