### Abstract

The spontaneous emission from nuclear spins has been observed at liquid-4He temperatures. The spins, Cl35 nuclei, are placed in the inductor of a tuned LCR circuit coupled to a dc superconducting quantum interference device used as a radio-frequency amplifier. When the spins are saturated and have zero polarization, the emission is observed at the nuclear quadrupole Larmor frequency as a bump in the spectral density of the Nyquist noise current in the tuned circuit. This bump arises from the temperature-independent fluctuations in the transverse component of the nuclear magnetization. When the spins are in thermal equilibrium, on the other hand, a dip in the spectral density of the current noise is observed, arising from an induced absorption of noise power from the circuit at the Larmor frequency. The standard circuit-coupled Blochs equation, modified to take into account radiation damping and transverse spin fluctuations, is consistent with the predictions of the Nyquist theorem and the Einstein equation for spontaneous emission. A spin-pendulum model for spin noise is described. The signal-to-noise ratio obtainable in a spin-noise measurement is discussed.

Original language | English (US) |
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Pages (from-to) | 1969-1980 |

Number of pages | 12 |

Journal | Physical Review B |

Volume | 36 |

Issue number | 4 |

DOIs | |

State | Published - 1987 |

### ASJC Scopus subject areas

- Condensed Matter Physics

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## Cite this

*Physical Review B*,

*36*(4), 1969-1980. https://doi.org/10.1103/PhysRevB.36.1969