Robust and Clean Majorana Zero Mode in the Vortex Core of High-Temperature Superconductor (Li0.84Fe0.16)OHFeSe

About 80 years ago, physicist Ettore Majorana proposed the existence of fermions that are their own antiparticles. Now, these Majorana fermions are attracting a lot of attention for their potential use in certain quantum computing applications. While it is possible to create Majorana-fermion-like states in a topological superconductor, it is very difficult to do so cleanly because of large Fermi energies or the required impurities in the material. Here, we experimentally break through this difficulty.

Majorana fermions can be realized as a bound state at zero energy, known as a Majorana zero mode (MZM). Using scanning tunneling spectroscopy, we detect the unique signature of a MZM in defect-free regions of the topological superconductor (Li0.84Fe0.16)OHFeSe. The intrinsic nontrivial topology of this system enables the realization of a MZM without the need to introduce impurities in the FeSe layer, which was a requirement in previous work.

Our work presents an ideal and practical platform to further study the properties of MZMs, explore their manipulation, and construct MZM-based quantum bits, all of which opens a new, clear route to rapid progress in the fundamental understanding and potential applications of MZMs.
This work has been published in Phys. Rev. X 8, 041056, 2018.

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Surface electronic structure and isotropic superconducting gap in (Li0.8Fe0.2)OHFeSe

Heavily electron-doped iron selenide superconductors, such as AxFe2-ySe2 (A = K, Rb, Cs, Tl/K) and single-layer FeSe on oxides (SrTiO3, BaTiO3), are currently the research focus in the field of iron-based superconductors. The absence of hole Fermi surfaces, together with the nodeless superconducting gap in these materials, pose great challenges on various pairing theories. Recently, a new intercalated FeSe-derived superconductor with Tc higher than 40 K, (Li0.8Fe0.2)OHFeSe, has been synthesized. The crystal structure consists of alternating stacking layers of FeSe and (Li0.8Fe0.2)OH [Fig. 1(a)], without antiferromagnetic phase or Fe-vacancy order in the FeSe layers. 

Using angle-resolved photoemission spectroscopy (ARPES), we found that (Li
0.8Fe0.2)OH layers dope electrons into FeSe layers. The electronic structure of surface FeSe layers in (Li0.8Fe0.2)OHFeSe resembles that of RbxFe2-ySe2 except that it only contains half of the carriers due to the polar surface, suggesting similar quasiparticle dynamics between bulk (Li0.8Fe0.2)OHFeSe and RbxFe2-ySe2 [Fig. 1(b-d)]. Superconducting gap (~10 meV) is clearly observed below Tc, with an isotropic distribution around the electron Fermi surface [Fig. 1(e, f)]. Compared with AxFe2-ySe2, the higher Tc in (Li0.8Fe0.2)OHFeSe could be due to the superior quality of FeSe layer in this material. The (Li0.8Fe0.2)OH layer may also play an important role for elevating Tc.

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X. H. Niu, et al., Surface electronic structure and isotropic superconducting gap in (Li0.8Fe0.2)OHFeSe, Physical Review B 92, 060504(R) (2015). editor’s choice)

Tuning the band structure and superconductivity in single-layer FeSe by interface engineering

The record of superconducting transition temperature (Tc) has long been 56 K for the
bulk iron-based high temperature superconductors. Recently, in single layer FeSe ontop of SrTiO
3 substrate, much enhanced superconductivity with a pairing temperature up to 65K has been observed. However, it remains mysterious how the interface affects the superconductivity. Besides, it encourages further effort to increase the pairing temperature above the liquid nitrogen boiling temperature, so that more cost-effective applications could be expected. research highlight

By molecular beam epitaxy, we changed the interfacial oxide material to Nb:BaTiO3 [Fig.1(a)]. Superconducting gap closing temperature up to 75 K is observed in extremely tensile-strained single-layer FeSe on Nb-doped BaTiO3 [Fig.1(b)], which sets a record high pairing temperature for both Fe-based superconductor and monolayer-thick films, providing a promising prospect on realizing more cost-effective superconducting device. Besides, using in situ angle-resolved photoemission spectroscopy, we compared the electronic structure and superconducting gap of various FeSe-based heterostructures with different interfacial oxides, strain and thicknesses. We have uncovered that electronic correlations and superconducting gap-closing temperatures are tuned by interfacial effects. Moreover, our results exclude the direct relation between superconductivity and tensile strain, or the energy of an interfacial phonon mode, and demonstrate the crucial and non-trivial role of FeSe/oxide interface on the high pairing temperature [Fig.1(c)]. We speculate some possible interfacial interactions that mediate/facilitate Cooper pairing [Fig.1(d)]. These results yield new microscopic insights into the high Tc and provide clues for further enhancing Tc through interface engineering.

