Pulsed Nuclear Magnetic Resonance on ~(3)He Adsorbed on Bare and ~(4)He Preplated MCM-41 Using DC SQUID Detection

摘要

We report low field DC SQUID NMR measurements down to 1.5 K of ~(3)He adsorbed in the pores of the mesoporous molecular sieve MCM-41. In the first experiment measurements were made on ~(3)He adsorbed onto the bare pore walls of MCM-41 with coverages ranging from n_(3_(He)) velence 0.86n_(1) to full pores at n_(3_(He)) velence 1.79n_(1), where n_(1) is the coverage for monolayer completion. A second experiment was performed with low ~(3)He coverages (n_(3_(He)) approx 0.01n_(1)) on ~(4)He preplated pores, where a crossover to a quasi-1D state is expected to occur at temperatures sufficiently below 700 mK. In both experiments relaxation times T_(1) and T_(2)~(*) were measured as a function of temperature and coverage at frequencies from 80 to 240 kHz. The frequency dependence of the linewidth in the pure ~(3)He experiment is extremely weak therefore T_(2)~(*) approx= T_(2). The 1.5 K isotherm shows a small minimum in T_(2)~(*) at a coverage corresponding to monolayer completion. In the experiment with ~(4)He preplating there was no significant change in T_(1) or T_(2)~(*) when the ~(3)He coverage was doubled from n_(3_(He)) velence 0.01n_(1) to 0.02n_(1) at a ~(4)He preplating of n_(4_(He)) velence 1.05n_(1). This suggests that the relaxation times are dominated by single particle effects in the low density regime.