Update And Debugging Of Combinatorial Laser Molecular Beam Epitaxy And Scanning Tunneling Microscopy System And Its Applications In Superconducting Research

Homemade combinatorial laser molecular beam epitaxy system integrated with specialized scanning tunneling microscopy（Combi-LMBE-STM）can synthesize high-quality films in parallel and characterize surface morphologies and electronic states in situ.This system could quickly establish detailed and reliable superconducting databases and accelerate the exploration of superconducting materials.In this thesis,we introduce the debugging and upgrading of this system to improve its performances on superconducting research.The main results are as follows:（i）Based on two open questions in iron-based 122 systems,we perform STM measurements on high-quality stoichiometric CaKFe₄As₄ single crystals to characterize the performance of our STM on superconducting research.We take full advantage of large range of XY coarse motion（10 mm×10 mm）and carry out surface morphological experiments.Besides√2×√2 and 1×2 surface reconstructions,we capture new-type morphologies containing the√2×√2 surface reconstruction,a disordered structure and 1×1 lattice.By analyzing the geometrical correlations of these morphologies,the1×1,1×2 and√2×√2 lattices are identified as original As,reconstructed As and reconstructed alkaline-earth-metal/alkalimetal（AE/A）layers,respectively.We also investigate the electronic structure of CaKFe₄As₄ and find that the tunneling spectra collected on the reconstructed As and AE/A layers have an almost identical superconducting energy gap as well as bosonic mode,which indicates that surface reconstructions of CaKFe₄As₄ do not change the electronic structure significantly.In addition,we carry out quasiparticle interference experiments and observe ring-like scattering structures in Fourier-transformed quasiparticle interference patterns.The estimated size of scattering vector is in good agreement with the diameter of Fermi pocket centered at theγpoint revealed by angle-resolved photoemission spectroscopy.These results demonstrate high stability and high energy and spatial resolution of our STM,and show great potential of our STM in superconducting research.（ii）We optimize the treatment process of the SrTiO₃（001）substrate to improve the surface roughness,and finally obtain an atomically flat surface.After that,we grow FeSe films by laser molecular beam epitaxy technique on SrTiO₃ substrates with different qualities,and study the growth kinetic of FeSe films via STM.It found that the growth mode and superconducting properties of FeSe films are dependent on the surface roughness of the substrates:（1）The morphologies of films grown on untreated substrates show obvious amorphous feature and the surface corrugation of films exceeds 6 nm.（2）By optimizing the roughness of SrTiO₃ substrate,the qualities of films are improved.The film grows in a screw dislocation mode and can capture large-scale atomic images.However,these FeSe films do not exhibit superconductivity at temperature above 2 K.（3）When the surface of the substrate is atomically flat,the growth mode of FeSe films changes to Frank-van der Merwe mode.Compare to bulk FeSe,our films exhibit enhanced superconductivity with transition temperature of above 20 K.In addition,we observe many interesting features such as domain boundaries and interstitial Fe in atomic resolved STM images.Next,we will investigate the origin of enhanced T_c,as well as the relationship between superconducting energy gap and morphologies which can further deepen our understanding of unconventional superconducting mechanisms.（iii）We design and built a probe array transport system（PAT system）to carry out in situ multi-channel electrical transport measurement of films with gradient composition/thickness.We complete the design and simulation of mechanical function,low-temperature part,multichannel acquisition function and build probe array part.Electrical transport measurements of FeSe and YBa₂Cu₃O_7-δfilms have been carried out successfully by this probe array part in air.The PAT system will be integrated into Combi-LMBE-STM system.Our PAT system could not only provide more intrinsic transport data for air-sensitive materials such as our high-quality FeSe film,but also enable rapid characterization and screening of superconducting materials and therefore has important prospects for application in high-throughput measurements.