Xiang Jiong

Ultrasonic investigation of lattice instability and superconductivity in high-Tc systems

Ultrasonic measurements have indicated that there are possible structural changes near 250 K, 160 K and just above Tc in the high-Tc Y-Ba-Cu-O and Y-Ba-Cu-Nb-O systems, which were confirmed to be first-order-like phase transitions by thermal analysis (DSC) results. It is suggested that the heavily suppressed pre-superconducting transition, which provides the lattice instability and the soft phonon modes, gives rise to the high Tc.

Lattice readjustment of YBa2Cu3O7-y between 150 and 300 K: a transmission-microscope study

The lattice instability of the high-Tc superconductor YBa2Cu3O7-y has been studied at temperatures between 150 and 300 K by transmission electron microscopy. Anomalous changes in the lattice constant c were observed near 170 and 250 K. The axial ratio b/a was found to vary in a range from 1.014 to 1.018 (error: +or-0.1%), and a tendency towards a reversible orthorhombic-tetragonal phase transition was observed between 170 and 200 K. Possible explanations are proposed.

Ultrasound anomalies in single phase (Bi1-xPbx)2Sr2CanCun+1O2n+6+y superconducting ceramics

Ultrasonic anomalies near 200 K in single-phase (Bi1-xPbx)2Sr2CanCun+1O2n+6+y (x=0-0.2, n=1, 2) superconducting samples were observed by velocity and attenuation measurements of both longitudinal and transverse sound waves. The remarkable isothermal-like anomalous changes near 200 K indicate that there are possible structural changes or phase transitions in the samples. On comparison with YBa2Cu3O7-y, the different behaviours in ultrasonic anomalies are attributed to the differences in structure. It is suggested, however, that the lattice instability appearing near T approximately=2Tc may be a common feature of the existing superconducting perovskites and may have some relation to their high Tc.

Ultrasonic investigations of the layered perovskite ceramic superconducting systems

Ultrasonic measurements have been carried out on three type high T c perovskite ceramic superconductors La-Sr-Cu-O, Y-Ba-Cu-O and Bi(Pb)-Sr-Ca-Cu-O. Anomalies in both sound velocity and attenuation of the above three systems were observed. The elastic constants and the Debye temperatures were also obtained and discussed.

NEUTRON DIFFRACTION STUDY ON (Bi−Pb)2Sr2Ca2Cu3O10

The polycrystalline sample of (Bi–Pb)2Sr2Ca2Cu3O10 with Tc=107 K was prepared. The X-ray diffraction proved that the sample is single phase. The crystal structure study on the sample has been carried out by neutron diffraction. The result shows that its structure is tetragonal body-centre structure with space group of I4/mmm, containing a few oxygen atoms at 4e site in Bi–Pb layers.

An ultrasonic study of energy-gap and granular characteristics of Y1Ba2Cu3O7-y superconducting ceramic

Ultrasonic attenuation measurements of Y1Ba2Cu3O7-y have been carried out between 100 and 10 K by using 10-25 MHz longitudinal acoustic waves. It was shown from the experimental results that when T<Tc the attenuation was apparently decreased and the granular superconducting characteristics of the samples were remarkable. It was indicated from the attenuation curves that the superconductivity of Y1Ba2Cu3O7-y is related to the electron pairing states. By means of introducing a transition volume distribution function fT, the calculated attenuation curves fitted the experimental ones very well, leading to a result of 2 Delta (0)=3.5kBTc. This suggests that the existence of a superconducting energy gap and its temperature dependence should be still the same with the BCS theory, although the mechanism of the electron coupling may be quite different.

Experimental evidence for the lattice instability of Bi-based superconducting systems

Abstract Ultrasonic measurements, specific heat and thermal analysis experiments, X-ray diffraction study and infrared investigation revealed that there are anomalous structural changes or lattice instabilities near 200 K in single 2212 or 2223 phase samples of Bi(Pb)-Sr-Ca-Cu-O system. Detailed study showed that anomalous changes or lattice instabilities are isothermal-like processes and have the characteristics of a structural phase transition, accompanying with increases in lattice constants. Possible mechanism for this lattice instability is discussed.

ULTRASONIC INVESTIGATIONS ON THE LATTICE INSTABILITY OF La 2-x Sr x CuO 4-y AND Nd 2-x Ce x CuO 4-y SUPERCONDUCTORS

Ultrasonic measurements were carried out on a series samples of La 2-x Sr x CuO 4-y and Nd 2-x Ce x CuO 4-y by 10 MHz longitudinal and transverse waves in the temperature range between 10 K to 300 K. A striking velocity softening was observed and this softening temperature T 0 decreases almost linearly with the increasing of x. A restiffening of velocity takes place at even lower temperature, implying another possible structural phase transition occurred. The anomalous changes in both attenuation and velocity around 170 and 200 K were found for the superconducting samples of Nd 2-x Ce x CuO 4-y (x=0.15) during cooling, whereas no anomalies can be found in this region for non-superconducting samples. The implication of these phenomenon is discussed.

Structure Refinement of Bi2Sr1.855Ca1.145Cu2O8 from Neutron Data

Neutron diffraction study on the structure of Bi2Sr1.855Ca1.145Cu2O8 was carried out. Data refinements show that the average subcell structure for Bi2Sr1.855Ca1.145Cu2O8 is in the best fit of the space group Fmmm and the structure is described by alternate stacking of SrO-CuO2-Ca-CuO2-SrO slab and a double BiO layers. An oxygen layer sandwiched by the bismuth layers was confirmed.

POSSIBLE STRUCTURAL PHASE TRANSITION OF SINGLE PHASE (Bi, Pb)2Sr2Ca2Cu3O10+y SUPERCONDUCTING CERAMIC: AN INFRARED STUDY

The infrared spectra of single phase (Bi, Pb)2Sr2Ca2Cu3O10+y polycrystalline samp were studied over a temperature range of 300–120 K. Three vibrational modes were found at ambient temperature. A new band appears at 900 cm−1 near 200 K and experi evidence proved that the appearance of the new band is actually through an isothermal-like process, indicating a structural phase transition taking place. Unstable fluctuations were also observed between 200 and 140 K. This phenomenon is consistent with our previously reported ultrasonic, specific heat and lattice constants anomalies of the same sample. A possible mechanism of this phase transition is also discussed.