Investigation On Lattice And Spin Dynamics Of Layered Materials CuP₂ And AgCrSe₂

Nowadays,energy shortage and environmental pollution have become the primary problems that limit the development of human society.However,with the progress of human industrialization,the energy and environmental problems are also increased rapidly,which means traditional energy materials and devices can no longer satisfy the objective requirements of sustainable development in the modern world.Thus,it is quite urgent to explore efficient,environmentally friendly and safe novel functional materials and devices for developing renewable and clean energy.In recent years,layered materials have attracted tremendous attention from both academy and industry in diverse fields such as energy storage and conversion,thermal management,and spintronics applications due to their intrinsic exotic physical properties.In the meanwhile,owing to the variety of properties of the diverse layered system,it is still a challenge to comprehensively understand the physical mechanism of their intriguing properties as well as limiting their development and applications.Therefore,in order to reveal the microscopic mechanism of the excellent physical properties in the layered system,the abnormal thermal transport of CuP2 and the coupling between lattice,phonon,magnon and electron of AgCrSe2 were investigated in this thesis,the main research results are shown as follows:(1)The abnormal thermal transport phenomenon with both high sound velocity and low thermal conductivity in CuP2 was investigated,and the physical origin of this anomaly was revealed by neutron scattering and theoretical calculation in lattice dynamics.In general,the lattice thermal conductivity of solid-state materials is proportional to the square of sound velocity.However,there is an exception CuP2,which possesses abnormal both the large mean sound speed of 4155 m·s-1 and very low lattice thermal conductivity of about 4 W·m-1·K-1.To unveil the mechanism of such a puzzling thermal transport behavior,the lattice dynamics was investigated by means of neutron scattering and first-principle simulations,etc.According to the research results,we found that Cu atoms formed dimers that are sandwiched in between the P atomic networks.And the dimers vibrate as a Rattling mode around 11 meV,which dramatically scatters the acoustic phonons and achieves low thermal conductivity.The discovery of this mechanism will provide a novel idea for optimizing the lattice thermal transport in solid-state materials.(2)The lattice dynamics of AgCrSe2 was studied using neutron scattering,and it was found that the ultrafast diffusion of Ag+at high temperatures leads to anisotropic liquid-like phonons in this system.Temperature dependent crystal structure of AgCrSe2 was studied by neutron powder diffraction,the in-plane magnetoelastic coupling below 200 K and strong in-plane anharmonicity of Ag+ were verified.Above phase transition temperature 475 K,we found that the transverse phonon propagated along c axis with in-plane vibration can be still observed.In contrast,the in-plane low energy transverse acoustic phonon with out-of-plane vibration is totally broadened which exhibits the liquid-like phonon.It indicates that the coupling between the dynamic disorder of Ag+and phonon is quite anisotropic.The inelastic neutron scattering analysis suggests that the appearance of liquid-like phonons along this direction is due to the ultrafast diffusion of Ag+in the disordered phase,which makes it migrate more quickly to other in-plane equivalent positions rather than transmitting the lattice wave,resulting in the suppression of transverse phonons at high temperature.(3)Research on the magnetic evolution induced by the freezing of Ag+ fluctuations in AgCrSe2.The existence of short-range magnetic order above 55 K was confirmed by the temperature dependence of spin magnetic excitations analyzed by inelastic neutron scattering.Our work suggests that the magnetic short-range order is prevented by significant magnetocrystalline anisotropy from decoupling,and the fluctuations of Ag+is gradually frozen as temperature decreases.More importantly,the freezing Ag+fluctuations were confirmed as a key role in connecting the adjacent CrSe6 layers through the indirect superexchange interactions formed with Se-Ag-Se bridge.(4)The study of anisotropic magnetoelectric transport in AgCrSe2 single crystals.The measurement of magnetic properties verified a magnetic phase transition at 32 K,and exhibited weak ferromagnetic properties with magnetic field along c axis.Moreover,the magnetoelectric transport measurements suggested that positive and negative magnetoresistance can be achieved by applying the magnetic field along in-plane and out-of-plane directions,respectively.It indicates that carrier mobility can be effectively controlled by both the lattice degree of freedom and spin degree of freedom,which lays the experimental foundation for manipulating the spin and electric transport through external fields.