Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe 1– x Se x ( x = 0.45; superconducting transition temperature T c = 14.5 kelvin) hosts Dirac-cone–type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below T c . Our study shows that the surface states of FeTe 0.55 Se 0.45 are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.

https://science.sciencemag.org/content/sci/early/2018/03/07/science.aan4596.full.pdf

Observation of topological superconductivity on the surface of an iron-based superconductor

A topological superconductor A promising path toward topological quantum computing involves exotic quasiparticles called the Majorana bound states (MBSs). MBSs have been observed in heterostructures that require careful nanofabrication, but the complexity of such systems makes further progress tricky. Zhang et al. identified a topological superconductor in which MBSs may be observed in a simpler way by looking into the cores of vortices induced by an external magnetic field. Using angle-resolved photoemission, the researchers found that the surface of the iron superconductor FeTe0.55Se0.45 satisfies the required conditions for topological superconductivity. Science, this issue p. 182 Angle-resolved photoemission spectroscopy indicates that FeTe0.55Se0.45 harbors Dirac-cone–type spin-helical surface states. Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe1–xSex (x = 0.45; superconducting transition temperature Tc = 14.5 kelvin) hosts Dirac-cone–type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below Tc. Our study shows that the surface states of FeTe0.55Se0.45 are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.

Observation of topological superconductivity on the surface of iron-based superconductor

Topological materials and unconventional iron-based superconductors are both important areas of study but, to date, relatively little overlap has been identified between these two fields. However, the combination of topological bands and superconductivity promises the manifestation of exotic superconducting states, including Majorana fermions, the central component of topological quantum computation. Here, using laser-based, spin-resolved and angle-resolved photoemission spectroscopy and density functional theory calculations, we have identified both topological insulator and Dirac semimetal states near the Fermi energy in different iron-based superconducting compounds. Carrier doping can tune these topologically non-trivial bands to the Fermi energy, potentially allowing access to several different superconducting topological states in the same material. These results reveal the generic coexistence of superconductivity and multiple topological states in iron-based superconductors, indicating that this broad class of materials is a promising platform for high-temperature topological superconductivity. Multiple different types of topological states are observed in iron-based high-temperature superconductors. This suggests that these may be a good place to try and engineer high-temperature topological superconductivity.

Multiple topological states in iron-based superconductors

Vortices in topological superconductors may host Majorana zero modes (MZMs), which have been proposed as the building blocks of fault-tolerant topological quantum computers. Recently, a new single-material platform with the potential for realizing MZMs has been discovered in iron-based superconductors, without involving hybrid semiconductor–superconductor structures. Here, we report a detailed scanning tunnelling spectroscopy study of a FeTe0.55Se0.45 single crystal and show that this material hosts two distinct classes of vortex. These differ by a half-integer level shift in the energy spectra of the vortex bound states. This level shift is directly tied to the presence or absence of a zero-bias conductance peak and also alters the ratios of higher energy levels from integer to half-odd-integer. Our model calculations fully reproduce the spectra of these two types of vortex bound state, suggesting the presence of regions with and without topological surface states, which coexist within the same crystal. Our findings provide strong evidence for the presence of MZMs in FeTe0.55Se0.45 and establish it as an excellent platform for further studies. The authors use STM to show that there are two different classes of zero-bias peak in vortex cores of Fe(Te,Se). One class is topological, one not. These are distinguished by a shift in the energy levels of the excited states.

Half-integer level shift of vortex bound states in an iron-based superconductor

Topological superconductors, whose edge hosts Majorana bound states or Majorana fermions that obey non-Abelian statistics, can be used for low-decoherence quantum computations. Most of the proposed topological superconductors are realized with spin-helical states through proximity effect to BCS superconductors. However, such approaches are difficult for further studies and applications because of the low transition temperatures and complicated hetero-structures. Here by using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we discover that the iron-based superconductor FeTe1-xSex (x = 0.45, Tc = 14.5 K) hosts Dirac-cone type spin-helical surface states at Fermi level, which open an s-wave SC gap below Tc. Our study proves that the surface states of FeTe0.55Se0.45 are 2D topologically superconducting, and thus provides a simple and possibly high-Tc platform for realizing Majorana fermions.

