Teen-Hang Meen

Suppression of Superconductivity in Y1-xPrxBa2Cu4O8 Prepared by Nitrite Pyrolysis Method

X-ray diffraction patterns show that samples contain the nearly single 124 phase for x<0.7. Lattice parameters a and b increase with Pr concentration. Thermal stability is reduced with increasing Pr concentration. Zero resistance temperature Tco decreases monotonically fiom 80 K at x=0 to 12 K at x=0.65. Room-temperature resistivity changes linearly to x=0.7 and increases abruptly at x=0.75. Thus the critical concentration xcr is estimated to be 0.7. Effective magnetic moments of Y1-xPrxBa2Cu4O8 are 3.63, 3.35 and 3.23µB for x=0.2, 0.4 and 0.6, respectively. In addition, the similarities and differences between Y1-xPrxBa2Cu4O8 and Y1-xPrxBa2Cu3O7-y are discussed.

Structure, superconductivity and magnetism of YBa2(Cu1−xMx)4O8 (MFe, Co, Ni, Zn and Ga)

Abstract YBa 2 (Cu 1− x M x ) 4 O 8 (MFe, Co, Ni, Zn and Ga; 0 ⩽ x ⩽ 0.05) have been investigated by means of X-ray diffraction, electrical-resistivity and magnetic-susceptibility measurements. X-ray-diffraction patterns show that all samples studied contain the nearly single 124 phase. They exhibit an orthorhombic-to-tetragonal structural transition at about x = 0.03 for MFe and Co; and at x = 0.04 for MGa; while the structure remains orthorhombic at x = 0.05 for MNi and Zn. These results suggest that Fe, Co and Ga substitute preferentially for [Cu(1)] sites, and the Ni and Zn prefer to occupy the [Cu(2)] sites. The initial depression rates of T c0 (temperature of zero resistance) with increasing doping concentration for various dopants are 16.85, 8.4, 23.75, 12 and 3.9 K/at.% for MFe, Co, Ni, Zn and Ga, respectively. Effective magnetic moments of YBa 2 (Cu 0.97 M 0.03 ) 4 O 8 are 5.63, 3.91 and 3.37 μ B for MFe, Co and Ni, respectively. In addition, the similarities and differences between YBa 2 (Cu 1− x M x ) 4 O 8 and YBa 2 (Cu 1− x M x ) 3 O 7− y are discussed.

Heat Treatment Effect on the Metal Contact of High-Tc (Pb, Bi)SrCaCuO Superconductor

Better metal electrode can be obtained by heat treatment which causes deeper diffusion of metal particles into the (Pb, Bi)SrCaCuO superconductor to improve contacts between metal electrodes and the sample. However, high-Tc phase of the specimen was destroyed at 700°C and the sample showed semiconducting behaviour. Low room temperature contact resistivity was obtained about 3.21×10-4 (Ω-cm2) and 2.34×10-6 (Ω-cm2) at 77 K for a sample with evaporating silver electrodes and annealed at 600°C for 2 hours. Non-ohmic contacts are observed for the indium and aluminum terminals.

Ionic-size effect on the structure and Tc of T′-(R1−xR′x)1.85Ce0.15CuO4 (R Pr, Nd, Sm and Eu; R′ Gd and Y)

Abstract (R 1− x R′ x ) 1.85 Ce 0.15 CuO 4 (R  Pr, Nd, Sm and Eu; R′  Gd and Y) have been investigated by means of X-ray diffraction, electrical-resistivity, and magnetic-susceptibility measurements. Lattice parameters a , c , and the unit-cell volume V decrease with increasing Gd or Y concentration x , and the decreasing rates with Y are larger than those with Gd. It is found that the critical concentration x c , at which the superconductivity disappears, depends on the ionic size of R, and the T c suppression rate for each system with Y doping is larger than that with Gd doping. Thus the effect of a small ionic radius for Y plays a much more important role than the large magnetic moment for Gd on the suppression of superconductivity in (R 1− x R′ x ) 1.85 Ce 0.15 CuO 4 . From the observations of the deviation of the lattice parameters in R 1.85 Ce 0.15 CuO 4 and the rapid increase of | dln T c /d x | in (Eu 1− x Y x ) 1.85 Ce 0.15 CuO 4 , the structural distortion boundar estimated to lie between Eu 1.85 Ce 0.15 CuO 4 and Gd 1.85 Ce 0.15 CuO 4 . This provides clear evidence that the absence of superconductivity for Gd 1.85 Ce 0.15 CuO 4 is due to weak ferromagnetism in the CuO planes resulting from the too small ionic radius of Gd, which induces a lattice distortion in the T′ structure.

Superconductivity and magnetic order in RSr2Cu2.7Mo0.3O7−δ (R = Y, Pr, Gd, and Tb)

Abstract Polycrystalline samples RSr 2 Cu 2.7 Mo 0.3 O 7−δ with R = Y, Pr, Gd, and Tb were synthesized and studied by means of powder X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific-heat measurements. All four samples form a perovskite-layer structure with a tetragonal symmetry. The calculated lattice constants using space group P4/mmm indicate that these rare-earth ions are basically in the 3+ valence state. Samples R = Y, Gd, and Tb are superconducting with transitions at around 30 K, while PrSr 2 Cu 2,7 Mo 0.3 O 7−δ remains semiconducting down to 2 K. Effective magnetic moments μ eff derived from a Curie-Weiss-behavior susceptibility for R = Gd and Tb are 7.92 μ B and 9.75 μ B , respectively, which are close to those of trivalent free ions. While the μ eff = 3.05 μ B for R = Pr is smaller than that expected for a Pr 3+ free ion. A magnetic ordering transition is observed by a λ-type anomaly in specific-heat measurements at 2.27 K and 5.43 K for R = Gd and Tb, respectively. In contrast, no significant feature was observed in specific heat for PrSr 2 Cu 2.7 Mo 0.3 O 7−δ in the temperature range 0.6–40 K. The similarities and differences in superconducting and magnetic properties between RSr 2 Cu 2.7 Mo 0.3 O 7−δ and RBa 2 Cu 3 O 7−δ are discussed.

