Thermal Detection of Nuclear Spin-Symmetry Conversion of Tunneling Methyl Groups

Nonequilibrium populations of proton spin-symmetry species of methyl groups undergoing tunneling rotation occur in a sample which has been thermally quenched to helium temperatures. When the external magnetic field is adjusted to bring the Larmor frequency of impurity electrons into coincidence with the methyl-group tunneling frequency, cross relaxation releases the stored mechanical energy of tunneling rotation to heat the sample.