High-Temperature Superconductivity: The Hole-Pairing Is s-Wave, and the Holes Are on the SrO, BaO, or Interstitial Oxygen

Muon spectroscopy experiments (⁺SR) on YBa₂Cu₃O₇ have shown since 1987 that the hole-pairing is s-wave rather than d-wave (as claimed in 1994 by Sonier et al.). New data taken for a high-quality crystal in a large applied field of 60 kOe show that the claims of d-wave pairing are invalid and based on a misinterpretation of what is actually fluxon-reordering. Several experiments have shown that the holes of high-T _c superconducting oxides reside in the SrO, BaO, or interstitial oxygen regions, and not in the cuprate-planes. These include: (i) successful predictions that PrBa₂Cu₃O₇ and three other compounds would superconduct, because the superconductivity is in the BaO or SrO layers (not in the cuprate-planes); (ii) successful demonstration that a Pr-on-a-Ba-site defect (above a cuprateplane) is a magnetic pair-breaker that kills superconductivity, but that Pron-a-Pr site (below the plane) does not destroy superconductivity, although the two sites are virtually the same distance from the cuprate-plane; (iii) evidence that cuprate-plane free Sr₂YRuO₆ has an onset of superconductivity at 49 K when doped with Cu on Ru sites, which nearly coincides with the onset temperatures of R_2–z Ce _z Sr₂Cu₂RuO₁₀ (for R=Eu or Gd) and GdSr₂Cu₂RuO₈, presumably because the SrO layers, not the CuO₂ planes, superconduct in all three types of compounds; (iv) data indicating that all high-temperature oxide superconductors are p-type and have electronically paired holes; and (v) evidence that the pairing is similar for the cuprates and the ruthenates, and for some organics where sulfur carries the holes.     

Correct Nature of High-Temperature Superconductivity

The correct nature of high-T _C superconductivity is outlined: (i) the cuprates do not superconduct in their cuprate-planes, but in their BaO, SrO, or interstitial-oxygen regions; (ii) doped Ba₂YRuO₆ and Sr₂YRuO₆, ruthenates without cuprate-planes, superconduct in their BaO or SrO layers; (iii) the rutheno-cuprates GdSr₂Cu₂RuO₈ and Gd_2−z Ce _z Sr₂Cu₂RuO₁₀ have cuprate-planes which do not superconduct, but superconduct in their SrO layers; (iv) LaFeAsO_1−x F _x superconducts via its oxygen ions, as do related compounds; and (v) the organic compound κ-[BEDT-TTF]₂Cu[NCS]₂ superconducts along the S chains of the molecule. In YBa₂Cu₃O₇, the superconductivity is consistent with observations by positive muon spin rotation and with analyses indicating an absence of Cu d-band contribution to the superconductivity. Hence only, the BaO layer superconducts. The superconductivity is s-wave in character (not d-wave), once fluxon depinning has been properly accounted for. The superconducting BaO layers are p-type and adjacent to the n-type cuprate-planes. The hole-pairing is not phononic, but Coulombic. Many experiments can be explained by understanding the characteristics of the high-T _C mechanism, which contradicts the theories with d-wave superconductivity in the cuprate-planes.     

Ruthenate and Cuprate High-Tc Superconductivity

The onset of superconductivity is 49 K in Cu-doped (but cuprate-plane-free) Sr₂YRuO₆, almost the same as the 45 K onsets in GdSr₂Cu₂RuO₈ and in R_{2 – z} Ce _z Sr₂Cu₂RuO₁₀ (for R = Gd or Eu), implying that the superconductivity in all four compounds originates in the SrO layers, not in the cuprate-planes. Muon studies show that the superconducting condensate in YBa₂Cu₃O₇ is either s-wave or extended s-wave, not d-wave, confirming earlier work.     

Sr2YRu1?uCuuO6: Evidence for SrO-Layer Superconductivity

Sr₂YRuO₆ doped on its Ru site by Cu superconducts at the below 45 K, although its Ru and Cu are magnetically ordered at 23 K and 86 K, respectively. The SrO layers superconduct. Ba₂GdRuO₆, when doped with Cu, does not superconduct, because L = 0 Gd is not crystal-field split, and so induces Cooper pair-breaking.     

Theory of sm1.5Ce0.5sr2Cu2Nbo10: Critical-temperature and doping effects

For Sm_1.5Ce_0.5Sr₂Cu₂NbO₁₀, the charge-reservoir oxygen model of superconductivity predicts: (i) T_c30 K (vs. the observed 28K); (ii) Roughly 1 percent Ni on Cu sites should drive T_c to zero; (iii) The superconductivity originates in the SrO layers; and (iv) The conduction, as with all high-temperature oxide superconductors, is necessarily p-type. The model also explains why this material can superconduct in the presence of the magnetic rare-earth ion Sm and why Gd on Sm sites does not destroy superconductivity, despite structural similarities to Sm_2-zCe_zCuO₄ and Gd_2-zCe_zCuO₄, although the former superconducts and the latter does not.     