Substitution Effects of Fe and Zn for Cu in YBa2(Cu1-xMx)4O8

YBa2(Cu1-xMx)4O8 (M=Fe, Zn; x=0 to 0.05) has been investigated by means of X-ray diffraction, thermogravimetric analysis, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all samples contain the nearly single YBa2Cu4O8 phase. The results of structural analysis indicate that the orthorhombic-to-tetragonal transition occurs at about x=0.03 for M=Fe, whereas the structure remains orthorhombic until x=0.05 for M=Zn. Thermal stability is reduced with increasing Fe concentration, but is nearly unchanged in the case of Zn substitution. The suppressed rates of Tco with increasing dopant content are 17.81 K/at.% and 15.69 K/at.% for Fe and Zn. Effective magnetic moments of YBa2(Cu1-xFex)4O8 are 5.99, 5.63 and 5.37 µB for x=0.01, 0.03 and 0.05, respectively. In addition, the similarities and differences between YBa2(Cu1-xMx)4O8 and YBa2(Cu1-xMx)3O7-y are discussed.

Synthesis of PrBa2Cu4O8 at ambient oxygen pressure

Abstract We report for the first time the successful synthesis of polycrystalline PrBa 2 Cu 4 O 8 (Pr124) at ambient oxygen pressure. Pr124 has been prepared by nitric pyrolysis and oxalate coprecipitation methods, and both methods yield qualitatively the same results. Powder X-ray-diffraction patterns of samples prepared by both methods show a nearly single “124” phase. Thermogravimetric analysis indicates no oxygen loss until the temperature is higher than 800°C, revealing that its thermal stability is distinct from that of PrBa 2 Cu 3 O 7 . Electrical resistivity ϱ ( T ) shows metallic behavior at temperatures. These results show that the quality of the samples prepared at ambient oxygen pressure is comparable to that by hot isostatic pressing technique.

Thin Copper Seed Layers in Interconnect Metallization Using the Electroless Plating Process

In this paper, we present a process for growing a Cu seed layer on a Ta/SiO2/Si substrate using an electroless plating (ELP) process at an extremely low temperature (~30°C). In this process, the activation treatment of the Ta/SiO2/Si substrate was carried out by immersion in a PdCl2/HCl solution prior to electroless Cu deposition. The optimum activation time for the substrate was clearly observed to be 7 min. The Cu seed layer was uniformly and smoothly deposited using a CuSO4 concentration of 30 mM for 80 s with an average roughness of 14 nm under a thin film of 50 nm thickness. The grain size of the Cu seed layer was 34 nm. After annealing in hydrogen ambience at 250–350°C, the average roughness of the Cu seed layer was reduced to 4 nm. A proposed mechanism for the ELP of Cu seed layers on Ta/SiO2/Si substrates is also presented.

Effect of Ag2O Addition in Y0.9Ca0.1Ba2Cu4O8 on the Structure and Contact Resistivity of Y0.9Ca0.1Ba2Cu4O8/Ag

Effects of Ag2O addition in Y0.9Ca0.1Ba2Cu4O8 on the structure and the contact resistivity of Y0.9Ca0.1Ba2Cu4O8/Ag interfaces have been investigated by the measurements of X-ray diffraction and contact resistivity. For Ag2O-free specimens, they must be sintered three times at 800° C for 24 h with intermediate grinding to obtain the pure 124 phase. However, with addition of 25 wt% Ag2O in Y0.9Ca0.1Ba2Cu4O8, the single 1–2–4 phase can be formed with sinted only one time at 800° C for 24 h. The lowest ohmic-contact resistivity about 4.55×10-5 Ω cm2 at 300 K and less than 10-8 Ω cm2 below T c for Ag2O-free system are observed by annealing at 600° C for 1 h under one oxygen atmosphere. With addition of 25 wt% Ag2O in Y0.9Ca0.1Ba2Cu4O8, the optimal annealing temperature to obtain the lowest ohmic-contact resistivity can be improved down to 400° C. In fact, a low-resistivity ohmic contact for the Y0.9Ca0.1Ba2Cu4O8/Ag interface can be achieved without any thermal annealing just by the addition of 25 wt% Ag2O in Y0.9Ca0.1Ba2Cu4O8. These results indicate that the addition of Ag2O can aid the formation of the 1–2–4 phase and upgrade effectively the characteristics of Y0.9Ca0.1Ba2Cu4O8/Ag interfaces to get better ohmic contact even without annealing, resulting in a lower annealing temperature and contact resistivity.

Magnetic Properties of Nonsuperconducting Pb2Sr2RCu3O8+y (R=Y, Pr, Gd and Tb)

X-ray diffraction patterns reveal that all samples contain the single Pb2Sr2YCu3O8+y phase. The results of electrical-resistivity measurements show that Pb2Sr2RCu3O8+y (R=Y, Pr, Gd and Tb) are not superconducting at temperatures above 10 K. The paramagnetic effective moments µeff and the magnetic transition temperatures TN derived from the magnetic susceptibility data are 3.11µB, 7.94µB and 10.04µB, and 8.5 K, 3 K and 5.5 K for R=Pr-, Gd- and Tb-containing compounds, respectively. The similarities and differences in terms of electrical and magnetic properties of these compounds compared with those of the corresponding RBa2Cu3O7-δ are briefly discussed.