Because of the great success in using endohedral muonium to probe the
electronic properties of A3C60 (Chapter 4),
attempts were made[168] to find
a similar signal in the metallic polymer phases of A1C60. The presence
of Mu is suggested by the missing fraction % observed in
transverse field in Rb1C60 and
K1C60, for T<250K. No measurement has yet been
attempted in Cs1C60.
An näive initial guess for the spin exchange rate of Mu@C60 in A1C60 is that it would simply be scaled down by 1/3 (for the lower conduction electron density) from the equivalent sized A3C60 system at the same temperature. However, modifications to the electronic structure due to polymerization and possibly electron-electron interactions will likely alter this. Initial attempts to identify Mu by its spin exchange relaxation in LF (see Section 3.1), using this guess as a rough guide, were unsuccessful[168].
In the following the observation of muonium in the first metallic system other
than the A3C60 materials is reported for the first time. In
zero field, the relaxation function is nearly temperature independent, and
contains a small approximately exponential relaxation superimposed on
a larger, very slowly relaxing signal (see squares in
Fig.6.36). This relaxation has been fit with the sum of an
exponential and a slow gaussian. The resulting relaxation rates are
temperature independent and are shown shown in Fig.6.37.
The relaxation rate of the fast component in K1C60 is considerably
less than that in Rb3C60 at 4K, which can be estimated from
the inset of Fig.4.27 to be about 2.5s-1.
In order to confirm that this signal is due to Mu and also in attempt to measure its hyperfine parameter, decoupling curve measurements (of the Mu asymmetry as a function of applied LF, Eq. (4.4)) were made. The best two curves are shown in Fig.6.38. The behaviour is clearly different from that of Mu@C60 in Rb3C60 (Fig.4.16), with the inflection occuring at much lower fields. The two data sets are fit to the isotropic expression Eq. (4.4), which yields hyperfine parameters of 500(100)MHz at 2K and 1070(100)MHz at 250K. This indicates that the chemical environment of the Mu is not the same as in Rb3C60, and, furthermore, it is temperature dependent. This is not really surprising, considering the distortion of the C60 molecule caused by polymerization.
The quenching of the zero field relaxation (see chapter 5)
shown in Fig.6.36 is likely due to the applied LF exceeding the
internal fields due to randomly oriented 13C and 39,40,41K
nuclear dipoles.
The exponential relaxation may be due to the rather dilute spatial
distribution of these moments.
Taking the quenching of the relaxation as a measure of the
width of the random field distribution, e.g. 5G, we estimate
an early time zero field exponential relaxation rate for diamagnetic muons sampling a
lorentzian field distribution with w=5G (Eq. (5.4)) of
s-1 (see [168]).
The observed
zero field relaxation exceeds this considerably. This may simply be due
to the enhanced effect of the field distribution because of the large
electron magnetic moment of Mu.
In addition, on the plateau of the decoupling curve, where the asymmetry is
high, i.e. in 1kG LF, a temperature scan of the very slow T1 relaxation
of Mu was made. The temperature dependence of the relaxation rate for an
exponential relaxation with amplitude fixed from the low temperature
decoupling curve is shown in Figure 6.39.
The value of the Korringa constant, (T1T)-1, for the linear
regime above 50K is s-1K-1. For
comparison the value of (T1T)-1 in K3C60,
which is obtained from Fig.4.15
by extrapolating down in field from 2T to 0.1T using Eq. (3.4), is
about 7.0
s-1K-1. Thus the Korringa relaxation rate
is
times slower than the näive guess discussed above.
The extremely small Korringa rate is also consistent with the ZF
relaxation rate being dominated by static fields. From this results
we expect that the
low temperature fast component in Rb3C60 (Fig.4.27),
is due to Mu@C60 relaxing via the (slow) low temperature Korringa
mechanism in addition to a contribution from the (more dense) 85,87Rb
moments.
The extremely slow LF relaxation observed here is likely the reason that
earlier attempts to find Mu in Rb1C60 were unsuccessful.
The deviation, below 50K, from linear temperature dependence of the LF relaxation rate at 1kG may be due to one of several factors. The first is simply that the rates are very close to the minimum rate measureable (due to the muon lifetime), and hence any slight late time distortion, due, for example, to imperfect background correction (see Eq. 2.4), may be mistaken for a slow relaxation. Physically more interesting explanations are that at low temperature, there is some degree oflocalization and/or the electrons begin to behave in a 1d manner. We note that in 1d, the Korringa relaxation mechanism leads to temperature independent T1 and is observed in the metallic polymer phases of both Rb1C60 and Cs1C60 [64]. At higher temperature, we note that 13C NMR finds Korringa behaviour in K1C60 down to 70K [64]. The results of Figure 6.39 suggest that similar NMR measurements be made below this temperature.
Finally, the results presented here are apparently inconsistent
with other published results on K1C60 [203].
I believe this inconsistency is entirely because the presence of
two components in the ZF relaxation was not recognized in [203].
In [203], the ZF relaxation was fit to a single component
gaussian relaxation. The temperature independence of the observed relaxation
guarantees that such a fit will yield a temperature independent
relaxation rate (such as that reported in [203]). A brief look
at the raw data of the measurements of [203] indicates that
there is no inconsistency between that data and the ZF data presented above.