6.3.2 High Transverse Field in Rb₁C₆₀

In this section TF measurements on Rb₁C₆₀ at low temperature (complementary to the ZF results of the previous section) are presented.

The temperature dependence of the transverse field damped precession has been analyzed using the same model for the relaxation as in ZF (i.e. Eq. (6.1)). In addition, to avoid overparametrizing, the phases and frequencies of the three signals were constrained to be equal for the three components. If the frequencies were free, a small frequency difference would be indistinguishable from a slow relaxation.

Fig.6.45a shows the two component nature of the relaxation in frequency space. The lineshape is approximately the superposition of two lines one narrow and one broad. This lineshape is quite similar to the observed ¹³C NMR lineshape in Cs₁C₆₀ shown in [64]. In Fig.6.45b, the mean frequency shift relative to a calibration run on high purity Ag is plotted. The calibration run was accomplished by affixing a 99.9985% Ag disk (0.25mm thick) to the front of the sample cell window with Apiezon N grease, and remounting the sample cell, thus reproducing the experimental conditions as closely as possible. The frequency shift is corrected for the known muon Knight shift in Ag (+94 ppm [73]) by the following,

where f_Ag is the observed temperature independent Ag frequency. We note that the frequency shift reported in [204] has the opposite sign, but these authors do not state how the reference frequency was measured. From this plot, the frequency exhibits a positive shift which increases gradually and continuously below 60K. The parameters describing the three component relaxation in 1.5T TF are shown in Fig.6.46. The qualitative agreement with the ZF results (Fig.6.44) is quite clear. Quantitatively, one expects any high TF relaxation rate to be less than the corresponding ZF rate, at least by the geometric factor $1/\sqrt{2}$, and by more in the case where ``non-secular'' relaxation is appreciable in the ZF.

Finally, in Fig.6.47, the results of fitting the three component model to the TF relaxation at 2.8K for various fields are presented. These results may bear on the proposed spin-flop transition which AFMR experiments indicate occurs below 2.7T [68]. Only two points (triangles and nablas in Fig.6.47a) were taken under field cooled (FC) conditions. The rest were zero field cooled (ZFC). No significant difference was found between the FC and ZFC results, so the points are not distinguished in Fig.6.47b. There is no strong systematic field dependence in the relaxation rates for either component. However, there is a strong field dependence at low field in the relative amplitudes. The analysis of the field dependent amplitude is somewhat complicated by the known effect of muon beam focusing in the HELIOS solenoid [77]. The strong field in the beam direction causes the beamspot to contract continuously up to about 2T (above which the beamspot expands). The effect of this is to change the relative magnitude of the non-relaxing signal (background) A_NR relative to the sample signal. This has been approximately accounted for by fixing A_NR(B) (stars in Fig.6.47b). From the plot, it can be seen that the magnitude of the slow component A_S decreases rapidly with field (from the ZF value of about 2A_F) to become equal to A_F by 1T. At higher fields, $A_S \approx A_F$.