Transport And Thermodynamics Properties In Three Typical Semimetals In Hight Magnetic Field

Semimetals have attracted much attention in condensed matter physics due to their low carrier concentration and abundant topological properties.The extremely low temperature environment can effectively reduce the influence of thermal disturbance for the electron-electron interaction,and strong magnetic field can adjust the fermi surface,so that the energy band can be quantized,thus induced novel quantum effects,such as quantum Hall effect,density wave and so on.In this paper,by studying the transport and thermodynamic properties of three typical semimetal materials（Graphite,Mo Al B,Mn₃Sn）under a strong magnetic field,it is found that they exhibit strange physical properties driven by electron electron interaction,such as exciton insulator,magnetic breakdown,linear magnetostriction,and skyrmions.The main contents and conclusions of this study include the following parts:The first chapter briefly outlines Boltzmann’s transport equation and quantum oscillation theory in resistance and magnetization under magnetic field,and then introduces thermodynamic equation and phenomenological theory under magnetic field.Finally,we focus on the research progress of three typical semimetals under strong magnetic field and the research purpose of this paper.The second chapter mainly introduces the equipment about low temperature、strong magnetic field and measurement used in the experiment,as well as the measurement methods used to study the properties of semimetal materials,including transport（Resistance,Nernst effect,Seebeck effect）and thermodynamics（Magnetization,Magnetic torque,Specific heat,Magnetostriction）measurements.In addition,we focus on the three aspects of sample installation,data acquisition and data analysis during the pulsed strong magnetic field measurement.In the third chapter,it is proved that phase B in graphite is a strong coupling exciton insulating phase by electrical transport measurements under a strong pulsed magnetic field.In addition,we first found new phase transition region in graphite at a higher magnetic field,which is named as phase C（magnetic field>75 T）and supplement the phase diagram of graphite in the magnetic field.By researching the angle dependent magnetoresistance of graphite in high magnetic fields,we found that except for theα^′transition,all the others（α,β,ζ）follow a perfect cosine behavior.The change trend of theα^′transition is similar to the ratio of Zeeman to orbital energies represented from the low field angle-dependent Nernst effect of graphite by(B_up-B_down)/(B_up+B_down).This indicates that the driving force behind theα^′transition is the Zeeman energy.Combined with the results of the angle and temperature dependent magnetoresistance of graphite under high field,it is proved that the B phase is generated because the electron-holes in different subbands form a strongly coupled exciton insulating phase under the action of the magnetic field.The chapter 4,the quasi-two-dimensional Fermi liquid behavior and the magnetic breakdown effect are found in the semimetal Mo Al B through transport and thermodynamic measurements under strong magnetic fields.When measuring the in-plane and out-of-plane resistance of Mo Al B,it was found to exhibit a huge anisotropy in the resistance.Combining first-principles calculations with ex-perimental results of the Kadowaki-Woods ratio（KWR）,it is shown that the huge anisotropy in Mo Al B originates from the quasi-two-dimensional electron Fermi pocket.By various measurements method（Magnetization,Magnetic torque,Magnetoresistance,Nernst effect and Seebeck effect）we have de-termined the main five oscillatory frequencies F_α、F_χ、F_γ、F_δ、F_φ.As well as,the four new oscillation frequencies F₁,F₂,F₃and F₄were also found in the measurement of the magnetoresis-tance under pulsed field through the Fourier transform results.The analysis results show that the four new frequencies are the superposition of the previous five main frequencies,indicating that there may be magnetic breakdown phenomenon in Mo Al B under high field.In the chapter 5,using magnetostriction measurement technology under strong magnetic field,linear magnetostriction behavior different from the traditional antiferromagnetic state is found in Mn₃Sn,and large magnetostriction induced by Berry curvature In addition,the magnetic field induced skyrmions is found in the spin glass state at low temperature.By measuring the magnetostriction and magneti-zation in the antiferromagnetic state of Mn₃Sn,it is found that the transverse and longitudinal mag-netostriction coefficient is linear in the field when the magnetic field along the x or y direction in the Kagome plane,which is contrary to the traditional antiferromagnetic theory.Furthermore,when Mn₃Sn enters the spin glass state at low temperature,the bipeak behavior observed on the magne-tostriction may have the same origin as topological Hall effect in the low temperature.The chapter 6 summarizes the research content of the full text,and proposes the research direc-tion of future work.