STM Investigation Of Fe(Te,Se) Thin Films And EuSn₂As₂ Single Crystals

Scanning tunneling microscopy(STM),which can probe atoms,has become a powerful experimental method in condensed matter physics since the 1980s.In this paper,I will introduce my engineering work in the building of a cryogenic strong magnetic field rotating STM/AFM system,and experimental results of three different materials with this STM/AFM.Conventional vector-field STM design is based on split magnet,which is difficult to achieve a vector-field higher than 3 T.We use the piezoelectric step motor to rotate the scanning head.And a 9 T‘vector-field'is achieved,which is far beyond the upper limit of the conventional vector-field STM.We also design a tip holder,which is compatible with the STM tip and the q Plus sensor.In addition,we design and manufacture a 1 K pot cryostat,which can achieve a base temperature of 1.25 K by He⁴only.In the building of this cryogenic strong magnetic field rotating STM/AFM system,the author's main contributions include the design of the positioning device of the rotating scanning head,the improvement of the q Plus sensor,and the elimination of Taconis oscillation of the 1 K pot cryostat,the design of the filter circuit,the design of the preamplifier,and the test and debugging of the entire system.Recently,Fe(Te,Se)/Sr Ti O₃ single-unit-cell film has attracted much research interest due to its high superconducting transition temperature(T_c).However,there is still no consensus on some fundamental problems,such as the relationship between post-annealing and superconductivity,the influence of crystal defects on superconductivity,and the microscopic structure of the crystal defects.With high spatial resolution and atomic manipulation ability,we utilize STM/STS to study the crystal defects and post-annealing procedure in Fe(Te,Se)/Sr Ti O₃ single-unit-cell films.We find the clover-shape defects,which were commonly attributed to surface Se atoms vacancies,is possible to have a more complex microscopic structure.The Fe lattice may be imperfect right beneath the clover-shape defects.Furthermore,we find the post-annealing procedure in MBE chambers equipped with Se source will increase the stoichiometric ratio of Se elements and finally induce a superconducting-like gap in Fe Te/Sr Ti O₃ single-unit-cell films.Thus a careful calibration of the stoichiometry of Fe(Te,Se)/Sr Ti O₃ single-unit-cell films is deserved.2H-Nb Se₂ and Fe(Te,Se)are typical conventional and unconventional superconductors,respectively.We successfully grow Fe(Te,Se)thin films on 2H-Nb Se₂single crystals with molecular beam epitaxial(MBE)technics,and study the interfacial properties of them with STM.We find moirépattern between film and substrate in both Fe Te/2H-Nb Se₂ and Fe Se/2H-Nb Se₂ thin films.The moirépattern can not be resolved in the atomic force microscopy(AFM)topography,indicating that lattice distortion is not the cause of the moirépattern in the STM topography.We find no 1×2 modulation in Fe Te/2H-Nb Se₂ single-unit-cell films,while clear 1×2 modulation in thicker films,suggesting suppression of magnetic order in the interface of Fe Te films and 2H-Nb Se₂substrates.In Fe Te single crystals and thin films,this 1×2 modulation was commonly attributed to the antiferromagnetic order.Meanwhile,we find evidence of superconductivity in Fe Se/2H-Nb Se₂ single-unit-cell films,with a T_c of about 7K.Intrinsic magnetic topological insulator(intrinsic MTI)is absent of magnetic dopants and disorder induced by magnetic dopants.Thus intrinsic MTI is predicted to hold some fancy macroscopic quantum states at a higher temperature,such as quantum anomalous Hall insulators(QAHI)and topological axion insulators(AXI).The research hotspot of intrinsic MTI is Mn Bi_2nTe_3n+1(n=1,2,3)family.While the study on other intrinsic MTI families is relatively rare.We use STM/STS to investigate another intrinsic MTI family,Eu Sn₂Pn₂(Pn=P,As)single crystal.We find the crystal defects have a strong influence on the local electrical properties in Eu Sn₂Pn₂ single crystals.Furthermore,we find a peak-dip feature in the measured STS.With STM/STS and susceptibility experiments at various temperatures and under different magnetic fields,we find the peak-dip feature only exists when bulk crystals are in AFM order.