Home Made Cryogenic Scanning Tunneling Microscopy With Vector Magnetic Field And Its Application On Toplogical Materials

Scanning tunneling microscope(STM)is a powerful equipment in condensed matter physics,because of its atomic resolution and ability to acquire local density of states(LDOS).Topological insulator is a flourishing research area for its possible application in low power consumption devices.Topological materials are characterized with bulk-boundary correspondence and spin-momentum lock,which need local sensitive technique to provide direct evidence.With its LDOS accessibility and state-of-the-art quasiparticle interference(QPI)technique,STM is an ideal probing method.As the study coming to magnetic topological insulator,lower temperature is needed for high energy resolution.And greater and rotatable magnetic field is required to change the spin alignment so that different topological properties emerge.Here,I will first give a brief introduction of the foundation of STM and research background of topological insulator.Then I will focus on our home-made low temperature high vector magnetic field STM-MBE combined system,and its application on the study of Iron-based superconductive thin film and magnetic topological materials.To fulfill the demand of science frontier,we develop and improve design of the Pan-Type STM,extend its functions.XY translational motion is added,this makes searching interested area more easily.The whole scanning head is rotatable in a single superconductor magnetic coil,drove by piezo actuators aligned in a circle.This is a more efficient and stable way to realize vector magnetic field.A longitudinal polarized piezo actuator is inserted in the scanning head so that the tip adapter is compatible with Q-plus AFM.A He4 refrigerator with 1K pot is designed and fabricated,the working temperature is as low as 1.2K.The cryogenic STM is combined with a molecular beam epitaxy(MBE)system.Therefore,we can investigate epitaxial films with in-situ STM and AFM.Magnetic topological insulator(MTI)hosts exotic quantum phenomena,such as anomalous quantum Hall effect,axion insulator state.The materials are usually realized in magnetically doped topological insulator,and heterojunction made of magnetic film and topological insulator.They suffer inhomogeneity or difficulty of construction,which hinder further exploration.Recently,several theoretical works predicted a series of MTI crystals with exact stoichiometric ratio.A systematic STM study of MTI candidate including EuSn2P2,EuSn2As2,EuIn2As2 is performed.Generally speaking,we find the materials are hole doped.However,we find defects can locally induce band bending toward either valence band or conduction band.And the concentration of the acceptors is superior than that of donors.By considering their symmetries,we can give a possible explanation of the origin of these defects.Furthermore,an asymmetrical peak-dip feature is found in the differential conductance spectrum near fermi energy.Applying magnetic field and elevating temperature can suppress that feature.And the critical fields and temperature of the peak-dip feature are consistent with the saturated fields and the Neel temperature in the magnetic measurements,which give hint that the surface magnetic order might be the same as in the bulk.Topological superconductor can host Majorana bond state on its surface or edge.To overcome the disadvantages of topological insulator+superconductor heterostructures,connate topological superconductor is proposed.FeTe1-xSex/STO is such a promising candidate with much higher superconducting transition temperature(Tc).Here,we successfully grow monolayer film,then we make STM and ARPES study.ARPES directly observe electron-like and hole-like band intersect at ? point,and STS find evidence of edge states,which is consistent with theorical calculation.These findings suggest that FeTe1-xSex/STO is a platform for realizing Majorana states.