Extended Exchange Interactions in Cr_1/3NbS_2,MnBi, and VTe: What is Nature saying?

Exchange interactions are well-known as fundamental to magnetism and magnetic phenomena, deriving ultimately from the interaction of the Pauli principle with electromagnetic and other atomic-level interactions. There is a community belief that such interactions generally fall off relatively quickly with interatomic distance (recognizing that at short distances, generally less than 3 Angstrom, an interaction sign change may occur). Here I present evidence from 3 recent theoretical studies that find evidence, generally supported by relevant experimental data, for substantial exchange interactions even at interatomic distances approaching 6 Angstroms, where one would ordinarily expect such exchange interactions to be minimal. What is Nature saying here? Is there operative a mechanism, or mechanisms, additional to the normal, well-known exchange, superexchange, and double exchange interactions? I will discuss these issues.

References:

1. N. Sirica et al, Communications Physics (Nature) 3, 1 (2020).
2. T.J. Williams et al, Applied Physics Letters 108, 192403 (2016).
3. D. S. Parker et al, "Insulating antiferromagnetism in VTe", under revision (2020).