Theory of Dipolar Echoes in Amorphous Matter at Low Temperatures:Analogies and Differences with the Nuclear Spin Echoes and the Role of an Applied Magnetic Field

The phenomenon of Nuclear Spin Echo (NSE) in Nuclear Magnetic Resonance (NMR) will be reviewed and its relevance for the MRI medical diagnostics technique will be emphasized. Further, the theory of the phenomenon of Polarization Echo (PE) (also known as dipole echo) in amorphous matter will be discussed and the similarities and differences between NSEs and PEs will be considered. The experimental and theoretical situation in the study of PEs in amorphous solids at very low temperatures will be reviewed, when the presence of even weak magnetic fields presents a puzzling response on the PE amplitude. Whilst the current viewpoint is in favour of the role played by nuclear electric quadrupole moments in the material's nuclei coupling to the solid's tunneling systems, on the one side, and to its nuclear magnetic dipole moments, on the other side, it will be shown that an entirely different - and simpler - explanation is possible. Namely, our explanation takes into account the multiphase (amorphous + nanocrystalline) nature of the materials and the existence of anomalous multi-level tunneling systems within the network-modifying component of these amorphous solids. A tentative explanation for the so-called isotope effect on the PE amplitude in the presence of a magnetic field will be also proposed.