Spin hall effect and origins of nonlocal resistance in adatom-decorated graphene

October 21, 2016 - In the recent Physical Review Letters article, Prof. Nikolic, visiting graduate student and Fullbright Scholar Juan Manuel Marmolejo-Tejada from Colombia and collaborators from France and Spain have performed detailed theoretical and computational analysis in order to resolve fierce controversy surrounding very recent experiments reporting an unexpectedly large spin Hall effect (SHE) in graphene decorated with adatoms. Due to the lightness of carbon atoms, relativistic quantum effects like spin-orbit coupling (SOC) are intrinsically small in graphene, so that observing sizable pure spin current driven by longitudinal charge current in the SHE phenomenology requires to employ adatoms which can locally enhance SOC or special substrates which can homogeneously enhance SOC by the proximity effect.

Since the spin current is not directly measurable, experiments rely on nonlocal signals where charge current is injected through one pair of electrodes and nonlocal voltage is detected at a remote pair of electrdes with spin Hall current is assumed to provide mediator of such signal. However, experiments attempting to reproduce originally reported measurements have found that nonlocal voltage is insensitive to the applied external magnetic field, thereby casting a doubt on the SHE interpretation of original measurments. 

Prof. Nikolic and collaborators have found multiple background contributions to the nonlocal resistance, some of which are unrelated to the SHE or any other spin-dependent origin, which could resolve the controversy. Their paper has also designed a multiterminal graphene circuit which suppresses these background contributions and could, therefore, quantify the upper limit for spin-current generation in two-dimensional materials.