Superconductivity without BCS.It was about 45 years from the discovery of superconductivity superconductivity, abnormally high electrical conductivity of certain substances. The phenomenon was discovered in 1911 by Kamerlingh Onnes, who found that the resistance of mercury dropped suddenly to zero at a temperature of about 4.2°K;. , the ability of some metals to pass electric currents without resistance when chilled to temperatures near absolute zero, to the formulation of a satisfactory theory of why it happens. The theory is called BCS (1) (The British Computer Society, Swindon, Wiltshire, England, www.bcs.org) The chartered body for information technology professionals in the U.K., founded in 1957. for its originators, John Bardeen Noun 1. John Bardeen - United States physicist who won the Nobel prize for physics twice (1908-1991) Bardeen , Leon Cooper Leon Nathan Cooper (born February 28, 1930) is an American physicist and winner of the 1972 Nobel Prize for Physics, along with John Bardeen and John Robert Schrieffer, for his role in developing the BCS theory (named for their initials) of superconductivity, work he did in his and Robert Schrieffer. Recently what seems to be a new kind of superconductivity has been found, which is called heavy-fermion superconductivity (SN:4/7/84, p. 212) and seems qualitatively different from the earlier known variety. Evidence is mounting that a new theory, not BCS, will be necessary to explain it. The latest such datum The singular form of data; for example, one datum. It is rarely used, and data, its plural form, is commonly used for both singular and plural. deals with the absorption of ultrasound by heavy-fermion superconductors. Brage Golding of AT&T Bell Laboratories in Murray Hill, N.J., and six others from Bell Labs, Los Alamos (N.M.) National Laboratories and the Swiss Federal Institute of Technology The Swiss Federal Institute of Technology may refer to one of two institutes of higher education in Switzerland:
The studied the absorption of sound frequencies between 0.9 and 2.4 gigahertz by the uranium-beryllium compound UBe.sub.13 when it is in the heavy-fermion superconducting state. Just below the critical temperature at which superconductivity sets in, they find the ultrasound absorption rising to a peak and then declining as the temperature drops further. In BCS superconductors this peak does not appear; the ultrasound absorption declines steadily from the critical temperature toward absolute zero. The experimenters suggest that the peak in UBe.sub.13 appears because the ultrasound triggers some kind of collective behavior by the electrons that form currents and supercurrents in the metal. BCS theory attributes the lack of resistance to a pariing of these conduction electrons: Paired, they can sail through the metal without resistance. However, the paired electrons in BCS theory are in a quantum state (L=0) that does not allow them the freedom for the suggested collective action. This finding seems to indicate that for heavy-fermion superconductivity such pairing has to take place in a higher state (L=1 or higher) that gives them the freedom, as some theorists have already suggested. |
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