摘要:

The subambient heat capacities of seven La‐Sr‐Cu‐O samples were measured by adiabatic calorimetry over the temperature range 5 to 350 K. The heat capacity of Sr14Cu24O41was derived from that of the mixed phase of nominal composition SrO:CuO=1:2. No anomalous contribution was identified in the heat capacities of SrCuO2, Sr2CuO3, Sr14Cu24O41, LaSrCuO3.5and La2CuO3. A small anomaly associated with the orthorhombic to tetragonal transformation was identified at the vicinity of 300 K in the heat capacity of La1.90Sr0.10CuO3.90. Onset of Meisner effects in the magnetic susceptibilities of La1.90Sr0.10CuO3.90and La1.4Sr0.6CuO3.7were identified at about 32 and 36 K respectively. The onset of superconductivity in these samples is not accompanied by a break in the heat capacity at the critical temperatures. A series of thermal history dependent anomalies were identified in the heat capacity, magnetic susceptibility and electrical conductivity of La1.4Sr0.6CuO3.7between 234 and 290 K. Annealing at 234 K causes the anomalies to shift to higher temperatures. Cooling below the transition temperature restores the anomalies. The anomalies tend to disappear after prolonged annealing at 380 K in air.

ISSN:0094-243X    

DOI:10.1063/1.40266

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

X‐ray absorption around oxygen K‐edge of Tl2Ba2Ca2Cu3O10high‐TCsuperconductors was measured by means of a bulk sensitive x‐ray fluorescence yield detection method. Three distinct pre‐edge peaks are revealed. They are ascribed to core‐level excitations of oxygen 1s electrons to empty states at Fermi level which have predominantly oxygen 2p character. These oxygen holes are located in CuO2, BaO and TlO planes. The strong dependence ofTCon oxygen holes concenration on O(1) site in CuO2layer is found.

ISSN:0094-243X    

DOI:10.1063/1.40281

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Although high Tc oxide superconductors have different chemical compositions, they are all perovskite‐type. In this paper, we define high Tc oxide superconductor‐forming elements, and show distribution of these elements in the periodic table. We present a novel expression from the high Tc perovskite‐type oxide superconductors, which is as simple as a chemical formula, but contains important structural information. Using this expression, we list most of known high Tc superconductors. We found that a perovskite‐type superconductor with certain layers of metallic atoms may reach a highest Tc, and the highest Tc is a function of total metallic atom layers: Tc(highest)=f×L (K) when L≤3×(E‐1), where E is the number of metallic elements, L is the number of metallic atom layers, and f is a coefficient. Therefore, higher Tc perovskite‐type superconductors should be sought in more complex systems.

ISSN:0094-243X    

DOI:10.1063/1.40267

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Mass‐spectroscopic studies on biased laser ablation of Ba2Y1Cu3O7−&dgr;have been carried out by using time‐of‐flight mass‐spectroscopy. The energies of the ions were hardly influenced by the electric field created by substrate biasing. This is probably because the ions in the plume are shielded from the external electric field by electrons. Energy differences between the atomic, monoxide and cluster ions have been observed. While the former has an energy of about 200 eV or more, the latter two have only a few eV. This implies that atomic ions are accelerated during the initial laser ablation process and monoxide and cluster ions are produced through plasma reactions in the plume. The multi‐valenced ion species (Cu2+and Cu3+) were observed under positively biased conditions. They should be one of the key factors in preparing high quality superconductive thin films at lower deposition temperatures due to their high internal energies.

ISSN:0094-243X    

DOI:10.1063/1.40268

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Critical current density is an important parameter for large scale application of superconductors. Oxide superconductors however carry only small critical currents though intra‐grain currents are estimated to be high. Rolling of oxide superconductors is one of the promising processes to achieve higher current densities as it improves the density, grain contact and texture of the material. Silver addition to the basic composition is known to improve the electrical properties of the oxide superconductors. Rolled tapes of Y‐Ba‐Cu‐O with 15% and 30% Ag additions were investigated for the effect of magnetic field and its orientation on resistive transition and critical current density. The measurements were carried out at 77 K and for fields up to 90 Oe in directions perpendicular and parallel to the sample. The observed shift in resistive transition for field orientation perpendicular and parallel to the current flow gives an understanding of the percolation path of currents. The nature of shift and Jc(B) curves is discussed in terms of the effectiveness of coupling across the grains. These measurements could lead to optimization of additive concentration for achieving maximum critical current density for a given process.

