Nobel prize in physics 1996
From nobelsrv@www.nobel.ki.seWed Oct 9 13:55:07 1996
Date: Wed, 9 Oct 1996 12:45:58 +0100
From: Nobel Foundation WWW Server
The Royal Swedish Academy of Sciences
Box 50005, S-104 05 Stockholm, Sweden. Ph: +46 8 673 95 00,
fax: +46 8 15 56 70, e-mail: rsas@kansli.kva.se
The Royal Swedish Academy of Sciences has decided to award the
1996 Nobel Prize in Physics jointly to
Professor David M. Lee, Cornell University, Ithaca, New York,
USA,
Professor Douglas D. Osheroff, Stanford University, Stanford,
California, USA and
Professor Robert C. Richardson, Cornell University, Ithaca, New
York, USA
for their discovery of superfluidity in helium-3.
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A breakthrough in low-temperature physics
When the temperature sinks on a cold winter's day water vapour
becomes water and water becomes ice. These so-called phase
transitions and the changed states of matter can be roughly
described and understood with classical physics. What happens
when the temperature falls is that the random heat movement in
gases, liquids and solid bodies ceases. But the situation
becomes entirely different when the temperature sinks further
and approaches absolute zero, -273.15 C. In samples of liquid
helium what is termed superfluidity occurs, a phenomenon that
cannot be understood in terms of classical physics. When a
liquid becomes superfluid its atoms suddenly lose all their
randomness and move in a coordinated manner in each movement.
This causes the liquid to lack all inner friction: It can
overflow a cup, flow out through very small holes, and exhibits
a whole series of other non-classical effects. Fundamental
understanding of the properties of such a liquid requires an
advanced form of quantum physics, and these very cold liquids
are therefore termed quantum liquids. By studying the
properties of quantum liquids in detail and comparing these
with the predictions of quantum physics low-temperature,
researchers are contributing valuable knowledge of the bases
for describing matter at the microscopic level.
David M. Lee, Douglas D. Osheroff and Robert C. Richardson
discovered at the beginning of the 1970s, in the
low-temperature laboratory at Cornell University, that the
helium isotope helium-3 can be made superfluid at a temperature
only about two thousandths of a degree above absolute zero.
This superfluid quantum liquid differs greatly from the one
already discovered in the 1930s and studied at about two
degrees (i.e. a thousand times) higher temperature in the
normal helium isotope helium-4. The new quantum liquid helium-3
has very special characteristics. One thing these show is that
the quantum laws of microphysics sometimes directly govern the
behaviour of macroscopic bodies also.
The isotopes of helium
In nature the inert gas helium exists in two forms, isotopes,
with fundamentally different properties. Helium-4 is the
commonest while helium-3 occurs only as a very small fraction.
Helium-4 has a nucleus with two protons and two neutrons (the 4
stands for the total number of nucleons, i.e. protons and
neutrons). The nucleus is surrounded by an electron shell with
two electrons. The fact that the number of particles
constituting the atom is even makes helium-4 what is termed a
boson. The nucleus of helium-3 also has two protons, but only
one neutron. Since its electron shell also has two electrons,
helium-3 consists of an odd number of particles, which makes it
what is termed a fermion. Since the two isotopes of helium are
built up of different numbers of particles, dramatic
differences in their behaviour arise when they are cooled to
temperatures near absolute zero.
The properties of the isotopes
Bosons such as helium-4 follow Bose-Einstein statistics which,
among other things, means that under certain circumstances they
condense in the state that possesses the least energy. A phase
transition process in which this occurs is termed Bose-Einstein
condensation. The first person to manage to cool helium-4 gas
to such low temperatures that it liquidised was Heike
Kamerlingh-Onnes (Nobel Prize in Physics 1913). This happened
at the beginning of the 1900s. He noted even then that when the
temperature came closer to absolute zero than about 2 degrees
something special happened in the liquid. But it was not until
the end of the 1930s that Pjotr Kapitza (Nobel Prize in Physics
1978) discovered experimentally the phenomenon of superfluidity
in helium-4, a phenomenon first explained schematically by
Fritz London and then in detail by Lev Landau (Nobel Prize in
Physics 1962). The explanations are based on the fact that the
superfluid liquid, which appears at a phase transition when the
temperature is only 2.17 degrees above absolute zero, is a kind of
Bose-Einstein condensate of helium atoms.
Fermions such as helium-3 follow Fermi-Dirac statistics and
should not actually be condensable in the lowest energy state.
For this reason superfluidity should not be possible in
helium-3 which, like helium-4, can be liquidised at a
temperature of some degrees above absolute zero. But fermions
can in fact be condensed, but in a more complicated manner.
This was proposed in the BCS theory for superconductivity in
metals, formulated by John Bardeen, Leon Cooper and Robert
Schrieffer (Nobel Prize in Physics 1972). The theory is based
on the fact that electrons are fermions (they consist of one
particle only, an odd number) and therefore follow Fermi-Dirac
statistics just as helium-3 atoms do. But electrons in greatly
cooled metals can combine in twos to form what are termed
Cooper pairs and then behave as bosons. These pairs can undergo
Bose-Einstein condensation to form a Bose-Einstein condensate.
Starting with the experience of superfluidity in helium-4 and
superconductivity in metals, it was expected that the fermions
in liquid helium-3 should be capable of forming boson pairs and
that superfluidity should be obtainable in very cold samples of
the isotope helium-3. Although many research groups had worked
with the problem for years, particularly during the 1960s, none
had succeeded and many considered that it would never be
possible to achieve superfluidity in helium-3.
