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Physical review. B, Condensed matter

Superradiation in magnetic resonance

C. L. Davis, I. V. Kaganov, Victor K. Henner

Published Nov 1, 2000• 10 citations
Publisher

Abstract

The phenomenon of superradiation due to a resonant circuit and the backward influence of the radiation field in a standard magnetic resonance experiment is investigated. It is shown that although the effect of this phenomenon in the radio-frequency region is small, it leads to a change in the shape of the absorption line, which may be possible to observe in an appropriately designed experiment. The predicted shape of the wings of the absorption curve is unusual in magnetic resonance. This analysis, using a traditional magnetic resonance approach, compares favorably with the results of a numerical simulation of superradiation from a system of spins.

DOI
10.1103/physrevb.62.12328

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Resonance Radiation and Excited Atoms

Allan C. G. Mitchell, Mark W. Zemansky, Sumner P. Davis

First printed in 1934, this book gives a critical survey of the principles and limitations of methods for studying resonance radiation that is the re-emission of absorbed radiation without change of wavelength and of the theoretical interpretation of the phenomenon. At the time of first publication, physicists had come to the limit of the then existing techniques in the field and the book was more useful to astronomers who were interested in line shapes existing in the spectra of stars. In past years other fields and techniques have developed for which the information in this book is of interest. These fields are: (1) those having to do with the determination of spins, magnetic moments and hyperfine structure separations by the methods of 'optical pumping' and 'optical double resonance': (2) optical MASERS; and (3) nuclear resonance absorption of gamma rays.

American Journal of Physics•Aug 1, 1972•1,527 citations
The phenomenon of superradiation due to a resonant circuit and the backward influence of the radiation field in a standard magnetic resonance experiment is investigated. It is shown that although the effect of this phenomenon in the radio-frequency region is small, it leads to a change in the shape of the absorption line, which may be possible to observe in an appropriately designed experiment. The predicted shape of the wings of the absorption curve is unusual in magnetic resonance. This analysis, using a traditional magnetic resonance approach, compares favorably with the results of a numerical simulation of superradiation from a system of spins.
We discuss the coherent behavior of a polarized, nuclear or electron, spin system for which the magnetic dipole radiation emitted in the radio-frequency region, has approximately quadratic dependence on the number of spins. An effective method of describing these phenomena is provided by computer simulation of a microscopic model of the spin system. Important aspects of this numeric simulation are described, together with a comparison with the theoretical predictions. The behavior of the transverse component of the magnetic moment, M+(t){M}^{+}(t)M+(t), in super-radiant conditions is studied. In addition, the role of dipole-dipole interactions in super-radiation phenomena is investigated in detail. It is shown that some important features of super-radiation cannot be described with the Bloch equations.
The peculiarities of coherent spin radiation by magnetic nanomolecules is investigated by means of numerical simulation. The consideration is based on a microscopic Hamiltonian taking into account realistic dipole interactions. Superradiance can be realized only when the molecular sample is coupled to a resonant electric circuit. The feedback mechanism allows for the achievement of a fast spin reversal time and large radiation intensity. The influence on the level of radiation, caused by sample shape and orientation, is analysed. The most powerful coherent radiation is found to occur for an elongated sample directed along the resonator magnetic field.
A comparative analysis is given of spin superradiance and atomic superradiance. Their similarities and distinctions are emphasized. It is shown that, despite a close analogy, these phenomena are fundamentally different. In atomic systems, superradiance is a self-organized process, in which both the initial cause, being spontaneous emission, as well as the collectivizing mechanism of their interactions through the common radiation field, are of the same physical nature. Contrary to this, in actual spin systems with dipole interactions, the latter are the major reason for spin motion. Electromagnetic spin interactions through radiation are negligible and can never produce collective effects. The possibility of realizing superradiance in molecular magnets by coupling them to a resonant circuit is discussed.
ABSTRACT It is shown that the magnetic field local inhomogeneous, caused by the stochastic spin distribution over the volume,effects significantly on the nuclear superradiation process. Keywords: Dicke superradiance, nuclear spin system 1. INTRODUCTION The possibility of the Dicke superradiance' in the nuclear spin system, placed in the magnetic field, was explained byBlombergen and Paund in 1954.2 Experimental researches of this effect related to more later time.3'4 The theoretical explanation of this experiment, based on the solution of the Maxwell - Bloch equations system was proposed (seealso7) in works.5'6 Further development of the nuclear spin superradiance theory is contained in works.8'4 A semi-classical approach,5'6 reproducing main features of the observed effect, contained a number of artificial assumptions. In particular, observation of the single pulses regime with high values of the initial spin polarizations couldn't beexplained with taking in account of Larmor frequency scanning, that took place in the experiment. To obtain thecorrespondence with the experimental data we had to artificially select the transverse relaxation time value.In the present paper we throw out these assumptions and show that the role of additional relaxation mechanismcan play line width, caused by the local magnetic field. Here we are not pretend to give the exact description of thementioned experiment,4 but only point to the sufficiently features of the phenomenon considered.
A spin system is considered with a Hamiltonian typical of molecular magnets, having dipole-dipole interactions and a single-site magnetic anisotropy. In addition, spin interactions through the common radiation field are included. A fully quantum-mechanical derivation of the collective radiation rate is presented. An effective narrowing of the dipole-dipole attenuation, due to high spin polarization is taken into account. The influence of the radiation rate on spin dynamics is carefully analysed. It is shown that this influence is completely negligible. No noticeable collective effects, such as superradiance, can appear in molecular magnets, being caused by electromagnetic spin radiation. Spin superradiance can arise in molecular magnets only when these are coupled to a resonant electric circuit, as has been suggested earlier by one of the authors in Laser Phys. 12, 1089 (2002).
First printed in 1934, this book gives a critical survey of the principles and limitations of methods for studying resonance radiation that is the re-emission of absorbed radiation without change of wavelength and of the theoretical interpretation of the phenomenon. At the time of first publication, physicists had come to the limit of the then existing techniques in the field and the book was more useful to astronomers who were interested in line shapes existing in the spectra of stars. In past years other fields and techniques have developed for which the information in this book is of interest. These fields are: (1) those having to do with the determination of spins, magnetic moments and hyperfine structure separations by the methods of 'optical pumping' and 'optical double resonance': (2) optical MASERS; and (3) nuclear resonance absorption of gamma rays.