The Novel Thermal Spin And Valley Transport Properties Based On Low-dimensional Nanoscale Materials

As a new subject,spin caloritronics has been a hot topic of condensed matter physics in recent decades.Different from traditional spintronics,it deals with the interactions of heat,spin and charge currents induced by the temperature gradient.It is well known that the spinSeebeck effect and thermoelectric conversion efficiency are its two important contents.The thesis includes two aspects: on the one hand,exploring the new mechanism of generating the spin-Seebeck effect;on the other hand,looking for new functional magnetic materials having a good symmetrical spin current together with high spin figure of merit.We use the density functional theory to calculate the band structures and spin density,and the non-equilibrium Green's function method to analysis the thermal spin transport properties.Meanwhile,valley as a new non-intrinsic degree of freedom,can also realize information encoding and data storage.In the last chapter of this paper,the temperature gradient will be considered in twodimensional nanostructures,while we use the k?p equations and transfer matrix methods to study valley-spin polarized currents transport properties and its regulation rules.The main findings are showning as follows:(i)Due to the anisotropy of gamma-graphyne(?-graphyne)two-dimensional nanosheet layer,we design four zigzag ?-graphyne nanoribbons cutting in different directions and found the four structures have different ground states including ferrimagnetic,antiferromagnetic and nonmagnetic state.These unique characteristics make it have a potential application in spin caloritronics.Based on above four homojunction systems,the thermal spin current is explored and our researches reveal that the ferromagnetic metallic ?-graphyne exhibits a good spinSeebeck effect similar to the ferromagnetic semiconductor one.That is,although the spinsplitting band gaps of the metallic ?-graphyne is closed,a good spin-Seebeck current is induced by the temperature gradient with the appearance of the on–off characteristic.Here,we proposed a new mechanism called the compensation effect originating from the cancellation of electrons and holes around the Fermi level to produce a net spin-up or spindown transport channel.Moreover,it is found that the symmetry of the net spin-dependent channels determines the symmetry of spin currents and the intersection region near the Fermi level dominates the threshold temperature.Meanwhile,the study show that the gate voltage is still an effective route to adjust the symmetry of spin-splitting bands of the metallic Z?GNRsto obtain pure spin currents and to realize spin-Seebeck rectifier and diode effects.(ii)To explore a feasible material platform and reveal the new mechanisms to realize thermal spin filtering effect(SFE)and spin-Seebeck effect(SSE),we construct several singlewall boron-nitrogen nanotubes(BNNTs)homojunctions,study the thermal spin-dependent transport properties in(5,5)amchair and(9,0)zigzag BNNTs with n boron(nitrogen)atoms substituted by carbons in every unit cell.The results show that for n=1,it contributes to produce a more symmetric up-spin and down-spin currents leading to a better SSE for magnetic BNNTs.However,as the n?2,increasing the carbon atoms concentration bring about some interesting physical phenomenons,we find the rotational symmetry of carbon substitutions determine the spin-splitting bands symmetry and the transformation between SSE and SFE,a high carbon dopant rotational symmetry is beneficial to generate the good SSE,otherwise towards the thermal SFE.In addition,due to the cancellation of the electrons and holes near the Fermi surface called the compensation effect,some metallic BNNTs can also generate the thermal SFE or the good SSE with a finite on-off temperature.Moreover,because of a larger Yong's modulus,a new mechanism is proposed that the compression strain engineering along the magnetic BNNTs radial direction is an effective method to realize and regulate the transformation of above the two effects,while it can also induce some novel thermoelectric effects such as temperature-dependent spin flipping or opposite spin currents.Finally,the PN junction consist of magnetic(9,0)BNNT also was investigated,some interesting transport phenomenons that spin filtering,charge diode and spin-Seebeck diode effect are induced by external bias or temperature difference.These theoretical results about the SSE in nanotubes enrich the spin caloritronics and put forward material candidates to realize the SSE and other inspiring thermospin phenomena.(iii)It is well known that low-dimensional materials nanoribbons and nanotubes have long been considered as two potential candidates to achieve a good SSE with high thermoelectric conversion efficiency.Therefore,it is vital to explore the factors influencing thermoelectric properties and thermal spin transports between nanoribons and nanotubes systematically.Here,we construct three spin caloritronics devices based on magnetic boronnitrogen nanotubes(BNNTs)and nanoribbons(BNNRs)with the same boron(B)and nitrogen(N)atoms,while some B atoms are substituted by carbon atoms and the BNNRs canbe rolled into the BNNTs.To obtain the same magnetism origins for comparison,the BNNRs edge magnetisms are removed by hydrogen atoms passivation(BNNR-H).The research results uncover that although the magnetic BNNT and hydrogenated BNNRs have the similar spin semiconductor properties,hydrogen passivation contribute to increase the spin figure of merit due to lower lattice thermal conductivity.However,the unique helical symmetry of nanotubes produce a good SSE with more symmetrical spin currents and the larger spin thermopower.As the edge magnetisms removed,the electronic state of BNNR-H is changed into the magnetic metallic,which suppresses the thermal spin currents and spin thermopower largely.In short,the above comparative studies will help us to choose a feasible route to optimize the SSE and spin figure of merit in nanoribbon and nanotube,and provide us profound understanding into the spin caloritronics device applications based on lowdimensional materials systems.(iv)To explore the coupling effect between valley and spin,we construct the three nonmagnetic/ferromagnetic/non-magnetic junction(NM/FM/NM)systems based on silicene,germanene and stanene nanoribbons,and make a comparative study to the thermal spin-valley polarized transport properties in systematically.Firstly,the research results reveal that different spin and valley transport channels can be open or closed by adjusting the on-site potential difference in NM or FM region.Secondly,the temperature gradient can realize the coexistence of valley and spin Seebeck effect.Finally,by controlling the gate voltage in central scattering region,the thermal valley-spin currents can be effective selected and filtered.