| Spin caloritronics is made up of thermoelectrics and spintronics,and introduces the spin degree of freedom,meanwhile,it regards charge and spin as carrier of energy and information transmission.It breaks the limitation of traditional thermoelectrics,is devoted to the conversion between heat and electricity,and has been widely used in the multi-functional devices.It promotes the development of science and technology,and meets the demand in the electronic information technology.Based on density functional theory and non-equilibrium green's function approach,in this thesis,we study the thermal spin transport properties of two-dimensional nanodevices constructed on zigzag graphene nanoribbon and silicon carbide nanoribbon,and has been specifically listed in the following three aspects:Firstly,we design a periodic graphene nanoribbon,which is composed of pentacene,a homojunction nano-device connected by a carbon tetragons.Without any applied electric field or bias,the temperature difference between the left and right electrodes was used to drive the thermal spin currents in the device.By comparing the total energy of three initial magnetic orders on the device,the result reveals that the ground state of the device is antiferromagnetic state.At the same time,we also calculate the band structure,transmission spectrum and the generated thermal spin currents in the antiferromagnetic and ferromagnetic states.In addition,in order to control the thermal spin current of the device,strain or gate voltage is applied.The research results show that the treatments are meaningless to thermal spin currents of the antiferromagnetic state,but it contributes to the spin-dependent Seebeck effect?SDSE?on the ferromagnetic state.From the current curves,the threshold temperatures are found and make the device to have wide practical application.In spite of absence of band gaps in the ferromagnetic state,but there is“compensate effect”in the transmission spectrum,leading to the on-off characteristic in metallic material and expand material candidates of SDSE.Secondly,based on the above device,we increase the width of the graphene nanoribbon and construct spin caloritronics device with both periodic and non-periodic structures.Due to the unpaired electrons in the edge carbon atoms attached to a hydrogen atom,the zigzag graphene nanoribbon has edge effect.Therefore,we construct three initial magnetic settings for the structure:antiferromagnetic-antiferromagnetic?AFM-AFM?state,antiferromagnetic-ferromagnetic?AFM-FM?state,ferromagnetic-ferromagnetic?FM-FM?state.The result reveals that the SDSE appears in the FM-FM state of non-periodic structure,and the AFM-FM and the FM-FM state of periodic structure.In addition,we also calculate the ZT value of the two devices in 300K.The comparison results show that the spin figure of merit ZTspp in the magnetic structure,which displays SDSE,is significantly larger than the charge figure of merit ZTch.Moreover,the ZT value of periodic structure is larger than that of non-periodic one,so it concludes that the thermoelectric conversion efficiency of periodic structure is higher than that of non-period one.Finally,in chapter 4,we choose silicon carbide nanoribbon to construct the devices.One type of device is similar to the periodic structure in previous chapter,and the other is the periodic structure connected by carbon-silicon bond?C-Si?.The calculation results show that the silicon carbide nanoribbon connected by tetragons exhibits a spin filtering and negative differential resistance effect.Finally,the strain or gate voltage is imposed to control the spin currents.Due to applying strain,the negative differential resistance effect disappears from the device.In addition,when the stretching reaches 1.5%,the Si-Si bond will appear in the junction of the tetragons,and there will still be spin filtering and negative differential resistance effect by controlling gate voltage,but the thermal spin current is significantly reduced. |
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