Spin Chirality And Topological Properties In Antiferromagnets

The 21 st century is an era which the rapid development of micro-nano electronic devices,the accelerated expansion of data volumes demand for faster and more stable,more miniaturization magnetic storage technology.In this condition,the disadvantages of traditional ferromagnetic materials are increasingly prominent,such as loss of storage data,low efficiency,High energy consumption and so on.However,antiferromagnetic materials are expected to change the epoch of the storage magnetism due to their many advantages especially some special physical properties.Compared with ferromagnetic materials,antiferromagnetic materials are not only insensitive to external magnetic field but also having strong anti-interference capability and fast dynamic conversion speed,hence it is expected to lead to the more stable and faster storage technology.So antiferromagnetic spintronics and relevant physical researches will help us to further understand the advantages and disadvantages of antiferromagnets as one kind of information carrier materials.According to Marshall's theorem,the ground state of antiferromagnetic must be spin singlet,while this theorem does not uniquely determine the ground state,there are many other total-spin singlets that are not the ground state,so the determination of the ground state of antiferromagnetic is still a fundamental problem in quantum magnetic system.There are strong quantum fluctuations in the low dimensional antiferromagnetic system,usually the classical description basically fails and the system is often accompanied with the quantum phase transformation,magnon excitation,and spin gap.Besides,when there exist magnetic frustration,spin orbit coupling or external field the system will possesses types of hidden multiple-spin ordering,such as spin-nematic state,spin liquid,valence bond solid,spin chiral states and so on,the spin chiral is concerned for some special physical properties.First of all,spin chirality can be directly coupled to the electricity,so we can control the electric polarization through the external electric field;In addition,when the spin chirality is not zero,the time inversion symmetry of the system is destroyed,there will be a fictitious magnetic flux which can induce the nontrivial topological Hall effect in magnon system;In our thesis,we start from an asymmetric frustrated one-dimensional chain,after investigating the time evolution of the spin chirality under different strength of spin–phonon coupling but with same initial state,we got the conclusion that although there is no long range magnetic order due to the quantum fluctuations in the low dimensional antiferromagnetic systems but the formation of vector-spin-chirality bound state due to spin–phonon interaction conditioned by the inverse Dzyaloshinskii–Moriya mechanism and the system undergoes a gapless first-order phase transition,the incoherent spin fluctuations can be strongly suppressed due to the formation of spin-chirality bound state driven by the spin–phonon coupling of DM type.The ratio between the spin fluctuations and the Debye frequency of phonons determines the critical point of spin–phonon interaction,beyond which the gapped spin-chiral state is resulted in.Based on Anderson's RVB(resonating valence bond)theory,we investigate the deformed structure of the frustrated antiferromagnetic kagome lattice.We select the finite clusters with higher symmetry then use the exact diagonalization method to solve the ground state energy of per spin,the numerical results show when there exists distortion in kagome lattice,the ground state energy becomes smaller,the triangular contraction distortion structure is the most stable both in the system S=1/2 and S=1.Besides,DM effect will make the triangular contraction distortion structure more stable with the system of S=1/2;On the other hand,for antiferromagnets such as frustrated kagome magnets,a noncoplanar spin configuration with finite averaged scalar spin chirality breaks time reversal symmetry and induces topological magnon excitation.However,a large magnon thermal Hall conductivity can be found even when the scalar spin chirality is very small,that's because the fluctuation of scalar spin chirality is a nonzero value,it will give rise to the fictitious magnetic flux,so the perturbation of the scalar spin chirality is the fundamental cause of the nontrivial topological properties in magnon system rather than the spin chirality that we thought before.