CaKFe_(4)As_(4) is a new-type superconductor with a relatively high transition temperature of 35 K among stoichiometric iron-based superconductors. Based on scanning tunneling microscopy/spectroscopy, the surface morp...CaKFe_(4)As_(4) is a new-type superconductor with a relatively high transition temperature of 35 K among stoichiometric iron-based superconductors. Based on scanning tunneling microscopy/spectroscopy, the surface morphology and electronic structure of CaKFe_(4)As_(4) single crystal were systematically investigated. The cleaved CaKFe_(4)As_(4) showed various morphologies, such as atomically resolved 1×1, 1×2, and √2×√2 lattices. By analyzing the geometrical correlations of these morphologies, the 1×1 and 1×2 lattices were identified as the original and reconstructed As layers, respectively, whereas the √2×√2 lattice was distinguished as the reconstructed alkaline-earth-metal or alkali-metal layer. The superconducting energy gap of 7.3 me V and bosonic mode of 12.7 me V were resolved in the scanning tunneling spectra. In addition, the superconducting energy gaps measured on different terminations were identical and consistent with the values obtained by bulk-sensitive techniques, indicating that the electronic structures of CaKFe_(4)As_(4) were insensitive to the surface reconstructions. Our study clarifies the relationships between complex surface reconstructions and surface terminations and preliminarily presents that there is no obvious effect of surface reconstructions on electronic states.展开更多
As a new type of iron-based superconductor, CaKFe_(4)As_(4) has recently been demonstrated to be a promising platform for observing Majorana zero modes (MZMs). The surface of CaKFe_(4)As_(4) plays an important role in...As a new type of iron-based superconductor, CaKFe_(4)As_(4) has recently been demonstrated to be a promising platform for observing Majorana zero modes (MZMs). The surface of CaKFe_(4)As_(4) plays an important role in realizing the MZM since it hosts superconducting topological surface states. However, due to the complicated crystal structure, the terminal surface of CaKFe_(4)As_(4) has not been determined yet. Here, by using scanning tunneling microscopy/spectroscopy (STM/S), we find that there are two types of surface structure in CaKFe_(4)As_(4). Bias-dependent atomic resolution images show an evolvement from 2–√×2–√ superstructure with respect to the As lattice into 1 × 1 when the tip is brought close to the surface, revealing the sublattice of missing As atoms. Together with the first-principles calculations, we show that the surface As layer has a buckled structure. Our findings provide insight to future surface study of CaKFe_(4)As_(4) as well as other iron-pnictide superconductors.展开更多
基金supported by the National Key Basic Research Program of China(Grant Nos.2017YFA0302902,2016YFA0300301,2017YFA0303003,and 2018YFB0704102)the National Natural Science Foundation of China(Grant Nos.11927808,11834016,118115301,119611410,11961141008,11822411,and 11961160699)+5 种基金the Key Research Program of Frontier Sciences,Chinese Academy of Sciences(Grant Nos.QYZDBSSW-SLH008,and QYZDY-SSW-SLH001)the Strategic Priority Research Program(B)of Chinese Academy of Sciences(Grant Nos.XDB25000000,and XDB33000000)the Beijing Natural Science Foundation(Grant Nos.Z190008,and JQ19002)the Key-Area Research and Development Program of Guangdong Province(Grant No.2020B0101340002)the CAS Interdisciplinary Innovation Teamsupport from the Youth Innovation Promotion Association of CAS(Grant No.Y202001)。
文摘CaKFe_(4)As_(4) is a new-type superconductor with a relatively high transition temperature of 35 K among stoichiometric iron-based superconductors. Based on scanning tunneling microscopy/spectroscopy, the surface morphology and electronic structure of CaKFe_(4)As_(4) single crystal were systematically investigated. The cleaved CaKFe_(4)As_(4) showed various morphologies, such as atomically resolved 1×1, 1×2, and √2×√2 lattices. By analyzing the geometrical correlations of these morphologies, the 1×1 and 1×2 lattices were identified as the original and reconstructed As layers, respectively, whereas the √2×√2 lattice was distinguished as the reconstructed alkaline-earth-metal or alkali-metal layer. The superconducting energy gap of 7.3 me V and bosonic mode of 12.7 me V were resolved in the scanning tunneling spectra. In addition, the superconducting energy gaps measured on different terminations were identical and consistent with the values obtained by bulk-sensitive techniques, indicating that the electronic structures of CaKFe_(4)As_(4) were insensitive to the surface reconstructions. Our study clarifies the relationships between complex surface reconstructions and surface terminations and preliminarily presents that there is no obvious effect of surface reconstructions on electronic states.
基金We thank Min Ouyang and Wu Zhou for helpful discussion. This work is supported by the National Key Research and Development Program of China (Nos. 2019YFA0308500 and 2018YFA0305800)the National Natural Science Foundation of China (Nos. 51922011, 51991340, and 61888102)+1 种基金the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB28000000)A portion of the research was performed in CAS Key Laboratory of Vacuum Physics. G. H. C. is supported by Funds for the Central Universities and the National Key Research and Development Program of China (Nos. 2019FZA3004, 2017YFA0303002, and 2016YFA0300202).
文摘As a new type of iron-based superconductor, CaKFe_(4)As_(4) has recently been demonstrated to be a promising platform for observing Majorana zero modes (MZMs). The surface of CaKFe_(4)As_(4) plays an important role in realizing the MZM since it hosts superconducting topological surface states. However, due to the complicated crystal structure, the terminal surface of CaKFe_(4)As_(4) has not been determined yet. Here, by using scanning tunneling microscopy/spectroscopy (STM/S), we find that there are two types of surface structure in CaKFe_(4)As_(4). Bias-dependent atomic resolution images show an evolvement from 2–√×2–√ superstructure with respect to the As lattice into 1 × 1 when the tip is brought close to the surface, revealing the sublattice of missing As atoms. Together with the first-principles calculations, we show that the surface As layer has a buckled structure. Our findings provide insight to future surface study of CaKFe_(4)As_(4) as well as other iron-pnictide superconductors.
