Korringa ratio

R =g/U )h/ r o(AgoP Korringa ratio X molecular field constant... 

The effect of interactions on the Korringa ratio 17 can be illustrated for the case of the Stoner model of susceptibility enhancement. The generalization of Eq. (3.6) for the nonuniform susceptibility is... 

With these characteristics of relaxation in normal metals in mind, let us now consider the behavior of the dynamic, nonuniform susceptibility in a low-density metal approaching the critical region. It follows from the foregoing discussion that, within the Stoner model, the Korringa ratio T7 should decrease as the static enhancement increases. This can be tested directly with the NMR data for cesium. It is evident in Fig. 3.6 that the prediction of the Stoner model is not borne out—17 increases in the low-density region where the static susceptibility enhancement also increases. Thus, we are again led to the conclusion that the susceptibility enhancement in the low-density metal is of a fundamentally different character than that of the normal, dense metal near T . 

A further piece of supporting evidence is to be found by considering the entrancement factor 17 >vhich is the ratio of the actual nuclear magnetic relaxation rate to that calculated with the aid of the Korringa relationship. The model proposed by Mott predicts that r o and this is again found to be the case (Figure 7.22). 

In liquid alloys of the M-M, M-S and S-SC it is possible to make measurements at compositions or temperatures in which a metallic description of the electron states begins to be valid. Mott (1971) advocates the use of Eq. (7.31) in these situations. The electron states at the Fermi level are no longer fully localised but both S and o respond to the value chosen for g. Warren s NMR work enables experimental nuclear relaxation rates to be compared with those predicted by the Korringa relationship. The ratio of the experimental to the predicted rate (denoted by Warren as ri) turns out to be > 10 for liquids in which a < 200" cm" and this result is consistent with the onset of Mott localisation. For metallic liquids, on the other hand, 17 1, so that in the intermediate regime 77 should fall in the range 1 <77 <10. This behaviour has, in fact, been observed for a variety of hquids (Warren (1971)), including In2 Te3, Ga2 Te3 and Sb2 Te3. 

The Korringa rate (l/7i)Korr. can be calculated directly from the measured Knight shift using Eq. (3.12) and compared with the value of l/Tj measured independently. It is convenient to consider the ratio... 

Fig. 3.6 Korringa enhancement ratio, defined in Eq. (3.13), as a function of density in liquid cesium at various constant pressures (El-Hanany et al., 1983 Warren et al., 1989). 68...

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