High-resolution nuclear magnetic resonance of superfluid ³B

High-resolution nuclear magnetic resonance measurements of bulk superfluid ³B have been performed at temperatures above 0.5 mK and at pressures from 0.3 to 21.7 bars. We have found that the resonance frequency is shifted from the Larmor frequency of the normal fluid. According to the theory of Greaves the shift at the superfluid transition determines a specific combination, β₃₄₅, of the 5 fourth-order coefficients of the order parameter invariants used in the Ginzburg-Landau description of superfluid ³He. We found that β₃₄₅ approaches the weak coupling limit at low pressure, and decreases at higher pressures qualitatively consistent with the theory of Sauls and Serene but in disagreement with the results of Tang et al.