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     

Copper and oxygen vibrations in La2CuO4 and YBa2Cu3O7

We perform eV neutron scattering experiments (neutron energy > 1 eV) on copper metal, CuO, La₂CuO₄ and YBa₂Cu₃O₇, and neutron absorption experiments of HoBa₂Cu₃O₇, and determined the mean kinetic energy of O, Cu and other atoms in these materials. The Debye temperatures of Cu atoms in La₂CuO₄ and YBa₂Cu₃O₇ were estimated to be extremely large, about 1500 and 2000 K, respectively.     

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)     

Crystal structures of high-T c oxides

A comprehensive review of structure work on high-T _c oxides as reported during the years 1987 and 1988 is given. Thirteen structures are refined from X-ray single-crystal and/or neutron powder diffraction data:I. (Ba_1–x ,K _x )BiO₃ (T _c =30 K),II. (La_2–x , Sr _x )CuO₄ (T _c =40 K),III. (Nd, Ce, Sr)₂CuO₄ (T _c =28 K),IV. (Nd_2–x , Ce _x )CuO₄ (T _c =24 K),V. YBa₂Cu₃O₇ (T _c =90 K),VI. YBa₂Cu₄O₈ (T _c =80 K),VII. Y₂Ba₄Cu₇O₁₄ (T _c =40 K),VIII. Pb₂Sr₂NdCu₃O₈ (T _c =70 K),IX. TlBa₂CaCu₂O₇ (T _c =103 K),X. TlBa₂Ca₂Cu₃O₉ (T _c =120 K),XI. Tl₂Ba₂CuO₆ (T _c =90 K),XII. Tl₂Ba₂CaCu₂O₈ (T _c =112 K),XIII. Tl₂Ba₂Ca₂Cu₃O₁₀ (T _c =125 K). Except forI (perovskite type),II (K₂NiF₄ type) andIV (Nd₂CuO₄ type) they represent new structure types. Structure data, bond distances, structure drawings and calculated X-ray powder diffraction patterns are given for each compound. Structural features and correlations with superconductivity are discussed. The review contains 301 citations.     

Muon spin rotation studies of local magnetism in magnetic and superconducting systems based on the highT c copper oxide structures

In the present paper we will review studies of local magnetism by the muon spin rotation technique in two classes of High-T_C superconductors, namely La_2-xSr_xCuO₄ and YBa₂Cu₃O_x. In addition we present some new results for the existence of local magnetism in hydrogen doped YBa₂Cu₃O₇.     

Investigation of novel highT c superconducting oxides

Our results of microscopic and macroscopic studies of novel highT _c superconductors are discussed. They permit a straightforward comparison of the nature of the superconductivity in 40 K range (La₂CuO₄ type) and 90 K range (YBa₂Cu₃O₇ type) superconductors.     

Influence of interatomic forces on the lattice dynamics of YBa2Cu3O7 (molecular dynamics method)

The method of molecular dynamics is used to study the dynamic properties of the high-temperature superconductor YBa₂Cu₃O₇. It is shown in the system La_2−x Sr_xCuO₄ that, due to the presence of strong anharmonism and a substantial difference in the interatomic forces, local “hot” regions arise around the Ba⁴⁺ charge defects in the CuO₂ and CuO planes, where the mean kinetic energy of the vibrations of the oxygen atoms reaches ∼0.5 eV. All other types of defects: oxygen and copper vacancies, substitutional atoms, and charge defects, do not lead to such effects. Fiz. Tverd. Tela (St. Petersburg) 41, 1729–1733 (October 1999)     

XPS studies of several high temperature superconductors: Y-Ba-Cu-O and La-Ba-Cu-O systems

Copper 2P core level X-ray photoelectron spectra on several high T_c superconductors La_1.85Ba_0.15CuO₄, Y_1.2Ba_0.8CuO_4−δ and Y_0.4B_0.6CuO_4−δ oxides have been measured. The results indicate the coexistence of Cu²⁺ and Cu³⁺ in these compounds.     

