Magnetic phase present in YBa2(Cu1−x Fe x )3O6+δ oxides

The Mössbauer study of⁵⁷Fe: YBa₂Cu₃O_6+δ oxides was very important to establish the preferential occupation of Cu(1) site by Fe at very low concentrations. Recent determination of antiferromagnetic ordering for Cu moments in Cu(2) sites (T≈450 K) and our early observation of a small proportion of a magnetic phase at room temperature for Fe:YBa₂Cu₃O₆ lead us to perform systematic studies of YBa₂(Cu_1?x Fe _x )₃O₆ withx=0.005, 0.03, 0.05, 0.10 and 0.15 in order to obtain information about the iron occupation of Cu(2) sites.     

高掺杂Sn对Y-Ba-Cu-O体系结构和性质的影响

通过对高掺杂Sn的YBa₂Cu_3-xSn_xO_7-y体系的穆斯堡尔谱测量,确定了掺杂的Sn全部以Sn⁴⁺离子形式存在,多数占据Ba,Y,Cu(2),少量占据Cu(1)位置,Sn含量x不同,其在各个位置上的占有比例也不同,超导电性测量表明,该系列样品T_c大都在77K以上,Cu(2)和Y位上的金属离子被替代对超导温度T_c影响不大,为Cu-O面不是超导的主要原因 关键词：     

89Y NMR probe of Zn induced local magnetism in YBa2(Cu1-yZny) 3O6+x

We present detailed data and analysis of the effects of Zn substitution on the planar Cu site in YBa₂Cu₃O_6+x (YBCO_6+x) as evidenced from our ⁸⁹Y NMR measurements on oriented powders. For x << 1x \ll 1 we find additional NMR lines which are associated with the Zn substitution. From our data on the intensities and temperature dependence of the shift, width, and spin-lattice relaxation rate of these resonances, we conclude that the spinless Zn 3d ¹⁰ state induces local moments on the near-neighbour (nn) Cu atoms. Additionally, we conjecture that the local moments actually extend to the farther Cu atoms with the magnetization alternating in sign at subsequent nn sites. We show that this analysis is compatible with ESR data taken on dilute Gd doped (on the Y site) and on neutron scattering data reported recently on Zn substituted YBCO_{6 + x}. For optimally doped compounds ⁸⁹Y nn resonances are not detected, but a large T-dependent contribution to the ⁸⁹Y NMR linewidth is evidenced and is also attributed to the occurence of a weak induced local moment near the Zn. These results are compatible with macroscopic magnetic measurements performed on YBCO_{6 + x} samples prepared specifically in order to minimize the content of impurity phases. We find significant differences between the present results on the underdoped YBCO_{6 + x} samples and ²⁷Al NMR data taken on Al³⁺ substituted on the Cu site in optimally doped La₂CuO₄. Further experimental work is needed to clarify the detailed evolution of the impurity induced magnetism with hole content in the cuprates.     

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.     

Superconductivity and magnetic order in RBa2Cu3Oz

Mössbauer studies of⁵⁷Fe in RBa_2?y K _y (Cu_1?x Fe _x )₃O_z, with R=Y and Pr;y=0 and 0.5;x=0.01, 0.05 and 0.1 andz between 5.9 and 7.1, have been performed. A minority of the iron ions enter the Cu(2) site and reveal its magnetic order. In nonsuperconducting YBa_1.5K_0.5(Cu_0.95Fe_0.05)₃O_6.1 two distinctly inequivalent magnetic iron sites are observed, probably corresponding to iron in the Cu(2) site with different Ba?K neighbours. In superconducting (T _c =60 K) YBa_1.5K_0.5(Cu_0.95Fe_0.05)₃O_6.5 one Cu(2) subsite remains magnetic (T _N=440 K). The implications of these findings on the valencies of the Cu ions are discussed.     

