The symbols in the CSC Logo refer to several aspects
of superconductivity whose scientific and technological
importance is such that most have been recognized
with the award of Nobel Prizes in physics.
The
relationships are described in the next section. The
relevance of the CSC Logo to other developments in
superconductivity and superfluidity that are not directly
represented in the Logo will also be discussed.
In
the last section, all the Nobel Prizes in Physics
associated with superconductivity and superfluidity
are listed in chronological order, since the developments
and the properties of superconductors and superfluids
are intimately related.
At
the very beginning, Heike Kamerlingh Onnes discovered
superconductivity while working on liquid helium.
Afterwards, Pyotr Leonidovich Kapitsa discovered superfluidity
in 4He. Then, the concept of a ground state with an
energy gap composed of even spin particles was developed
by Lev Davidovich Landau to describe the properties
of superfluid 4He. At the same time, theorists were
trying to extend these ideas to odd spin particles
or Fermions. By considering phonon interaction of
a pair of electrons with opposite spins and with equal
and opposite momenta, Cooper was able to postulate
pairs of electrons with zero net spin that in a many
body formalism could undergo a Bose-Einstein condensation,
with an energy gap. When superfluid 3He was discovered,
theorists tried to formulate a theory invoking Bose-Einstein
condensation of these nuclei with odd spins. They
succeeded by considering pairs of nuclei with a net
spin of 1 and an angular orbital momentum of 1. Since
then, theorists have successfully extended these concepts
in attempts to explain the properties of heavy Fermion
superconductors.
The
list also includes the Meissner effect. Although this
effect is a cornerstone of superconductivity, its
discoverer did not receive a Nobel Prize.
Finally,
it should be noted that the quotations included in
the description of the Nobel Prizes given below are
taken from the Nobel Prize Announcements, presentations,
biographies and Nobel Laureate lectures.
CSC
Logo Symbolism
The CSC Logo depicts phenomena associated with three
Nobel
Prizes in Superconductivity and with the Meissner
Effect.
- The
red arrow on the right represents the persistent
electric current resulting from the zero resistance
of a superconductor. Heike Kamerlingh Onnes received
the Nobel Prize in Physics in 1913 for his studies
at liquid helium temperatures which included the
discovery of superconductivity.
- The
four golden horizontal lines symbolize the exclusion
of magnetic field lines from the interior of a superconductor.
This is known as the Meissner Effect, which was
discovered by Fritz Meissner and Robert Ochsenfeld
in 1933. It is associated with a perfectly diamagnetic
state in a Type I superconductor, and is evidence
that superconductivity is a thermodynamic phase
independent of the path chosen to reach it.
- The
two opposing black arrows, one with a yellow dot
and the other with a yellow "x" at the
center represent electrons with opposite spins but
with equal and opposite momenta. These "paired
electrons" are known as Cooper pairs. The many
body properties of these Cooper pairs led to the
development of a microscopic theory of superconductivity
by John Bardeen, Leon Cooper and J. Robert Schrieffer,
who received the Nobel Prize in Physics in 1972
for their theory, which is commonly known as the
"BCS Theory."
- The
green "X" on the left of the diagram is
the symbol for a Josephson tunneling junction in
electric circuits. Brian David Josephson received
the Nobel Prize in Physics for his theoretical prediction
that, under certain conditions, a supercurrent could
flow through a tunnel barrier when no voltage is
applied.
As
mentioned in the introduction, the logo is relevant
to other superconducting effects that are not directly
illustrated in the Logo. The simplest connection is
to single quasi-particle tunneling into a superconductor,
discovered by Ivar Giaever (Nobel Prize 1973). If
sufficient current is passed through a Josephson junction
or a superconducting tunnel junction, a voltage will
appear across the tunnel junction and the current
vs. voltage curve can be used to measure the superconducting
energy gap.
High
temperature ceramic oxide superconductors were synthesized
in 1986 by J. Georg Bednorz and K. Alexander Müller
(Nobel Prize 1987). Superconductivity was confirmed
in these oxides when the effects described in the
CSC Logo, specifically zero resistance, the Meissner
Effect and tunneling of both paired and single quasi
particles, were established.
Magnetization
curves similar to those found for Type II superconductors,
whose magnetization curves are fully diamagnetic up
to a lower critical field, Hc1, whereupon magnetic
field lines penetrate the superconductor up to an
upper critical field, Hc2, where superconductivity
disappears, (see 2003 Nobel Prize), but which were
irreversible had been obtained for superconducting
materials that were inhomogeneous, and therefore,
were assumed not to be thermodynamic phases. It was
not until Alexei A. Abrikosov developed his theory
for Type II superconductors, that the possibility
was recognized that these would display reversible
magnetization curves that were not fully diamagnetic.
Once the reversible character of their magnetization
curves was established, Type II superconductors satisfied
the principal conditions implied by the CSC Logo.
Namely, superconductivity is a reversible thermodynamic
phase whose properties do not depend on the path taken
to achieve the superconducting state.
Nobel
Laureates
1913
Nobel
Prize
In 1913, Heike Kamerlingh Onnes received the Nobel
Prize in Physics "for his investigations on the
properties of matter at low temperatures, which led,
inter alia, to the production of liquid He4",
and the discovery of superconductivity.
In
1908, Kamerlingh Onnes successfully liquefied helium.
This allowed him to investigate the thermodynamics
properties of helium in the liquid and gas phase.
He also investigated the electrical properties of
metals down to about 1K. In 1911, he observed the
transition to a "zero resistance state"
in a pure mercury sample as the temperature of the
sample was lowered to below 4.2K. He labeled this
"zero resistance state" as "superconductivity."
