First-principles Studies Of Novel Topological Electronic And Phononic Materials

Information storage and renewable energy are two problems that we will face and need to solve in the age of the big data.In conventional electronic devices,the use of the charge freedom of the electron to store information has been subject to quantum mechanical limits.Recently,valleytronics and topological materials show that valley and spin,as new degree of freedom,can also be used to store information.At present,valleytronics and topological materials are hot research topics in condensed matter physics.Since the energy consumption of spin is one thousandth of that of charge,topological materials also have broad application prospects in low-power quantum devices.After the concept of topological insulators was proposed,topological materials have been extended to many aspects,including 2D magnetic topological materials,topological semimetals,topological phonons,Weyl phonons and so on.On the other hand,topological materials and thermoelectric materials have a considerable degree of overlap in the macroscopic characteristics of materials,for example heavy element and norrow gap.Thermoelectric materials are one kind of renewable energy materials,thus that searching thermoelectric materials in topological materials has begun to draw more and more attention.In this thesis,we use first-principles calculations to study two-dimensional magnetic materials with novel quantum properties,Weyl phononic materials,and thermoelectric materials closely related to phonons.The main contents of this thesis are listed as below:i)When the crystal structure is limited to honeycomb structure,the energy degeneracy of the valley is protected by the time-reversal symmetry?TRS?.Therefore,if the TRS is broken,the degeneracy of valley will no longer remain,thereby realizing the desiered valley polarization.In previous studies,polarized light,external magnetic fields,and magnetic substrates were used to break the TRS in non-magnetic materials.In addition,previous calculations also show that vanadium-sulfur compounds have intrinsic ferromagnetism.By calculating the electronic band structure of VSe₂,we found that ferromagnetic VSe₂ monolayer achieved intrinsic valley splitting after spin-orbit coupling was included.The intrinsic valley splitting can realize valley polarization under polarized light,hole doping,and linear light of specific energy.ii)In traditional view,the anomalous Hall conductance is related to the net magnetic moment of a material,so the anomalous Hall conductance in the antiferromagnetic material is considered to be zero.However,in recent years,many theoretical calculations and experiment results have found that there can be large anomalous Hall conductance in noncollinear coplanar antiferromagnetic materials,due to the fact that the spin-orbit coupling?SOC?breaks the combined TRS.More importantly,noncollinear noncoplanar antiferromagneitc?nc-AFM?structure can give rise to quantum anomalous Hall effect.We found that 2D antiferromagnetic monolayer was rarely reported to host the complex antiferromagnetic state.Our calculations show that the layered material NiTl₂S₄ has a weak interlayer interaction and is therefore likely to be exfoliated into a 2D material.Further research found that 2D NiTl₂S₄ could have many antiferrmomagnetic structures,and the most stable magnetic structure was noncollinear coplanar antiferromagnetic.Although nc-AFM is a magnetic metastable state,its magnetic moment can be maintained in optimization and therefore it may exist at high temperatures.Interestingly,the electronic bands of nc-AFM NiTl₂S₄ show a significant band inversion at the Fermi surface.The Chern number inside the gap is=-4,so four conducting channels are oberseved on the surface of the material.We explain=-4 from the view of Skyrmion in momentum space.In addition,we find that there is another topological nontrivial state below the Fermi level 0.52 eV,which is a coexistent state of the quantum anomalous Hall effect and the quantum spin Hall effect with=3 and=1.iii)Ideal Weyl phase is favorable for the observation of the surface state in experiment.Previous work have reported many ideal type-I Weyl materials,however ideal type-II Weyl materials have not been reported to date.Our calculations show that wurtzite CuI is an ideal type-II Weyl phonon materials.In the phonon spectrum of CuI,there are six pairs of Weyl points?WPs?in6)=0.0 plane.All these WPs are related to each other by symmetry and therefore locate at the same energy.More importantly,an obvious band gap is formed between the bulk bands away from WPs.The above two reasons reduce the hybridization between the type-II Weyl point and the bulk state to a minimum,thereby obtaining an ideal type-II Weyl phase.On the I-termitated surface,we can easily observe very clean and open surface arcs.The distance between two WPs of opposite chirality connected by the surface arc is 0.26?^-1,which is more than ten times longer than that in TaAs.iv)Combining the first-principles calculations with the Boltzmann theory,we studied the thermoelectric properties of the ternary compound KZnP.We find that the lattice thermal conductivity of KZnP show strong anisotropy.For example,the in-plane and out-of-plane lattice thermal conductivities at room temperature are 5.24 and 2.58Wm^-1K^-1,respectively.We have also analyzed the contribution of electron-phonon?e-ph?scattering from each phonon mode and found that optical branch contributes at least45%to the e-ph scattering.Thus,deformation potential theory should greatly overestimate the ZT value of KZnP.For n-type doped KZnP,the maximum ZT calculated from our first-principles results is 0.52.Although the ZT value of KZnP is smaller than that of most classical thermoelectric materials with narrow-band gap?¹.0?,the lattice thermal conductivity in the out-of-plane direction is on the order of¹.0 Wm^-1K^-1,thus KZnP still has good potential in the thermoelectric applications.In this thesis,new types of valley electronic materials,2D magnetic topological materials,Weyl phononic materials and thermoelectric materials are predicted and explored.Firstly,we predict that monolayer VSe₂ can achieve intrinsic valley polarization,in which the valley degree of freedom is easily controlled and thus have potential application in information coding.Secondly,we predict that monolayer NiTl₂S₄ can have rich noncollinear antiferromagnetic states.Specially,nc-AFM state,one of magnetic metastable state,can realize quantum anomalous Hall effect and TRS broken quantum spin Hall effect.Thirdly,we propose that wurtzite CuI can realize ideal type-II Weyl phonons,which provides a platform for the experimental detection of surface states and the study of the novel properties in type-II Weyl phase.Finally,we predict that KZnP family materials have excellent lattice thermal conductivity and thus shoud have good potential in thermoelectric applications.In conclusion,our results provide potential candidate and open up new ideas for finding new spintronic devices and energy materials.