Preparation of high J c YBa2Cu3O7−δ superconducting thin films by ion beam sputtering deposition

Preparation of high T _c and high J _c YBa₂Cu₃O_7– superconducting thin films by ion beam sputtering deposition is reported. The main factors affecting the composition of the films and the orientation of the crystal grains have been examined. Experimental results show that the Y, Ba and Cu composition of as-deposited films can be conveniently and accurately adjusted by a combined sputtering target which consists of a large sintered target of YBa₂Cu₃O_7– and a small one that is Ba and Cu rich (YBa_2.5Cu_3.3O_x). Fabrication conditions of highly oriented superconducting thin films are described. YBa₂Cu₃O_7– superconducting films with zero resistance at 88–90.5K and critical current density J _c (at 77K) of 1.5×10⁵ A/cm² are obtained.     

Electric quadrupole interaction measurements in YBa2Cu3O x and related compounds

In this work we present Electric Quadrupole Interaction (EQI) measurements, made by Time Differential Perturbed Angular Correlation (TDPAC), on¹¹¹Cd in YBa₂Cu₃O _x and some related compounds. These studies were intended to determine the relationship between the EQI and the actual probe site. The probes were introduced into the materials as a diluted¹¹¹In-complex or via In(¹¹¹In)₂O₃. Our observations indicated that there is no need to suppose the presence of many probe sites in YBa₂Cu₃O _x to explain the experimental results.     

Low-temperature growth of high-Tc superconducting films by plasma-enhanced metal-organic chemical vapor deposition

Plasma-enhanced MOCVD in which metal-organic compounds are sublimated directly into the growth chamber is studied for the first time as a new low-temperature process for growing superconducting YBa₂Cu₃O-_-x thin films. Y(THD)₃, Ba(THD)₂, Cu(THD)₂ and oxygen are used as metal sources and oxydizing agent. Emission spectroscopy reveals that activated metal-organic compounds and activated oxygen species are present during film growth. Superconducting YBa₂Cu₃O_7-x films whose zero-resistivity temperature are 50 K and 82 K are grown at 550°C and 600°C.     

High temperature study of yttrium--barium--copper oxides near the YBa2Cu3Ox composition by 111In/Cd time differential \gamma--\gamma perturbed angular correlation spectroscopy

The local atomic order around very dilute indium impurities in c-axis-oriented YBa₂Cu₃O₇ and YBa₂Cu₃O_6.25 films at room temperature and in YBa₂Cu₃O_x bulk powders held in air or oxygen over a wide temperature range were investigated using ¹¹¹In/Cd \gamma--\gamma perturbed angular correlation (PAC) spectroscopy. The probe atoms occupy a single substitutional lattice position in YBa₂Cu₃O_x, and evidence reported here strongly supports previous claims that this is the yttrium position. In YBa₂Cu₃O_x powders, the local atomic order changes continuously with temperature. At room temperature the electric field gradient measured by PAC in bulk YBa₂Cu₃O_x powder is indistinguishable from that in the fully oxygenated YBa₂Cu₃O₇ film. Near the decomposition temperature, the bulk YBa₂Cu₃O_x powder data are semiquantitatively similar to data in the room temperature YBa₂Cu₃O_6.25 film, with quantitative differences that we attribute to temperature averaging in the former. Other sites usually found in PAC spectra of YBa₂Cu₃O_x bulk samples arise from ¹¹¹In dissolved in Y₂BaCuO₅ and Y₂Cu₂O₅ impurity phases. The latter phase apparently arises because of significant barium loss during processing; the barium deficiency is clearly demonstrated by comparison of PAC data with the alloy phase diagram above the decomposition temperature. PAC data on pure Y₂Cu₂O₅ are reported here also. This revised version was published online in July 2006 with corrections to the Cover Date.     

