The slow and sometimes very intermittent
progress of research during the last century, nevertheless, brought
a number of advances in the fundamental knowledge of the superconductor
phenomenon and its unique properties. It was in 1933 that the Germans
Walter Meissner and Robert Ochsenfeld demonstrated the surprising
diamagnetic effect (now known as the Meissner effect) by which a
current passing through a superconductor exercises a strong repelling
force on an external magnetic field. This property causes a magnet
brought close to a material in a superconductive state to levitate
spectacularly. The Meissner effect has consequently become the best
way of testing whether a material superconducts or not. It is also
the principle behind the current development of prototypes of the
first Maglev (Magnetic Levitation) trains.
Scientists have been carrying out a wide range of full-scale experiments.
These include attempts to discover all the elements that could become
superconductors, identify their critical performances and investigate
possible applications for them. Scientists have so far experimented
on a series of metals and natural metalloids (numbering about 30
at ambient pressure), alloys and combinations of materials of all
Since the 1940s, a number of alloys have been developed –
in particular the family of niobium nitrides and niobium titanium
– showing excellent superconductive properties, at critical
temperatures between 15 and 22 K, while attaining higher critical
magnetic fields. But it was not until 1962 that the US firm Westinghouse
first developed long lengths of superconducting electric wire produced
from NbTi alloys on an industrial scale.
The super magnets
Meanwhile, liquid helium cryogenic technologies had made considerable
progress and it became possible to use superconducting coils developed
at temperatures close to absolute zero. This made it possible to
pass currents of unprecedented intensities, in the region of a million
amperes per square cm, which in turn generated uniquely intense
It is in the field of these ‘super’ magnets that most
of today's superconductive applications have been perfected. In
particular, it has given rise to a boom in medical diagnosis technologies
employing magnetic resonance imaging (MRI). These devices are now
widely used in hospitals and represent a global market of almost
€2.5 billion annually.
Other high-tech applications in the field of high magnetic fields
are also of interest to the scientific world. These include the
particle accelerators used at the European Organisation for Nuclear
Research, CERN, the experimental nuclear fusion installations at
the Wendelstein Stellarator in Germany, and the various spectrometric
technologies used in the molecular study of materials or living
The Josephson junction
Another very interesting field of application for superconductivity
was opened up in 1962 when a brilliant young doctorate student at
Cambridge University, Brian Josephson, demonstrated the existence
and very particular characteristics of a 'tunnel’ effect that
can be produced between two superconducting materials separated
by a thin insulating layer. In such an interference device (also
known as the Josephson junction), the flow of electrons –
even in the absence of any external voltage – can pass from
one material to another, crossing the insulating barrier. This provides
the capacity to detect even the weakest magnetic field.
This discovery has since come to acquire considerable importance.
It is at the core of a revolutionary technique known as SQUID (Superconducting
Quantum Interference Device). SQUID works by placing in parallel
two Josephson junctions connected in a loop by a superconducting
link. It was discovered that even a very weak magnetic field, known
as a magnetic quantum, produced at the centre of the superconducting
loop causes the latter to generate an electrical signal.
SQUID is the most 'sensitive' magnetic detection system available
to scientists today. The technique is giving rise to highly advanced
applications in medicine, particularly in the fields of neurological
imaging by magneto-encephalography and cardiology. Important applications
are also emerging in the field of metrology and the physico-chemical
The chips of the future
The unique properties of Josephson junctions and the phenomena
of quantum interference devices have also opened the door to a potential
revolution in superconductive computing. They were used in designing
what is known as the Single Flux Quantum (SFQ) device, which acts
like a new generation of switches for binary logic with a reaction
time in the area of one picosecond.
The absence of resistance in a superconductor circuit minimises
energy loss, which is currently one of the major obstacles to boosting
computer performance. Superconductive electronics would, therefore,
make it possible to produce processors 500 times faster than present
semi-conductor components, while producing 200 times less heat.
A whole new ballgame
However, even if prototypes of these chips of the future have already
been developed, the first generation of superconductive supercomputers
has, to date, remained very much a promise rather than a reality.
In the early 1980s, superconductivity seemed to have reached the
limit of its potential as its applications were limited by the need
to work at temperatures approaching absolute zero. The record, obtained
from a niobium germanate alloy, was 23 K.
The cost and technological complexity of helium cryogenics closed
the door to many 'dream' applications, whether in the field of electrotechnics
or computing. It was against this backdrop that, in 1986, came a
major breakthrough: the high temperature superconductors, which
suddenly opened up a whole new world of possibilities.