The Topological State Of ZrTe₅

Topological insulator(TI)is a band insulator with gapless surface state.The surface state has a Dirac-like despersion.In a 3D Dirac semimetal,the bulk state is 3D Dirac cone,while the surface state is Fermi arcs.In the history of TI,ZrTe5 is quite strange one.The bulk state of ZrTe5 is not normal band insulator,but behaves like a 3D Dirac semimetal with definite gap.The energy gap is less than 100meV.For Iodine-chemical-vapor-transport ZrTe5 single crystal,the Fermi level lies in conduction band at low temperature,making it a good platform to study the bulk state of ZrTe5.For Te-flux ZrTe5 single crystal,the Fermi level is very close to Dirac point,makes it possible to observe the surface state.In this thesis,we study the topological nature of both the bulk and surface states of ZrTe5 by transport,magnetic measurements and microwave technique.The main results include:The transport and magnetic properties of Iodine-transport ZrTe5.The resistivity anomally of Iodine-ransport ZrTe5 occurs at TP=143K.The hall carrier is n-type at low temperature.The Fermi level is about 70meV upon the Dirac point,the 3D Fermi surface has a very tiny area,and the effect mass of electron is 0.015me.The magnetoresistance(MR)strongly depends on the temperature,and shows a competition among linear MR,3D WAL and normal MR.MR has a maxima value at TP where the electron and hole become degenerate.When the magnetic field is parallel to the electric field,ZrTe5 presents the chiral magnetic effect.The bulk electrons of Iodine-transport ZrTe5 behave like 3D Dirac fermions.The size effect of Iodine-ransport ZrTe5.The transport properties of ZrTe5 nanosheets rely on the thickness(t).When t>40nm,TP decreases with decreasing t.When t<40nm,a resistivity peak with different origin shows up,and moves to higher temperature region when thining ZrTe5 nanosheets.As t<20nm,ZrTe5 shows a metallic behavior.By analyzing the revolution of MR and carrier denisty,the Fermi level is found to be shifted from conduction band(t>40nm)to valance band(t<40nm)when decreasing thickness.The transport and magnetic properties of Te-flux ZrTe5 single crystal,and its 2D Fermi surface.The resistivity peak of Te-flux ZrTe5 occurs below 70K.The hall carrier is p-type from 2-300K.MR can be as large as 25000%.The diamagnetism become divergent at low temperature.ZrTe5 shows significant quantum oscillation in microwave cavity,by studying the oscillation behavior with varying magnetic angle,we unveil a 2D Ferimi surface of ZrTe5.The Fermi level is very close to Dirac point,its value can be as small as 3.5meV.The effect mass of hole is 0.003me.Anomalous microwave resonance of Te-flux ZrTe5.ZrTe5 shows anomalous resonace in microwave frequence.The resonance is highly sensitive to the dimensions of samples and the magnetic field strength.The resonance becomes stronger when the dimensions of samples become larger,meanwhile the magnetic field of resonance become larger.By studying the resonance with different magnetic angle,we exclude the possibility of surface plasmon resonance.We also exclude the possibility of cyclotron resonance and spin resonance.