ISSN:0094-243X    

DOI:10.1063/1.40269

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Low‐field ac magnetic properties of high‐Tc superconductors are studied using a model of a granular system with weak links between grains. The existence of closed loops consisting of grains connected by small area Josephson contacts is assumed. The periodicity condition of the superconducting order parameter phase leads to the family of the energy branch corresponding to different phase ‘winding’ numbers. When the loop self‐inductance L is taken into account, the hysteresis occurs for the applied magnetic flux close to half‐integer values of flux quantum &Fgr;0. The width of the hysteresis range is proportional to LI0/N&Fgr;0, whereNis the number of weak links in the loop, Ic is the contact critical current. In this model the ac susceptibility is found, its imaginary part is due to the energy dissipation in the branch switching processes. The dependence of the susceptibility on the ac field amplitude, the second harmonic power of the magnetization vs dc magnetic field, temperature variation of the dissipative part of the susceptibility are calculated. Statistical properties of the medium are taken into account by an average in loop areas.

ISSN:0094-243X    

DOI:10.1063/1.40270

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Nonlinearities in the conductance of point contacts between a superconductor and a normal metal are considered for applied voltages larger than the energy gap &Dgr;. Voltage dependence of the conductance and the second derivative of the current‐voltage characteristic, resulting from the local electron‐phonon interaction in the contact region, are obtained when electrons move diffusively through the contact, their mean free path being much shorter than the contact dimension d. The conductance nonlinear structure corresponding to peaks of the Eliashberg function is calculated for simple model examples. These results are valid when the inequality &lgr;d2&Dgr;/nD<1 holds, where &lgr; is the electron‐phonon coupling constant and D is the electron diffusion coefficient. The second derivative, d2‖/dV2, is obtained for voltages near the values corresponding to Van Hove singularities of the phonon density of states.

ISSN:0094-243X    

DOI:10.1063/1.40271

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We have measured oxygen K edge absorption spectra in Tl2Ba2Ca2Cu2O10and Bi2Sr2CaCu2O8layered copper oxide superconductors. We have tried to isolate the contribution of Tl‐O and Bi‐O bonds to the spectra by measuring O K edge absorption spectra in Tl2O3and Bi2O3. Since Tl and Bi are determined to be in +3 valence state in both of the superconducting compounds, we attribute the peaks below 530 eV in the absorption spectra to holes in the Cu‐O bonds. We compare the angular dependence of holes in oxygen by using polarized x‐rays.

ISSN:0094-243X    

DOI:10.1063/1.40272

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Ion channeling measurements on 90‐K and 60‐K YBCO provide evidence of an abrupt synchronization of certain lattice vibrations as T is lowered through Tc. This evidence supports a theoretical model in which kinetic energy is stored in pi‐electron holes circulating in Mo¨bius orbits; for T<Tcthese orbital excitations become locked together with the aid of synchronous lattice vibrations. A pattern of electric potential peaks and valleys driven by the synchronized orbiting holes carries mobile charges as singlet pairs, whose net flow in the presence of a vector potentialAconstitutes the observed supercurrent,J=−(e2‐n/mc)A.

ISSN:0094-243X    

DOI:10.1063/1.40274

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The near‐edge structure (XANES) of the Au L3and Cu K edges of YBa2Au0.3Cu2.7O7−&dgr;has been studied. X‐ray diffraction suggests that Au goes on the Cu(1) site and XANES shows that this has little effect on the oxidation state of the remaining copper. The gold L3edge develops a white line feature whose position lies between that of trivalent gold oxide (Au2O3) and monovalent potassium gold cyanide (KAu(CN)2) and whose intensity relative to the edge step is smaller than in the two reference compounds. The L3EXAFS for Au in the superconductor resembles that of Au2O3. However, differences in the envelope of the Fourier filtered component for the first shell suggest that the local structure of the AU in the superconductor is not equivalent to Au2O3.

ISSN:0094-243X    

DOI:10.1063/1.40275

出版商:AIP    

年代:1991

数据来源: AIP