POSITRON LIFETIME STUDIES OF Y1-xCaxBa2Cu3O7-δ AND Y1-xCaxBa2Cu3-xFexO7-δ

In this paper it is reported that the measurement of the bulk positron lifetime as a function of substitution content x in the temperature range from 70 to 220K was performed in two high-T_c superconducting systems, Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ and Y_1-xCa_xBa₂Cu₃O_7-δ. It was found that τ_B of both systems decreases significantly with x and the temperature dependence of τ_B is very weak in normal state. In lower temperature region (Tc), a dramatic x-dependent temperature variation of τ_B was observed in the Ca-substituted system: from a decrease of τ_B with decreasing T to an increase of τ_B. With increasing x, the temperature dependence of τ_B remains weak in the Ca- and Fe- substituted systems. Compared with the experimental data of positron lifetime in other substituted systems and the calculation of the positron density distribution, the authors suggest that positron bulk lifetime spectra behaviour can be interpreted by the physical model based on the transfer of electron density between the CuO₂ planes and Cu-O chains. Therefore, the study of positron lifetime spectra provides a useful means to detect the local charge density and to study the correlation between the electronic structure and the high-T_c superconductivity.     

Ca掺杂氧化物超导体正常态电阻反常及其磁输运行为

本文研究了对Y1-xCaxBa2Cu3O7-δ进行Ca掺杂后样品的输运特性.实验中发现:随Ca含量增加,样品正常态电阻随之下降.出现反常电阻现象原因可能是Ca替代Y位,导致体系载流子浓度发生变化,从而使得晶界附近的电子能带结构发生改变.另外;低温下的样品伏安特性在外磁场下出现反常,动态电阻dV/dI随磁场增加而下降.有关该现象的物理机制有待于进一步研究.     

Magnetoresistance of lightly doped TmBa2Cu3Ox crystals. Reorientation of the antiferromagnetic structure in a magnetic field

The magnetoresistance of lightly doped TmBa₂Cu₃O_x single crystals is investigated in the temperature range 4.2–300 K for magnetic fields up to 12 T. For the antiferromagnetic sample (x=6.3), when the current and field lie in the ab plane, the magnetoresistance is the sum of an anisotropic and a background component. The existence of the anisotropic component is attributed to the restructuring of the antiferromagnetic domain structure in a magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 5, 350–355 (10 September 1999)     

Possible antiferromagnetic ordering in Y2Cu2O5. Paramagnetic behaviour of BaCuO2

The susceptibility anomaly observed at 13 K for Y₂Cu₂O₅ strongly suggests the occurrence of antiferromagnetic Cu spin coupling in this compound. Only paramagnetic behaviour was found for BaCuO₂.     

Fe-site assignments in Y1Ba2Cu3O7 revisited

A new symmetric Fe³⁺ site (labelled E) (δ=0.28(2) mm/s and Δ=0.15 mm/s) is observed in the R₁Ba₂Cu₃O₇ family of superconductors for R=Y and Eu. The decay and growth of site E are established in two seemingly unrelated experiments, one as a function of room temperature aging for R=Y, and the other as a function of oxygen loading at elevated temperatures and pressures for R=Eu. In each experiment, the results show that site E is genetically related to the well documented chainsites A(Δ=1.9 mm/s), C(Δ=1.1 mm/s) and D(Δ=1.6 mm/s). We propose that site E represents a quasi-octahedral chain-site having pairs of O(1), O(4) and O(5) nearestneighbor oxygen sites. The two remaining doublets, B(Δ≈0.4 mm/s) and B′(Δ=0.78 mm/s) represent Fe³⁺ dopant sites present in the all-important CuO₂ planes, possessing coordination numbers of 6 and 5, respectively.     

The nature of anomalies in the magnetoresistance of antiferromagnetic YBa2Cu3O6+x

The characteristic d-type angular dependence of the in-plane magnetoresistance on the orientation of the external magnetic field in the (ab) plane for the antiferromagnetic tetragonal crystal YBa₂Cu₃O_6+x (x ～ 0.3) is considered theoretically. This dependence is interpreted in terms of the efficient hole transfer through the low-lying purely oxygen O 2pe _u doublet, which is not hybridized with the $b_{1g} (d_{{}_x2_ - {}_y2} )$ ground state. The external magnetic field determines the orientation of the strong exchange field acting on the b _1g e _u : ³ E _u triplet state of the hole-type CuO₄ center. The spin-orbit interaction results in orbital polarization of the E _u doublet, which is responsible for the d-type spatial anisotropy of the hole transport. The available experimental data make it possible to evaluate the parameter of the effective spin Hamiltonian. The influence of spin-vibronic effects on the hole transfer is analyzed.     

Study of the magnetic phase diagram of Y1-x Cax Ba2 Cu3 O6

We have studied the magnetic phase diagram for the bilayer compound Y_1-x Ca_x Ba₂ Cu₃ O₆ in the regime of low doping (hole concentration within a CuO₂ plane, p_sh < 0.1). For p_sh < 0.03, the data demonstrate the freezing of the spin degrees of freedom associated with the doped holes into a spin‐glass‐like state which is superimposed on the preexisting long range order of the Cu²⁺ spins. Only a single spin‐glass like transition at a temperature T_g is observed for samples in the hole concentration range 0.03 < p_sh < 0.1. While the threshold for the occurrence of superconductivity is almost the same (p_sh > 0.06) for the Y_1-x Ca_x Ba₂ Cu₃ O₆ and the La_2-x Sr_x CuO₄ systems, the magnetic correlations that coexist with superconductivity are considerably stronger in the bilayer Y_1-x Ca_x Ba₂ Cu₃ O₆ system as indicated by significantly increased transition temperatures T_g. This revised version was published online in August 2006 with corrections to the Cover Date.