Measurements of Nuclear Spin-Lattice Relaxation Time of Solid Hydrogen in the Ordered State

Nuclear spin lattice relaxation time ( T 1 ) of solid hydrogen with the fractional ortho content ( X ) greater than 0.6 has systematically been measured at temperatures ( T ) between 4.2 K and 0.5 K by using a pulsed NMR spectrometer of ν=22 MHz. As temperature decreases, T 1 decreases monotonically until the phase transition takes place, and at the transition T 1 increases abruptly by a factor 5 for X =0.71. Below the transition, T 1 shows strong dependence both on T and X . These results are analyzed by using a formula derived from the libron theory in the ordered state. The minimum excitation energy of librons is estimated to be Δ L =(8.5±2.0)(1.8 X -1) K.