Valence states of copper atoms in cuprates determined from the change in the electron-capture constant of 64Cu

The change in the ⁶⁴Cu electron-capture constant for the compounds Cu₂O, YBa₂Cu₃O₇, YBa₂Cu₃O₆, Nd₂CuO₄, Nd_1.8Ce_0.2CuO₄, La₂CuO₄, La_1.7Sr_0.3CuO₄, Tl₂Ba₂CaCu₂O₈, and Bi₂Sr₂CaCu₂O₈ has been measured. It is concluded that the copper in YBa₂Cu₃O₇, Nd₂CuO₄, La₂CuO₄, La_1.7Sr_0.3CuO₄, Tl₂Ba₂CaCu₂O₈, and Bi₂Sr₂CaCu₂O₈ is exclusively divalent, whereas in Cu₂O, YBa₂Cu₃O₆, and Nd_1.8Ce_0.2CuO₄ univalent copper is present. Fiz. Tverd. Tela (St. Petersburg) 39, 1793–1794 (October 1998)     

Superconductivity in Tl2Ba2−xSrxCaCu2O8 solid solutions: Tc≈44 K for a composition with x=2.0

Solid solutions in the Tl₂Ba_2−xSr_xCaCu₂O₈ system were prepared and characterized by X-ray powder diffraction, resistivity and magnetic susceptibility measurements. Systematic changes in T_c and lattice parameters with x are reported. A sample with nominal composition of Tl₂Sr₂CaCu₂O₈ (x=2.0) is superconducting with T_c≈44 K.     

X-ray Fluorescence Spectroscopic Studies of High TeSuperconductors

The Cu-Lβ/Cu-Lα x-ray emission intensity ratio of the following compounds have been found to show an interesting consistency: Cu, Cu₂O, CuO, La_1.85Ba_.15CuO₄ (vacuum annealed), La_1.85Ba_.15CuO₄ (air annealed, T_C = 26K), La_1.85Sr_.15CuO₄ (T_C = 35K), YBa₂Cu₃O_7-Δ (T_C = 91K), ErBa₂Cu₃O_7-Δ (T_C = 81K), and La₂CuO₄. The interesting consistency is that all of the last six compounds, all superconductors or superconductor related, have the same Lβ/Lα intensity ratio, namely, 0.33±0.01. The Cu-Lα,β spectra of CuO, air and vacuum annealed La_1.85 Ba_.15CuO₄ and YBa₂Cu₃O_7-Δ are presented. Also presented are O-K x-ray fluorescence spectra of air and vacuum annealed La_1.85 Ba_.15CuO₄, and of “new” and “old” YBa₂Cu₃O_7-Δwhich seem to show a correspondence between spectral sharpness and high T_C behavior.     

Coexistence of superconductivity and magnetic order in YBa2Cu4O8

⁵⁷Fe Mössbauer spectroscopy, magnetic susceptibility and powder x-ray-diffraction measurements were used to study superconductivity and magnetic order in YBa₂(Cu_1?xFe_x)₄O_8+δ. T_c is decreasing with x, disappearing for x>x_c≈0.04. For xc iron substitutes Cu, predominantly in the Cu(1) site exhibiting a single quadrupole Mössbauer spectrum at 90 K. For x>x_c magnetic order is observed in the Cu(2) site, T_N=380 (5) K for x=0.1 and H_eff (Cu(2), 4.2 K)=510(2) kOe. However, the most surprising discovery is that for x=0.025, for which T_c=27(2) K, the Fe in the Cu(1) site orders magnetically at T_N=30(2) K and H_eff (Cu(1), 4.2 K)=461(2) kOe. The coexistence and competition between superconductivity and magnetic order in the Cu(1) and Cu(2) sites in YBa₂Cu₄O₈ are discussed in terms of the previously observed phase diagrams for Y_1?xPr_xBa₂(Cu_1?yFe_y)₃O_z.     

