Positive muon sites and possibility of anyons in the YBa2Cu3O x system

Possible positive muon sites in YBa₂Cu₃O _x were determined from the observedμ ⁺ hyperfine fields in antiferromagnetically ordered YBa₂Cu₃O _x and GdBa₂Cu₃O₇. After determining theμ ⁺ sites, the possibility of anyons or chiral spin ordering in the superconducting YBa₂Cu₃O₇ is discussed. Positive muon implanted in YBa₂Cu₃O₇ feel static magnetic fields of average 1.4G, which are explicable in terms of nuclear magnetic dipolar fields. Non observation of static local magnetic fields of electronic origin (the upper limit is the order of 0.1 G) means that anyons or chiral spin ordering might not exist in superconducting YBa₂Cu₃O₇.     

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 .     

Spin-lattice relaxation in cuprate superconductors

This presentation gives a personal review of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spin-lattice relaxation studies in cuprate superconductors mainly dealing with the YBa₂Cu₄O₈ compound with many examples from the Zürich laboratory. The studies were performed in both the normal and the superconducting state with various NMR isotopes (e.g.,¹⁷O,^63,65Cu,^135,137Ba). The relatively broad signals were mostly obtained by a phase-alternating add-subtract spin-echo technique. We will discuss the general behavior of spin-lattice relaxation in the normal state and the calculation of the dynamic spin including an approach (on the basis of thet-J model) to calculate the relaxation for plane copper, oxygen, and yttrium. An application of the Luttingerliquid model to the relaxation of chain copper in YBa₂Cu₃O₇ and YBa₂Cu₄O₈ is also given. We then will deal with characteristic features of the YBa₂Cu₄O₈ structure: the spin gap, an electronic crossover in the normal state, the single-spin fluid model, and the d-wave pairing.     

63Cu and17O NMR studies on the magnetic and superconducting properties of YBa2Cu3O7−y

Results of⁶³Cu and¹⁷O NMR experiments in YBa₂Cu₃O₇ and YBa₂Cu₃O_6.63 are reviewed. The normal state data revealed two important aspects of the magnetic properties of these materials, namely, the temperature dependent antiferromagnetic Cu spin correlations and the spin gap behavior, the latter being observed in the reduced oxygen material. These features appear to be the general properties of many high-T _c cuprates. Anomalous temperature dependence of the anisotropy of the Cu relaxation rate was found in the superconducting state of YBa₂Cu₃O₇, which can be explained by a d-wave pairing model.     

Theory of63Cu and17O quadrupole interactions in copper oxide high-T c systems

Electronic wave-functions obtained by the first-principles Unrestricted Hartree-Fock Cluster procedure are utilized to evaluate the electric field-gradient tensors at the⁶³Cu and¹⁷O nuclei in YBa₂Cu₃O₇. Our results provide satisfactory explanations of the available nuclear quadrupole interaction data for this system.     

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.     

Identification ofμ +-sites in the 1-2-3 compoundin the 1-2-3 compound

Possibleμ ⁺ sites have been identified by a comparison of measured spontaneous internal fields in YBa₂Cu₃O_6+δ and REBa₂Cu₃O_7−δ and of measured second moments and width of transverse field powder spectra with the corresponding calculated quantities. In the YBa₂Cu₃O₇ system only one possibleμ ⁺ site emerges at a distance of ∼1.05 A from a chain oxygen O(4) at the position (0.15(1), 0.44(1), 0.071(1)). In the system YBa₂Cu₃O₆ the only possibleμ ⁺ sites are near to an oxygen O(1) with z/c=0.133 and the same distance as above. The analysis leads also to improved valuesμ _Ho = 2.2(1)μ _B andμ _Cu = 0.67(4)μ _B and to a determination of the latters direction:μ _Cu ‖〈110〉.     

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.     

EPR study of the high-TC superconductor YBa2Cu3O7−δ

EPR spectra of superconducting YBa₂Cu₃O_7-δ and insulating Y₂BaCuO₅ have been examined. The intensity of Cu²⁺ ions in YBa₂Cu₃O_7-δ is about 1/3000 of that in Y₂BaCuO₅ at room temperature. Owing to the Meissner effect in the superconducting state, the EPR signal of Cu²⁺ ions in YBa₂Cu₃O_7-δ disappears; moreover the EPR signal of DPPH deposited on the YBa₂Cu₃O_7-δ specimen is shifted. An additional EPR line is found for YBa₂(Cu_2.94Cr_0.06)O_7-δ at temperatures below 113K; this can be related to the phenomenon of superconductivity.     