High-Temperature Superconductivity Without Cuprate Planes

Sr₂YRuO₆ and Ba₂YRuO₆, both doped with Cu on Ru sites, superconduct at onset temperatures of ∼49 and ∼93 K, despite having no cuprate planes. Sr₂YRuO₆ has two sister cuprate compounds, GdSr₂Cu₂RuO₈ and Gd_2-z Ce _z Sr₂Cu₂RuO₁₀, that begin superconducting at nearly the Sr₂YRuO₆ temperature, which suggests that all of these materials superconduct not in their cuprate planes (which do not exist in Sr₂YRuO₆ or Ba₂YRuO₆), but in their SrO layers (BaO layers for Ba₂YRuO₆). Generalization of this result to all high-T _c superconductors implies that the cuprate planes do not superconduct, and that the SrO layers (or equivalent layers) do.Sr₂YRuO₆ superconducts with less than 1% Cu, too few Cu atoms to form a significant number of CuO₂ planes [1].Cu-doped Sr₂YRuO₆ becomes fully superconducting at 23 K. See Ref. [2].     

Ruthenate and Cuprate High-T c Superconductivity

The onset of superconductivity is ≈49 K in Cu-doped (but cuprate-plane-free) Sr₂YRuO₆, almost the same as the ≈45 K onsets in GdSr₂Cu₂RuO₈ and in R_{2 ? z} Ce _z Sr₂Cu₂RuO₁₀ (for R = Gd or Eu), implying that the superconductivity in all four compounds originates in the SrO layers, not in the cuprate-planes. Muon studies show that the superconducting condensate in YBa₂Cu₃O₇ is either s-wave or extended s-wave, not d-wave, confirming earlier work.     

Pair breaking as a probe of d-wave high-temperature superconductivity

A unified picture is obtained of the Cooper pair-breaking data by Cu-site Zn and Ni in Nd_2–z Ce _z CuO₄, La_2–SrCuO₄, Bi₂Sr₂CaCu₂O₈, Bi_1.8Pb_0.2Sr₂Ca₂Cu₃O₁₀, YBa₂Cu₃O₇, and YBa₂Cu₄O₈. The data are generally inconsistent with spin-fluctuation d-wave pairing mechanisms of superconductivity and with all two-dimensional cuprate-plane models. The data are consistent with superconductivity in the charge reservoirs.     

Sr2YRu1−uCuuO6: Evidence for SrO-Layer Superconductivity

Sr₂YRuO₆ doped on its Ru site by Cu superconducts at the below ～45 K, although its Ru and Cu are magnetically ordered at ～23 K and ～86 K, respectively. The SrO layers superconduct. Ba₂GdRuO₆, when doped with Cu, does not superconduct, because L = 0 Gd is not crystal-field split, and so induces Cooper pair-breaking.     

Superconducting Properties of the 3-K Phase of Sr2RuO4 near Hc2

The superconducting transition temperature, T _c , of the impurity-free, intrinsic Sr₂RuO₄ is as high as 1.50 K. However, we recently showed that T _c is remarkably increased up to 3 K in the Sr₂RuO₄–Ru eutectic system, in which plate-like microdomains of Ru metal are embedded in the primary-phase Sr₂RuO₄. The phase diagram of the anisotropic upper critical field of the 3-K phase indicates that H _c2 for the field parallel to the RuO₂ plane is strongly suppressed at low temperatures. We argue that the reorientation of the Cooper-pair spin direction near the Sr₂RuO₄–Ru interface may be responsible for this suppression. In addition, we observed unusual hysteresis in the out-of-plane resistivity, _c , at low temperatures and near H _c2, only when the field was applied parallel to the RuO₂ plane.     

Comparison of Superconductivity and Structure for Lanthanum Replacing for Barium and Copper in YBa2Cu3Od

X-ray diffraction studies and the resistivity measurements are used to characterize the structure and the superconductivity of the nominal composition of YBa₂Cu_{3 – x} La _x O _d (YBCLO) cuprates with x 0.30. There was a BaCuO₂ impurity phase detected with x 0.10. The structure of the main phase (123) has the orthorhombic form with Pmmm symmetry in the whole doping range. With increasing content of lanthanum, x, the lattice constants increase for x < 0.04, and decrease for x 0.04. Rietveld refinements for X-ray diffraction show that the dopant of lanthanum substitutes for copper in the lower doping level, and replaces for both barium and copper in the high doping level. The zero-resistance temperature T _c0 first increases with the increase of the content of lanthanum in YBCLO as x 0.04 and then decreases with x as x 0.04. We compared the results with those of La-doped YBa_{2 – z} La _z Cu₃O _y cuprates. The different relationship in superconductivity dependence of lanthanum content may result from the strains due to the different occupancy of lanthanum in the unit cell of YBa₂Cu₃O _d .     

