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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Computer simulation of spin superradiance

Tatyana Belozerova, Victor K. Henner, Vyacheslav I. Yukalov

Published May 3, 1994• 0 citations
Publisher

Abstract

Coherent effects in polarized systems of nuclear or electron spins using a microscopic model are studied, but not the Bloch equations. It is shown that the spin system in a resonance circuit can emit a coherent superradiant pulse when the system is initially in an essentially unstable state, at a time much shorter than the spin dephasing time T2 when the coupling of the system with the circuit is sufficiently strong, and at the time longer than T2 when this coupling is weak. The important role if dipole spin-spin interactions is elucidated.

DOI
10.1117/12.175149

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Coherent effects in polarized systems of nuclear or electron spins using a microscopic model are studied, but not the Bloch equations. It is shown that the spin system in a resonance circuit can emit a coherent superradiant pulse when the system is initially in an essentially unstable state, at a time much shorter than the spin dephasing time T2 when the coupling of the system with the circuit is sufficiently strong, and at the time longer than T2 when this coupling is weak. The important role if dipole spin-spin interactions is elucidated.
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 question addressed here is: What originates pure spin superradiance in a polarized spin system placed inside a resonantor? The term ``pure'' means that no initial coherence is imposed on spins, and its appearance manifests a purely self-organized collective effect. An accurate solution of evolution equations for a microscopic model is given. The results show that the resonator Nyquist noise does not play any role in starting spin superradiance, but the emergence of the latter is initiated by local spin fluctuations due to nonsecular dipole interactions.
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.
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.