We performed angle-resolved photoemission spectroscopy studies on a series of FeTe1−xSex monolayer films grown on SrTiO3. The superconductivity of the films is robust and rather insensitive to the variations of the band position and effective mass caused by the substitution of Se by Te. However, the band gap between the electron- and hole-like bands at the Brillouin zone center decreases towards band inversion and parity exchange, which drive the system to a nontrivial topological state predicted by theoretical calculations. Our results provide a clear experimental indication that the FeTe1−xSex monolayer materials are high-temperature connate topological superconductors in which band topology and superconductivity are integrated intrinsically.

FeTe 1− x Se x monolayer films: towards the realization of high-temperature connate topological superconductivity

We performed angle-resolved photoemission spectroscopy studies on a series of FeTe$_{1-x}$Se$_{x}$ monolayer films grown on SrTiO$_{3}$. The superconductivity of the films is robust and rather insensitive to the variations of the band position and effective mass caused by the substitution of Se by Te. However, the band gap between the electron- and hole-like bands at the Brillouin zone center decreases towards band inversion and parity exchange, which drive the system to a nontrivial topological state predicted by theoretical calculations. Our results provide a clear experimental indication that the FeTe$_{1-x}$Se$_{x}$ monolayer materials are high-temperature connate topological superconductors in which band topology and superconductivity are integrated intrinsically.

FeTe$_{1-x}$Se$_{x}$ monolayer films: towards the realization of high-temperature connate topological superconductivity

With increasing attention to environmental safety and health, long-chain biologically-based polyamides (PA) are considered to be promising materials owing to their biocompatibility and combining the dual advantages of polyolefins and short-chain petroleum-based polyamides. However, poor flame retardancy severely impedes long-chain biologically-based polyamides’ application in versatile high-standard fields. Herein, we introduced Fe element into the polyhedral oligomeric silsesquioxane (POSS) cage to obtain iron-containing hepta-phenyl POSS (Fe-Ph-POSS), deriving from the chemical reaction between Li-Ph-POSS and FeCl3. The chemical structure of Fe-Ph-POSS was fully characterized and confirmed by FTIR, 1H NMR, 13C NMR, Solid state 29Si NMR, XPS and MALDI-TOF MS. Thereafter, Li-Ph-POSS and Fe-Ph-POSS were adopted to flame retarded biologically-based PA1010. The addition of 10 wt% Fe-Ph-POSS distinctly enhanced the thermal stability without affecting the melting and crystallization temperature of PA1010 composites. Additionally, the introduction of Fe-Ph-POSS did not cause embrittlement for PA1010 and almost completely maintained its excellent mechanical properties (elongation at break, tensile and impact strength were reached 473.8%, 46.87 MPa and 4.52 kJ/m2, respectively). The burning tests results suggested that 10 wt% Fe-Ph-POSS increased the LOI value of PA1010 from 22.6 to 28.4%, and the generation of red-brown char layers rich in iron-containing silicates and carbonates, which effectively hindered the release of heat, smoke and toxic volatiles. Thereby imparting the resultant PA1010 composites to superior flame-retardant performances. Therefore, our work offers an exceptional thought for construction of high-performance biologically-based polyamide composites based on environmentally friendly organic-inorganic hybrid nanomaterials. 

Designing Fe-containing polyhedral oligomeric silsesquioxane to endow superior mechanical and flame-retardant performances of polyamide 1010

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Enhanced superconductivity accompanying a Lifshitz transition in electron-doped FeSe monolayer

Copper-containing tetra-phenyl polyhedral oligomeric silsesquioxane endows epoxy resin with reinforcements in the thermal stability, flame retardancy and smoke suppression

Zhiqing Han

Papers

FeTe 1− x Se x monolayer films: towards the realization of high-temperature connate topological superconductivity

FeTe$_{1-x}$Se$_{x}$ monolayer films: towards the realization of high-temperature connate topological superconductivity

Designing Fe-containing polyhedral oligomeric silsesquioxane to endow superior mechanical and flame-retardant performances of polyamide 1010

Enhanced superconductivity accompanying a Lifshitz transition in electron-doped FeSe monolayer

Copper-containing tetra-phenyl polyhedral oligomeric silsesquioxane endows epoxy resin with reinforcements in the thermal stability, flame retardancy and smoke suppression