Structure and superconductivity studies on (Y1-xLa x )Ba2Cu3O7 (0 ≤ x ≤ 1)

Studies on (Y_{1 - x}La _X )Ba₂Cu₃O₇, x = 0.0 - 1.0, in steps of 0.1, have been carried out. Results show that for x ≤ 0.4, orthorhombicity and high T_c of 90 K are retained. T_c decreases gradually for x ≥ 0.4. Pure LaBa₂Cu₃O₇ shows a T_C ^ZERO of 73 K. XRD data do not reveal any clearcut orthorhombic distortion for pure LaBa₂CuO₇ after annealing at 350°C for 4 days.     

Electronic properties of YBa2Cu4O8 and Y2Ba4Cu7O15 as probed by NMR/NQR

Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) allow the investigation of electronic properties at the atomic level. We will report on such studies in typical members of the Y-Ba-Cu-O family of high-temperature superconductors, namely YBa₂Cu₄O₈ and Y₂Ba₄Cu₇O₁₅. We will deal with temperature and orientational dependences of Cu and O Knight shifts and spin-lattice relaxation. Topics to be discussed are: (1) Existence of a common electronic spin-susceptibility (a single-spin fluid) in the planes and in the chains of YBa₂Cu₄O₈. (2) Evidence for the existence of a pseudo spin-gap in the anti-ferromagnetic fluctuations in YBa₂Cu₄O₈ and Y₂Ba₄Cu₇O₁₅. (3) Evidence for non-s symmetry of the pair wave function in YBa₂Cu₄O₈. (4) First NQR observation of a strong coupling of inequivalent Cu-O planes in Y₂Ba₄Cu₇O₁₅. (5) Alternative explanations of relaxation and Knight shift.     

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.     

Superconductivity in the LnBaCuO system (Ln = Y,Ho, Er)

Superconductivity around 90K is observed in mixed and single phase system of LnBaCuO (Ln = Y, Ho, Er). Resistivity, X-ray powder diffraction and AC susceptibility measurements were carried out on samples of the mixed phase systems Ln_2−xBa_xCuO_4−y (Ln = Ho, Er), Y_1.2Ba_0.8Cu_1+xO_4+x−y(0<x<5) and single phase system Ba₂LnCu₃O₇ (Ln = Y, Ho, Er). Ln_2−xBa_xCuO_4−y consists of Ba₂LnCu₃O₇, BaCuLn₂O₅, Cu₂Ln₂O₅ and starting materials and shows a small diamagnetic signal below T_c in the temperature dependent AC susceptibility. Y_1.2Ba_0.8Cu_1+xO_4+x−y is a mixture of Ba₂YCu₃O₇, BaCuY₂O₅ and CuO and also indicates a small diamagnetic signal at 92K. Interesting resistivity anomalies occur in the Y_1.2Ba_0.8Cu₄O_7−y and Ho_0.8Ba_1.2CuO_4−y composites including large drops in the resistivity at ∼220 and ∼120K, respectively. The pure compounds, Ba₂LnCu₃O₇ (Ln = Y, Ho, Er) show sharp resistive superconducting transitions and bulk Meissner effects of 90%.     

High temperature superconductivity in (M1−xM′x)2CuO4−δ and related compounds (M = Y,La, Eu,Sm; M′ = Ba,Sr)

Compounds of the form (M_1−xM′_x)₂CuO_4−δ and related compounds where M and M′ are Y, various rare earths from La to Lu, and the alkaline earths Sr and Ba, have been investigated in connection with high temperature superconductivity. High temperature superconductivity is confirmed for the system (La_1−xBa_x)₂CuO_4−δ, (La_1−xSr_x)₂CuO_4−δ and (Y_1−xBa_x)₂CuO_4−δ with superconducting transition temperature T_c onsets of 30 K, 38 K and 90 K, respectively. We have found that the related systems (Eu_1−xBa_x)₂CuO_4−δ and (Sm_1−xBa_x)₂CuO_4−δ also exhibit high temperature superconductivity with T_c onsets of 95 K and 65 K, respectively. The highest T_c onset observed in this investigation was 97 K for a sample with the nominal composition of the spinel structure Y_0.33Ba_0.67Cu₂O_4−δ. Measurements of the specific heat C as a function of temperature T on a La_0.8Sr_0.2CuO_4−δ sample reveal a break in slope in the C/T vs T curve at the T_c midpoint, but no clearly discernable jump in C at T_c. A linear term ≈ λ′T in C was observed at low temperature in the superconducting state.     

