Nuclear exchange coupling and electronic structure of low-dimensional semiconductors

We review the nuclear magnetic resonance (NMR) studies of the indirect nuclear exchange coupling and electronic structure of the chain and layered semiconductors Tl(I)M(III)X-2 (M = Tl, Ga, In, X = Se, S, Te) and some other low-dimensional Tl-contained semiconducting compounds. Both univalent and trivalent Tl atoms in these compounds show essential chemical shielding anisotropy despite their formal spherically symmetric 5d(10)6s(2) and 5d(10) electron configurations. Such a behavior results from the sp-hybridization of the Tl electron wave functions. Strong exchange coupling among the spins of Tl3+ and M3+ ions, which reside in neighboring chains or layers, is observed. Such coupling is realized due to the overlap of the Tl3+ and M3+ electron wave functions across the intervening chalcogen atom. This overlap is the important mechanism in the formation of the valence and conduction bands and determines the electronic structure and properties of the compounds. The long-range indirect nuclear exchange coupling via a chalcogen atom is an analog of the Kramers mechanism of electron spin exchange via a nonmagnetic bridge ion. Recent photoemission spectroscopy studies and band-structure calculations of several aforementioned compounds have confirmed the NMR results on the interchain and interlayer overlap.