Valley caloritronics and its realization by graphene nanoribbons

We propose and theoretically investigate an idea of valley caloritronics where quantum transport of the valley degrees of freedom is thermally induced. Valley caloritronics addresses questions such as thermal generation of valley polarized current and, more importantly, pure valley current without an accompanying charge current. After establishing a general physical picture, we show that heat-induced pure valley current can be generated by virtue of wedge-shaped graphene nanoribbons in a two-probe device setup. We discover that the quantum transport properties of the valley degree of freedom can be very different when driven by a voltage bias or by a temperature bias. A very surprising result is that an alternating valley current can be thermally generated via gate control: namely the heat-induced valley current changes its flow direction in some quasiperiodic manner versus the value of a gate voltage at a fixed polarity. Our results indicate a vast potential for developing valley caloritronic devices.