The search for Majorana bound states (MBSs) has been fueled by the prospect of using their non-Abelian statistics for robust quantum computation. Two-dimensional superconducting topological materials have been predicted to host MBSs as zero-energy modes in vortex cores. By using scanning tunneling spectroscopy on the superconducting Dirac surface state of the iron-based superconductor FeTeSe, we observed a sharp zero-bias peak inside a vortex core that does not split when moving away from the vortex center. The evolution of the peak under varying magnetic field, temperature, and tunneling barrier is consistent with the tunneling to a nearly pure MBS, separated from nontopological bound states. This observation offers a potential platform for realizing and manipulating MBSs at a relatively high temperature.

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Vortices in topological superconductors host Majorana zero modes (MZMs), which are proposed to be building blocks of fault-tolerant topological quantum computers. Recently, a new single-material platform for realizing MZM has been discovered in iron-based superconductors, without involving hybrid semiconductor-superconductor structures. Here we report on a detailed scanning tunneling spectroscopy study of a FeTe 0.55 Se 0.45 single crystal, revealing two distinct classes of vortices present in this system which differ by a half-integer level shift in the energy spectra of the vortex bound states. This level shift is directly tied with the presence or absence of zero-bias 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 states, suggesting the presence of topological and conventional superconducting regions that coexist within the same crystal. Our findings provide strong evidence for the topological nature of superconductivity in FeTe 0.55 Se 0.45 and establish it as an excellent platform for further studies on MZMs.Majorana zero modes (MZMs) are proposed to be building blocks of fault-tolerant topological quantum computation 1 due to their non-Abelian statistics. Several systems are predicted to host MZMs, such as intrinsic p-wave superconductors 2,3 , and multiple heterostructures combining strong spin-orbital coupling (SOC) and superconductivity 4-12 .Recently, a new single-material platform of iron-based superconductors (FeSC) has been discovered 13-15 , in which topological nontrivial bands and high-T c superconductivity coexist naturally 16 without the need of proximity effect common to other proposals. This has led to the observation of a pronounced zero-bias conductance peak (ZBCP) in vortices of FeTe 0.55 Se 0.45 17 and a related compound 18 .While a ZBCP that does not split across the vortex core is regarded as a strong indication of MZM and topological nature of the superconducting vortex 4,17-19 , the observation of ZBCP alone is not enough to prove it. Although several pieces of evidence including spatial profile, tunneling barrier dependence, magnetic field dependence and temperature evolution are fully consistent with MZM in FeTe 0.55 Se 0.45 17 , more convincing verification requires demonstration of the nontrivial topology of the superconducting vortex and underlying band structure. The single crystal of FeTe 0.55 Se 0.45 is a unique platform to demonstrate the fundamental distinction between the trivial and topological vortices. Its large ratio 17,20 of Δ /E F enables realization of the quantum limit 21 , where the low-lying quasiparticle bound states, the so-called Caroli-de Gennes-Matricon bound states (CBSs) 22 , become discrete levels observable separately within the hard superconducting gap. These bound states are the eigenstates of the vortex planar angular momentum 21-23 with the eigenvalue determined by topological phase of the host superconductor 4,24 . Even thou...

Majorana zero modes (MZMs) are spatially-localized zero-energy fractional quasiparticles with non-Abelian braiding statistics that hold promise for topological quantum computing. Owing to the particle-antiparticle equivalence, MZMs exhibit quantized conductance at low temperature. By utilizing variable-tunnel-coupled scanning tunneling spectroscopy, we study tunneling conductance of vortex bound states on FeTe0.55Se0.45 superconductors. We report observations of conductance plateaus as a function of tunnel coupling for zero-energy vortex bound states with values close to or even reaching the 2e2/h quantum conductance (here e is the electron charge and h is Planck’s constant). In contrast, no plateaus were observed on either finite energy vortex bound states or in the continuum of electronic states outside the superconducting gap. This behavior of the zero-mode conductance supports the existence of MZMs in FeTe0.55Se0.45.

Purpose: The signal transducer and activator of transcription 3 (STAT3) is frequently overexpressed in most cancers, propagates tumorigenesis, and is a key regulator of immune suppression in cancer patients. We sought to determine the incidence of phosphorylated STAT3 (p-STAT3) expression in malignant gliomas of different pathologic types, whether p-STAT3 expression is a negative prognostic factor, and whether p-STAT3 expression influences the inflammatory response within gliomas. Methods: Using immunohistochemical analysis, we measured the incidence of p-STAT3 expression in 129 patients with gliomas of various pathologic types in a glioma tissue microarray. We categorized our results according to the total number of p-STAT3^expressing cells within the gliomas and correlated this number with the number of infiltrating T cells and T regulatory cells. We then evaluated the association between p-STAT3 expression and median survival time using univariate and multivariate analyses. Results: We did not detect p-STAT3 expression in normal brain tissues or low-grade astrocytomas. We observed significant differences in the incidence of p-STAT3 expression between the different grades of astrocytomas and different pathologic glioma types. p-STAT3 expression was associated with the population of tumor-infiltrating immune cells but not with that of T regulatory cells. On univariate analysis, we found that p-STAT3 expression within anaplastic astrocytomas was a negative prognostic factor. Conclusions: p-STAT3 expression is common within gliomas of both the astrocytic and oligodendroglial lineages and portends poor survival in patients with anaplastic astrocytomas. p-STAT3 expression differs significantly between gliomas of different pathologic types and grades and correlated with the degree of immune infiltration.A key transcription factor, signal transducer and activator of transcription (STAT3), has been shown to drive the fundamental components of tumorigenesis and metastasis by preventing apoptosis (by increasing survivin, BCL-XL, and MCL1 expression) and enhancing proliferation (by increasing c-Myc and cyclin D1/D2 expression; ref. 1), angiogenesis (by increasing vascular endothelial growth factor and hypoxia-inducible factor-1a expression), invasion (by increasing matrix metalloproteinase-2 and matrix metalloproteinase-9 expression), and metastasis (2, 3). Growth factors and cytokines, including interleukin (IL)-6, can activate Janus kinase 2, which subsequently activates STAT3 by phosphorylating the tyrosine residue in the STAT3 transactivation domain (4). Phosphorylated STAT3 (p-STAT3) then translocates into the nucleus and induces the expression of a variety of target genes. IL-6, which is expressed in the central nervous system (CNS) under a variety of conditions, such as hypoxia (5), traumatic and metabolic injury (6), and inflammation (7), has been shown to attract T cells to the CNS (8). Specifically, IL-6 signaling by means of STAT3 is tightly linked to the homing and migrational capacity of T cells (8)...

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Photoemission established KHgSb as a nonsymmorphic topological insulator, which hosts hourglass-shaped surface states.