氧缺位对单相YBa2Cu3O7-x热电势率的影响

研究了氧缺位对单相YBa₂Cu₃O_7-x体系超导电性及热电势率的影响。结果表明,超导转变温度随氧缺位的增加而下降,热电势率随氧缺位的增加而增大。由实验结果推断,单相YBa₂Cu₃O_7-x的多数载流子为空穴。存在较大的声子曳引热电势表明。在YBa₂Cu₃O_7-x超导体中存在较强的电-声子相互作用。 关键词：     

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.     

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.     

Identification of two-electron centers with a negative correlation energy in high-temperature superconductors

On the basis of comparison of experimental and calculated parameters of the electric field gradient tensor, it is shown that holes in the YBa₂Cu₃O₇ and YBa₂Cu₄O₈ lattices are localized on oxygen ions in the CuO₃ chains. These results agree with the model suggesting that the mechanism responsible for the high-temperature superconductivity in the YBa₂Cu₃O₇ and YBa₂Cu₄O₈ compounds is the interaction of electrons with two-atom two-electron copper centers with a negative correlation energy.     

Crystallographic study of tetragonal,superconducting YBa2(Cu0.93Fe0.05)3O7

Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal- and defect-structure of YBa₂(Cu_0.93Fe_0.05)₃O₇. Crystals of this compound are superconducting (T _c 80 K) and appeart to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa₂Cu₃O₇ phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site too, leading to the approximate formula YBa₂(Cu_0.97Fe_0.03)₂(Cu_0.86Fe_0.10.04)O₇. High resolution electron microscopy reveals the existence of orthorhombic micro-domains, 20–30 Å in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.     

On the structure of non-superconducting YBa2Cu3O6+ɛ

The crystal structure of non-superconducting, tetragonal YBa₂Cu₃O₆₊ ( close to 0) (s.g.P4/mmm) was studied at room temperature by means of single crystal X-ray diffraction, electron microscopy and electron diffraction. Crystals may easily be obtained by heating YBa₂Cu₃O₇ samples under vacuum. For 0, the tetragonal single crystals are chemically and crystallographically homogeneous — without any twin domains. The only defects immediately apparent are stacking faults alongc. The structure is very closely related to that of the high-T _c superconductor YBa₂Cu₃O₇. The most important difference is, that the oxygen on theb-axis (O(1)), which together with Cu(1) forms Cu–O–Cu–O-chains alongb in YBa₂Cu₃O₇, is vacant in YBa₂Cu₃O₆. Some crystals show a superstructure with superstructure reflections which are sharp in thea ^*–a ^*-plane but form diffuse streaks alongc ^*. We propose a model which relates these superstructure reflections to a 2-dimensional ordering of residual oxygen atoms on the O(1) site (corresponding to >0 in the chemical formula YBa₂Cu₃O₆₊).     

Y—Ba—Cu—Al—O体系的X射线光电子能谱研究

对单相样品YBa₂Cu_3-xAl_xO_7-y进行了X射线光电子能谱(XPS)、O缺位以及电阻-温度关系的测量。结果发现,Al在1-2-3相中有可能部分地替代了Cu和Y的位置。由于Al³⁺的存在,使得含量增加,0的环境趋于一致,XPS图谱上O₁₅两个状态的区别很不明显。而明显地观祭到了Cu³⁺的存在。 关键词：     

Calculation of EFGs in high Tc superconductors

The electric field gradients (EFGs) at all atomic positions of YBa₂Cu₃O₇ and YBa₂Cu₃O₆ are calculated on the basis of full potential linearized augmented plane wave calculations by an ab initio method. Cood agreement with experimental EFG_s is found for all oxygen positions in YBa₂Cu₃O₇ and the Cu1 position in both compounds, while for the Cu2 position in the superconductor the symmetry of the EFG agrees with experiment but the magnitude of our theoretical EFG is only half the experimental value. A small transfer of 0.07 electrons from d(x²−y²) to d(z²) symmetry would be sufficient to achleve agreement with experiment. Thus the underlying local density approximation is not perfect, but remains a very accurate method for describing ground state properties.     

Stability of muon-oxygen bond sites in RBa2Cu3O7

Relative energies of muon probe sites in the chain region of RBa₂Cu₃O₇ (RBCO) are investigated using a molecular quantum chemistry calculation for (Oμ)⁻ embedded in a cluster of point charges to simulate local charge distributions in RBCO. Partial Cu−O chain covalency results in a O-μ...O muon site between the chain and bridging oxygens. However, Cu-μ “hydride”-like sites are suggested by results for nominally ionic clusters.     

YBa2Cu3O7超导体中空穴状态的X射线光电子能谱研究

对正交单相YBa₂Cu₃O₇样品进行了X射线光电子能谱(XPS)测量。明显观察到Cu的类高价状态。讨论了Cu_2p和O_1s的XPS中各峰对应的电子状态,认为由于过量的O而引入的额外空穴产生在O_2p轨道上。Cu的类高价状态包含Cu3d⁹L和Cu3d¹⁰L²两种状态的组合。 关键词：