INTRODUCTION TO SUPERCONDUCTIVITY ROSE INNES PDF

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. O n n e s , 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 w a s mercury and, in an a t t e m p t to dis- cover the behaviour of a very pure metal, O n n e s measured the resistance of pure mercury. H e found t h a t at very low t e m p e r a t u r e s the resistance became immeasurably small, which w a s not surprising, b u t he soon dis- covered t h a t the m a n n e r in which the resistance disappeared w a s completely unexpected. F u r t h e r m o r e , this sudden transition to a state of no resistance w a s not confined to the pure metal b u t occurred even if the mercury w a s q u i t e impure. It w a s later discovered that superconductivity could be destroyed i.

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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. O n n e s , 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 w a s mercury and, in an a t t e m p t to dis- cover the behaviour of a very pure metal, O n n e s measured the resistance of pure mercury. H e found t h a t at very low t e m p e r a t u r e s the resistance became immeasurably small, which w a s not surprising, b u t he soon dis- covered t h a t the m a n n e r in which the resistance disappeared w a s completely unexpected.

F u r t h e r m o r e , this sudden transition to a state of no resistance w a s not confined to the pure metal b u t occurred even if the mercury w a s q u i t e impure. It w a s later discovered that superconductivity could be destroyed i. Recently , h o w - ever, it has been discovered t h a t s o m e ceramic metallic oxides b e c o m e superconducting at m u c h h i g h e r temperatures, i. T h o s e materials w h i c h exhibit s u p e r c o n d u c t i v i t y w h e n sufficiently cooled are called superconductors.

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 t h e t w o types separately. T h e first part of this b o o k deals w i t h t y p e - I superconductors and t h e second part w i t h t y p e - I I superconductors.

Electrons have, of course, a wave-like n a t u r e , and an electron travelling t h r o u g h a metal c a n b e represented b y a plane w a v e progressing in t h e same direction. A metal h a s a crystalline structure w i t h t h e a t o m s lying on a regular repetitive lattice, and it is a property of a plane w a v e t h a t it can p a s s t h r o u g h a perfectly periodic structure w i t h o u t being scattered into other directions. I n other w o r d s , if in a perfect crystal w e start a current flowing which is equivalent t o giving t h e conduction electrons a net m o m e n t u m in t h e direction of the current t h e current will experience n o resistance.

H o w e v e r , any fault in t h e periodicity of t h e crystal will scatter t h e electron w a v e and introduce s o m e resistance. T h e r e are t w o effects which can spoil t h e perfect periodicity of a crystal lattice and so introduce resistance. At t e m p e r a t u r e s above absolute zero the a t o m s are vibrating and will b e displaced b y various a m o u n t s from their equilibrium positions; furthermore, foreign a t o m s or other defects randomly distributed can interrupt t h e perfect periodicity.

B o t h t h e ther- mal vibrations and any impurities or imperfections scatter the moving conduction electrons and give rise t o electrical resistance.

W e can n o w see w h y t h e electrical resistivity decreases w h e n a metal or alloy is cooled. W h e n t h e t e m p e r a t u r e is lowered, t h e thermal vibrations of t h e a t o m s decrease and t h e conduction electrons are less frequently scattered.

T h e decrease of resistance is linear d o w n to a t e m p e r a t u r e equal to about one-third of the characteristic D e b y e t e m p e r a t u r e of t h e material, b u t below t h i s the resistance decreases progressively less rapidly as t h e t e m p e r a t u r e falls Fig. T h i s zero resistance w h i c h a hypothetical " p e r f e c t " specimen would acquire if it could b e cooled t o absolute zero, is not, however, t h e p h e n o m e n o n of superconductivity.

Any real specimen of metal c a n n o t b e perfectly p u r e and will c o n t a i n some impurities. A s a result, there is a certain "residual resistivity" p F i g.

Temperatur e FIG. Variatio n of resistanc e of metal s wit h temperature. C e r t a i n metals, however, show a very r e m a r k a b l e b e h a v i o u r ; w h e n they are coolea their electrical resistance decreases in t h e usual way, b u t on reaching a t e m p e r a t u r e a few degrees above absolute zero they suddenly lose all trace of electrical resistance Fig.

T h e y are t h e n said t o have passed into t h e superconducting state. W e us e th e adjectiv e superconducting t o describ e it whe n it is exhibitin g superconductivi - ty , an d normal whe n it is not exhibitin g superconductivit y e. Superconductin g Transitio n Temperatur e T h e t e m p e r a t u r e at w h i c h a superconductor loses resistance is called its superconducting transition temperature or critical temperature; this t e m p e r a t u r e , w r i t t e n T , is different for each metal.

W e shall see in C h a p t e r 9 t h a t ferromagnetism, in w h i c h the spins of electrons are aligned parallel t o each other, is incompatible w i t h superconductivity. Such elements, therefore, only exhibit superconduc- tivity if they are extremely pure, and specimens of these m e t a l s of nor- mal commercial purity are not superconductors. N o t all p u r e metals h a v e been found t o b e superconductors; for example, copper, iron and sodium have not shown superconductivity d o w n to the lowest t e m p e r a t u r e t o which they h a v e so far been cooled.

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