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Coherent Nuclear Radiation  [PDF]
V. I. Yukalov,E. P. Yukalova
Physics , 2004,
Abstract: The main part of this review is devoted to the comprehensive description of coherent radiation by nuclear spins. The theory of nuclear spin superradiance is developed and the experimental observations of this phenomenon are considered. The intriguing problem of how coherence develops from initially incoherent quantum fluctuations is analysed. All main types of coherent radiation by nuclear spins are discussed, which are: free nuclear induction, collective induction, maser generation, pure superradiance, triggered superradiance, pulsing superradiance, punctuated superradiance, and induced emission. The influence of electron-nuclear hyperfine interactions and the role of magnetic anisotropy are studied. Conditions for realizing spin superradiance by magnetic molecules are investigated. The possibility of nuclear matter lasing, accompanied by pion or dibaryon radiation, is briefly touched.
Small Kerr-anti-de Sitter black holes are unstable  [PDF]
Vitor Cardoso,Oscar J. C. Dias
Physics , 2004, DOI: 10.1103/PhysRevD.70.084011
Abstract: Superradiance in black hole spacetimes can trigger instabilities. Here we show that, due to superradiance, small Kerr-anti-de Sitter black holes are unstable. Our demonstration uses a matching procedure, in a long wavelength approximation.
Relaxation Regimes of Spin Maser  [PDF]
V. I. Yukalov
Physics , 1997,
Abstract: Spin relaxation in a microscopic model of spin maser is studied theoretically. Seven qualitatively different regimes are found: free induction, collective induction, free relaxation, collective relaxation, weak superradiance, pure superradiance, and triggered superradiance. The initiation of relaxation can be originated either by an imposed initial coherence or by local spin fluctuations due to nonsecular dipole interactions. The Nyquist noise of resonator does not influence processes in macroscopic samples. The relaxation regimes not initiated by an imposed coherence cannot be described by the standard Bloch equations.
Comment on cond-mat/0406502 by V. Yukalov and E. Yukalova on Coherent Nuclear Radiation  [PDF]
E. M. Chudnovsky,D. A. Garanin
Physics , 2004,
Abstract: We show that the argument of Yukalov and Yukalova that dipole-dipole interaction prevents a system of magnetic dipoles from exhibiting superradiance unless assisted by a resonator is incorrect.
Generalised superradiant scattering  [PDF]
Mauricio Richartz,Silke Weinfurtner,A. J. Penner,W. G. Unruh
Physics , 2009, DOI: 10.1103/PhysRevD.80.124016
Abstract: We analyse the necessary and sufficient conditions for the occurrence of superradiance. Starting with a wave equation we examine the possibility of superradiance in terms of an effective potential and boundary conditions. In particular, we show that the existence of an ergoregion is not sufficient; an appropriate boundary condition, e.g. only ingoing group velocity waves at an event horizon, is also crucial. After applying our scheme to the standard examples of superradiance, we show that analogue models of gravity without an event horizon do not necessarily exhibit superradiance. Particularly, we show that the superradiant phenomenon is absent in purely rotating inviscid fluids with vorticity. We argue that there should be a catalogue of superradiant systems that can be found by focusing on the necessary and sufficient conditions outlined below.
Comment on the paper "Electromagnetic Wave Dynamics in Matter-Wave Superradiant Scattering" by L. Deng, M.G. Payne, and E.W. Hagley  [PDF]
Wolfgang Ketterle
Physics , 2010,
Abstract: The paper by Deng et al. (Phys. Rev. Lett. 104, 050402 (2010)) presents an analytic theoretical description of matter-wave superradiance (Phys. Rev. Lett. 85, 4225-4228 (2000)) which claims to go beyond previous theoretical frameworks. I show here that the theory presented in this paper is not a description of superradiance per se, but rather an elegant perturbative description of a Raman amplifier far away from the superradiant threshold. As such, it merely is a limiting case of previously known treatments of superradiance. Two additional new findings of the paper are incorrect: (1) The claim that adiabatic elimination of the excited state of the atoms is only possible when the probe pulse propagates slowly. (2) The prediction that superradiance has a dependence on the sign of the detuning of the pump laser due to a phase-matching condition.
