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Linear Response

  • Laurent-Patrick Lévy
Chapter
Part of the Texts and Monographs in Physics book series (TMP)

Abstract

In a sufficiently weak magnetic field (i.e., such that the Zeeman energy is small compared with k B), an ensemble of paramagnetic spins does not manifest much alignment. Many spins are in reversed states with energies well above the ground state energy. What processes lead to this thermal disorder? We can cite dipolar interaction as an example: a Bohr magneton creates a magnetic field of order 1 T at a distance of 3 Å (the field of a nuclear magneton at this distance is a thousand times weaker). In such conditions, quantum states of spins coupled by dipolar interactions cannot be those of isolated spins. Nevertheless, at high temperatures, spatial correlations between spins become negligible. This is because dipolar fields are then fluctuating rapidly and can be considered as random ‘forces’ (equivalent to Langevin forces [178, 179]). Such effects bring the system towards thermodynamic equilibrium. Fluctuations then have only a slight effect on average with regard to thermodynamic quantities. Fluctuations in dipolar fields can nevertheless induce transitions between the various states accessible at neighbouring sites. In this way, they decrease temporal correlations between spins. It is the correlation functions and response functions which are sensitive to fluctuations.

Keywords

Correlation Function Linear Response Larmor Frequency Bloch Equation Knight Shift 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2000

Authors and Affiliations

  • Laurent-Patrick Lévy
    • 1
  1. 1.MPI für Festkörperforschung, Laboratoire des Champs Magnétiques IntensesCNRSGrenoble Cedex 9France

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