Effect of iron doping and oxygen stoichiometry on infrared absorption in Y1Ba2Cu3O7−δ

We report infrared absorption of Y₁Ba₂Cu₃O_7−δ as a function of oxygen stoichiometry (0<δ<1) and copper substitution by iron in the spectral range of 450–700 cm⁻¹. The strong bands associated with Cu-O vibrations undergo significant changes in their frequencies and intensities asδ is varied across the orthorhombic to tetragonal phase. These changes coupled with those arising as a result of doping with iron has helped in identifying the nature of the vibrational modes.     

Kinetics of oxygen desorption in Y Ba2Cu3O7-δ

In this paper the kinetics of oxygen desorption from Y Ba₂Cu₃O_7-δ has been investigated and measurements carried out in the temperature range 350–750 C. It is shown that the oxygen desorption is diffusion-controlled (and not surface-controlled), and characteristic of diffusion from small grains in a loosely coupled porous matrix. At every temperature a number of time constants with well-defined ratios appear and have been identified and measured. These time constants can be related to the average bulk diffusion constant which shows an Arrhenius behaviour with deviations around 750°C which are ascribed to the well-known orthorhombic to tetragonal transition. In this paper we establish the basic underlying physical mechanism of oxygen desorption in Y Ba₂Cu₃O_7-δ.     

退火氧压对YBa2 Cu3O7-x薄膜中的激光感生热电电压效应的影响

测量了在不同氧压下退火生长的YBa2 Cu3O7-x(YBCO)薄膜中的激光感生热电电压(LITV)信号,发现随退火氧压的增大可使LITV信号的峰值有2-4倍的增强,并且变化趋势与薄膜热电势的各向异性随氧含量的变化规律相同.波长在473-808 nm范围内的连续激光辐照,在5000 Pa的氧压下退火生长的YBCO薄膜中探测到的LITV信号最大;而紫外脉冲激光辐照时,IJTV信号的最大值出现在退火氧压为105 Pa的薄膜中.LITV信号的光响应时间随退火氧压的增大而减小.理论分析表明,这一结果是由于退火氧压的提高导致薄膜材料的热扩散系数增大所致.在退火氧压为5000 Pa的YBCO薄膜中探测到响应最快的LITV信号,对紫外脉冲激光的响应时间为78 ns,并且信号的上升时间只有29 ns,与激光的脉冲宽度(约为28 ns)相当.     

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.     

Electron tunneling into thin films of Y1Ba2Cu3O7

We have fabricated tunnel junctions on thin films of Y₁Ba₂Cu₃O₇ which were epitaxially grown by magnetron sputtering on (100) and on (110) oriented SrTiO₃ substrates. These junctions of the type Y₁Ba₂Cu₃O₇/barrier/Pb or In showed with high reproducibility in the conductance a gap-like structure with the maxima near±16mV. We supplied experimental arguments that this structure reflects properties of the quasi particle excitation spectrum of Y₁Ba₂Cu₃O₇. The gap-like structure was found to disappear atT _c mainly by weakening and not by a shift of the peaks to lower voltages.     

Metallic normal state of Y1Ba2Cu3O(7-delta)

Flux flow was studied over an entire temperature range down to T approximately 2% of T(c) by using intense pulsed current densities to overcome flux-vortex pinning. The resistivity at high vortex velocities is proportional to B and roughly follows rho approximately rho(n)B/H(c2), with a prefactor of order unity. Contrary to some speculation, rho(n) saturates to a finite residual value as T-->0, indicating a metallic (rho-->finite) rather than insulating (rho-->infinity) normal state, and the vortex dissipation continues to be conventional as T-->0.     