NMR study of magnetic moments arising in superconducting cuprates under electron/neutron irradiation

The measurements of direct-current magnetic susceptibility and nuclear magnetic resonance (NMR) were performed in the superconducting cuprates YBa₂Cu₃O_6.9 (⁸⁹Y,⁶³Cu), Tl₂Ba₂CaCu₂O₈ (¹⁷O,²⁰⁵Tl), irradiated by fast neutrons (E _n = 1 MeV) and electrons (E _e = 5 MeV). The influence of the radiation-induced structural disorder on the spin susceptibility and magnetic state of atoms in the CuO₂ layer is considered. The possible effect onT _c of the spin-exchange scattering of carriers by localized moments arising under irradiation was estimated following the Abrikosov-Gorkov approach and its contribution was found to be too small in comparison with the real suppression ofT _c under irradiation.     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

Approaches to the synthesis of High-TC superconducting oxides in La–Ba–Cu–O and Y–Ba–Cu–O systems

Abstract

High-T_C superconducting oxides of nominal La_1.85Ba_0.15 CuO₄ and YBa₂ Cu₃ O₇ have been prepared by using nitrate, carbonate, oxalate/malonate and citrate precursors. While the samples in the Y-system are generally monophasic YBa₂Cu₃O_7?δ with T_C around 90K, the preparations in the La-system are biphasic containing K₂NiF₄-like La_1.85Ba_0.15 CuO₄ (T_C = 30K) and a perovskite-like phase with' a much higher T_C (200–300K). Effect of Ca, Zr, Ce as well as S substitution in YBa₂Cu₃O_7?δ has also been investigated     

Transport properties of a and c axis oriented (Y1?xCax)Ba2Cu3Oy thin films

Anisotropy and Hall effect measurements have been performed in calcium-doped, i.e., overdoped YBa₂Cu₃O_y ((Y_1−xCa_x)Ba₂Cu₃O_y) thin films witha andc axis orientations. In highly overdoped films (x=0.4), the anisotropy of the normal resistivity decreases and a drastic change in Hall conductivity in the mixed state is observed. The change in Hall conductivity in the overdoped region is consistent with recent experimental results for La_2−xSr_xCuO₄ films and seems to be common in highT _c superconductors.     

Weak link problem and intragrain current density in polycrystalline Bi1Ca1Sr1Cu2Ox and Tl2Ca2Ba2Cu3O10

Polycrystalline sintered specimen were investigated by means of ac susceptibility and inductive critical current measurements. Main objects are whether the new highT _c oxides have a weakly coupled intergranular structure and to get information about the intragrain critical current density. The Tl₂Ca₂Ba₂Cu₃O₁₀ specimen shows a connective nature similar to YBa₂Cu₃O₇ with low intergrain currents (100 A/cm² at 100K,B=0 T) and high intragrain ones (4.5·10⁵ A/cm² at 100K,B=0 T). The investigation of the Bi₁Ca₁Sr₁Cu₂O_x specimen becomes complex due to the presence of two superconducting phases and the low intragrain critical current density of the lowerT _c phase.     

Temperature dependence of surface impedance of Tl2Ba2CaCu2O8−δ and YBa2Cu3O6.95 single crystals measured in the microwave band

The real part R _s and the imaginary part X _s of the surface impedance Z _s=R _s+ iX _s of Tl₂Ba₂CaCu₂O_8−δ and YBa₂Cu₃O_6.95 single crystals have been measured with high precision at frequency ω/2π=9.4 GHz in the temperature range 0<T<140 K. In the Tl₂Ba₂CaCu₂O_8−δ crystal a linear temperature dependence R _s(T) has been found for T⩽50 K, and the magnetic field penetration depth λ(4.2 K)=X _s(4.2 K)/ω μ ₀≈3760 Å has been measured. Along with well known features of the function Z _s(T) in high-quality YBa₂Cu₃O_6.95 single crystals, such as the linearity of λ(T) and R _s(T) for T<T _c/3 and a maximum of R _s(T) at T∼T _c/2, the linearity range of λ(T) extends to T≃50 K, and this curve has a plateau in the range 60<T<85 K. The curve of R _s(T) in both the superconducting and normal states of YBa₂Cu₃O_6.95 is well described by a two-fluid model with the electron-phonon mechanism of quasiparticle relaxation. A formula describing the curve of λ ²(0)/λ ²(T) throughout the studied temperature range is also given. Zh. éksp. Teor. Fiz. 112, 2210–2222 (December 1997)     

Investigations of the interlayer coupling in high-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> cuprates

Low-field magnetizationM(H) measurements can be used to probe the nature of the screening currents and the interlayer coupling in high-T _c cuprates. Here we compare theM(H) behaviour of single crystals of Bi₂Sr₂CaCu₂O₈ and fully oxygenated and oxygen reduced YBa₂Cu₃O_7??. In YBa₂Cu₃O₇, theM(H) behaviour is consistent with anisotropic 3D superconductivity whilst in Bi₂Sr₂CaCu₂O₈, the surface screening currents are strongly affected by the presence of vortices, implying that the CuO₂ planes are coupled via a weak Josephson interaction. In oxygen-deficient YBa₂Cu₃O_6.7 (T _c =63K), theM(H) behaviour at low temperatures is similar to that found for Bi₂Sr₂CaCu₂O₈, implying that the removal of oxygen from the chains has resulted in a dimensional crossover of the superconducting state in YBa₂Cu₃O_7??. As the temperature approachesT _c , the 3D behaviour is eventually restored as thec-axis coherence length ξ _c becomes comparable with the interlayer spacingd.     

