Nuclear Magnetic Resonance in Layered Diselenides
The layered dichalcogenides TX2 (where T is a transition metal and X = S, Se, Te) have been of recent interest since their electronic properties are dominated by the two-dimensional geometry of their crystal structures.1 The compounds containing group VB transition metals (V, Nb, Ta) exhibit striking variations in their electronic properties. One finds among them superconductors (2H-NbSe2, 4H-NbSe2), good metals (VSe2), and semiconducting materials (1 T-TaSe2). Attempts to characterize the band structure of these materials by measurement of transport properties,2,3 photoemission,4 and electron paramagnetic resonance,5 as well as recent studies of the solid solutions TixTa1−xS2 6,7, have illustrated the remarkable sensitivity of these materials to the details of the crystal structure. We have attempted to augment the current experimental understanding of these systems by conducting a systematic study of the Knight shifts of the 77Se nucleus in the diselenides containing transition metals of group IVB, VB, and VIB of the periodic table. Since the 77Se nucleus has an intrinsic spin of 1/2, it is relatively unaffected by the low symmetry site of the Se atoms and can be observed without difficulty in all of the materials mentioned. Shifts in the position for resonance of the 77Se nucleus arise from the polarization of Fermi surface electrons and the magnitude of the shift can be related to an average over all such electrons. In this sense such measurements compliment the studies of transport properties, which may be significantly affected by a certain subset of carriers. The availability of this universal probe permits the study not only of the variation occuring between transition metals of various groups and periods within the periodic table, but also very striking electronic differences among the various polytypes (different crystal structures) of the same compound.
KeywordsElectron Paramagnetic Resonance Knight Shift Anisotropic Chemical Shift Intrinsic Spin Remarkable Sensitivity
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