Coating processes – Electrical product produced – Superconductor
Reexamination Certificate
1996-08-26
2001-07-24
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Superconductor
C505S121000
Reexamination Certificate
active
06265019
ABSTRACT:
TECHNICAL FIELD
The present invention relates to superconductors with a high critical temperature according to the first part of claims comprising an artificial heterostructure made by first portions of a superconducting material of a size L pushed toward the atomic limit. Moreover the invention relates to a method to realize these superconductors.
It is well known that a wide industrial application of superconductivity is limited by the fact that the superconducting metals available on the market exhibit very low critical temperatures (the critical temperature indicates the temperature for the onset of the superconductivity, i.e., for the zero resistivity and diamagnetism). These very low temperatures, of the order of few degree's Kelvin, can be reached by using low temperature technologies, that imply high cost and difficulties in the operation. Therefore the research in this field is addressed to realize new superconducting materials that will show higher critical temperatures.
BACKGROUND ART
A recent innovation in this field is the discovery of superconductivity in cuprate perovskites that show critical temperatures of the order of hundred degree's Kelvin. However this class of ceramic superconductors has a limited industrial application because these materials are fragile, they are difficult to shape and it is difficult to keep under control the oxygen content. These difficulties generate limits for the industrial applications of superconducting electronic devices using these ceramic materials.
Another difficulty for the advances in this field is due to the lack of a theory that explains the superconductivity at high temperature in these materials, therefore the progress in these materials advances by empirical approaches.
Superconducting cuprate perovskites have been described as natural superlattices of first superconducting CuO
2
layers with a two dimensional electronic structure, sandwiched by second blocking layers made of insulators, or metals with lower T
c
, stacked in the c axis direction and similar artificial superlaltices have been synthesized (Physical Review Letters Vol 63, No. 9 August 1989, pages 1016-1019; J. -M. Triscone, M. G. Karkut, L. Antognazza, O. Brunner, Ø. Fisher: “Y-Ba-Cu-O/Dy-Ba-Cu-O Superlattices: A First Step towards the Artificial Construction of High T
c
Superconductors”) and it has been also discussed in a recent patent application (Toray Industries, European Patent Appication EPA 0 502 204 A1, International application number PCT/JP91/01255, International publication number WO 92/0514 (02.04.92 92/08)); and in some papers (Physica C, Vol. 190, ISSN 0921-4534 Nos. 1/2 Decmber 1991, pages 22-26; Qi Li, T. Venkatesan and X. X. Xi: “Growth and superconducting properties of YBa
2
Cu
3
O
7
-based superlattices”); (Physica C, Vol. 185-189, ISSN
0921-4534,
Dec. 1, 1991, pages 1747-1748; M. Holcomb, J. P. Collman, and W. A. Little: “New phenomena in proximity effect tunneling of high T
C
superconducting cuprates”). Moreover also superconducting films of doped fullerene, such as K
3
C
60
, can be made for the production of intricately layered microelectronic devices (Scientific American (Int. Edition) October 1991, USA, vol. 265, no. 4, ISSN 0036-8733, pages 32-41 R. F. Curl and R. E. Smalley “Fullerenes”).
DISCLOSURE OF INVENTION
The present invention as claimed is intended to overcome said difficulties by definition of the physical parameters for the design of new superconducting heterostructures with a critical temperature larger by several times than that of the bulk metallic superconducting materials.
In accordance with the present invention I provide a superconductor with high critical temperature formed by a plurality of first portions of a superconducting material, forming a lattice of substantially equal elements with constant period &lgr;
p
, at least in a first direction y, separated by a second material with different electronic structure; said plurality of second portions having a size W of the order of the superconducting Pippard coherence length &xgr;
0
of the superconducting material (W≦&xgr;
0
) forming said plurality of first portions; characterized in that said plurality of first portions have a size L, a period &lgr;
p
, measured in said first direction y, and a charge density to satisfy the “shape resonance” condition of the electrons at the Fermi level.
The present invention is based on the discovery that the superconductivity with high critical temperature is related to a particular heterogeneous structure of the superconducting material; said heterogeneous structure can be found in a cuprate perovskite. This heterogeneous structure determines an “amplification” of the critical temperature in comparison with that of the homogeneous superconducting material. Therefore by making artificial heterogeneous structures including any superconducting metal with a low critical temperature this is multiplied by an “amplification factor” related to the selected heterostructure, providing g a superconductor with an artificial ordered heterogeneous structure with a critical temperature several times, up to 100, larger than that of the homogeneous material.
The invention relates to a method to realize superconducting heterostructures with high critical temperature according with the principal claim characterized by the following phases:
to prepare a substrate made by a material that can be a metal, or a non metal.
formation on the substrate of a plurality of first portions of a superconducting material, which have at least in a direction y a selected dimension L and a selected spacing W.
formation on said first portions of second portions of a material with different electronic structure, that play the role of spacers. The spacing W should be of the order of the superconducting Pippard coherence length &xgr;
0
of the superconducting material (W≦&xgr;
0
) forming said plurality of first portions.
repetition of said phases of formation by a selected number of times up to realize a three-dimensional heterostructure; said dimensions L and spacing W are selected in order to realize for the electrons at the Fermi level in the superconducting metal a “shape resonance” condition. In particular said heterostructure can be realized by molecular beam epitaxy. A second method to realize superconductors with high critical temperature according with the principal claim, but more simple, relates to the formation of a three-dimensional heterostructure made by a plurality of films of a superconducting material, first portions, separated by a plurality of non superconducting films (or superconductors with a lower critical temperature), second portions; the thickness L of the superconducting films, and the thickness W of the non superconducting films are such to realize a “shape resonance” condition for the electrons at the Fermi level in the superlattice of superconducting films. In order to realize a “shape resonance” condition the thickness of the superconducting metal film will be of the order of the thickness of few atomic layers, therefore the film thickness should be controlled to be uniform at the atomic limit.
REFERENCES:
patent: 0 502 204 A1 (1992-09-01), None
J.M. Triscone, et al., “Y-Ba-Cu-O/Dy-Ba-Cu-O Superlattices: A First Step towards the Artificial Construction of High-TcSuperconductors,”Physical Review Letters,vol. 63, No. 9, Aug. 28, 1989, pp. 1016-1019.
Qi Li, T. Venkatesan, et al., “Growth and superconducting properties of Yba2Cu3O7-based superlattices,”Physica C,190, 1991, pp. 22-26.
M. Holcomb, et al., “New Phenomena in Proximity Effect Tunneling of High TcSuperconducting Cuprates,”Physica C,185-189, 1991, pp. 1747-1748.
Robert F. Curl and Richard Smalley, “Fullerenes,”Scientific American,vol. 265, No. 4, Oct. 1991, pp. 32-41.
Flehr Hohbach Test Albritton & Herbert LLP
Talbot Brian K.
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