Giant room-temperature spin caloritronics in spin-semiconducting graphene nanoribbons

Spin caloritronics refers to generating spin current by thermal gradient. Here we report a theoretical study demonstrating giant spin caloritronic effects in a new class of materials, called spin semiconductors, which are characterized with a spin gap, the energy gap between spin-up and -down channels. Generally, spin Seebeck coefficient (S-s) is shown to increase linearly with the spin gap. Specifically, unprecedented large S-s similar to 3.4 mV/K and spin figure of merit Z(s)T similar to 119 were found in spin-semiconducting graphene nanoribbons (GNRs) with sawtooth (ST) zigzag edges, based on first-principles calculations. Such giant spin caloritronic effects are shown to originate from a large spin gap of ST GNRs, in addition to two other spin-independent features of large band gap and narrow bandwidth which are commonly known for good thermoelectric materials. Our studies suggest that spin-semiconducting nanostructures, such as ST GNRs, are promising candidates for room-temperature spin caloritronics with high efficiency.