Preparation of Bi2Sr2CaCu2O8 + x-Matrix Composites Containing Fine Strontium Calcium Indate Inclusions via Glass Crystallization

The crystallization of glass with a composition of Bi₂Sr₂CaCu₂O_{8 + x} + 0.25Sr_0.6Ca_0.4In₂O₄was studied in air between 400 and 800°C. Below 700°C, crystalline phases were formed in the following sequence: (Sr,Ca)_0.9Bi_1.1O_2.55, Cu₂O, Bi-2201, (Sr,Ca)In₂O₄, (Sr,Ca)₃Bi₂O₆, and Bi-2212. Above 700°C, the predominant phases were Bi-2212 and (Sr,Ca)In₂O₄. The introduction of In into Bi–Sr–Ca–Cu–O was shown to reduce the glass-forming capability of this system, without suppressing Bi-2212 formation. The Bi₂Sr₂CaCu₂O_{8 + x} + 0.25Sr_0.6Ca_0.4In₂O₄composites prepared by annealing the precursor glass contained 0.2- to 0.4-m inclusions and possessed enhanced superconducting properties.     

Magnetic field dependence of the density of states of YBa2Cu3O6.95: Implications for the vortex structure

The total specific heat of YBa₂Cu₃O_6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T² in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO₂ planes. In agreement with d-wave superconductivity, we find that _n/T_c and (H)_n(H/H_c2)^1/2, where _n is the coefficient of the normal-state linear term. The H^1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core.     

Anisotropy in the resistive superconducting transition under magnetic fields in single crystal Pb2Sr2Ho0.5Ca0.5Cu3O8

Anisotropie properties of the single crystal Pb₂Sr₂Ho_0.5Ca_0.5Cu₃O₈ have been investigated by measuring the electrical resistivity in theab-plane _ab (H, ,T), which depends on the angle between theab-plane and the magnetic-field direction, in various constant fieldsH perpendicular to the current direction. All the angle-dependent values of _ab (H, ,T) at a constant temperature are scaled to be on one curve as a function of reduced field. The anisotropic parameter (m _c ^* /m _ab ^* )^1/2 is estimated as 12–13, which is larger than that of YBa₂Cu₃O₇ and much smaller than that of Bi₂Sr₂CaCu₂O₈. It has been concluded that the anisotropy does not always depend on the thickness of the blocking layer but seems to depend on the overlap of the electronic wave functions along thec-axis. Anisotropy in the pinning potential has also been discussed from the resistive tail in the temperature dependence of _ab (H,,T).     

Chemical Control of Underdoped and Overdoped States in Y(Ba2 ? ySry)Cu3O6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba_2–y Sr _y )Cu₃O_{6 +} (0.9) compounds gives rise to high hole concentrations within both the CuO₂ planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.     

Studies on (Hg0.7Mo0.3)Sr2(Sr1?xLax)Cu2Oz with a Wide Range of Doping State

The single-phased series of Sr-bearing Hg-1212 superconducting cuprate,(Hg_0.7Mo_0.3)Sr₂(Sr_1–x La _x )Cu₂O _z has been prepared. X-ray diffraction showed that, the obtained samples belong to the 1212-structure with tetragonal space group P4/mmm, similar to that of (Hg, Mo)Sr₂(Ca, Y)Cu₂O_z, and stabilized in a wide compositional range of 0.25x0.75. Refinements of the structure are carried out in which the oxygen atoms at the (Hg, Mo) layer is shifted from high-symmetry position (0.5, 0.5, 0) to (x, x, 0). Magnetization and electrical resistivity measurements show that the as-prepared samples exhibit evidence for superconductivity and their superconducting properties were improved after O₂ annealing with T ^onset _c as high as 92 K.     

Correlation Between Superconducting Transition Temperature and Bond Length Ba–O in High-Tc Cuprate Superconductors

A well-known correlation of the maximum superconducting transition temperature (T _c,max) with ionic radius of rare earths and Y (R) in the series RBa₂Cu₃O_{7 –} is converted to the intrinsic correlation of T _c,max with bond length between Ba and oxygen in CuO₂ plane, being valid not only for RBa₂Cu₃O_{7 –} but also for other cuprate superconductors containing BaO plane such as HgBa₂CaCu₂O_{6 +} . It is pointed out that this correlation places a constraint on possible mechanisms inducing the high-T _c superconductivity.     

Infinite-layer,T′-Phase,and 1-D-ladder copper oxide compounds

Results of recent research on electron-doped infinite-layer compounds (e.g., Sr_1–xLa_xCuO₂), T-phase compounds (e.g., Nd_2–xCe_xCuO₄), and spin-1/2 quasi-1-D ladder compounds (e.g., Sr₁₄Cu₂₄O₄₁) are presented, including structural and magnetic measurements. Studies of steric effects indicate that superconductivity disappears in both electron-doped systems for values of the in-plane lattice constant below a critical value, a_cr 3.92 Å. Attempts to hole dope both the infinite-layer and T phases are described. For the quasi-1-D ladder compound Sr₁₄Cu₂₄O₄₁, the anisotropic susceptibility of this system and its small gap are discussed. Collaborative studies of these systems using SR, NMR, and other spectroscopies are reviewed.This research was supported by NSF Grant DMR-9158089 and Welch Foundation Grant F-1191.