NMR Knight shift

These include, again in the approximate order of decreasing usefulness, (a) optical spectroscopy, (b) density measurements, (c) NMR (Knight shifts), (d) conductivity measurements (mobilities and temperature coefficients), (e) ESR, and (f) thermochemistry. There is every reason to believe that in the future other techniques will be applied to these studies, and that a number of subtle features of the solutions will be uncovered. 

The pressure dependence of N(EF) has been measured recently through the pressure dependence of the 13C-NMR Knight shift in K3Q0 [94]. In Fig. 25, a plot of In T P) versus K(P) is presented. As shown by this plot, linear behavior is effectively observed, which intersects the y axis at flph = 600 K and = N(EF)V = 0.3 at ambient pressure [94]. Thus the value of Tc appears to be governed by N(EF) and the pressure data suggest that high-frequency intraball phonons are likely to be involved in the superconductivity of fullerenes [20,94]. 

For CeBe, however, even two local probes of magnetism (p,SR and NMR) differ in their results. Feyerherm et al. (1994c) and Amato (1997) point out that pSR Jinight shift data for N < r < Fg appear to be incompatible with NMR Knight shifts imder comparable conditions (in particular under comparable applied fields). The quadrupolar ordering in combination with possible CEF-strain interactions are certainly a severe complicatioiL But the question of whether the differences in spin structure are sample dependent or an intrinsic property, as well as the question of which spin structure (iSR actually sees, remain open at this point. Further studies by different methods are clearty indicated. 

It was Yasuoka et al. (1989) who suggested for the first time the presence of the spin gap based on experiment. They observed a sharp increase in the nuclear spin relaxation lifetime below T (T > T ) in the underdoped region. This was attributed to the opening of a spin gap . They presumed that the spin fi eedom of electrons dies down below T. Alloul et al. (1989) soon also assumed the spin gap fi om NMR Knight shift measurements. 

Fig. 3.5. NMR Knight shift in liquid cesium (El-Hanany et al., 1983 Warren et al., 1989) as a function of temperature at constant pressures of 90 bar closed circles) and 120 bar open squares).

Fig. 4.6. NMR Knight shift as a function of density for liquid mercury close to the liquid-vapor coexistence line (El-Hanany and Warren, 1975 Warren and Hensel, 1982). Upper scale shows the corresponding values of the DC electrical conductivity.

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