摘要:

In this work we present the results of the application, in a novel way, of an automatic proportional-internal-derivative controller to the measurements of the irreversibility line in high temperature superconductors. The system operates by applying to the sample a controlled dc field that maximizes the imaginary (out-of-phase) component of the ac susceptibility, commonly used as a criterion for the determination of such dependencies. Through this technique, for the first time, direct and continous determination of irreversibility lines are possible, allowing the measurement of a whole curve in only one temperature sweep, such in a typical resistivity versus temperature curve. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59622

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Polarized x-ray absorption fine structure (XAFS) measurements at the La and Sr sites inLa2−xSrxCuO4(0.075≲x≲0.35)indicate that doped holes introduced with Sr are not uniformly distributed in theCuO2planes but reside in impurity states with the majority of charge located onCuO6octahedra coupled to the Sr dopants by an apical oxygen (denoted as Sr octahedra). A model based on doped hole-induced Jahn Teller (JT) distortions of these Sr octahedra indicates the impurity states are overlapping singlet and triplet ones resulting from pairings of intrinsic and extrinsic holes. The more mobile singlet pairs reside in theCuO2planes, are bound by more than 0.1 eV and could Bose-condense into a superconducting state. The predominant appearance of (mostly) out-of plane triplet states at highxintimately related to the 2D-3D crossover in transport properties and could relate to the observed loss of highTcin the overdoped regime, as confinement of carriers to theCuO2planes is reduced. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59594

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We have used a scanning SQUID microscope to image vortices emerging parallel to the planes in the high-Tccuprate superconductorLa2−xSrxCuO4.These images provide a value for thec-axis penetration depth,&lgr;c∼5&hthinsp;&mgr;m,which is in reasonable agreement with previous measurements using optical techniques. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59595

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

This paper discusses the electronic structure in the vortex core in relation to the pinning potential under the influence of the pseudo-gap which opens in the normal state in the high-temperature superconductor (HTS). It is argued that the condensation energy may be significantly reduced in the under-doped regime where the pseudo-gap becomes pronounced and hence it should be deeply related with the small pinning force in this regime. The empirical scaling laws observed for the vortex behaviors in the mixed state phase diagram are also discussed in terms of the condensation energy and the anisotropy factor. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59623

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Li &etal; found that the critical current densityJcJacross atomically cleanc-axis twist junctions ofBi2Sr2CaCu2O8+&dgr;is the same as that of the constituent single crystal,JcS,independent of the twist angle&fgr;0,even atTc.We investigated theoretically if adx2−y2-wave order parameter might twist by mixing indxycomponents, but find that such twisting cannot possibly explain the data near toTc.Hence, the order parameter contains ass-wave component, butnotanydx2−y2-wave component. In addition, thec-axis Josephson tunneling is completely incoherent. We also propose ac-axis junction tricrystal experiment which does not rely upon expensive substrates. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59596

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

High pressure X ray diffraction studies up to 15GPa were performed on the Y doped Bi2212 compound with the rare earth concentrationx=0.1at the calcium site. From the studies it has been found that the system remains in the orthorhombic structure up to 15GPa. The relative volume is found to decrease around 15GPa. This may be a positive indication for a pressure induced structural transition in the system beyond 15GPa. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59624

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We studied thec-axis transport in the high qualityBi-2212stacked junctions fabricated by the double-sided focused ion beam (FIB) processing of the single crystal whiskers. We found the value of the critical current to be considerably (by factor 30) below the value given by the Ambegaokar-Baratoff (AB) relation in the BCS model at low temperatures. The results are discussed as a possibility of thed-wave partially coherent interlayer tunneling. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59597

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We report on thec-axis-polarized electronic Raman scattering ofBi2Sr2CaCu2O8+&dgr;single crystals with various oxygen concentrations. BelowTc,there is a low-energy redistribution of the electronic continuum and the presence of a 2&Dgr; peak-like feature for all doping levels studied. The superconductivity-induced effect is also accumulated from frequencies as high as 40&Dgr;. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59626

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

The inelastic neutron scattering spectra were measured for several Sr concentrations of polycrystallineLa2−xSrxNiO4.We find that the generalized phonon density-of-states is identical forx=0andx=1/8.Forx=1/3andx=1/2,the band of phonons corresponding to the in-plane oxygen vibrations (>65 meV) splits into two subbands centered at 75 meV and 85 meV. The lower frequency band increases in amplitude for thex=1/2sample, indicating that it is directly related to the hole concentration. These changes are associated with the coupling of oxygen vibrations to doped holes which reside in theNiO2planes and are a signature of strong electron-lattice coupling. Comparison ofLa1.9Sr0.1CuO4andLa1.875Sr0.125NiO4demonstrates that much stronger electron-lattice coupling occurs for particular modes in the cuprate for modest doping and is likely related to the metallic nature of the cuprate. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59627

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

A photoinduced IR absorption peak centered at ∼5000 cm−1was observed in(La1−xSrxMn)1−&dgr;O3at low hole doping. The peak diminishes and softens as hole doping is increased. The peak is attributed to absorption due to the anti-Jahn-Teller polaron. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59628

出版商:AIP    

年代:1999

数据来源: AIP