Thermoelectric And Spin Transport Properties Of New 2D Systems

Electrons have two degrees of freedom,charge and spin.By using the heat transport properties of charge,the mutual conversion between heat and electric power can be realized,and it has important application in energy field.The magnetic storage can be realized by using spin property,which has an important application prospect in spintronics.There are also correlations between heat and spin,such as the spin Seebeck effect and the anomalous Nernst effect.The research on heat transport and thermal spin transport is an important direction in condensed matter physics.How to improve the efficiency of thermoelectric conversion and the performance of spintronic devices is the key problem to be solved in this field.For low dimensional systems especially the two-dimensional ones with atomic-layer thickness,their quantum confinement effect can bring about novel thermal transport properties and thermal spin transport properties,which can be used to construct thermoelectric devices and spintronic devices.In this thesis,the thermoelectric and thermal spin transport properties of the new experimental two-dimensional semiconductors and two-dimensional magnetic materials such as magnetic half-metals and Dirac spin gapless semiconductors are studied by using density functional theory,Boltzmann transport theory and nonequilibrium Green’s function.The main results are as follows:1.Inspired by recent reports about the successful growth of two-dimensional layered semiconductor Nb₂Si Te₄ in experiments,we study the electronic structure and thermoelectric transport properties of monolayer Nb₂XTe₄（X=Si,Ge）.The results show that Nb₂XTe₄ monolayers have the characteristics of narrow band gaps and"pudding-mold-type"band structures,which means potential thermoelectric performance.The calculated binding energy,molecular dynamics simulations and phonon spectrum indicate the highly structural stability,thermal stability and dynamic stablity for both monolayers.Importantly,they also show high carrier mobility and strong anisotropy,and the highest mobility can reach 4344 and 2131 cm^-2V^-1s^-1,respectively.The strong coupling of optical and acoustic phonon branches,the low phonon group velocity,the low relaxation time and the large Grüneisen parameter make the lattice thermal conductivity very low.At room temperature,the thermoelectric figures of merit in monolayer Nb₂Si Te₄ and Nb₂Ge Te₄ can reach 1.4and 2.2,respectively,indicating good thermoelectric performance.Furthermore,we build the Au/Nb₂Si Te₄/Au tunneling field effect transistor device,and explore the device thermoelectric properties under the gate voltage.Results show that the gate voltage is an effective method to improve the thermoelectric performance.Under the regulation of negative gate voltage,the device exhibits excellent thermoelectric figure of merit（ZT>0.6）in a wide temperature range of 500～1000 K.These studies indicate that monolayer Nb₂XTe₄ have important applications in the thermoelectric field.2.We use transition metal atoms（TM=Sc～Zn）to dope nonmagnetic semiconductor of blue phosphorous monolayer,and find that the Mn-doped system exhibits excellent half-metallicity and long-range ferromagnetic ordering whther in a wide range of doping concentration,in HSE or in the Coulomb interactions.The Curie temperatures estimated by the mean-field approximation and the Monte Carlo simulations are higher than the room temperature.We use the crystal field,atomic orbital arrangement and partial density of states to explain the magnetic mechanism.In addition,we design new types of spintronic and thermal spintronic devices based on the Mn-doped monolayer blue phosphorous.The results show that the devices have perfect spin filtering effect,giant magnetoresistance effect（10⁶%）and novel square wave signals,etc.The research of thermal spin transport shows that the device has a perfect thermal spin filtering effect,and interestingly,the spin thermoelectric ZT_sp value（160）is much higher than the charge thermoelectric ZT_ch value（40）.These results indicate that a multifunctional electronic device can be designed based on Mn-doped monolayer blue phosphorous which can exhibit 100%spin filtering and high spin thermoelectric conversion efficiency.3.We design a new type of metal-organic framework（MOF）monoayers Mn₂C₆Se₁₂ and Mn₂C₆S₆Se₆,and both the generalized gradient approximation and hybrid functional methods show that monolayer Mn₂C₆Se₁₂ is a spin gapless semiconductor,and monolayer Mn₂C₆S₆Se₆ is a nearly spin gapless semiconductor.The calculated phonon spectra,molecular dynamics simulations,Young’s modulus and Poisson’s ratio show high stability and flexible plasticity for both monolayers.The estimated Curie temperatures exceed 400 K based on the Monte Carlo method.Interestingly,the calculated Berry curvature,anomalous Hall conductance,and nozero Chen number show that monolayer Mn₂C₆Se₁₂ has topologically non-trivial properties,which exhibit the peculiar quantum anomalous Hall effect and topologically protected boundary states.Finally,we design the planar homojunction devices,planar tunnel junctions and vertical van der Waals tunnel junction devices based on monolayer Mn₂C₆Se₁₂,and find that these devices have rich transport characteristics,such as the spin filtering,the diode effect,the negative differential resistance,and the tunneling magnetoresistance effect,etc.These results demonstrate the potential applications in spintronics and topological field for the spin gapless semiconductor with MOF structure.