Nuclear spin dephasing in the frozen core

This paper reports on optical Raman heterodyne detection (RHD) of nuclear magnetic resonance (NMR) in low temperature solids demonstrated to be a sensitive and powerful spectroscopic technique. Recently the authors have extended RHD from its original application to hyperfine (hf) spectra of rare earth ions to superhyperfine (shf) spectra of Al in Ruby. For shf spectra, the radio frequency and optical photons are absorbed by different atoms (Al and Cr respectively). The Ram transitions are activated by the exchange and dipole coupling between Al and Cr ions. Excellent agreement of theoretical and experimental Al spectra was obtained using earlier measured values of the various coupling and quadrupole constants. The authors have now extended this work to observation of Al NMR spin echoes. This is of interest, since for the first time, the authors are able to measure dephasing times in the frozen core. The concept of the frozen core or diffusion barrier was introduced by Bloembergen in 1949 to describe spin diffusion and later dephasing phenomena. According to this picture, nuclei close to a paramagnetic ion will not undergo spin flips with those in the bulk because of detuning. Thus spin flips with those in the bulk because of detuning. Thus the authors expect the dephasing time of Al ions in the frozen core to be much longer than those in the bulk. The authors' echo measurements confirm this picture