Coherent dynamics of radiating atomic systems in pseudospin representation  [PDF]
V. I. Yukalov
Physics , 2014, DOI: 10.1088/1054-660X/24/9/094015
Abstract: The aim of this review is twofold. First, a general approach is presented allowing for a unified description of dynamics in radiating systems of different nature. Both atomic systems as well as spin assemblies can be treated in the frame of the same mathematical method based on pseudospin (or spin) representation of evolution equations. The approach is applicable to all stages of radiation dynamics, including the most difficult initial quantum stage, where coherence is not yet developed. This makes it possible to study the process of coherent self-organization from the chaotic quantum stage. Second, the approach is illustrated by applying it for the description of several coherent phenomena. Different types of superradiance are characterized: pure superradiance, triggered superradiance, pulsing and punctuated superradiance. The theory is presented of such interesting effects as triggering dipolar waves, turbulent photon filamentation, collective liberation of light, pseudospin atomic squeezing, and operator entanglement production.
Asymmetric Superradiant Scattering Patterns from Bose--Einstein Condensates
Asymmetric Superradiant Scattering Patterns from Bose-Einstein Condensates

CHEN Yuan-Kai,ZHOU Xiao-Ji,YANG Fan,CHEN Xu-Zong,
陈元凯
,周小计,杨帆,陈徐宗

中国物理快报 , 2008,
Abstract: The asymmetric patterns of superradiance from Bose-Einstein condensates are studied for the spatially inhomogeneous pump pulse with the semiclassical Maxwell-Schrodinger equations. The coupling dynamics between the optical field and condensate in the strong pulse and a faded wing in the weak coupling regime are discussed, which not only explain the spatial effects in the process of superradiance, but also supply a new method to control its patterns.
Controlled Dicke Subradiance from a Large Cloud of Two-Level Systems  [PDF]
Tom Bienaime,Nicola Piovella,Robin Kaiser
Physics , 2012, DOI: 10.1103/PhysRevLett.108.123602
Abstract: Dicke superradiance has been observed in many systems and is based on constructive interferences between many scattered waves. The counterpart of this enhanced dynamics, subradiance, is a destructive interference effect leading to the partial trapping of light in the system. In contrast to the robust superradiance, subradiant states are fragile and spurious decoherence phenomena hitherto obstructed the observation of such metastable states. We show that a dilute cloud of cold atoms is an ideal system to look for subradiance in free space and study various mechanisms to control this subradiance.
Cooperativity of a few quantum emitters in a single-mode cavity  [PDF]
Eduardo Mascarenhas,Dario Gerace,Marcelo Fran?a Santos,Alexia Auffèves
Physics , 2013, DOI: 10.1103/PhysRevA.88.063825
Abstract: We theoretically investigate the emission properties of a single-mode cavity coupled to a mesoscopic number of incoherently pumped quantum emitters. We propose an operational measure for the medium cooperativity, valid both in the bad and in the good cavity regimes. We show that the opposite regimes of subradiance and superradiance correspond to negative and positive cooperativity, respectively. The lasing regime is shown to be characterized by nonnegative cooperativity. In the bad cavity regime we show that the cooperativity defines the transitions from subradiance to superradiance. In the good cavity regime it helps to define the lasing threshold, also providing distinguishable signatures indicating the lasing regime. Increasing the quality of the cavity mode induces a crossover from the solely superradiant to the lasing regime. Furthermore, we verify that lasing is manifested in a wide range of positive cooperative behavior, showing that stimulated emission and superradiance can coexist. The robustness of the cooperativity is studied with respect to experimental imperfections, such as inhomogeneous broadening and pure dephasing.
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