Inverse quantum spin Hall effect generated by spin pumping from precessing magnetization into a graphene-based two-dimensional topological insulator

摘要

We propose a multiterminal nanostructure for electrical probing of thequantum spin Hall effect (QSHE) in two-dimensional (2D) topological insulators.The device consists of a ferromagnetic (FM) island with precessingmagnetization that pumps (in the absence of any bias voltage) pure spin currentsymmetrically into the left and right adjacent 2D TIs modeled as graphenenanoribbons with the intrinsic spin-orbit (SO) coupling. The QSH regime of thesix-terminal TI|FM|TI nanodevice, attached to two longitudinal and fourtransverse normal metal electrodes, is characterized by the SO-coupling-inducedenergy gap, chiral spin-filtered edge states within finite length TI regions,and quantized spin Hall conductance when longitudinal bias voltage is applied,despite the presence of the FM island. The same unbiased device, but withprecessing magnetization of the central FM island, blocks completely pumping oftotal spin and charge currents into the longitudinal electrodes whilegenerating DC transverse charge Hall currents. Although these transverse chargecurrents are not quantized, their induction together with zero longitudinalcharge current is a unique electrical response of TIs to pumped pure spincurrent that cannot be mimicked by SO-coupled but topologically trivialsystems. In the corresponding two-terminal inhomogeneous TI|FM|TInanostructures, we image spatial profiles of local spin and charge currentswithin TIs which illustrate transport confined to chiral spin-filtered edgesstates while revealing concomitantly the existence of interfacial spin andcharge currents flowing around TI|FM interfaces and penetrating into the bulkof TIs over some short distance.