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Register a new userObservation of a Robust Zero-energy Bound State in Iron-based Superconductor Fe(Te,Se)
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A robust zero-energy bound state (ZBS) in a superconductor, such as a Majorana or Andreev bound state, is often a consequence of non-trivial topological or symmetry related properties, and can provide indispensable information about the superconducting state. Here we use scanning tunneling microscopy/spectroscopy to demonstrate, on the atomic scale, that an isotropic ZBS emerges at the randomly distributed interstitial excess Fe sites in the superconducting Fe(Te,Se). This ZBS is localized with a short decay length of ~ 10 {AA}, and surprisingly robust against a magnetic field up to 8 Tesla, as well as perturbations by neighboring impurities. We find no natural explanation for the observation of such a robust zero-energy bound state, indicating a novel mechanism of impurities or an exotic pairing symmetry of the iron-based superconductivity.
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Majorana quasiparticles (MQPs) in condensed matter play an important role in strategies for topological quantum computing but still remain elusive. Vortex cores of topological superconductors may accommodate MQPs that appear as the zero-energy vortex bound state (ZVBS). An iron-based superconductor Fe(Se,Te) possesses a superconducting topological surface state that has been investigated by scanning tunneling microscopies to detect the ZVBS. However, the results are still controversial. Here, we performed spectroscopic-imaging scanning tunneling microscopy with unprecedentedly high energy resolution to clarify the nature of the vortex bound states in Fe(Se,Te). We found the ZVBS at 0 $pm$ 20 $mu$eV suggesting its MQP origin, and revealed that some vortices host the ZVBS while others do not. The fraction of vortices hosting the ZVBS decreases with increasing magnetic field, while chemical and electronic quenched disorders are apparently unrelated to the ZVBS. These observations elucidate the conditions to achieve the ZVBS, and may lead to controlling MQPs.
Recent experiment reported the evidence of dispersing one-dimensional Majorana mode trapped by the crystalline domain walls in FeSe0.45Te0.55. Here, we perform the first-principles calculationsto show that iron atoms in the domain wall spontaneously form the ferromagnetic order in line withorientation of the wall. The ferromagnetism can impose a $pi$ phase difference between the domain-wall-separated surface superconducting regimes under the appropriate thickness and magnetization of the wall. Accordingly, the topological surface superconducting state of FeSe$_{0.45}$Te$_{0.55}$ can give rise to one-dimensional Majorana modes bounded by the wall. More importantly, we further propose a topological phase battery junction in the form of FeSe$_{0.45}$Te$_{0.55}$/ferromagnet/FeSe$_{0.45}$Te$_{0.55}$, which can be adopted to create and fuse the Majorana zero modes through controlling the thickness or magnetization of the interior ferromagnetic barrier. The braiding and readout of Majorana zero modes can easily be realized by the designed device. Such topological phase battery junction has the potential application in the superconducting topological quantum computation.
The iron-chalcogenide high temperature superconductor Fe(Se,Te) (FST) has been reported to exhibit complex magnetic ordering and nontrivial band topology which may lead to novel superconducting phenomena. However, the recent studies have so far been largely concentrated on its band and spin structures while its mesoscopic electronic and magnetic response, crucial for future device applications, has not been explored experimentally. Here, we used scanning superconducting quantum interference device microscopy for its sensitivity to both local diamagnetic susceptibility and current distribution in order to image the superfluid density and supercurrent in FST. We found that in FST with 10% interstitial Fe, whose magnetic structure was heavily disrupted, bulk superconductivity was significantly suppressed whereas edge still preserved strong superconducting diamagnetism. The edge dominantly carried supercurrent despite of a very long magnetic penetration depth. The temperature dependence of the superfluid density and supercurrent distribution were distinctively different between the edge and the bulk. Our Heisenberg modeling showed that magnetic dopants stabilize anti-ferromagnetic spin correlation along the edge, which may contribute towards its robust superconductivity. Our observations hold implication for FST as potential platforms for topological quantum computation and superconducting spintronics.
Topological superconductivity is one of the frontier research directions in condensed matter physics. One of the unique elementary excitations in topological superconducting state is the Majorana fermion (mode) which is its own antiparticle and obeys the non-Abelian statistics, and thus useful for constructing the fault-tolerant quantum computing. The evidence for Majorana fermions (mode) in condensed matter state is now quickly accumulated. Here we report the easily achievable zero-energy mode on the tunneling spectra on Bi islands deposited on the Fe(Te,Se) superconducting single crystals. We interpret this result as the consequence of proximity effect induced topological superconductivity on the Bi islands with strong spin-orbital coupling effect. The zero-energy mode is argued to be the signature of the Majorana modes in this size confined system.
The search for Majorana bound state (MBS) has recently emerged as one of the most active research areas in condensed matter physics, fueled by the prospect of using its non-Abelian statistics for robust quantum computation. A highly sought-after platform for MBS is two-dimensional topological superconductors, where MBS is predicted to exist as a zero-energy mode in the core of a vortex. A clear observation of MBS, however, is often hindered by the presence of additional low-lying bound states inside the vortex core. By using scanning tunneling microscope on the newly discovered superconducting Dirac surface state of iron-based superconductor FeTe1-xSex (x = 0.45, superconducting transition temperature Tc = 14.5 K), we clearly observe a sharp and non-split zero-bias peak inside a vortex core. Systematic studies of its evolution under different magnetic fields, temperatures, and tunneling barriers strongly suggest that this is the case of tunneling to a nearly pure MBS, separated from non-topological bound states which is moved away from the zero energy due to the high ratio between the superconducting gap and the Fermi energy in this material. This observation offers a new, robust platform for realizing and manipulating MBSs at a relatively high temperature.
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