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-O compound system at ambient pressure. An estimated upper critical field H c2(0) between 80 and 180 T was obtained.

https://link.aps.org/pdf/10.1103/PhysRevLett.58.908

Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure

Many materials systems are currently under consideration as potential replacements for SiO2 as the gate dielectric material for sub-0.1 μm complementary metal–oxide–semiconductor (CMOS) technology. A systematic consideration of the required properties of gate dielectrics indicates that the key guidelines for selecting an alternative gate dielectric are (a) permittivity, band gap, and band alignment to silicon, (b) thermodynamic stability, (c) film morphology, (d) interface quality, (e) compatibility with the current or expected materials to be used in processing for CMOS devices, (f) process compatibility, and (g) reliability. Many dielectrics appear favorable in some of these areas, but very few materials are promising with respect to all of these guidelines. A review of current work and literature in the area of alternate gate dielectrics is given. Based on reported results and fundamental considerations, the pseudobinary materials systems offer large flexibility and show the most promise toward success...

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High-κ gate dielectrics: Current status and materials properties considerations

The oxide superconductors, particularly those recently discovered that are based on La2CuO4have a set of peculiarities that suggest a common, unique mechanism: they tend in every case to occur near a metal-insulator transition into an odd-electron insulator with peculiar magnetic properties. This insulating phase is proposed to be the long-sought “resonating-valence-bond” state or “quantum spin liquid” hypothesized in 1973. This insulating magnetic phase is favored by low spin, low dimensionality, and magnetic frustration. The preexisting magnetic singlet pairs of the insulating state become charged superconducting pairs when the insulator is doped sufficiently strongly. The mechanism for superconductivity is hence predominantly electronic and magnetic, although weak phonon interactions may favor the state. Many unusual properties are predicted, especially of the insulating state.

http://science.sciencemag.org/content/sci/235/4793/1196.full.pdf

The resonating valence bond state in La2CuO4 and superconductivity

A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77 kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are discussed.

http://science.sciencemag.org/content/sci/264/5157/413.full.pdf

Thousandfold Change in Resistivity in Magnetoresistive La-Ca-Mn-O Films

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

We have prepared and identified as a single phase the high-temperature superconducting compound in the chemical system Y-Ba-Cu-O, an orthorhombic, distorted oxygen-deficient perovskite of stoichiometry ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{9\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (\ensuremath{\delta}\ensuremath{\simeq}2.1). Samples exhibit zero resistance at 91 K, with a transition width of 1.5 K. The Meissner effect attains a value of 76% of the independently measured diamagnetic susceptibility. We estimate parameters that characterize this superconductor, e.g., \ensuremath{\gamma}\ensuremath{\simeq}3--5 mJ (mole Cu${)}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}2}$. The critical current density at 77 K and H=0 exceeds 1100 A/${\mathrm{cm}}^{2}$.

https://link.aps.org/pdf/10.1103/PhysRevLett.58.1676

Bulk superconductivity at 91 K in single-phase oxygen-deficient perovskite Ba 2 YCu 3 O 9 − δ

We report the results of resistivity and magnetic susceptibility measurements in La/sub 2-//sub x/Sr/sub x/CuO/sub 4/ for xless than or equal to0.3. The x = 0.2 sample shows a superconducting transition at 36.2 K with a width of 1.4 K. The associated dc diamagnetic susceptibility (Meissner effect) is a large fraction (60%--70%) of the ideal value. We estimate the density of states from critical-field and resistivity data and suggest, by analogy to BaPb/sub 1-//sub x/Bi/sub x/O/sub 3/, that conventional phonon-mediated superconductivity can account for the high T/sub c/ in this class of materials.

Bulk superconductivity at 36 K in La1.8Sr0.2CuO4.

Melt‐textured growth of polycrystalline YBa2Cu3O7−δ superconductor using directional solidification created an essentially 100% dense structure consisting of long, needle‐ or plate‐shaped crystals preferentially aligned parallel to the a‐b conduction plane. The new microstructure, which completely replaces the previous granular and random structure in the sintered precursor, exhibits dramatically improved transport Jc values at 77 K of ∼17 000 A/cm2 in zero field and ∼4000 A/cm2 at H=1 T (as compared to ∼500 and ∼1 A/cm2, respectively, for the as‐sintered structure), with the severe field dependence of Jc (‘‘weak‐link’’ problem) no longer evident in the new melt‐textured material. The improvement in Jc is attributed to the combined effects of densification, alignment of crystals, and formation of cleaner grain boundaries. Microstructure and distribution of various phases present in the melt‐textured material are discussed in relation to the superconducting properties.

High critical currents in Y‐Ba‐Cu‐O superconductors

We have extended Bean’s critical state model to explicitly include anisotropic critical currents. Measurements at 30 K of the critical currents parallel to the Cu‐O planes but with vortex motion either parallel or across twin boundaries show twin boundaries are probably not an important cause of vortex pinning. In the critical state, current flow perpendicular to the Cu‐O planes is about 30 times smaller than flow parallel to these planes.

Anisotropic critical currents in Ba2YCu3O7 analyzed using an extended Bean model

THE attainment of unprecedentedly high transition temperatures (Tcs) in the copper oxide superconductors illustrates how working with more complex chemical systems allows greater opportunity to balance opposing forces within a single chemical compound, leading to a better optimization of physical properties. For many desired properties, materials with optimal chemical complexity have undoubtedly not yet been found. This appears to be the case for the intermetallic superconductors, whose study has languished in recent years, and which almost never show Tcs above 15 K. These are almost all binary compounds with substitution-type additives, or, rarely, true ternary compounds such as LuRh4B4 (Tc = 11.7 K; refs 1, 2). If, as some argue (refs 3, 4), materials such as AXC60 (ref. 5) and Ba0.6K0.4BiO3 (refs 6, 7) are conventional electron–phonon superconductors with Jcs of ~30 K, then the absence of higher Tcs in intermetallic compounds may mean only that more complex materials have not been sufficiently explored. We have recently found superconductivity at 23 K (a Tc equal to that of the previous intermetallic record holder, Nb3Ge; ref. 9) in the quaternary intermetallic system yttrium–palladium–boron–carbon8, but we were unable to identify the superconducting phase. Here we report superconductivity at temperatures up to 16.6 K for the single-phase quaternary intermetallic compounds LnNi2B2C (where Ln stands for Y, Tm, Er, Ho or Lu). The presence of the 3d transition metal nickel, and the layered crystal structure10 raise intriguing questions about the origin of the superconductivity, and the relatively high Tcs of these and the Y–Pd–B–C superconductor suggest that there may yet be more surprises in store.

R. B. van Dover

Papers

Bulk superconductivity at 91 K in single-phase oxygen-deficient perovskite Ba 2 YCu 3 O 9 − δ

Bulk superconductivity at 36 K in La1.8Sr0.2CuO4.

High critical currents in Y‐Ba‐Cu‐O superconductors

Anisotropic critical currents in Ba2YCu3O7 analyzed using an extended Bean model

Superconductivity in the quaternary intermetallic compounds LnNi 2 B 2 C