On the strength of electron correlations in high-T c superconducting copper oxides

We analyze the strength of electronic correlations in the half-filled antibonding Cu–O orbitals of high-T _c superconducting copper oxides by considering a Cu₁₂O ₁₇ ^n– cluster withn=8 or 10, respectively. The correlated ground state is calculated by the method of the local approach (LA) in a version which allows the treatment of stronger correlated electrons. As mean-field basis a semiempirical Hamiltonian of the ZDO (zero differential overlap) type has been adopted. It is found that the correlations are particularly strong in the Cu 3d _x ²–y² orbitals. The nonintegral orbital occupation allows for valence fluctuations between Cu⁺ and Cu²⁺ in spite of the remarkable correlations. According to the present model excess holes are located at the oxygen sites. The theoretical findings are compared with the results of spectroscopic investigations. The present electronic-structure analysis allows for a straighforward rationalization of previous experimental measurements. The strong connection between the importance of electronic correlations and the symmetry properties of the electronic wave function is emphasized.     

Correlations between frequency of infra-red active vibrational modes and copper-oxygen distance in copper oxides. Application to superconductors

The infra-red spectra of a large number of ternary Cu(II) oxides with at least a quasi square-planar coordination of oxygen around the copper ions have been studied. The frequency of the bands with the highest frequency,v _max, is found to correlate extremely well with the shortest Cu–O distance.v _max increases at an impressive rate of 20 cm^–1 per 0.01 Å when the Cu–O distance becomes less than 1.97 Å, which is the Cu²⁺–O^2– distance in square-planar CuO₄ complexes as obtained from empirical ionic radii considerations. The marked sensitivity may be used as a titration procedure not only to assign bands but also to obtain diagnostic information about local coordination in compounds derived, for example, from the YBa₂Cu₃O_7–d structure such as LaCaBaCu₃O_7–d . The only example where this correlation fails is in the two-layer non-superconducting oxides derived from La₂(Ca, Sr)Cu₂O₆. The significance of this result is discussed. The marked dependence of frequency on the bond-distance is qualitatively examined in terms of an increased electron-phonon coupling to account for the observed tendency of the superconducting transition temperature to go through a maximum as the average basal plane Cu–O distance is decreased.     

Electronic structure of high-temperature superconductors II. La-Ba-Cu-O systems

Electronic structure of La_2–x Ba _x CuO₄ superconductors has been studied. Model systems La₁₀CuO ₂₂ ^–12 , La₁₀cuO ₁₄ ⁺⁴ and La_gBaCuO ₁₄ ⁺³ with various Ba positions have been investigated using the quasirelativistic INDO molecular orbital method. The oxidation state of copper atoms corresponds to Cu(II). The influence of Ba/La substitution on copper atoms is much lower than on their oxygen ligands sphere.     

Non-superconducting and rendered superconducting YBa2Cu3O7−x . An in-situ photoemission study

We present X-ray photoemission spectroscopy (XPS) results obtained on polycrystalline YBa₂Cu₃O_7–x in the non-metallic (x>0.6) and metallic state (x<0.6) by an in-situ oxygen deloading/loading procedure, thus avoiding uncontrolled surface contaminations. The transition to the metallic (superconducting) state is characterized by corresponding changes in the O1s and Cu2p photoelectron lineshapes being in accordance with charge transfer from CuO₂ planes to CuO chains. In particular, a peak at 531.1 eV binding energy is related to the presence of oxygen hole states (O^–).     

Superconductivity at 40 K in La1.8Sr0.2CuO4

High-T _c superconductivity withT _c onsets up to 42 K (midpoint: 37 K, zero resistance: 34 K) is observed in X-rays homogeneous single phase La_1.8Sr_0.2CuO₄. The quarternary compounds La_2–x Ba _x CuO₄ and La_2–x Sr _x CuO₄ (0x0.3 for Ba and 0x<1 for Sr, depending on the heating conditions) are mixed-valence oxygen defect oxides, characterized by the presence of Cu²⁺ and Cu³⁺ simultaneously. These oxides have a tetragonal symmetry (space group:I 4/mmm) similar to that of K₂NiF₄. We report the synthesis, characterization, and superconducting properties of various high-temperature superconducting La–Ba–Cu–O and La–Sr–Cu–O compounds. Through the substitution of Sr for Ba in these oxygen defect compounds an increasing superconducting transition temperature onset from 28 K to 35 K for La_1.8Sr_0.1Ba_0.1CuO₄ was observed. A positive initial pressure coefficient ofdTc/dp=290 (mK/kbar) has been found for La_1.8Sr_0.2CuO₄ with a magnetic susceptibility change consistent with the 100% diamagnetic expectation value.     

