凝聚态物理导论-中科院课件-苏刚Chapter7.pdfVIP

Chapter 7 Superconductivity of Matter Superconductors, materials that have no resistance to the flow of electricity, are one of the last great frontiers of scientific discovery. Not only have the limits of superconductivity not Heike yet been reached, but the theories that explain Kamerlingh superconductor behavior seem to be constantly under review. Onnes In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University. When he cooled it to the temperature of liquid helium, 4 degrees Kelvin (-452F, -269C), its resistance suddenly disappeared. The Kelvin scale represents an absolute scale of temperature. Thus, it was necessary for Onnes to come within 4 degrees of the coldest temperature that is theoretically attainable to witness the phenomenon of superconductivity. Later, in 1913, he won a Nobel Prize in physics for his research in this area. Brief history of discovery of superconductors 1911: Superconductivity with Tc=4.2 is observed in solid mercury by H.K. Onnes. 1911-1975: Superconductivity observed in several metals and alloys. Their properties are well described by BCS theory and its strong-coupling extension, Migdal-Eliashberg theory. The increase of the critical temperature with time is linear. Semi-empirical arguments suggest that Tc cannot be raised above 25 K. 1957: The Bardeen-Cooper-Schrieffer (BCS) theory is formulated: superconductivity is caused by the condensation of pairs of electrons (Cooper pairs), which are held together by an attractive force provided by phonons. 1986: Bednorz and Mueller observe superconductivity with a Tc of 35 K in LaBaCuO. 1987: A Tc of 92 K, i.e. above the freezing point of nitrogen, is observed in YBCO. Bednorz and Mueller receive the Nobel prize. In the next few years, even higher Tcs are observed in other cuprates. It is very soon realized that cuprates do not