One hundred years of superconductors

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15 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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One hundred years

of s
uperconduct
ors


This

week

marks the centenary

of the discovery of
supe
rconductivity
,
one

of the
great puzzles
of
modern science
. A superconductor is a material that, under certain circumstances,
behaves like a
perfect conductor of electricity i.e.
offer
s

no resistance
what
ever
to the passage of electric current

through it
.
Such materials are of imm
ense interest to

society a
s they
hold out the p
romise of extremely
cheap electricity

(
a material with zero resistance can
conduct electr
icity indefin
itely, even after

its
power
supply

is removed
)
. H
owever,
progress in superconductor sci
ence has been frustrating
ly slow,

despite a

number of

breakthrough
s

during the 20
th

century
.

The phenomenon
of

superconductivity was first observed in
April
1911
by the Dutch physicist
Heike Kamerlingh Onnes . On
investigating how

different materials

behave when cooled to

extremely
low
temperatures,
Onnes

observe
d

that the electrical
resistance
of
the element

mercury

disappeared

completely
at a temperature of four degrees above absolute zero

(4 Kelvin or
-

269
°

Celcius)
.
This
was a
great surprise
, but

the
discovery did not seem to be of

immediate practical

value
as

work
ing
at

such
temperature
s
presented great technical challenges
.

Indeed
, Onnes
was awarded the

1913
Nobel Prize in
physics for
his

work

in
low
-
temperature physics
.




It soon transpired that some

ot
her

element
s could become

superconducting at
very
lo
w
temperature
s
,

but no
-
one had an

e
xplanation for the effect. Th
e puzzle

deepened

in 1933
when the

German researchers Walther Meissner and Ro
bert O
chsenfeld discovered that superconductors had
other

bizarre

properties
;

a

material

in superconducting state

could
expel

any
magnetic field

within the
material,
and

even
repulse

a
n external
magnetic field

brought into the

vicinity
. This effect
, now
known
as

t
he Meissner effect
,
i
s

so s
trong
that i
t can

cause
a magne
t to

levitate

above

a

superconductor
.

By the
end of the
1950s, it had been discovered that many metal alloys become

superconducting if cooled to low
enough
temperature
. More importantly, a

theoretical explanation
had
finally
emerged
. T
he American physicists John Bardeen, Leon
Co
oper

and John Schrieffer
provided a
beautiful

explanation
for
the effect in terms
of quantum ph
ysics (now known as B
C
S

theory)
;
they were
later awarded a
Nobel P
rize in physics

for this work
.


By the 1980s,

numerous
applications of
superconductivity

exis
te
d
, from
sophisticated
devices for measuring minute

magnetic fields to giant

electro
magnets that consume

almost no

power. (The latter proved to be

ideal
for

the hug
e

magnetic
fields required by
hi
gh
-
energy
particle

accelerator
s
)
.

And then, a
long came
superconductivity

mark

II
.
I
n 1986,
Alex Müller and Georg Bednorz, two
researchers at a
n IBM research laboratory in
Switzerland, created a

ceramic
compound that
became
superconducting
at the
relatively high temperature of
30 K
elvin
.
This was a completely
unexpected

breakthrough
,

and r
esearch
ers around the world began cooking

up ceramics of every imaginable
combination in a quest for
materials that w
ould superco
nduct at even

higher temperatures. B
y
January
1987
,

a research team at the Un
iversity of Alabama

had synthesised a material that exhibited

zero
electrical
resistance at 92 K
.
This
was an important milestone as it was a temperature
that
was
easily
achievable in

the

laboratory
,

and
the pro
sp
ect of widespread application of

superconductor technology

seem
ed within reach
. B
y th
e year 2000, materials that beco
me superconducting at temperatures over
100K had been discovered.



However,
these

exciting advances

in experiment were

not matched by advances in theory
. I
t

soon
became apparent
that

good old
BCS theory could not account for the
new class of
superconductor
s. Indeed,
a

problem
of fundamental importance
had
emerged
;
b
ecause
superconductivity
involved

the co
-
operative

behaviour
of a vast number of atoms
, it constituted

a
complex system

t
hat

c
ould
not

be modelled
simply
from a knowledge of the
behaviour of individual
atoms
.
This discovery
, that
certain
phenomena
in nature
are too complex to

be
successfully explained

from
the bottom up
, was a new challenge
to

science
; such
phenomena

are now known as

emerg
ent’
.

What is the state of play
with supercondu
c
tivity today
?

In the absence of
an overarching theory,
empirical
work
has
continued in a hit
-
and
-
miss manner

quite unusual in modern science
.
The current
world record i
s a compound

that becomes superconducting when c
ooled to a temperature of 138 K
.
S
uperconduc
t
or technology has found
important application
in society, from the

superconducting
magnets of

MRI scanners
in hospitals to the famous

‘MAGLEV’ levitating
trains
in
Japan. How
ever, the
holy grail of this field, a materi
al that

exhibit
s

superconductivity
at r
oom temperature

-

and
hence
can
deliver electricity in a manner th
at is both cheap and convenient
-

remains as
tantalisingly
elus
i
ve as
ever.


Dr Cormac O’Raifeartaigh

lectures in physics at Waterford Ins
titute of Technology
and is the author of
the science blog ANTIMATTER. He is
currently a

research f
ellow with

the
Science, Technology and Society
Group at the
Kennedy School of Government of Harvard University.



Fig Ca
ption

The M
eissner effect:

a magnet levitates over a superconductor