CNDO studies of Y-Ba-Cu-O superconductors II. Simple models of YBa2Cu3Ox

The electronic structure of YBa₂Cu₃O_x (x=6 and 7) is investigated using the CNDO molecular orbital method. Electronic structures of model clusters [Cu₃O₁₀]^–15, [Ba₈Cu₃O₁₀]⁺¹, [Y₈Cu₃O₁₀]⁺⁹ of the non-superconducting (x=6) and [Cu₃O₁₂]^–17, [Ba₈Cu₃O₁₂]^–1, [Y₈Cu₃O₁₂]⁺⁷ of the superconducting (x=7) phases are compared. Y and Ba layers cause a considerable electron density transfer from the central Cu(1) region.     

Antiferromagnetic order in PrBa2Cu3O x (x≈6, 7)

Zero and weak transverse fieldμ ⁺ SR measurements on PrBa₂Cu₃O _x samples withx≈6 andx≈7 show ordered magnetism in both oxygen concentration limits. As expected for equivalent doping (nominally PrBa₂Cu₃O _x ∼YBa₂Cu₃O _x −0.5), neither sample is superconducting. Two muon signals are observed in thex≈6 sample, as in YBa₂Cu₃O₆, but only one (the weaker) has the same local magnetic field as in YBa₂Cu₃O₆. In thex≈7 sample, only one site is observed; its local field is reduced with respect to that of the primary site in YBa₂Cu₃O₆ by a factor roughly consistent with the carrier-density dependence of the Cu ion moment in antiferromagnetic YBa₂Cu₃O _x .     

Y1-xPrxBa2Cu3O7+δ的红外反射谱

测量了温度范围为4.2—300K的Y_1-xPr_xBa₂Cu₃O_7+δ(x=0,0.1,0.2,0.4,0.5,0.61.0)的红外反射谱,发现在低温下Ba模有双模行为,随着x值增大双模强度反转,在YBa₂Cu₃O_7+δ中已被判定为Y模的位于194cm^-1的反射峰的位置与Pr的含量无关,即用Pr部份或全部替代Y时此峰不发生频移。建议Pr在Y_1-xPr_xBa₂Cu₃O_7+δ,中是正4价,Pr的替代使原来可移动的空穴被定域在Pr的周围,使超导电性受到压制。 关键词：     

Effect of heavy atom disorder on the intensities of powder XRD lines of YBa2Cu3O7-y

Abstract

It is shown that heavy atom disorder resulting from Y ? Ba, Ba ? Cu and Y ? Cu interchanges due to local stoichiometric constraints can cause significant changes in the intensities of X-ray powder diffraction lines of YBa₂Cu₃O_7-y . A comparison of the theoretically predicted intensities with published patterns for specimens prepared by the conventional dry route reveals that Ba ? Cu interchanges involving copper atoms in the CuO₂ planes can occur quite frequently and may be mistaken for (00l) texture.     

57Fe and57Co Mössbauer studies of highT c Y-Ba-Cu oxides

The⁵⁷Co emission Mössbauer spectra from YBa₂Cu₃O_6.92 (1-2-307) and YBa₂Cu₃O_6.00 (1-2-306) have been measured and compared with the⁵⁷Fe absorption spectra from YBa₂Cu_2.95Fe_0.05O_7?δ in order to clarify decisively the site assignments for the⁵⁷Fe quadrupole-split doublets in these compounds. Mössbauer spectra obtained from both specimens consist of four components whose hyperfine interaction parameters well agree with each other. It is shown that the Co and Fe atoms mainly substitute at Cu1 chain sites in 1-2-307, but in 1-2-306 the Co atoms occupy randomly the Cu2 plane sites and indicate magnetically-split sextet which converts to a paramagnetic doublet of S-state Fe³⁺ in 1-2-307 by a post-annealing in O₂ gas.     