Microhardness of the compounds La2CuO4 and La1.85Sr0.15CuO4 at temperatures from 81 to 292 K

The microhardness of La₂CuO₄ and La_1.85Sr_0.15CuO₄ crystals is measured in the temperature range 81–292 K, and the influence of various factors on it is investigated. The thermal activation parameters of the plastic deformation process in the vicinity of the indenter impression are estimated. The change of the phase state of the compound La_1.85Sr_0.15CuO₄ at the temperature T ₀=180 K and the appearance of domain (twin) boundaries formed in the ferroelastic tetraortho transition are not seen in the temperature dependence of the microhardness. The results of the study are compared with previously published results for YBa₂CuO_7−x crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 493–497 (March 1998)     

Oxygen stoichiometry,doping and the role of finite Cu-oxide features in high Tc superconductors

We consider the effect of the doping parameter x and the oxygen nonstoichiometry parameter y (in La_2−xBa_xCuO_4+y and in YBa₂Cu₃O_7−y) on the short range structure in high T_c cuprates and argue that both x and y tend to create finite Cu-O structures (FCuOS) e.g., chainlets, in these materials. This is shown to be consistent with the details of their structures, optical properties, Hall effect data etc. The electronic excitations between the confined states of these FCuOS provide a plausible pairing mechanism for superconductivity in the 2-D CuO planes present in these materials.     

Infrared spectra of superconducting La2−xSrxCuO4 and YBa2Cu3O7−δ

Abstract

Abstract Infrared spectra of La_2?xSr_xCuO₄ and YBa₂Cu₃O_7?δ are discussed, specially with respect to the disappearance of the high-frequency (～650 cm) band of the superconducting compositions at 300K. Some of the bands persist at 300K eventhough the materials are fairly conducting. YBa₂Cu₃O_7?δ does not show evidence in the far IR spectrum for the presence of an optical gap.     

μ− SR studies of oxide-superconductorsSR studies of oxide-superconductors

The negative muon spin rotation method (μ⁻ SR) has been applied to studies of electronic states at oxygen sites of oxide superconductors YBa₂Cu₃O₇, Nd_2−x Ce _x CuO_4−δ (x=0.15, oxygen reduced), LiTi₂O₄ and related oxide-insulators La₂CuO_4−δ, CuO, Cu₂O. The paramagnetic shifts of μ⁻ trapped at oxygen nuclei in these polycrystalline powder samples have been measured at 300 K. All the measured shifts are positive. In copper-oxides the paramagnetic shifts are of the order 10⁻³, while in LiTi₂O₄ is very small (8.4±3.34×10⁻⁵). In YBa₂Cu₃O₇, a fast μ⁻ spin relaxation timeT ₂ ^* (∼ 200 ns) has been observed; the reason for this is unknown and further studies are now in progress.     

Above-T c anomalies of the infrared-active phonons in RBa2Cu4O8 (R=Dy,Ho) and Y2Ba4Cu7O15−δ superconductors

We report a far-infrared reflectivity study of DyBa₂Cu₄O₈, HoBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– superconductors in the temperature range 10–300 K. The assignment of thez-polarized infrared-acitve vibrations of the rare-earth-ion (R) was verified by the substitution ofR. Since the structure of the Y₂Ba₄Cu₇O_15– compound inz-direction is an ordered sequence of YBa₂Cu₃O_7– and YBa₂Cu₄O₈ blocks, the phonon spectrum of Y₂Ba₄Cu₇O_15– is a superposition of their respective phonon modes. The temperature dependence of the infrared-active phonons in RBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– was analyzed. A narrowing of the plane-oxygen phonon upon sample cooling was found to occur atT _c , as it was reported for single-chain YBa₂Cu₃O₇ materials; however a pronounced frequency softening of this mode for both RBa₂Cu₄O₈ and Y₂Ba₄Cu₇O_15– was found to start well aboveT _c , suggesting the interaction of infrared-active phonons with an excitation gap similar to that observed in spin-dependent measurements.     

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.     

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.