Layered copper oxides designed by the combination of Pb compound and fluorite blocking layers

According to the principles proposed for the design of possible new superconductors, we have synthesized two kinds of copper oxides which consist of Pb based layers and fluorite Ln₂O₂ layers as blocking layers. One consists of SrO/PbO–Cu–PbO/SrO and Ln₂O₂ as blocking layers inserted by Cu–O₂ sheets. The other consists of SrO/(Pb, Cu)O/SrO and Ln₂O₂ as blocking layers inserted by Cu–O₂ sheets. These copper oxides have common characteristics for the candidates of a high temperature superconductor.     

Roles of the blocking layer in high temperature copper-oxide superconductors

Roles of the blocking layer that we named the layer which separates Cu–O₂ layers more than 6Å have been studied by comparing two kinds of highT _c copperoxide superconductors such as Bi(2212) and La(2126) compound. These following results have been obtained. The hole concentration decreases when Sr is substituted by La and increases when Bi is substituted by Pb in Bi(2212), and it can be optimized by these substitution. In La(2126)T _c becomes up to 43K and the hole concentration (p;[Cu–O]^+p) increases to 0.09 by the substitution of Ca for La and heat treatment under high oxygen partial pressure. The distance between Cu–O₂ layers in both Bi(2212) and La(2126) are not changed by these substitution and heat treatment. We have found that the blocking layer has not direct roles for the maximumT _c value of the material though by supplying carriers to Cu–O₂ layers, it affects the actualT _c value.     

Electronic structure of square planar CuO 4 6− clusters

The results of spin-polarized MSX calculations show that the ground state of the CuO ₄ ^6– cluster is essentially non-magnetic in spite of odd number of electrons in the system for short Cu–O distances (1.90 Å) as found in the highT _c superconductors. This arises due to the fact that the unpaired electron resides in a molecular orbital with primarily oxygen 3s character. The stability of this molecular orbital is found to be sensitive to the cluster geometry and thus, increase in Cu–O distance (as well as other changes affecting oxygen-oxygen distance) tend to favour a magnetic state. From these calculations we have also estimated the Coulomb correlation strength within the Cu 3d to be about 5.3 eV.     

Valence fluctuations and oxygen dimerization in high-temperature superconductors from X-ray photoemission spectroscopy

The high-temperature superconducting Cu oxides — which are intrinsically Mott insulators — are strongly correlated metals because of valence fluctuations involving Cud ⁹–d ¹⁰ configurations. Temperature-dependent X-ray photoemission spectroscopy of theO 1s and Cu 2p levels in the range 10–300 K provides evidence for O-dimerization, with corresponding increase of the Cud ¹⁰O 2p ⁵ well-screened XPS-final state belowT _c . This supports the suggestion of dynamical mixed-valence and double-exchange antiferromagnetic interaction as an important mechanism for the origin of highT _c superconductivity.     

Fourier Transform Spectroscopy of theYΣ–XΠiTransition of CuO

TheY²Σ⁺–X²Π_inear-infrared electronic transition of CuO was observed at high resolution for the first time. The spectrum was recorded with the Fourier transform spectrometer associated with the McMath–Pierce Solar Telescope at Kitt Peak. The excited CuO molecules were produced in a low pressure copper hollow cathode sputter with a slow flow of oxygen. Constants for theY²Σ⁺states of CuO are:T₀= 7715.47765(54) cm⁻¹,B= 0.4735780(28) cm⁻¹,D= 0.822(12) × 10⁻⁶cm⁻¹,H= 0.46(10) × 10⁻¹⁰cm⁻¹, γ = −0.089587(42) cm⁻¹, γ_D= 0.1272(79) × 10⁻⁶cm⁻¹,b_F= 0.12347(22) cm⁻¹, andc= 0.0550(74) cm⁻¹. ImprovedX²Π_iconstants are also presented.     