Interface and grain boundary structures in YBa2Cu3O7-x and YBa2Cu4O8 materials

Y₂BaCuO₅YBa₂Cu₃O_7-x (Y211/Y123) interfaces in melt-processed YBa₂Cu₃O_7-x were studied by high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Yttrium enrichment and barium depletion were observed locally at the Y211/Y123 interfaces where Y123 (001) facets were present. This effect may be interpreted as the result of lattice substitution of Ba by Y near these interfaces. Cation nonstoichiometry was found near Y211/Y123 interfaces where liquid phases (Cu-Ba-O) were present. This chemical disorder introduces numerous point defects in the Y123, and these defects may act as additional pinning sites alongwith stacking faults. A comparison of grain boundary (GB) chemical composition in polycrystalline YBa₂Cu₃O_7-x and YBa₂Cu₄O₈(Y124), studied using nanoprobe parallel-detection electron energy-loss spectroscopy (EELS), is presented. The studies of Y124 show that stoichiometric grain boundaries can also form weak links between superconducting grains. It is suggested that weak-link behavior is determined largely by misorientation at grain boundaries.     

Mössbauer studies of magnetic behavior of 3d-doped REBa2Cu3−x Fe x O7+δ (RE=Y,Er, Dy,Gd)

A Mössbauer study has been made on⁵⁷Fe ions substituted into the Cu(1) site of REBa₂Cu_3?x Fe _x O_7+δ (RE=Y, Er, Dy, Gd;x=0.15, 0.30). At low temperature, the iron atoms antiferromagnetically order with a transition temperature which is dependent on the Fe concentration. The temperature dependence of the magnetic subspectra representing Fe ions with various local oxygen environments in YBa₂Cu_3?x Fe _x O_7+δ and ErBa₂Cu_3?x Fe _x O_7+δ fit a 2D-Ising model with a ratio of the anisotropic exchange between the two directions on the order of 0.5–1.0(10^?3) for the Y-compounds and on the order of 1 for the Er-compounds. The magnitude of the local dopant magnetization is related to a short-range chemical order which determines the magnetic chain size and defines the correlation lengths. For the Y-compound, the order is quasi-1D with strong intrachain but very weak interchain coupling. For the Er-compounds, the magnetic coupling is Ising 2D. The strong fluctuation behavior expected in low dimensional systems above and belowT _N is observed via characteristic relaxation in the Mössbauer linewidth nearT _N. For both the Dy- and Gd-compounds, the magnetic order is 3D. The magnitude of the rare-earth magnetic moments appears to affect the character of the magnetic interaction in the Cu(1)-site. However, a Mössbauer effect measurement at¹⁵⁵Gd nuclei in GdBa₂Cu_2.85Fe_0.15O_7+δ (T _N(Fe)～14 K) shows paramagnetic behavior at 4.9 K.     

TixY1-xBa2Cu3O7-δ超导体的红外、远红外光谱

本文报道了Ti_xY_1-xBa₂Cu₃O_7-δ(x=0.2和0.4)在4.2—300K的红外、远红外光谱。在50—360cm^-1波段内发现七个反射峰。它们分别与Ba,Cu,O离子团,Y,O离子团。Ti,O离子团,以及Cu—O键的振动有关,对于x=0.2的样品,存在两个反转结构。对于x=0.4的样品,则仅发现一个反转结构。在红外光谱中观察到六个反对峰和三个吸收峰,它们的强度大多随Ti含量的增加而增强。与Y₁Ba₂Cu₃O_7-δ的光谱结果比校,讨论了声子峰及反转结构的物理起因。 关键词：     

Mössbauer absorption in some high Tc superconductors:170Yb in YbBa2Cu3O7−x,166Er in ErBa2Cu3O7−x,and57Fe in YBa2Cu3O7−x

For the high T_c compound YbBa₂Cu₃O_7−x, Mossbauer absorption measurements show that magnetic ordering occurs within the Yb³⁺ sublattice at 0.35K (μ_sat=1.7μ_B). For ErBa₂Cu₃O_7−x the Er³⁺ sublattice orders at 0.7K (μ_sat=4μ_B). Measurements on⁵⁷Fe diluted into YBa₂Cu₃O_7−x shows that no ordered magnetic moments exist within the Cu sublattices down to at least 4.2K. Instituto de Fisica, Porto-Alegre, Brasil (supported by CNPq)     

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.