Yu-Fen Zhang

Superconductivity and antiferromagnetic ordering in (Gd,Nd,Ce)2 CuO4

Abstract Magnetic and superconducting properties of Gd 1.85− x Nd x Ce 0.15 CuO 4 and Gd 2− x Ce x CuO 4 have been studied. The antiferromagnetic ordering temperature of Cu decreases with increasing Ce content in Gd 2− x Ce x CuO 4 and further decreases when Nd is substituted for Gd. In Gd 1.85− x Nd x Ce 0.15 CuO 4 , the antiferromagnetic ordering of Cu disappears at about x = 5 and superconductivity appears at about x = 0.8.The temperature - Nd concentration phase diagram for Gd 1.85− x Nd x Ce 0.15 CuO 4 is reported. Resistivity measurements reveal that the substitution of Nd for Gd results in an increase of charge carrier number. The antiferromagnetic ordering of Gd coexists with superconductivity at low temperatures. A possible explanation for the effect of Nd substitution is discussed.

Josephson effects observed in constricted YBaCuO films at liquid nitrogen temperature

Abstract The DC and AC Josephson effects have been observed in constricted superconducting Y-Ba-Cu-O films at liquid nitrogen temperature. The temperature dependence of the critical current was studied. It was found that I c ∝ (T c -T) 3 2 . The induced steps were seen in the I-V characteristics when the sample was irradiated by a microwave field. Separations between steps agrees with the Josephson relation.

Properties of multilayer samples with interface of superconducting YBa2Cu3O7 and ferromagnetic Pr0.7Sr0.3MnO3

Abstract YBa 2 Cu 3 O 7 /Pr 0.7 Sr 0.3 MnO 3 /Ag sandwiches and YBa 2 Cu 3 O 7 /Pr 0.7 Sr 0.3 MnO 3 /YBa 2 Cu 3 O 7 step-edge-junction structure samples with interfaces of superconducting YBa 2 Cu 3 O 7 (YBCO) and ferromagnetic Pr 0.7 Sr 0.3 MnO 3 (PMSO) layers have been prepared by pulsed laser deposition. The curves of resistivity versus temperature are used to discuss the quality and the interaction of the interfaces between the ferromagnetic PSMO and superconducting YBCO.

Observations of the ac Josephson effect in Y-Ba-Cu-O superconductors at 77 K

Abstract The observations of the ac Josephson effect in Y-Ba-Cu-O superconducting weak link were reported. The Y-Ba-Cu-O sample had a Tc =90 K . The I–V characteristics of the weak link junction were mearsured at 77 K. The junction had a Ic =0.43 mA and the normal resistance Rn =0.38 ohm. 10.15 GHz microwave was applied to the junction and the rf-induced steps up to n =10 were observed. The dependence of the step height on the rf level was in agreement with ordinary Josephson tunneling behavior.

Superconductivity and antiferromagnetic ordering in (Gd,R)1.85Ce0.15CuO4 with R = Pr and Sm

Abstract Magnetic and superconducting properties of (Gd,R) 1.85 Ce 0.15 CuO 4 with R = Pr and Sm have been studied. Antiferromagnetic ordering of Cu is observed in Gd-rich samples, and superconductivity appears in R-rich samples. With increasing R content, antiferromagnetism - superconductivity transition occurs. The temperature - R concentration phase diagram for (Gd,R) 1.85 Ce 0.15 CuO 4 is reported. Resistivity measurements reveal that the substitution of R for Gd results in an increase of effective charge carrier number. The antiferromagnetic ordering of Gd and Sm coexist with superconductivity at low temperature. The magnetic behavior of Gd and R ions varies due to the interaction between Gd and R ions. A possible explanation for the effect of R substitution is discussed.

Structural and magnetic properties of (Gd, La)1.85Ce0.15CuO4

Abstract Structural and magnetic properties of Gd 1.85− x La x Ce 0.15 CuO 4 are studied. The samples re single phase with the T′ structure when x ≤1.2. Lattice parameters increase linearly with increasing La content. Weak ferromagnetism associated with the canting of the copper moments is observed in the compounds with x ≤0.2. The weak ferromagnetism disappears when the lattice parameter is a≥3 A . The structure of Gd 1.85− x La x Ce 0.15 CuO 4 is unstable at high temperature. No superconductivity can be induced down to 1.5 K by reduction annealing in N 2 .

Weak ferromagnetism and superconductivity in (Gd,R)1.85Ce0.15CuO4 with R = Pr, Nd, and Sm

Abstract Gd 1.85− x R x Ce 0.15 CuO 4 compounds with R = Pr, Nd, and Sm are prepared by regular solid state reaction. Structure deformation, weak ferromagnetism, and superconductivity are studied. Experimental results showed that weak ferromagnetism appears in the deformed compounds. Superconductivity appears in the composition where weak ferromagnetism disappears, but there is no direct correlation between structure deformation and superconductivity. Structure instability prevents a proper reduction to introduce superconductivity.

Raman spectra of superconducting and non-superconducting phase of Nd2−xCexCuO4

Abstract The phonon Raman spectra of Nd 2- x Ce x CuO 4 polycrystals before and after reduction were investigated. After the reduction, the frequencies for phonon Raman peaks at 125 cm -1 and 230 cm -1 associated with the vibration of Nd(Ce) ions changed, and the peak at 582 cm -1 possibly associated with the vibration of O(1,2) and Cu in the CuO 2 -plane sharpened. Some concise discussions about the results are given.

Effect of Ni and Zn substitution on magnetic ordering in Gd2CuO4

Abstract Ceramic samples of Gd 2 Cu 1−x Ni x O 4 with x =0, 0.03, 0.05, 0.07, 0.09 and 0.11, and Gd 2 Cu 1−x Zn x O 4 with x = 0.01, 0.03 and 0.05, were prepared. Magnetic measurements showed that the antiferromagnetic ordering temperature of Cu decreased due to the substitution of Cu by Ni or Zn. The effect of Ni on suppressing antiferromagnetic ordering is larger than that of Zn. The results are compatible with the substitution effect on superconductivity in Nd 1.85 Ce 0.15 CuO 4 .

Effects of isochronal thermal annealing and hydrogenation on photoluminescence from n-type GaN films grown by metalorganic vapor phase epitaxy

Abstract We have investigated the behavior of three emission bands located at around 375, 580 and 750 nm in photoluminescence (PL) spectra from n -type GaN films grown by the metalorganic vapor phase epitaxy method in the isochronal annealing process and in a similar process after the films have been hydrogenated using a hydrogen plasma. The PL intensities of the two bands located at around 375 nm and 750 nm vs isochronal annealing temperature show similar rules, but the isochronal annealing behavior of PL intensity for the band located at around 580 nm differs obviously from that of the two bands mentioned above. Hydrogenation has an obvious effect on the isochronal annealing behavior of PL intensity, e.g. decreases the number of peaks in the curves of PL intensity vs isochronal annealing temperature.