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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1ft 'J .Ó AP E418 Superconductivity ExperimentS uperconductivity may beome one of the most signifcant inovations of mom times. Ths is a result of very recent discoveries of high temperature superconductorswhich may revolutionize everything from electrcal generation and transmission to high-speed cQmputers to controlled fusion generatig plants.In ths experiment, you will investigate the superconductivity of the new Y -Ba-Cu-O compound system as a function of temperature and magnetic field. The most important question should should answer is "What is the resistance of the superconductor as the temperatu and applied magnetic field ar changed?" Other questions which you wil not be able to answer are related to the determination of curent density which can pass through the material and the mechanical strength and elasticity of the material.Attached to these notes are some articles which describe aspects of superconductivity and the results of recent measurements of the Y -Ba-Cu-O compound made by Prof. M. K. Wu and co-workers. These wil provide background for the experiments that you wil be penormg. Pay paricular attention to Prof. Wu's aricle. Their measurements are identical to those that you can obtan with the equipment supplied in the laboratory. Next, look over the description of tye 1 and j , ty.e 2 superconductors given by Gennes in Superconductivity in Metals and Alloys.o.f\1. , l' -Type 1 superconductors have a "correlation length" between. conduction electrons ((sheri (denoted by the symbol, ÇO) much longer than the magnetic field penetration length ( denoted by Â.). They generally have a very abrupt crtical field, He, below which the P-i1lèt material is superconducting and excludes all magnetic flux. (H e is a function of temperature.) This "penect diamagnetîsm" is charaèteristic of the superconducting state. Type 2 superconductors have Â. ~ Ço and have a more gradual transition to this superconducting state. They are described by thee critical magnetic fields. Below Hci (the lowest critical field), a typ 2 superconductor has no resistace and, like a type 1 superconductor, excludes flux. Between H c1 and H e2, the resistance is very low, but finite, and the superconductor parally excludes flux. Between He2 and He3, the resistance is low, but no flux is excluded. Finally, above the highest of the three crtical fields, He3. the material is no longer superconducting. DescrI ptionThe Y -Ba-Cu-O compound becomes superconducting between 77 and 85 eK.This makes the cryogenic aspects of these experiment very easy compared with experiments using more conventional superconductors that require temperatures below 22 eK. Liquid nitrogen is relatively inexpensive and a styrofoam thermos is al that is required to store it for several hours.-We have built three LN2 containers for you, and these are large enough to hold stainless-steel and copper "sample stands". The sample stands cool the 1 \, W£t.~ i-superconducting sample and provide a stable base to make your measurements. Also, attached to each sample is a calibrated t...

We have measured thermoelectric power (TEP) as a function of hole concentration per CuO 2 layer P pl in Y 1−x Ca x Ba 2 Cu 3 O 6 ͑P pl = x /2͒ with no oxygen in the Cu-O chain layer. The room-temperature TEP as a function of P pl , S 290 ͑P pl ͒, of Y 1−x Ca x Ba 2 Cu 3 O 6 behaves identically to that of La 2−z Sr z CuO 4 ͑P pl = z͒. We argue that S 290 ͑P pl ͒ represents a measure of the intrinsic equilibrium electronic states of doped holes and, therefore, can be used as a common scale for the carrier concentrations of layered cuprates. We shows that the P pl determined by this new universal scale is consistent with both hole concentration microscopically determined by NQR and the hole concentration macroscopically determined by the formal valency of Cu. We find two characteristic scaling temperatures, T S * and T S2 * , in the TEP versus temperature curves that change systematically with doping. Based on the universal scale, we uncover a universal phase diagram in which almost all the experimentally determined pseudogap temperatures as a function of P pl fall on two common curves; lower pseudogap temperature defined by the T S * versus P pl curve and upper pseudogap temperature defined by the T S2 * versus P pl curve. We find that while pseudogaps are intrinsic properties of doped holes of a single CuO 2 layer for all high-T c cuprates, T c depends on the number of layers, therefore, the inter layer coupling, in each individual system.

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