Mechanism of high Tc superconductivity and and anti-ferromagnetic order of high Tc oxides

We propose, here, new theory of high T_c superconductivity mechanism. The hole-Cooper pair formation and the Bose condensation of the Cooper pairs occur in the high T_c superconducting oxides such as Bi–Sr–Ca–Cu–O. The atractive interaction is due to the antiferromagnetic order and its fluctuation of CuO₂ plane. Dependence of T_c on the numbers of CuO₂ layers is quantitatively analized.     

High temperature superconductivity research at the IBM thomas J. Watson and Almaden research centers

Complementing the work of Bednorz and Müller, which started the field of high temperature oxide superconductivity, the IBM Research laboratories at Yorktown Heights in Almaden have made contributions also via many outside collaborations across the full scope of this field. Materials work includes the identification, separation and structural characterization of the YBa₂Cu₃O_7–x phase, the detection of superconductivity and antiferromagnetism in La₂CuO₄, and the deposition of YBaCuO films with transition temperatures above 77 K by electron-beam deposition and plasma spraying. Characterization work includes measurement of the superconducting energy gap by far-infrared and tunneling measurements, study of Meissner and diamagnetic shielding, observation of superconducting fluctuations, correlation of [Cu–O]⁺ complex concentration withT _c, and determination of valence electronic structure by photoemission. Theoretical work includes proposal of an interband scattering mechanism for superconductivity, a mean-field solution to an electron-interaction model, band-structure calculations, and a phenomenological model for lattice parameters and their influence onT _c. YBa₂CU₃O_7–d single crystals have been synthesized and shown to exhibit up to 70:1 magnetic anisotropy. In work of relevance to applications, the first thin film SQUIDs have been operated in temperatures up to 68 K, epitaxial films on SrTiO₃ have been found to sustain critical currents of more than 10⁵ A/cm² at 77 K, and coatings, superconducting above 77 K, have been deposited by plasma spraying.     

Crystal chemistry of the Tl2−xBa2Ca n Cu n+1O2n+6 (n=0, 1, 2) high-T c superconductors

We report on a comparative crystal chemical study of the Tl_2–xBa₂Ca _n Cu _n+1O_2n+6(n=0, 1, 2) high-T _c superconductors. Superconducting properties were determined by DC-conductivity and-susceptibility measurements. An X-ray diffraction analysis reveals the nonstoichiometry of the compounds as a link between structural and superconducting behavior. A comparison of the different structures gives evidence that T1 deficiency in the TlO double layers increases with increasingn; this deficiency causes apparent Cu valences larger than 2+in the superconducting phases thus maintaining the charge balance. By extracting appropriate chemical formulas and performing a bond strength calculation we obtained the mixed-valent oxidation state of copper for the superconducting tetragonal phases as being 2.17+(n=0), 2.25+(n=1) and 2.31+(n=2) respectively; for the non-superconducting orthorhombic phase Tl_2–xBa₂CuO_6–y a Cu valence of about 2+is suggested.     

Oxidation states of Cu,Ba and Y in superconducting YBa2Cu3O7−x

A high temperature (91.2 K) Y–Ba–Cu–O superconductor has been studied by monochromatized XPS core level measurements. The position, shape and satellite emission of Cu 2p indicate a mixture of Cu valencies 2+ and 1+ with some indication for the existence of a third oxidation state of 3+. The two component structure of the Ba 3d line suggests two principal Ba–O configurations due to oxygen vacancies. The shape and position of the Y 3d doublet imply only weak Y–O interactions.     

X-ray absorption spectroscopy of the 90K superconductor YBa2Cu3O7−δ

X-ray absorption spectroscopy has been used to probe the Cu environment of the new highT _c YBa₂Cu₃O_7– superconductor. Evidences of other valencies than two are found for copper. Besides Cu¹⁺ which is localized on Cu₁ sites when the unit cell is oxygendeficient, one invokes a ground state which merges bivalent 3d ⁹ to 3d ⁹ L configurations as proposed in a recent paper (L stands for a ligand hole in the CuO bond) very similar to that observed in many light rare earth oxides.     

