A stable and reproducible superconductivity transition between 80 and 93 K has been unambiguously observed both resistively and magnetically in a new Y-Ba-Cu-0 compound system at ambient pressure. An estimated upper critical field H, 2(0) between 80 and 180 T was obtained.

An apparent superconducting transition with an onset temperature above 40 K has been detected under pressure in the La-Ba-Cu-0 compound system synthesized directly from a solid-state reaction of La203, CuO, and BaCO3 followed by a decomposition of the mixture in a reduced atmosphere. The experiment is described and the results of effects of magnetic field and pressure are discussed.

Abstract: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 Å, 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.Main Text: Superconductivity arises from the macroscopic quantum condensation of electron pairs. The symmetry of the wave-function of these pairs is one of the most essential aspects of the microscopic pairing mechanism. Since the impurity-induced local density of states (DOS) is sensitive to the pairing symmetry, it can be used to test the symmetry of the order parameter and to probe the microscopic pairing mechanism. Being a local probe with atomic resolution, scanning tunneling microscopy/spectroscopy (STM/S) (1) has played a key role in this respect, especially in the study of high-TC cuprate superconductors (2,3).Since its discovery, new compounds of iron-based superconductor (IBSC) continue to be found. However, the pairing symmetry remains a central unresolved issue. So far,

We have analyzed various characteristic temperatures and energies of hole-doped high-Tc cuprates as a function of a dimensionless hole-doping concentration (pu). Entirely based on the experimental grounds we construct a unified electronic phase diagram (UEPD), where three characteristic temperatures (T * 's) and their corresponding energies (E * 's) converge as pu increases in the underdoped regime. T * 's and E * 's merge together with the Tc curve and 3.5kBTc curve at pu ∼ 1.1 in the overdoped regime, respectively. They finally go to zero at pu ∼ 1.3. The UEPD follows an asymmetric half-dome-shaped Tc curve in which Tc appears at pu ∼ 0.4, reaches a maximum at pu ∼ 1, and rapidly goes to zero at pu ∼ 1.3. The asymmetric half-dome-shaped Tc curve is at odds with the well-known symmetric superconducting dome for La2−xSrxCuO4 (SrD-La214), in which two characteristic temperatures and energies converge as pu increases and merge together at pu ∼ 1.6, where Tc goes to zero. The UEPD clearly shows that pseudogap phase precedes and coexists with high temperature superconductivity in the underdoped and overdoped regimes, respectively. It is also clearly seen that the upper limit of high-Tc cuprate physics ends at a hole concentration that equals to 1.3 times the optimal doping concentration for almost all high-Tc cuprate materials, and 1.6 times the optimal doping concentration for the SrD-La214. Our analysis strongly suggests that pseudogap is a precursor of high-Tc superconductivity, the observed quantum critical point inside the superconducting dome may be related to the end point of UEPD, and the normal state of the underdoped and overdoped high temperature superconductors cannot be regarded as a conventional Fermi liquid phase.

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