摘要:

We consider the Anderson lattice for finiteUusing a novel equation of motion technique. The method employs a truncation scheme based on a projection operator which projects out of any operator its exact thermal average in the space ofcorrelatedsingle‐particlelike excitations, i.e., excitations both in the presence and absence of the opposite spin state. Thus, this method is good for allU. This is important in these superconducting transition metal oxide systems since experimental evidence has shown that the bandwidth/Uis not small. For the single‐particle spectrum we naturally generate the anomalous correction induced pairing and resulting gap equation for finiteU, in contrast to previous treatments based on the large‐Ulimit, where on‐site double occupancy is excluded. The magnetic and pairing susceptibilities are calculated in a similar manner, again for finiteU, and the competition between these instabilities will be discussed.

ISSN:0021-8979    

DOI:10.1063/1.340235

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A new class of Fe‐, Co‐, Ni‐, Cu‐, and Ge‐based ductile ribbons in which an amorphous matrix containing less than 12 vol. % of a fine dispersion of 0.1–3‐&mgr;m size particles of immiscible elements M=Pb, Bi, Sn, or In have been produced by a modified melt‐quenching technique. The Fe‐ and Co‐based alloys containing Bi and Pb are found to be ferromagnetic above room temperatureandalsosuperconductingat temperatures in a range 7–9 K depending on the alloy. The saturation magnetization in these usual alloys range from 92 emu/gm for Co‐Si‐B‐Pb‐Bi to 15 emu/g for Fe‐P‐Pb‐Bi at 77 K. At the superconducting transition the resistivity typically is found to drop by 2–3 orders of magnitude with a transition width ∼0.5 K before the ‘‘zero’’ resistance value is reached. In all the alloys studied theTdependence of the resistivity shows a typical metallic behavior despite its high magnitude (a few thousand &mgr;&OHgr; cm in some cases). Extremely high values are found for the upper critical fields (1.6–1.72 T at 4.2 K) with critical current densities typically around 1.3×107A m−2at 4.2 K. For Ge‐Pb‐Bi‐SnHc2values of 5 T are obtained at 3 K. Along with detailed studies of magnetic and superconducting properties, extensive TEM, and x‐ray characterization data will also be presented. Attempts to explain the above described behavior in terms of proximity, percolation, and surface properties at the dispersed Pb, Bi particles are found to be unsatisfactory. As far as we know there exists no theory that can explain the co‐existence of ferromagnetism and superconductivity of the type described above. Time permitting, we will also discuss our recent work on highTcoxide materials produced by melt‐quenching techniques. Thus amorphous composite materials exhibiting unique and useful characteristics which cannot be achieved for homogeneous amorphous alloys can be produced by melt quenching. These materials are of importance both for scientific and engineering points of view.

ISSN:0021-8979    

DOI:10.1063/1.340236

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A series of studies carried out at IBM on the magnetic, transport, and optical properties of single crystal Y1Ba2Cu3O7−xdemonstrate the overriding importance of anisotropy in its superconducting and normal‐state properties.

ISSN:0021-8979    

DOI:10.1063/1.340237

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We present high‐precision results on the electronic band structure and properties of YBa2Cu3O7−&dgr;, YB2Cu3O6, GdBa2Cu3O7−&dgr;, and La2−xMxCuO4as obtained from highly precise state‐of‐the‐art local density calculations. The results obtained demonstrate the close relation of the band structure to the structural arrangements of the constituent atoms and provide an integrated chemical and physical picture of the interactions and their possible relation to superconductivity. The ionic character of the Y is proven by similar detailed highly precise local density calculations for highTCGdBa2Cu3O7, and explains the coexistence of magnetism and superconductivity in the highTCrare‐earth superconductors. Surprising features are the low density of states (DOS) atEF, especially for &dgr;≥0.1 which is lower per Cu atom than that in La2−xSrxCuO4—in agreement with experiment and a relatively large magnetic Stoner factor. Strong indications are demonstrated for the inadequacy of a conventional phonon mechanism for obtaining the higherTC. Charge transfer excitations of occupied Cu1−Odp&pgr; antibonding orbitals into their empty Cu1‐Odp&sgr; antibonding orbital partners, result in poorly screened ‘‘Cu3+‐Cu4+’’ ‐like charge fluctuations. These charge transfer excitations (excitons) thus lead to strong polarization effects in these poorly screened (highly ionic) materials and induce attractive interactions among the 2D electrons. Thus, these interactions via exchange of excitons enhance the electron pairing and serve to enhance theTCproposed for the quasi‐2D superconductors.

ISSN:0021-8979    

DOI:10.1063/1.340238

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The one‐dimensional Hubbard model with first‐ and second‐neighbor hopping,t1andt2, witht2≊t1/(2)1/2and large repulsionU, is proposed as a testing ground of Anderson’s idea that the existence of an resonating valence bond ground state in the half‐filled‐band case will show superconductivity when a finite concentration of holes is introduced. Interestingly, our preliminary calculations in theU=∞ limit for small chains have revealed anewmechanism for enhancement of singlet pairing: We find that fort2<0 the ground state is a singlet, and, fort2<0 and ‖t2/t1‖≪1, the nearest‐neighbor extended‐singlet‐pairing correlation function becomes enhanced appreciably as the temperature decreases belowt2.