R. Peng, H. C. Xu, S. Y. Tan, H. Y. Cao, M. Xia, X. P. Shen, Z. C. Huang, C. H. P. Wen, Q. Song, T. Zhang, B. P. Xie, X. G. Gong, D. L. Feng, Tuning the band structure and superconductivity in single-layer FeSe by interface engineering, Nature Comm. 5, 5044 (2014).

Extraordinary doping effects on quasiparticle scattering and bandwidth in iron-based superconductors

Chemical substitution plays an important role in tuning properties and inducing emergent phenomena in correlated materials. The high-temperature superconductivity in the iron-based superconductors could be obtained by chemical substituting. Like cuprates, the undoped compounds of iron-based superconductors are intimately related to magnetic orders. With carrier doping, the magnetic order is suppressed and superconductivity emerges. Phenomenologically, the superconducting transition temperature (TC) is very sensitive to the doping level and the superconducting region exhibits a dome-like shape in the phase diagram. Comprehensively understanding the role of dopants on the electronic structure and further figuring out their effects on the superconductivity are of great importance for uncovering the mechanism of iron-based superconductivity.
We have systematically studied the effects of chemical substitution in various iron-based superconductors. In addition to the control of Fermi surface topology with heterovalent doping, we found two more extraordinary effects: 1. the site and band dependencies of quasiparticle scattering; and more importantly 2. the ubiquitous and significant bandwidth-control by both isovalent and heterovalent dopants in the iron-anion layer. We found that such a bandwidth-control could be achieved by either applying the chemical pressure or doping electrons, but not by doping holes. More intriguingly,
when the bandwidth is increased beyond a common range, the superconductivity would disappear. On the other hand, after examining a large variety of Fermi surfaces of iron-based superconductors, we suggest that the Fermi surface topology should play a secondary role on the superconductivity. Our results complete the microscopic picture of the electronic effects of dopants, which facilitates a comprehensive understanding of the diversified phase diagrams of various iron-based superconductors. In addition, it suggests that higher TC can be achieved by introducing moderate electronic correlations while minimizing impurity in the iron-anion layer.
Z. R. Ye, Y. Zhang, F. Chen, M. Xu, J. Jiang, X. H. Niu, C. H. P. Wen, L. Y. Xing, X. C. Wang, C. Q. Jin, B. P. Xie, and D. L. Feng, Extraordinary doping effects on quasiparticle scattering and bandwidth in iron-based superconductors. Phys. Rev. X. 4, 031041 (2014). Link


Angle-resolved Photoemission Spectroscopy Study on the Surface States of the Correlated Topological Insulator YbB6

Topological insulator (TI) is a new class of matter with topologically protected surface states that possess unique electronic and spin properties. Recently, how the topological order interplays with the electronic correlation has attracted a lot of theoretical considerations. As materials with significant correlations, the rare-earth borides are interesting with a variety of correlated phenomena. Especially, samarium hexaboride (SmB6) has recently been predicted to be a topological Kondo insulator, which fueled intense effort in search for topological orders in correlated systems. YbB6 is a related rare-earth hexaborides which shares the same structure with SmB6, and is predicted to be a moderately correlated TI with a larger band gap than that of SmB6, YbB6 might be another promising candidate for applications based on topological surface states. However, despite the previous successes of density functional theory in predicting TIs, the correlations in rare-earth compounds pose challenges to an accurate calculation. Therefore, it is crucial to experimentally determine the electronic structure and topological nature of YbB6.
With angle-resolved photoemission spectroscopy, we studied the electronic structure of YbB
6 single crystals. Linearly dispersive bands were observed in its insulating gap around Γ and Χ with negligible kz dependences, indicative of their surface origins. The Circular dichroism (CD) of these in-gap states at various photon energies show patterns consistent with the locked spin- momentum texture of TIs. The CD pattern of the α band around Γ shows obvious two-fold symmetry, while the β band around Χ presents anti-symmetric pattern about the Γ-Χ axis in the surface Brillouin zone (BZ). Moreover, we found that the chemical potential varies up to about 500 meV, from one cleaved surface to another, or sometimes even across the same surface. The bulk bands often coexist with the topological surface states, however, we also identified a bulk insulating gap of about 100 meV, much larger than predicted. It is thus possible to tune the chemical potential into such a gap and realize the surface states dominated transport.


This work has been published as Xia, M. et al. Sci. Rep. 4, 5999; DOI:10.1038/srep05999 (2014).