Photoemission study of the Ba core levels in YBa2Cu3O7-x

Angle dependent X-ray photoemission spectra of epitaxial thin YBa₂Cu₃O_7-x films show characteristic differences after various heat and oxygen pretreatments. The component with the lowest binding energy in Ba 3d _5/2 and 4d _5/2 peaks results from bulk Ba in YBa₂Cu₃O_7-x . A component with 2 eV higher binding energy results from Ba in the surface unit cell with different bonding if compared with the bulk superconductor. This surface structure is observed for all YBa₂Cu₃O_7-x surfaces which have been exposed to oxygen. It is not observed for UHV prepared surfaces after mechanical scraping or fracturing.     

Effect of pressure on the electrical resistivity and thermoelectric power of the compounds Ba(Pb1-xBi x )O3 and K y Ba1-yPb0.5Cu0.5O3-x

High pressure studies on the electrical resistivity and thermoelectric power of BaPb_1-xBi _x O₃ and K _y Ba_1-yPb_0.5Cu_0.5O_3-x show that many compounds in these systems exhibit qualitatively the same type of behaviour as observed in the superconducting oxides (La, Sr)CuO₄ and YBa₂Cu₃O_7-x. The compounds K _y Ba_1-yPb_0.5Cu_0.5O_3-x show temperature-independent resistivity over a wide range of temperatures, but are not found to be superconducting down to 77 K.     

High temperature crossover in paraconductivity of granular Y1Ba2Cu3O7-y

We have examined the deviation from linearity of the temperature dependence of the electrical resistivity of a YBaCuO ceramics. We have observed at high temprature a crossover behavior toward a logarithmic temperature dependence at lower temperature. It seems relevant to attribute the origin of such a term to a pair breaking mechanism contribution, thus to give a lower bound to the existence of superconductivity fluctuations onset temperature. Our findings also implies that carriers are bosons aboveT _c .     

Dynamics of fluxons in Y1−Ba2−Cu3−O7 single crystals

Summary Evidence is given that in Y₁−Ba₂−Cu₃−O₇ single crystals a critical state is developed on increasing (or decreasing) the applied fieldH ₀, only whenH ₀ is parallel to the crystalc-axis. An ?irreversibility? line in the (H, T)-plane can be drawn where the irreversibility of the second-harmonic response sets in. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Positron annihilation measurements across the superconducting transition in Y1Ba2Cu3O7−x

Positron lifetime and Doppler broadened annihilation radiation lineshape measurements have been carried out in Y₁Ba₂Cu₃O_7−x as a function of temperature in the range of 300 K to 58 K. The positron lifetime and the peak parameter of the annihilation radiation lineshape are observed to decrease on lowering the temperature without showing any discontinuous change across the superconducting transition temperature of 90 K as determined by susceptibility measurements. The variation of positron annihilation parameters with temperature in the superconducting state is significantly larger than that in the normal state. This is qualitatively explained in terms of the dimerization of oxygen ions in the superconducting state of Y₁Ba₂Cu₃O_7−x .     

Study of highT c superconductivity in Y1Ba2Cu3O7−x by PAC

HighT _c superconductivity in the YBaCuO superconductor has been investigated through the quadrupole interaction of the probe nuclei⁹⁹Tc. The quadrupole interactions were measured by the TDPAC method from 77 to 296 K. The probe nuclei⁹⁹Tc were introduced into the YBaCuO superconducting specimen by diffusion. The derived electric quadrupole interaction parameters show that the probe nuclei are subject to a unique EFG interaction and occupy a substitutional lattice site in the YBaCuO superconductor. A strong EFG of 10¹⁹ V/cm² was observed. The temperature dependence of the EFG exhibits a linear decrease with temperature increase. Anomalies of both EFG and ν were found in the superconducting transition temperature region. The role of the oxygen vacancies in the Cu−O chains is discussed.     

A universal trend in the structural correlation with oxygen content in RBa2Cu3O x systems

It is shown that the lattice parametersa, b andc in the superconducting perovskites RBa₂Cu₃O _x are inter-related and well correlated to the total oxygen content. The correlation shows a universal trend.