ISSN:0021-8979    

DOI:10.1063/1.340239

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The superconducting YBa2Cu3O7−xphase has been produced through a novel rapid solidification processing route yielding a high‐quality granular superconducting phase. Features in the magnetic susceptibility as measured at various fields can be attributed to important transitions in the superconducting behavior of these materials. At low fields (∼20 Oe) anomalies in the susceptibility arise from decoupling of the grains in these polycrystalline materials. Susceptibility response at higher fields (∼100 Oe) is representative of randomly oriented superconducting grains which show local superconducting current densities comparable to those of single crystals. At even higher fields features consistent with Muller’s superconducting glass state model are observed.

ISSN:0021-8979    

DOI:10.1063/1.340240

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Field ion microscopy (FIM) observations (using either hydrogen or neon gas) were performed on both the highTcsuperconducting oxide Ba2YCu3O7−x(0<x<0.5) and the related nonsuperconducting oxide Ba2YCu3O6. For the superconducting material, ac magnetic susceptibility measurements determined a transition temperature of 92 K. At 30 K strong preferential images of atomic or molecular layers were observed for Ba2YCu3O7−xperpendicular to the longcaxis, but not for Ba2YCu3O6. The preferential field evaporation or field ionization responsible for the layered images (possessing a spacing of approximately 12 A˚ which is equivalent to thec‐axis lattice parameter) in the superconducting oxide below its transition temperature is interpreted as possible evidence for the existence of relatively highly conducting atomic layers in the unit cell of this material at low temperatures. Computer simulations of FIM images expected under various assumptions of preferential imaging are most consistent with the layered images being due to the copper‐oxygen end planes (those located between the planes of Ba atoms). FIM observations of Ba2YCu3O7−xperformed above the superconducting transition temperature (at 100 K) showed primarily a highly intense disordered image similar to that obtained for the structurally related nonsuperconducting oxide Ba2YCu3O6at low temperatures, and faintly discernible parallel rows of atoms. Both twins and grain boundaries in Ba2YCu3O7−xhave been observed in the FIM at low temperatures; and no special images related to the superconducting state have been found to be associated with these structural features. To date, consistent observations have been performed on approximately 100 specimens, thereby assuring that the present observations are not artifacts, but representative of the bulk material. Similar FIM observations have also been obtained for the related highTcsuperconductor Ba2YbCu3O7−x.

ISSN:0021-8979    

DOI:10.1063/1.340241

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Since the 1950’s there have been a number of experimental observations involving small single domain (SD) magnetic particles (≲200 A˚ diameter) which have been attributed to what we’d today call ‘‘macroscopic quantum tunneling’’ (MQT) of the magnetization in its switching from one easy axis direction to another. A comprehensive study of this phenomenon is not only of great interest for the sake of fundamental physics: It is of utmost practical importance in thatMQTsetsalimitonthelifetimeofinformationstorageon magnetic tapes and films. In this paper we present a brief summary of some of the latest results of our theoretical and experimental studies of this phenomenon.

ISSN:0021-8979    

DOI:10.1063/1.340216

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have prepared controlled arrays of submicron permalloy particles as part of a program to determine the magnetic properties of verifiable single‐domain particles (SDP) and the effects of interactions between SDPs. We have measured the hysteresis loops of these arrays as a function of size, aspect ratio, and spacing. The arrays are fabricated utilizing electron beam lithography and liftoff techniques. Each array consists of approximately 106identical particles with identical spacing between the particles. Particle size and spacing [center‐to‐center distance (ccd)], vary from 5 &mgr;m×15 &mgr;m (width by length) with ccd of 25 &mgr;m to 0.07 &mgr;m×0.1 &mgr;m with ccd of 0.25 &mgr;m. The thickness of the films varied from 0.072 to 0.049 &mgr;m. Hysteresis loops are obtained utilizing a SQUID magnetometer. For well‐separated particles we observe an order of magnitude increase inHcwith decreasing particle size, and then a subsequent decrease inHcas the ccd is reduced. However, even for the smallest particles, the ratio of the remanence to saturation magnetization indicates that they are not single domain, and the loop shape is not indicative of Stoner–Wohlfarth behavior.

ISSN:0021-8979    

DOI:10.1063/1.340217

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Magnetic granular solids consist of ultrafine metal granules of nanometer sizes embedded in an insulating medium. The magnetic properties are dictated by the microstructure such as particle size, metal volume fraction, which are process controlled. We report the results of a series of granular Fe60(SiO2)40films in which the particle size has been systematically varied. The magnetic coercivity, ranging from 500 to 2200 Oe, increases with particle size. The ferromagnetic‐superparamagnetic transition had been studied by SQUID magnetometry and Mo¨ssbauer spectroscopy. The value of the magnetic anisotropy energy is found to be much larger than that due to magnetocrystalline anisotropy.

ISSN:0021-8979    

DOI:10.1063/1.341126

出版商:AIP    

年代:1988

数据来源: AIP