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 Physics , 2005, DOI: 10.1016/j.nimb.2005.11.073 Abstract: We report on the observation of a strong perturbation of the electron gas induced by 20 MeV/u U$^{91+}$ ions and 13 MeV/u Pb$^{81+}$ ions channeled in silicon crystals. This collective response (wake effect) in-duces a shift of the continuum energy level by more than 100 eV, which is observed by means of Radiative Electron Capture into the K and L-shells of the projectiles. We also observe an increase of the REC probability by 20-50% relative to the probability in a non-perturbed electron gas. The energy shift is in agreement with calculations using the linear response theory, whereas the local electron density enhancement is much smaller than predicted by the same model. This shows that, for the small values of the adiabaticity parameter achieved in our experiments, the density fluctuations are not strongly localized at the vicinity of the heavy ions.
 Physics , 2006, DOI: 10.1103/PhysRevLett.96.093601 Abstract: We show that in atomic gases cooperative effects like superradiance and subradiance lead to a potential between two atoms that decays like $1/r$. In the case of superradiance, this potential is attractive for close enough atoms and can be interpreted as a coherent mesoscopic effect. The contribution of superradiant pairs to multiple scattering properties of a dilute gas, such as photon elastic mean free path and group velocity, is significantly different from that of independent atoms. We discuss the conditions under which these effects may be observed and compare our results to recent experiments on photon transport in cold atomic gases.
 Physics , 2002, DOI: 10.1103/PhysRevB.68.134301 Abstract: We systematically study the fluorescence of low density Frenkel excitons in a crystal slab containing $N_T$ V-type three-level atoms. Based on symmetric quasi-spin realization of SU(3) in large $N$ limit, the two-mode exciton operators are invoked to depict various collective excitations of the collection of these V-type atoms starting from their ground state. By making use of the rotating wave approximation, the light intensity of radiation for the single lattice layer is investigated in detail. As a quantum coherence effect, the quantum beat phenomenon is discussed in detail for different initial excitonic states. We also test the above results analytically without the consideration of the rotating wave approximation and the self-interaction of radiance field is also included.
 Physics , 2005, Abstract: We discuss the ideal gas like models of a trading market. The effect of savings on the distribution have been thoroughly reviewed. The market with fixed saving factors leads to a Gamma-like distribution. In a market with quenched random saving factors for its agents we show that the steady state income ($m$) distribution $P(m)$ in the model has a power law tail with Pareto index $\nu$ equal to unity. We also discuss the detailed numerical results on this model. We analyze the distribution of mutual money difference and also develop a master equation for the time development of $P(m)$. Precise solutions are then obtained in some special cases.
 Soumen Basak Physics , 2005, Abstract: We investigate the scattering of phase oscillation of Bose-Einstein Condensate by a 'draining of bathtub' type fluid motion. We derive a relation between the reflection and transmission coefficients which exhibits existence of analog of 'Superradiance effect' in BEC vortex with sink.
 A. P. Saiko Physics , 2003, DOI: 10.1134/1.1503754 Abstract: The Dicke superradiance on vibronic transitions of impurity crystals is considered. It is shown that parameters of the superradiance (duration and intensity of the superradiance pulse and delay times) on each vibronic transition depend on the strength of coupling of electronic states with the intramolecular impurity vibration (responsible for the vibronic structure of the optical spectrum in the form of vibrational replicas of the pure electronic line) and on the crystal temperature through the Debye-Waller factor of the lattice vibrations. Theoretical estimates of the ratios of the time delays, as well as of the superradiance pulse intensities for different vibronic transitions well agree with the results of experimental observations of two-color superradiance in the polar dielectric KCl:O2-. In addition, the theory describes qualitatively correctly the critical temperature dependence of the superradiance effect.
 Physics , 2005, DOI: 10.1103/PhysRevB.72.214430 Abstract: We discuss the effects caused by a resonant cavity around a sample of a magnetic molecular crystal (such as Mn${}_{12}$-Ac), when a time dependent external magnetic field is applied parallel to the easy axis of the crystal. We show that the back action of the cavity field on the sample significantly increases the possibility of microwave emission. This radiation process can be supperradiance or a maser-like effect, depending on the strength of the dephasing. Our model provides further insight to the theoretical understanding of the bursts of electromagnetic radiation observed in recent experiments accompanying the resonant quantum tunneling of magnetization. The experimental findings up to now can all be explained as being a maser effect rather than superradiance. The results of our theory scale similarly to the experimental findings, i.e., with increasing sweep rate of the external magnetic field, the emission peaks are shifted towards higher field values.
 Physics , 2013, DOI: 10.1103/PhysRevA.89.033833 Abstract: Recently, Yao et al. demonstrated the creation of coherent emissions in nitrogen gas with two-color (800 nm + 400 nm) ultrafast laser pulses [New J. Phys. 15, 023046 (2013)]. Based on this two-color scheme, here we report on systematic investigation of temporal characteristics of the coherent emission at 391 nm by experimentally examining its evolution with the increase of the plasma channel induced by the intense 800 nm femtosecond laser pulses at a nitrogen gas pressure of ~25 mbar. We reveal unexpected temporal profiles of the coherent emissions, which show significant superradiance signatures owing to the quantum coherence via cooperation of an ensemble of excited N2+ molecules. Our findings shed more light on the mechanisms behind the laser-like emissions induced by strong-field ionization of molecules.
 Physics , 2007, Abstract: We summarize the experimental observations of three case studies on the long-lived rhodium Mossbauer Effect. Extraordinary observations reported in this work manifest the open-up of photonic band gap in analogy to the superconducting gap. Observable gravitational effect is manifested by the superradiance of different sample orientations corresponding to the earth gravity. These observations are of potential importance for detecting gravitational waves and development of the two-photon gamma laser.
 Physics , 2015, DOI: 10.1007/978-3-319-19000-6 Abstract: Superradiance is a radiation enhancement process that involves dissipative systems. With a 60 year-old history, superradiance has played a prominent role in optics, quantum mechanics and especially in relativity and astrophysics. In General Relativity, black-hole superradiance is permitted by dissipation at the event horizon, that allows for energy, charge and angular momentum extraction from the vacuum, even at the classical level. Black-hole superradiance is intimately connected to the black-hole area theorem, Penrose process, tidal forces and even Hawking radiation, which can be interpreted as a quantum version of black-hole superradiance. Various mechanisms (as diverse as massive fields, magnetic fields, anti-de Sitter boundaries, nonlinear interactions, etc...) can confine the amplified radiation and give rise to strong instabilities. These "black-hole bombs" have applications in searches of dark matter and of physics beyond the Standard Model, are associated to the threshold of formation of new black hole solutions that evade the no-hair theorems, can be studied in the laboratory by devising analog models of gravity, and might even provide a holographic description of spontaneous symmetry breaking and superfluidity through the gauge-gravity duality. This work is meant to provide a unified picture of this multifaceted subject, which was missing in the literature. We focus on the recent developments in the field, and work out a number of novel examples and applications, ranging from fundamental physics to astrophysics.
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