不对称石墨烯纳米带的热自旋输运性质研究

Thermal Spin Transport Properties of Asymmetric Graphene Nanoribbons

采用非平衡态格林函数方法结合密度泛函理论的第一性原理,研究了非对称结构石墨烯纳米带的热自旋输运性质。发现在源极,漏极之间施加温度场,在带宽N=4,6,8,10,12的石墨烯纳米带中可以获得方向相反、大小几乎相同的不同自旋极化取向电流,即体系中存在自旋赛贝克效应。对比不同宽度纳米带运输性质,发现带宽对自旋赛贝克效应具有调制作用,即随着带宽N增大,热激发自旋流增大,自旋赛贝克效应增强,而热激发净电流从正向变为负向。此外,当纳米带带宽为4,6,8和12时,体系中还同时存在热负微分电阻效应,这些发现对制备低能耗的热自旋电子学器件具有指导意义。

By using the nonequilibrium Green ＇s function（ NEGF） combined with the density functional theory（ DFT） method,the thermal spin transport properties of asymmetric zigzag graphene nanoribbons（ N-ZGNR） were studied,where N denotes the number of zigzag chains across the ZGNR. The device proposed here is composed of single-hydrogen-terminated ZGNR（ sp2 hybridization） on the upper edge of ZGNR,while double-hydrogen-terminated ZGNR（ sp3 hybridization） on the lower edge of ZGNR. These devices could be hopefully prepared in experiment because the composition of the sp2 and sp3-like bonds at the edges might be feasible experimentally by regulating the chemical potential of hydrogen by pressure of H2gas and temperature. The asymmetric graphene nanoribbons are stable ferromagnetic semiconductor at room temperature,which indicates that it is possible to achieve the perfect spin Seebeck effect. Our calculated results show that by applying a temperature gradient between the source and the drain of the graphene nanoribbons,spin-up and spin-down currents with nearly equal magnitudes are generated and flow in opposite directions,indicating that spin Seebeck effect exists in these systems. We also find that the total spin currents increased with increasing the width of the graphene nanoribbons,indicating that quantum size（ the number of zigzag chains） can modulate the spin Seebeck effect. Moreover,the negative differential thermoelectric resistance can be found in the N = 4,6,8 and 12 systems due to the competitive effect of spin up and spin down current. These results indicating that these devices can be used in low-power-consumption spin caloritronics devices.