I N T R O D U C T I O N
SUPERCONDUCTIVIT Y is the name given to a remarkable combination of electric and magnetic properties which appears in certain metals when they are cooled to extremely low temperatures. Such very low temperatures first became available in 1908 when Kamerlingh O n n e s at the University of Leiden succeeded in liquefying helium, and by its use w a s able to obtain temperatures down to about 1°K.
O n e of the first investigations which O n n e s carried out in the newly available low-temperature range w a s a study of the variation of the elec- trical resistance of metals with temperature. It had been known for many years that the resistance of metals falls when they are cooled below room temperature, but it was not known what limiting value the resistance would approach if the temperature were reduced towards 0°K. Onnes, experimenting with platinum, found that, when cooled, its resistance fell to a low value which depended on the purity of the specimen. At that time the purest available metal was mercury and, in an attempt to dis- cover the behaviour of a very pure metal, O n n e s measured the resistance of pure mercury. H e found that at very low temperatures the resistance became immeasurably small, which w a s not surprising, but he soon dis- covered (1911) that the manner in which the resistance disappeared w a s completely unexpected. Instead of the resistance falling smoothly as the temperature w a s reduced towards 0°K, the resistance fell sharply at about 4°K, and below this temperature the mercury exhibited no resistance whatsoever. Furthermore, this sudden transition to a state of no resistance was not confined to the pure metal but occurred even if the mercury w a s quite impure. O n n e s recognized that below 4°K mercury passes into a new state with electrical properties quite unlike those previously known, and this new state was called the "superconducting state".
It was later discovered that superconductivity could be destroyed (i.e.
electrical resistance restored) if a sufficiently strong magnetic field were applied, and subsequently it w a s found that a metal in the superconduc-
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xviii INTRODUCTION
ting state has very extraordinary magnetic properties, quite unlike those known at ordinary temperatures.
U p to the present time about half of the metallic elements and also a n u m b e r of alloys have b e e n f o u n d to b e c o m e s u p e r c o n d u c t i n g at low temperatures, that is to say below about 25°K. Recently (1987), h o w - ever, it has been discovered that some ceramic metallic oxides become superconducting at m u c h higher temperatures, i.e. at about 100°K (see C h a p t e r 14).
T h o s e materials w h i c h exhibit superconductivity w h e n sufficiently cooled are called superconductors. For m a n y years it was t h o u g h t t h a t all s u p e r c o n d u c t o r s b e h a v e d according t o a basically similar p a t t e r n . However, it is n o w realized that there are two kinds of superconductor, which are known as type-I and type-II. M o s t of those elements which are s u p e r c o n d u c t o r s exhibit t y p e - I superconductivity, w h e r e a s alloys generally exhibit type-II superconductivity. T h e t w o types have m a n y properties in c o m m o n b u t show considerable differences in their magnetic behaviour. T h e s e differences are sufficient for us to treat the two types separately. T h e first part of this book deals w i t h type-I superconductors and the second part with type-II superconductors.
