The Majorana fermion, which is its own anti-particle and obeys non-abelian statistics, plays a critical role in topological quantum computing. It can be realized as a bound state at zero energy, called a Majorana zero mode (MZM), in the vortex core of a topological superconductor, or at the ends of a nanowire when both superconductivity and strong spin orbital coupling are present. A MZM can be detected as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. However, in practice, clean and robust MZMs have not been realized in the vortices of a superconductor, due to contamination from impurity states or other closely-packed Caroli-de Gennes-Matricon (CdGM) states, which hampers further manipulations of MZMs. Here using scanning tunneling spectroscopy, we show that a ZBCP well separated from the other discrete CdGM states exists ubiquitously in the cores of free vortices in the defect free regions of (Li0.84Fe0.16)OHFeSe, which has a superconducting transition temperature of 42 K. Moreover, a Dirac-cone-type surface state is observed by angle-resolved photoemission spectroscopy, and its topological nature is confirmed by band calculations. The observed ZBCP can be naturally attributed to a MZM arising from this chiral topological surface states of a bulk superconductor. (Li0.84Fe0.16)OHFeSe thus provides an ideal platform for studying MZMs and topological quantum computing. 2

ABSTRACT:Previous experimental results have shown important differences between iron selenide and arsenide superconductors, which seem to suggest that the high temperature superconductivity in these two subgroups of iron

The Majorana zero mode (MZM), which manifests as an exotic neutral excitation in superconductors, is the building block of topological quantum computing. It has recently been found in the vortices of several iron-based superconductors as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. In particular, a clean and robust MZM has been observed in the cores of free vortices in (Li0.84Fe0.16)OHFeSe. Here using scanning tunneling spectroscopy (STS), we demonstrate that Majorana-induced resonant Andreev reflection occurs between the STM tip and this zero-bias bound state, and consequently, the conductance at zero bias is quantized as 2e 2 /h. Our results present a hallmark signature of the MZM in the vortex of an intrinsic topological superconductor, together with its intriguing behavior.A peak at zero energy in tunneling spectroscopy is an important hallmark, but not sufficient proof, for identifying a MZM. For example, although the existence of a MZM is predicted at the ends of a strongly spin-orbital-coupled semiconductor nanowire in the presence of a proximate superconductor and a large Zeeman field [1-7], there are alternative interpretations for the experimental observations of zero energy peaks in these systems [8][9][10][11][12]. Morecompelling evidence for a MZM is that the zero-bias peak possesses the quantized universal conductance, G0 = 2e 2 /h, in tunneling experiments, due to resonant Andreev reflection and the particle-hole symmetry of the MZM [13]. At zero temperature, this Majorana-induced resonant Andreev reflection (MIRAR) would give the quantized conductance regardless of the coupling strength [13]; and at finite temperature, the quantized conductance may be observed when the tunneling coupling is sufficiently strong [1,14]. The quantized conductance of a ZBCP was first observed in the tunneling spectrum of a hybrid device between superconducting aluminum and an InSb nanowire [15]. It thus strongly supports the existence of MZMs in semiconductor nanowire devices, although the ultimate proof of Majorana physics would be a demonstration of non-Abelian statistics [16][17][18]. *

The superconducting film of (Li 1-x Fe x )OHFeSe is reported for the first time.The thin film exhibits a small in-plane crystal mosaic of 0. Moreover, a large critical current density (J c ) of a value over 0.5 MA/cm 2 is achieved at ~20 K. Such a (Li 1-x Fe x )OHFeSe film is therefore not only important to the fundamental research for understanding the high-T c mechanism, but also promising in the field of high-T c superconductivity application, especially in high-performance electronic devices and large scientific facilities such as superconducting accelerator.

The superconducting and magnetic properties of single crystalline (Li 0.84 Fe 0.16)OHFe 0.98 Se with the transition temperature T c 40 K were studied by means of muon-spin rotation (μSR). The zero-field and field-shift μSR experiments confirm the homogeneity of the sample and the antiferromagnetic ordering within the (Li 0.84 Fe 0.16)OH layers below T m 10 K. The temperature dependence of the in-plane component of the magnetic penetration depth (λ ab) was found to be consistent with gap opening within the superconducting FeSe planes, and it is well described within either the single s-wave gap or two s-wave gaps scenario. The opening of an additional small superconducting gap within the insulating (Li 1−x Fe x)OH layers was detected from the temperature evolution of the out-of plane component of the magnetic penetration λ −2 c (T). The superconductivity in (Li 0.84 Fe 0.16)OH is most probably induced by the proximity to the superconducting FeSe layers.

Stabilized FeSe thin films in ambient pressure with tunable superconducting critical temperature would be a promising candidate for superconducting electronic devices. By carefully controlling the depositions on twelve kinds of substrates using a pulsed laser deposition technique single crystalline FeSe thin films were fabricated. The high quality of the thin films was confirmed by X-ray diffraction with a full width at half maximum of 0.515° in the rocking curve and clear four-fold symmetry in φ-scan. The films have a maximum Tc ~ 15 K on the CaF2 substrate and were stable in ambient conditions air for more than half a year. Slightly tuning the stoichiometry of the FeSe targets, the superconducting critical temperature becomes adjustable below 15 K with quite narrow transition width less than 2 K. These FeSe thin films deposited on different substrates are optimized respectively. The Tc of these optimized films show a relation with the out-of-plane (c-axis) lattice parameter of the FeSe films.

In order to elucidate pressure-induced second superconducting phase (SC-II) in AxFe2−ySe2 (A = K, Rb, Cs, and Tl) having an intrinsic phase separation, we perform a detailed high-pressure magnetotransport study on the isoelectronic, phase-pure (Li1−xFex)OHFe1−ySe single crystals. Here we show that its ambient-pressure superconducting phase (SC-I) with a critical temperature Tc ≈ 40 K is suppressed gradually to below 2 K and an SC-II phase emerges above Pc ≈ 5 GPa with Tc increasing progressively to above 50 K up to 12.5 GPa. Our high-precision resistivity data uncover a sharp transition of the normal state from Fermi liquid for SC-I to non-Fermi liquid for SC-II phase. In addition, the reemergence of high-Tc SC-II is found to accompany with a concurrent enhancement of electron carrier density. Without structural transition below 10 GPa, the observed SC-II with enhanced carrier density should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction.

The magnetic order induced by the pressure was studied in single crystalline FeSe by means of muon-spin rotation (µSR) technique. By following the evolution of the oscillatory part of the µSR signal as a function of angle between the initial muon-spin polarization and 101 axis of studied crystal it was found that the pressure induced magnetic order in FeSe corresponds either to the collinear (single-stripe) antiferromagnetic order as observed in parent compounds of various FeAsbased superconductors or to the Bi-Collinear order as obtained in FeTe system, but with the Fe spins turned by 45 o . The value of the magnetic moment per Fe atom was estimated to be 0.13−0.14 µB at p 1.9 GPa.