Preparation and structural investigation of normalconducting and high-T c superconducting (Tl,Pb)Sr2(Ca,Tl)Cu2O7−y

We report on the synthesis and a structural investigation of two isomorphous variants of (Tl, Pb)Sr₂(Ca, Tl)Cu₂O_7–y with different conductive properties. Our analysis of their tetragonal crystal structure (space groupP4/m m m) from X-ray powder data revealed characteristic features such as Tl off-centering, multiple cations and partly vacant oxygen sites. We found that substitution of about 35% Pb²⁺ for Tl³⁺ in the TlO monolayers of the structure, accompanied by replacement of about 23% Tl³⁺ for Ca²⁺ in the perovskite-like layers, changed normalconducting into superconducting behavior, with a transition to zero resistivity at 76 K; normalconducting behavior was found for 25% Pb on Tl sites. Subtle differences in the comparably high levels of doping of the Cu–O pyramidal sheets with holes are assumed to be responsible for the different conductive properties observed. We suggest that an excessive hole concentration turns the compound from superconducting into metallic behavior.     

Possible highT c superconductivity in the Ba−La−Cu−O system

Metallic, oxygen-deficient compounds in the Ba–La–Cu–O system, with the composition Ba _x La_5–x Cu₅O_5(3–y) have been prepared in polycrystalline form. Samples withx=1 and 0.75,y>0, annealed below 900°C under reducing conditions, consist of three phases, one of them a perovskite-like mixed-valent copper compound. Upon cooling, the samples show a linear decrease in resistivity, then an approximately logarithmic increase, interpreted as a beginning of localization. Finally an abrupt decrease by up to three orders of magnitude occurs, reminiscent of the onset of percolative superconductivity. The highest onset temperature is observed in the 30 K range. It is markedly reduced by high current densities. Thus, it results partially from the percolative nature, bute possibly also from 2D superconducting fluctuations of double perovskite layers of one of the phases present.     

Superconductivity in LaSr(Ba)CuO and related systems

Systematics in the superconducting behaviour of Ln _2−xSr _x(Ba _x)CuO ₄, La _2−xSr _x−yBa _y(Ca _y)CuO ₄, (La _1−xLn _x) _2−ySr _y(Ba _y)CuO ₄ with Ln = Pr, Nd or Y and related systems are presented. The role of oxygen stoichiometry and lattice parameters of these mixed valent copper oxides is indicated. These high T _c superconducting oxides exhibit high-temperature resistivities in the borderline of the metal-nonmetal transition.     

Superconducting properties of gold powder mixed Bi−Pb−Sr−Ca−Cu−O composites

Two kinds of Au powder having different particle size were mixed with a Bi–Pb–Sr–Ca–Cu–O superconductor, purpose being to improve the superconducting characteristics, especiallyJ _c. By conventional Au powder addition,J _c appreciably increased and a maximumJ _c of 900 A/cm² was obtained on the Au 30 wt% mixed composite. On the contrary,J _c decreased by mixing with ultra fine Au powder. ThisJ _c deterioration was predominantly ascribed to the formation of microcracks and poses by the segregation of Au ultra fine powder.     

X-ray photoelectron spectroscopy of the high-temperature superconductor YBa2Cu3O7−x and the copper oxides

The X-ray photoelectron (XPS) core-level spectra of Cu-metal, Cu₂O, CuO, NaCuO₂ and YBa₂Cu₃O_7–x have been determined. It has been shown, that the Cu³⁺ ions in NaCuO₂ are reduced to lower valence states by X-rays in ultrahigh vacuum (UHV). The determination of the electron binding energy of the Cu 2 _{p 3/2} level in NaCuO₂ yields a value of 934.1 eV. The shape of the Cu 2 _{p 3/2} peak as well as the appearence of a shake-up satellite in YBa₂Cu₃O_7–x which is smaller than in CuO show that YBa₂Cu₃O_7–x contains Cu³⁺ ions. The behaviour of YBa₂Cu₃O_7–x also was studied during X-ray irradiation and a reduction was noticeable.