The discovery
The researchers at Cornell University were low-temperature
specialists and had built their apparatus themselves. With it
they could produce such low temperatures that the sample was
within a few thousands of a degree of absolute zero. David Lee
and Robert Richardson were the senior researchers while Douglas
Osheroff was a graduate student in the team. Actually they were
looking for a different phenomenon: A phase transition to a
kind of magnetic order in frozen helium-3 ice. To find this
phase transition, they were studying the pressure measured
within the sample as a function of the time during which the
volume was slowly increased and reduced. It was Osheroff's
vigilant eye that noted small extra jumps in the curve measured.
It is easy to consider such small deviations as more
or less inexplicable characteristics of the apparatus, but this
student and his older co-workers became convinced that it was a
true effect. In a first report published in 1972 the result was
interpreted as a phase transition in the solid helium-3 ice
which can also form at these low temperatures. But since the
interpretation did not correspond precisely with the results of
measurement, a rapid series of supplementary measurements was
undertaken and in the same year the researchers were able to
show in a second publication that there were in fact two phase
transitions in liquid helium-3. The discovery heralded the
start of intensive research on the new quantum liquid. A
particularly important contribution was made by the
theoretician Anthony Leggett, who assisted in the
interpretation of the discovery. This thus assumed great
significance for our knowledge of how the laws of quantum
physics, formulated for microscopic systems, sometimes directly
govern macroscopic systems also.
Superfluidity in helium-3
That the new liquid really was superfluid was confirmed soon
after the discovery, among others by a research team under Olli
Lounasmaa at the Helsinki University of Technology. They
measured the damping of an oscillating string placed in the
sample and found that the damping diminished by a factor of one
thousand when the surrounding liquid underwent the phase
transition to the new state. This shows that the liquid is
without inner friction (viscosity).
Later research has shown that helium-3 has at least three
different superfluid phases, of which one occurs only if the
sample is placed in a magnetic field. As a quantum liquid
helium-3 thus exhibits a considerably more complicated
structure than helium-4. It is, for example, anisotropic, which
means that it has different properties in different spatial
directions, which does not occur in classical liquids but more
resembles the properties of liquid crystals (cf. Nobel Prize in
Physics 1991 to Pierre-Gilles de Gennes).
If a superfluid liquid is caused to rotate at a speed exceeding
a critical value, microscopic vortices arise. This phenomenon,
which is also known from superfluid helium-4, has in helium-3
led to extensive research since its vortices can assume more
complicated forms. Finnish researchers have developed a
technique using optical fibres to observe directly how vortices
affect the surface of rotating helium-3 at temperatures only
one thousandth of a degree from absolute zero.
A fascinating application of superfluidity in helium-3
The phase transitions to superfluidity in helium-3 have
recently been used by two experimental research teams to test a
theory regarding how what are termed cosmic strings can be
formed in the universe. These immense hypothetical objects,
which are thought possibly to have been important for the
forming of galaxies, can have arisen as a consequence of the
rapid phase transitions believed to have taken place a fraction
of a second after the Big Bang. The research teams used
neutrino-induced nuclear reactions to heat their superfluid
helium-3 samples locally and rapidly. When these were cooled
again, balls of vortices were formed. It is these vortices that
are presumed to correspond to the cosmic strings. The result,
which must not be taken as proof of the existence of cosmic
strings in the universe, is that the theory tested appears to
be applicable to vortex formation in superfluid helium-3.
Further reading
Additional background material on the Nobel Prizes in Physics
1996, the Royal Swedish Academy of Sciences
Superfluid helium 3, by N.D. Mermin and D.M. Lee, Scientific
American, December 1976, p. 56.
Low temperature science - what remains for the physicist?, by
R.C. Richardson, Physics Today, August 1981, p. 46.
Special Issue: He3 and He4, Physics Today, February 1987,
including among other articles Novel magnetic properties of
solid helium-3, by M.C. Cross and D.D. Osheroff, p.34. The 3He
Superfluids, by O.V. Lounasmaa and G.R. Pickett, Scientific
American, June 1990.
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David M. Lee
born 1931 in Rye, NY, USA. American citizen. Doctoral degree in
physics 1959, Yale University. Lee has received among other
awards the Institute of Physics Sir Francis Simon Memorial
Prize 1976 and the Oliver E. Buckley Solid State Physics Prize
(American Physical Society) 1980 for the discovery of
superfluidity in helium-3.
Professor David M. Lee
Department of Physics
Cornell University
Ithaca, NY 14853
USA
Douglas D. Osheroff
born 1945 in Aberdeen, WA, USA. American citizen. Doctoral
degree in physics 1973 at Cornell University. Osheroff has
received among other awards the Institute of Physics Sir
Francis Simon Memorial Prize 1976 and the Oliver E. Buckley
Solid State Physics Prize (American Physical Society) 1980 for
the discovery of superfluidity in helium-3.
Professor Douglas D. Osheroff
Department of Physics
Stanford University
Stanford, CA 94305
USA
Robert C. Richardson
born 1937 in Washington, DC, USA. American citizen. Doctoral
degree in physics 1966 at Duke University. Richardson has
received among other awards the Institute of Physics Sir
Francis Simon Memorial Prize 1976 and the Oliver E. Buckley
Solid State Physics Prize (American Physical Society) 1980 for
the discovery of superfluidity in helium-3.
Professor Robert C. Richardson
Department of Physics
Cornell University
Ithaca, NY 14853
USA
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