Electronic Structure And Heat Transport Properties Of Pyrolytic Boron Materials

As a special lattice with three-dimensional frustration,pyrochlore crystal lattice has been studied extensively in its magnetic properties and flat bands in the electronic structure.From spin glass,spin liquid and spin ice in spin frustration,to flat bands,Wigner lattices and degenerate energy levels in orbital frustration,the pyrochlore lattice has very interesting Physical properties.However,so far,all the materials with pyrochlore lattice characteristics that have been studied have been rare earth metal oxides?A₂B₂O₆O'?.Rare earth elements occupy the pyrochlore lattice,but oxygen atoms have an important influence on the properties of this type of structure,and thus cannot reveal the essential characteristics of the pyrochlore lattice.In this paper,taking the pyrochlore structure of PL-B4 and PL-B8 as the examples,the thermal transport and topological properties of the pyrochlore lattice are studied,aiming to explore the underlying physics in the pure pyrochlore lattice.phenomenon.This paper is divided into five chapters.In Chapter 1,we introduce the research status of geometric frustration and pyrochlore lattice,as well as some allotropes and developments of boron structure.In Chapter 2,we introduce the tight-binding model,k·p perturbation theory,density functional theory,and Boltzmann transport equations.In Chapter 3,the two pyrochlore boron structures proposed are analyzed from symmetry to phonon dispersion characteristics.It is found that the quasi-flat band in PL-B4 promotes the three-phonon scattering process in phonon transport.Due to the emergence of the flat band,phonons on the flat band are used as the source of scattering,which will make the scattering process only need to consider the conservation of energy and not consider the conservation of momentum conditions too much,which will induce lower thermal conductivity.In addition,the appearance of phonon band gap in PL-B8,which also has flat bands,inhibited the scattering of low-frequency phonons and induced the appearance of higher lattice thermal conductivity?13.75w/mK at room temperature?.At the same time,a structural regulation method for the possible construction of band gap was proposed.In Chapter 4,we predicted three types of quadratic degeneracy points of quadratic curves in the pyrochlore lattice by simulating the hopping energies between different lattice points through the tight-binding model method.In addition,the possibility of these three types of degenerate points was proved by the k·p perturbation theory,and the evolution and coupling phenomenon of the triple degenerate points and node lines of the quadratic curve in the pyrochlore system under different stresses was abstracted on this basis.Finally,we verified the above-mentioned work through the two DFT calculations of PL-B4 and PL-B8.At this point we provide a reliable reference for finding a new class of quasi-fermions.In Chapter 5,we summarize this work,and put forward the prospect of the future development of pyrochlore lattice and geometric frustration.