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.     

NMR study of the spin dynamics in full over-doped Tl2Ba2CuO y (T c=0 K)

The temperature dependence and the anisotropy of the Knight shift and the spin-lattice relaxation time (T ₁) at the Cu and the O sites of the full over-doped Tl₂Ba₂CuO _y (T _c=0 K: Tl2201L) were measured. The Knight shift values at the Cu and the o sites in the CuO₂ plane are larger in Tl2201L compared to those in Tl₂Ba₂CuO _y (T _c=85 K: Tl2201M). This indicates the increase of the spin part of the Knight shift due to the over-doping. Based on the comparison between theT ₁ in Tl2201L and Tl2201M, the enhancement of the antiferromagnetic correlation still remains in the whole over-doped region.     

NMR in highly correlated superconductors

Results of our systematic NMR study in highT _c cuprates are reviewed. The antiferromagnetic spin fluctuations (AFSF) decrease in the order of La_1.85Sr_0.15CuO₄. YBa₂Cu₃O₇ and Tl₂Ba₂CuO_6+y . 1/T ₁ of⁶³Cu in the CuO₂ plane in the normal state follows essentially a Curie-Weiss law at high temperature andT ₁ T = const. law at low temperature. The temperature dependence of 1/T ₁ and the Knight shift together with their impurity effect in the superconducting state strongly suggest d-wave pairing implying the AFSF to be responsible for the occurrence of superconductivity. From the NQR frequency measurement the density of Cu 3d and O 2p holes decreases and increases, respectively, in the order of La, Y and Tl compounds, which is consistent with the change of AFSF. The relation betweenT _c andv _Q, and their pressure dependence suggest that there exists and optimum value of the ratio of Cu 3d and O 2p hole density to give a maximum inT _c.     

Correlations betweenT c andn s/m * (carrier density/ effective mass) in high-T c and organic superconductors

We update our muon spin relaxation studies of the magnetic field penetration depth which show the correlations betweenT _c and the relaxation rate σ αn _s/m ^* (carrier density/effective mass) of hole-doped high-T _c cuprate superconductors (La₂, Sr₃)CuO₄, YBa₂Cu₃O₇ (Y_1−xPr_x)Ba₂Cu₃O₇, and other double and triple layer systems. These studies are extended to the organic superconductor (BEDT-TTF)₂Cu(NCS)₂.     

Superconducting interaction charge in thallium-based high-TC cuprates: Roles of cation oxidation state and electronegativity

Superconductivity in the Tl-based cuprates encompasses a notably broad range of measured optimal transition temperatures T_C0, ranging from lowest in the charge-depleted Tl-1201 compounds (Tl_1–x(Ba/Sr)_1+yLa_1–yCuO_5–δ), such as Tl_0.7LaSrCuO₅ (37 K) and TlBa_1.2La_0.8CuO₅ (45.4 K), to highest in the Tl-1223 compound TlBa₂Ca₂Cu₃O_9±δ (133.5 K). Seven Tl-based cuprates are considered and compared using the model of superconductive pairing via electronic interactions between two physically separated charge reservoirs, where T_C0∝(ση/A)^1/2ζ⁻¹ is determined by the superconducting interaction charge fraction σ, the number η of CuO₂ layers, and the basal-plane area A, each per formula unit, and the transverse distance ζ between interacting layers. Herein it is demonstrated that σ follows from the elemental electronegativity and the oxidation state of Tl, and other structurally analogous cations. The comparatively lower elemental electronegativity of Tl, in conjunction with its oxidation state, explains the higher σ and T_C0 values in the Tl-based compounds relative to their Bi-based cuprate homologs. A derivation of σ is introduced for the optimal Tl₂Ba₂Ca_η–1Cu_ηO_2η+4 (for η=1, 2, 3) compounds, which exhibit a Tl oxidation state at or near +3, obtaining the fundamental value σ₀=0.228 previously established for YBa₂Cu₃O_6.92. Also reported is the marked enhancement in σ associated with Tl⁺¹ and analogous inner-layer cations relative to higher-valence cations. For a model proposition of σ=σ₀, the fractional Tl⁺¹ content of the mixed-valence compound, TlBa₂Ca₂Cu₃O_9±δ, is predicted to be 1/3 at optimization, in agreement with existing data. Charge depletion is illustrated for the two Tl-1201 compounds, where σ<σ₀ values are determined according to substitution of Ba⁺² or Sr⁺² by La⁺³, and/or Tl depletion. Additionally, statistical analysis of calculated and experimental transition temperatures of 48 optimal superconductors shows an absence of bias in determining σ, A, and ζ.     

