Mar 13, 2020  
Discovery of zeroenergy bound states at both ends of a onedimensional atomic line defect(Nanowerk News) In recent years, the development of quantum computers beyond the capability of classical computers has become a new frontier in science and technology and a key direction to realize quantum supremacy. 

However, conventional quantum computing has a serious challenge due to quantum decoherence effect and requires a significant amount of error correction in scaling quantum qubits.  
Therefore, the exploration of faulttolerant quantum computation using quantum states topologically protected against local environmental perturbations is an important endeavor of both fundamental value and technological significance for realizing largescale quantum computation.  
Majorana zeroenergy bound states (ZEBSs) in condensed matter systems such as superconductors are such rare quantum states with topological protection against local perturbations. These so called Majorana zero modes (MZMs) are charge neutral and obey nonabelian exchange statistics and serve as the building block of topological qubits.  
MZMs are theoretically predicted to exist in the vortex core of pwave topological superconductors or at the ends of onedimensional (1D) topological superconductors. Being a ZEBS, one of the main characteristics of the MZM is the differential conductance peaks for tunneling at zero bias voltage.  
Experimentally, the current Majorana platforms include the following. One is using a threedimensional (3D) topological insulator proximitycoupling to an swave superconductor to realize the superconducting topological surface states and detect the vortex states by applying a magnetic field. The other one is using a 1D spinorbit coupling nanowire proximitycoupling to an swave superconductor to detect zerobias conductance peaks at the ends under an external magnetic field.  
However, the complicated fabrication of the hybrid structures, the extremely low temperature and the applied magnetic field required for observation present great challenges to the possible application of MZMs.  
Recently, Professor Wang Jian's group at Peking University, in collaboration with Professor Wang Ziqiang's group at Boston College, discovered MZMs at both ends of 1D atomic line defects in twodimensional (2D) ironbased hightemperature superconductors and provided a promising platform to detect topological zeroenergy excitations at a higher operating temperature and under zero external magnetic field (Nature Physics, "Atomic line defects and zeroenergy end states in monolayer Fe(Te,Se) hightemperature superconductors").  
Wang Jian's group successfully grew largearea and highquality oneunitcellthick FeTe0.5Se0.5 films on SrTiO3(001) substrates by molecular beam epitaxy (MBE) technique, which show Tc (∼62 K) much higher than that (ˆ∼14.5 K) in bulk Fe(Te,Se).  
By in situ lowtemperature (4.2 K) scanning tunneling microscopy/spectroscopy (STM/STS), the 1D atomic line defects formed by the missing topmost Te/Se atoms can be clearly identified on the monolayer FeTe0.5Se0.5 films.  
Figure 1. ZEBSs at the ends of a long atomic line defect (about 15 Te/Se atoms in length). a, An STM topographic image of the long 1D atomic line defect. b, Spatial zeroenergy mapping. c, Tunnelling spectra measured at the lower end and in the middle of the atomic line defect. d, Tunnelling spectra taken along the red arrow direction in a. e, The temperature evolution of the ZEBS at the bottom end of the line defect. The coloured curves are normalized tunnelling spectra and the grey curves are the 4.2K spectra convoluted by the FermiDirac distribution function at higher temperatures. f, The tunnelling barrier dependence of the ZEBS at the bottom end of the line defect. (Image: School of Physics, Peking University) (click on image to enlarge)  
The ZEBSs are detected at both ends of the 1D atomic line defect (Figure 1), while the tunneling spectra in the middle of the line defect recover to the fully gapped superconducting states. As the temperature increases, the ZEBS reduces in intensity, and finally vanishes at a temperature (around 20 K) far below Tc.  
The ZEBS does not split with increasing tunneling barrier conductance and becomes sharper and higher as the tip approaches the film, showing the robust property.  
Moreover, on the shorter defect chain, the coupling between the ZEBSs at both ends leads to reduced zerobias conductance peaks even in the middle section of the atomic line defect chain (Figure 2).  
The positive correlation between the zerobias conductance and line defect lengths can be deduced from the statistics. The spectroscopic properties of the ZEBSs, including the evolution of the peak height and width with temperature, the disappearing temperature of ZEBS, the tunneling spectra in tipapproachingsample process, as well as unsplit property are found to be consistent with the MZMs interpretation.  
Figure 2. ZEBSs at the ends of a short atomic line defect (about 8 Te/Se atoms in length). a, An STM topographic image of the short 1D atomic line defect. b, Spatial zeroenergy mapping. c, Tunnelling spectra measured at the upper end and in the middle of the atomic line defect. d, Tunnelling spectra taken along the red arrow direction in a. e, The temperature evolution of the ZEBS at the top end of the line defect. The coloured curves are normalized tunnelling spectra and the grey curves are the 4.2K spectra convoluted by the FermiDirac distribution function at higher temperatures. f, The tunnelling barrier dependence of the ZEBS at the top end of the line defect. (Image: School of Physics, Peking University) (click on image to enlarge)  
Other possibilities such as Kondo effect, conventional impurity states or the Andreev zeroenergy bound states in nodal hightemperature superconductors can be excluded in general.  
Professor Wang Ziqiang's group at Boston College proposed a possible theoretical explanation by extending the band theory of the Shockley surface state to the case of superconductors. Due to the large spinorbit coupling, the 1D atomic line defect in monolayer FeTe0.5Se0.5 film may become an emergent 1D topological superconductor and a Kramers pair of MZMs appearing at the ends of the line defect protected by timereversal symmetry.  
Even without timereversal symmetry along the line defect, the 1D topological superconductor can also be realized with a single MZM located at each end of the chain. This work, for the first time, reveals a class of topological zeroenergy excitations at both ends of 1D atomic line defects in 2D hightemperature superconducting monolayer FeTe0.5Se0.5 films, which show the advantages of being a single material, higher operating temperature and zero external magnetic field, and may offer a new platform for future realizations of applicable topological qubits. 
Source: Peking University  
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