NMR Anomalies and Superconductivity in Transition Metals: Vanadium
One of the major predictions of BCS theory is that the spin susceptibility in a superconductor should decrease exponentially with temperature, falling to zero at absolute zero.1 An indirect way of testing this prediction is the Knight shift experiment in which one expects that, like the spin susceptibility, the Knight shift K for a superconductor should also vanish exponentially at absolute zero. The nontransition metal superconductors (Hg, Sn, Al) do show a decrease in K with temperature, though they are not in perfect agreement with the BCS prediction.‡ On the contrary, the Knight shift result in transition elements is quite anomalous.3 For vanadium no change in the Knight shift value from the normal (K n ) to the superconducting (K s ) state could be detected.3 A number of theoretical attempts to explain this anomaly invoked the idea that in the presence of an external magnetic field the electronic ground state of superconductors is not a spin-zero singlet state. It was pointed out that the spin—orbit scattering at surface atoms,4 spin—orbit scattering with the periodic lattice,5 or triplet-state pairing6 may result in a nonzero spin electronic ground state. For vanadium the spin—orbit effects are quite small7 and, to the author’s knowledge, there is no experimental indication of the striking consequences of a triplet-state pairing theory of superconductivity in any system.
KeywordsHyperfine Field Knight Shift Spin Susceptibility Electronic Specific Heat Vleck Paramagnetism
Unable to display preview. Download preview PDF.
- A. A. Abrikosov and L. P. Gor’kov, Soviet Phys-JETP 15, 752 (1962).Google Scholar
- 6.M. A. Jensen and H. Suhl, Magnetism, G. T. Rado and H. Suhl, eds., Academic Press, New York (1966), Vol. IIB, p. 199;Google Scholar
- 8.G. M. Eliashberg, Soviet Phys-JETP 12, 1000 (1961);Google Scholar
- 15.N. F. Berk and J. R. Schrieffer, in Proc. 10th Intern. Conf. Low Temp. Phys., 1966, VINITI, Moscow (1967), Vol. I IA, p. 150.Google Scholar