The Study Of Spin Caloritronics In Spin-Semiconducting Silicene And Graphene Nanoribbons

Spin caloritronics,a new electronics of studying the relationship between spintronics and thermoelectrics,which has been a popular research direction in condensed matter physics.In recent years,the rapid development of low-dimensional materials open a new direction towards the choices of the future devices'materials.For graphene and silicene,they have been arousing universal attention because of their novel physical and chemical properties.In this paper,we designed several devices which are composed of spin-semiconducting silicene and graphene nanoribbons.Using density functional theory based on first principles and nonequilibrium Green's function method,we have investigated the thermal spin transport properties of these devices.Our main results are listed as follows in details.Firstly,we study a sawtooth-like silicene nanoribbons?ST SiNRs?.Our results show that the single-hydrogen-terminated ST SiNRs have subbands with opposite spins across the Fermi level,which is a characteristic of spin semiconductor.We investigated the thermally induced spin transport through ST SiNRs.The device is a homojunction consisting of left electrode and right electrode with singled-hydrogen-terminated ST SiNRs.The flowing of thermally excited carriers may produce a net spin current due to the special band structure of ST SiNRs when temperature gradient is applied across the ribbons.Particularly,the flow directions of spin-up and spin-down electrons are opposite and the current magnitudes are nearly equal.As a result,the perfect spin-dependent Seebeck effect?SDSE?occurs.As for total spin current,it shows spin-dependent Seebeck diode?SDSD?effect.In addition,we also study the spin caloritronics of the ST SiNRs homojunctions with different widths.For even-width ribbons,the spin-up and the spin-down currents are still symmetrical with each other about the zero-current axis,which means that even-width ones tend to exhibit better SDSD and SDSE than the odd-width ones.And the charge currents in even-width ST SiNRs show a typical negative differential thermal resistance?NDTR?.In contrast,odd-width ST SiNRs manifest features of a thermoelectric diode and can be used to produce both charge and spin currents produced by temperature gradient.Furthermore,we study thermal spin transport in another structure of silicene nanoribbons with“T”shape called TSiNRs.In this device,the SDSE and SDSD effect still emerge.However,we find that the appearance of these effects does not depend on the parity of ribbon width.These findings can be extended to other spin semiconductors and open a door for designs of new materials and spin caloritronic devices.Secondly,we study a new zigzag graphene nanoribbons?ZGNRs?.This nanoribbon is composed of narrowest ZGNRs which are connected by square-shaped carbon tetragons in the proper place and in zigzag direction the width is 5 carbon atoms,which is called?5,5?-ZGNRs.We study the thermal transport properties of a new spin caloritronics device based on the new ZGNRs homojunction.The homojunction consisting of single-hydrogen-terminated?5,5?-ST SiNRs as the left and right electrode.Our results show that?5,5?-ST SiNRs have an antiferromagnetic ground state,showing as a spin semiconductor.By applying temperature gradient between the source and the drain,spin-up and spin-down currents are driven and flow just in opposite directions.As a result,the spin-dependent Seebeck effect?SDSE?emerges.In addition,we can obtain the perfect SDSE and pefect thermal spin filtering effect by applying gate voltage.Thus,the potential value of(n_1,n₂)-ZGNRs will have significant impact on the design and development of new electronic devices in the future.