摘要:

We review recent progress in the understanding of the quasiparticle properties of d-wave superconductors in a magnetic field within a semiclassical treatment. We demonstrate that there exists quantitative agreement between theory and experimental data onYBa2Cu3O7−&dgr;(YBCO) for the field applied along thec-axis. We suggest that the field dependence of the NMR magnetization in the regime where the single relaxation time approximation breaks down can be used as a probe of the quasiparticle properties. We also show that for the field applied in the a-b plane the density of states and thermodynamic properties depend on the angle between the applied field and the order parameter nodes, resulting in fourfold oscillations of the electronic specific heat and anisotropy of the thermodynamic coefficients. We argue that this anisotropy can serve as a test of gap symmetry in the bulk, and as a further probe of the nodal quasiparticles. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59600

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Microscopic analyses have been performed on YBCO superconductors with Ca-doping and Gd-doping as a function of temperature by employing a low-temperature scanning electron microscope (LTSEM). On lowering temperature of the sample from 300 K to 90 K, the brightness of the SEM image changes due to the change in resistance of the sample. For the underdoped cupratesY1−xCaxBa2Cu3O7−&dgr;,withx=0.2,the pseudo gap in the normal state is opened at a temperatureT*which far below the critical temperatureTcof the superconductor. The opening of the pseudo gap has directly been observed under LTSEM (temperature fluctuating in the range of ±5 K). At temperatureT*the formation of quasi-particles takes place, thus a sudden brightness change in the SEM image is observed. The results of these measurements are compared with the four-point probe measurements. It is proved that the data of these two measurements are quite in agreement with each other. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59652

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We report on two tunneling experiments studying the pairing symmetry inYBa2Cu3O7−&dgr;(Tc=90&hthinsp;K)at 4.2 K. First, directional tunneling and point-contact spectroscopy measurements were made on the {100}, {110} and {001} faces of single crystal samples. The conductance spectra showed either quasiparticle tunneling, Andreev-reflection or zero-bias peak characteristics, depending on the junction orientation and impedence. Quantitative spectral analysis using the generalized Blonder, Tinkham and Klapwijk theory indicated a predominantlydx2−y2pairing symmetry, with less than 5&percent;s-wave component in either thed+sord+isscenario. Second, scanning tunneling spectroscopy was performed on epitaxial superconductor/ferromagnet heterostructures comprisingYBa2Cu3O7−&dgr;andLa0.7Ca0.3MnO3.Quasiparticle tunneling and zero-bias peak characteristics observed on the cuprate were consistent withd-wave pairing symmetry, which appeared to be invariant under a spin-polarized quasiparticle current injected from the manganite, up to at least7×103&hthinsp;A/cm2.©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59637

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Some preliminary discussion is presented concerning predicting superconductivity in general and the properties of existing fullerene superconductors. This is followed by a consideration of the likelihood of superconductivity inC36solids. Our present knowledge aboutC36and the similar moleculeC24N12is reviewed along with the relevant arguments used as a basis for suggesting that these systems are good candidates for superconductivity at relatively high temperatures. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59602

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Local density functional results are presented for the electron-doped metallochloronitridesAxZrNClandAxHfNCl,A=Lior Na, which superconduct up to 25 K. The alkali non-stoichiometry is treated in a virtual crystal approximation. The electronic structure is strongly two dimensional, especially in the conduction band region occupied by the carriers, because the states are formed from the in-plane orbitalsdxy,dx2−y2of the metal ion and thepx,pyorbitals of the N ion. We predict a change of behavior at a doping level ofx=0.3.©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59601

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

The superconductivity in bothSr2RuO4and Bechgaard salts is most likely of p-wave. Here we review the effect of impurity scattering and the features of the vortex state in p-wave superconductors. There are striking similarities between those properties in p-wave superconductors and the ones in d-wave superconductors. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59638

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Using synchrotron x-ray scattering on single crystals ofNd1/2Sr1/2MnO3andLa5/3Sr1/3NiO4,we have demonstrated the existence of charge stripe structures in both compounds. InNd1/2Sr1/2MnO3charge ordered satellite reflections were located at positions(h/20l),h=oddandl=even.In addition we have also observed some weak reflections at other positions(h/2kl/2)and(h/20l/2),h&hthinsp;&&hthinsp;l=oddandk=1.The result is in accord with the CE-type antiferromagnetic structure as reported by neutron powder diffraction. The transition was observed to be at a transition temperatureTCO≈159&hthinsp;Kand to be strongly first order in nature. By contrast, studies ofLa5/3Sr1/3NiO4showed a second order transition atTCO≈240&hthinsp;K.This produced charge stripe satellites with a wavevectorq=(h±2&Vegr;0l),h=evenandl=odd.Critical exponents of the transition were obtained and show that the system is in the 2D universality class. The two dimensional nature of the charge stripes is clearly shown in their markedly different correlation lengths at low temperatures e.g.,&xgr;a≈110&hthinsp;Å,&xgr;b≈130&hthinsp;Å,whist&xgr;cis only about 17 Å. These results demonstrate clearly that the charge stripes are themselves two dimensional in character. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59649

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

TheAMO3perovskites containing a first-row transition-metal atom M allow study of the cross-over from localized to itinerant electronic behavior in a single-valentMO3array dominated by (180°-&fgr;) M-O-M interactions. TheLnNiO3perovskites with Ln a lanthanide have one electron per low-spin Ni(III) in a narrow &sgr; band that approaches cross-over from the itinerant-electron side. Transport measurements are reported that add to the mounting evidence for a heterogeneous electronic transition in the perovskites rather than the homogeneous transition assumed in conventional treatments of the Mott-Hubbard transition. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59639

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

We present an electronic Raman scattering study of the colossal magnetoresistance manganese perovskites as a function of temperature, magnetic field, symmetry, and doping. The low-frequency electronic Raman spectrum of these materials exhibits a distinctive change through the paramagnetic insulator/ferromagnetic metal transition, characterized by a transformation from a diffusive scattering response to a flat continuum response. We also find that the magnetic-field dependence of electronic Raman scattering in the manganese perovskites is both highly anisotropic and indicative of an inhomogeneous ferromagnetic phase. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59603

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Neutron diffraction measurements were used to investigate the local atomic structure of manganese and cobalt oxides. Static Jahn-Teller (JT) distortions present in the lightly doped perovskite manganates were found in metallic compositions as well. The cooperativeness of the distortions is however lost as the doping is increased. In the two-layer manganates, the existence of a local JT effect helps explain the similarities in the properties between cubic and layered systems. In the cobalt system, the coupling strength of the lattice to theegstates during the thermal activation from the ground, low-spin (LS) state to an excited, intermediate (IS) or high-spin (HS) states for Co inLaCoO3changes as a function of temperature. The introduction of extra carriers by doping ofLa1−xSrxCoO3stabilizes the IS JT states, populated at a rate proportional to the charge density. The JT distortions induced in this system are dynamic in nature if compared to the ones in the manganates. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.59604

出版商:AIP    

年代:1999

数据来源: AIP