The possibility of introducing additional states in the conduction band of YBa2Cu3Oy by Ca doping

Measurements of the temperature dependences of the resistivity and Seebeck coefficient S, as well as of the Hall coefficient R _H at T=300 K are reported for two series of ceramic Y_1−x Ca_xBa₂Cu₃O_y samples with oxygen contents in the initial sample close to stoichiometric and then lowered by annealing. It has been found that an increase in calcium content in the first series results in a drop of the critical temperature T _c , a weak variation of S (300 K), and an increase of R _H (300 K), whereas in the second series T _c increases, S (300 K) decreases, and R _H (300 K) remains practically constant. The S (T) relations acquire additional features with increasing doping level, which are not typical of the YBa₂Cu₃O_y system for other types of substitution. An analysis of the data obtained made in terms of the phenomenological band model permits a conclusion that calcium is capable of introducing additional states in the conduction band. Based on this assumption, we have succeeded in providing a qualitative explanation for the unusual features in the behavior of the transport coefficients and critical temperature in Y_1−x Ca_xBa₂Cu₃O_y. Fiz. Tverd. Tela (St. Petersburg) 40, 2145–2152 (December 1998)     

In-plane London penetration depths near the critical temperature of Tl2Ba2Ca n−1Cu n O2n+4 and (Bi,Pb)2Sr2Ca n−1 Cu n O2n+4 (n=2,3)

From isothermalM(H) curves nearT _c , measured on polycrystalline Tl₂Ba₂Ca _n–1Cu _n O_2n +4 and (Bi, Pb)₂Sr₂Ca _n–1Cu _n O_2n+4 (n=2,3) samples, we deduce the in-plane penetration depths _ab as functions of temperature. An estimate according to the BCS weak-coupling clean-limit fit, which produces the data nearT _c very well, yields _ab(0)=3100 Å, 2320 Å 2210 Å, and 1960 Å for Bi₂Sr₂CaCu₂O₈, Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀, Tl₂Ba₂CaCu₂O₈, and Tl₂Ba₂Ca₂Cu₃O₁₀, respectively. A comparison between strong-coupling and weak coupling fitting curves clearly favours the weak-coupling temperature dependence of _ab nearT _c .     

High-temperature superconductors YBaCu<Subscript>2</Subscript>O<Subscript>5</Subscript> and Tl<Subscript>1.5</Subscript>BaCa<Subscript>2</Subscript>Cu<Subscript>2.5</Subscript>O<Subscript>8</Subscript> with a decreased content of heavy metals (Ba and Tl)

New high-temperature superconductors YBaCu₂O₅ and Tl_1.5BaCa₂Cu_2.5O₈ have been synthesized. They can be considered as a result of the removal of barium cuprate BaCuO₂ from YBa₂Cu₃O₇ and barium cuprate-tallate BaCu_0.5Tl_0.5O₂ from Tl₂Ba₂Ca₂Cu{ia3}O₁₀. The synthesized compounds contain a smaller amount of toxic Ba and Tl. Investigation of electrical conductivity has demonstrated that the values of T _c of synthesized materials are close to T _c of starting materials. The Meissner effect observed for these materials has confirmed their belonging to high-temperature superconductors.     

Cu nuclear spin-spin relaxation in YBa2Cu3O6.98 and YBa2Cu4O8

We have measured the temperature (T) dependence of the transverse relaxation rate (T _G ^–1 ) of the Cu(1) nuclear spin in YBa₂Cu₃O_6.98 (T _c=92 K) and YBa₂Cu₄O₈ (T _c=82 K). From the scaling ratio ofT _G ^–2 (Cu1) toT _G ^–2 (Cu2), we have estimated the strength of a covalent bonding between the CuO₂ plane and the CuO chain to be B0.38×A _zz. The experimentalT _G ^–1 (Cu1) in YBa₂Cu₄O₈ was of the same order of magnitude as the estimated one fromT _G ^–1 (Cu2). These results appear to indicate that the electrons in the CuO₂ plane fairly spread out of the plane in both compounds.