Introduction

1 . 1 Superconductivity

A superconductor exhibits two characteristic properties, namely zero electrical resistance and perfect diamagnetism when cooled below a critical temperature, Tc. Above this temperature it behaves as a conventional metal, is said to be in the normal state and ordinarily is not an exceptional conductor. For example, tin and lead become superconducting upon cooling whilst copper, silver and gold, which are

excellent room-temperature conductors, do not.

1.1.1 History o f superconductivity

Superconductivity was discovered in 1911 when Kammerlingh Onnes observed the d.c. resistance o f mercury dropping to zero at 4.15K^^^ (Fig. 1.1). The following year Onnes discovered that by applying a strong magnetic field the resistance could be restored to its normal value and the superconductivity destroyed. For more than half a century research focused on elemental and binary metal alloys and a record Tc for this group o f materials was reached in 1973 with NbsGe (Tc = 23K).

In 1986 a brief article entitled ‘Possible High Tc Superconductivity in the Ba-La-C u-O System’ written by Bednorz and Mullo^^ was met with much scepticism. However the results were reproduced by groups in Japan and the USA within a week and the birth o f high Tc superconductivity was taken seriously.

Soon after this discovery, a Tc o f 93K was achieved in the copper oxide material YBaiCusO?^^^ and in 1988 the record Tc was increased further to 125K by the thallium

based cuprate material TbBazCazCusOio^^l The current record I t stands at I56K in

HgBazCazCusOgfS under pressure'll

Q^ers

0,00

Fig. 1.1 The discover)' of superconductivity in mercur) by Kammerlingh Onnes in 1911.

Superconductivity in an organic solid was first achieved in 1980 by Klaus Bechgaard

and Denis Jerome in the charge-transfer salt (TM TSF)2PF6 with a Tc o f 0.9K at

llkbad^l Shortly afterwards (TMTSF)2C104'^ became the first organic

superconductor at ambient pressure with a Tc o f 1.3K.

Many new organic superconducting charge-transfer salts were prepared through the

1980’s from organic donors with complex inorganic anions o f different sizes, shapes,

charges and magnetism. This has led to many new ambient pressure organic

Introduction

tetramethyltetrasele^nafiilvalene, or BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene. The highest ambient pressure T« for charge-transfer salts to date is 11.8K for

K-{BEDT-TTF)2Cu[N(CN)2]Br^*^, whilst under an applied pressure o f 0.3kbar the

charge-transfer salt k-(BEDT-TTF)2Cu[N(CN)2]CF^^ has a Tc o f 12.8K.

These compounds were, principally synthesized in a search for the superconducting ground state but they have also yielded many other interesting states; metals which are not superconducting, spin-Pierls systems and molecular solids which undergo transitions between the insulating, metallic and superconducting states as a function o f temperature and pressure. These compounds have therefore provided a rich source o f physical behaviours to be investigated.

In 1991 the fullerene salt K3C60 was found to be a superconductor with a Tc o f 18K,

which led to preparation o f various fullerides o f the general formula where

the metal atoms were combinations o f potassium, rubidium or cesium. The highest Tc for this group is 33K for Cs2RbC6o^'^'^

For both the cuprate and organic superconductors new compounds with higher T c’ s were discovered in rapid succession but in both cases progress has abruptly stopped. Whether these groups o f compounds can give still higher values o f Tc remains to be seen.

1.1.2 Electrical properties of superconductors

In the metallic state resistance is caused by the random scattering o f conduction electrons as they pass through the lattice. As a metal is cooled, lattice vibrations are reduced so the scattering, and therefore resistance, is reduced to a limiting value. On cooling a metal it may undergo a transition to the superconducting state once its temperature is below a critical temperature, Tc where the current is carried by Cooper

pairs o f electrons which conserve their total momentum in the scattering process^’^l

This perfect conduction o f superconductors can be exploited by passing large

currents through a superconducting sample and generating a large magnetic field

ow ing to the ‘zero resistance’ o f the sample

1.1.3 Magnetic properties of superconductors

The second characteristic property o f superconductors is perfect diamagnetism,

whereby an externally applied field is unable to penetrate the interior o f a sample.

Superconductors expel magnetic flux when in the superconducting state so in an

external magnetic field they behave as if B is zero inside the superconductor so H =

-TttM, and thus the magnetisation is equal and opposite to the field strength o f the

external field Diamagnetism may be exploited for use in frictionless bearings,

s q u i d’s or levitation (for example in levitating trains with magnetic tracks and

superconducting material on the base o f the train).

All superconductors so far investigated fall into one o f tw o categories depending on

their behaviour in a magnetic field Type I superconductors behave as perfect

Applied Field

diamagnets up to a critical field, At fields less than He they are superconducting

and expel all magnetic flux, at He and above the superconducting state is destroyed

(Fig 1.2).