1962 Nobel
Prize
In 1962, Lev Davidovich Landau received the Nobel
Prize in Physics "for his pioneering theories
for condensed matter, specially liquid helium."
In
1937, Landau proposed a theory of phase transitions,
in which he introduced a main variable called the
"order parameter", which was finite below
the transition and zero above it. This theory has
been applied to ferromagnetic, superfluid and superconducting
transitions, among others. Different phase transitions
have different order parameters. For superfluid helium
(see 1978 Nobel Prize), Landau started by considering
the state of the fluid at absolute zero as the ground
state. The excited states were the quasi particles
(see 1972 Nobel Prize), whose dispersion relations
were determined in 1957 by neutron scattering measurements
at the Atomic Energy Ltd. in Stockholm.
1972
Nobel
Prize
In 1972, John Bardeen, Leon N. Cooper and J. Robert
Schrieffer received the Nobel Prize in Physics "for
the jointly developed theory of superconductivity,
usually called the BCS theory."
In 1957, Bardeen, Cooper, and Schrieffer developed
a microscopic theory of superconductivity in which
pairs of electrons with anti-parallel spins and opposite
momenta, known as Cooper pairs, are attracted to each
other via the exchange of lattice vibrations, phonons.
When the energy of the many body system comprising
these Cooper Pairs is computed, they found that a
temperature dependent energy gap is produced at the
Fermi surface of a material that becomes superconducting.
Since Cooper pairs have a spatial extent of 10-4 cm.,
they cannot be considered non-interacting zero spin
particles, Bosons. Therefore, a quasi particle formalism
proposed independently by both N. N. Bogoliubov and
by J. G. Valatin is used to describe both the electromagnetic
and thermodynamic properties of superconductors. In
this treatment, excited quasi particles are principally
responsible for superconducting thermodynamic properties
and their temperature dependences, while quasi particle
pairs in the ground state do not interact with the
lattice and are principally responsible for the superconducting
transport properties
1973
Nobel
Prize
In 1973, Leo Esaki, and Ivar Giaever received one
quarter each of the Nobel Prize in Physics "for
their experimental discoveries regarding tunneling
phenomena in semiconductors and superconductors, respectively";
and, Brian David Josephson received one half of the
Nobel Prize in Physics "for his theoretical predictions
of the properties of a supercurrent through a tunnel
barrier, in particular those phenomena which are generally
known as the Josephson Effects.
In
1958, Esaki "performed some deceptively simple
experiments, which gave convincing evidence for tunneling
of electrons in solids." These measurements could
be used to determine the energy gap of semiconductors
and to scrutinize their electronic states; and, also,
to investigate the interaction of tunneling electrons
with phonons, photons, plasmons and vibrational modes
of the molecular species in the tunneling barriers.
These studies led to development of the tunnel diode
or the Esaki Diode.
In
1960, Giaever studied the tunneling of electrons,
quasi particle excitations, through a "thin sandwich
of evaporated metal films insulated by the natural
oxide of the film first evaporated", where one
or both of the metallic films were superconducting.
Thus, he was able to measure the superconducting energy
gap from the current versus voltage curves across
these tunneling junctions, and also, the density of
the phonon states in the superconductor by taking
the derivative of these curves.
In
1962, Josephson developed a theory that predicted
that under certain experimental conditions, ground
state quasi particle pairs, Cooper Pairs, could tunnel
through a barrier even when no voltage is applied.
This is known as the DC Josephson Effect. Josephson
also proposed that, if a voltage could be applied
across the junction, an alternating current would
flow across the junction with the ratio of frequency
to applied voltage being a multiple of the quantity
h/2e, which is called the magnetic flux quantum, where
h is Planck's constant and e is the charge of the
electron. This is called the "AC Josephson Effect",
which has been used to define the International Voltage
Standard
1978
Nobel
Prize
In 1978, Pyotr Leonidovich Kapitsa received one half
of the Nobel Prize in Physics "for his basic
inventions and discoveries in the area of low temperature
physics," which included the discovery of superfluidity
in He. The other half of the Nobel Prize in 1978 was
awarded to Arno Allen Penzias and Robert Woodrow Wilson
"for their discovery of cosmic microwave background
radiation."
Starting
in 1921, while working with Ernest Rutherford at the
Cavendish Laboratory, Cambridge University, Kapitsa
"made the first experiment in which a cloud chamber
was placed in a strong magnetic field and observed
the bending of alpha-particle paths." In 1924,
he developed methods for producing magnetic fields
as high as half a Megagauss, which were not surpassed
in strength until 1956, which he used to discover
the linear dependence of the resistivity of various
metals in strong magnetic fields.
In
1932, the Royal Society Mond Laboratory was created
specially for Kapitsa. By 1934, he had developed there
"an ingenious (adiabatic expansion) device for
liquefying helium in large quantities - a pre-requisite
for the great progress made in low temperature physics.."
Kapitsa
returned to Russia in 1934 and was detained there.
He was made director of the Institute of Physical
Problems of the Soviet Academy of Sciences in Moscow.
Through the intercession of Rutherford, Kapitza was
permitted to have the equipment from the Mond Laboratory
shipped to Moscow. While investigating the heat conduction
properties of helium, he discovered "the superfluidity
of (liquid) helium, implying that the internal friction
(viscosity) of the fluid disappears below 2.2K (the
so called lambda-point of helium)..Kapitsa stands
out as one of the greatest experimentalists of our
time (1978), in his domain the uncontested pioneer,
leader and master."
1987 Nobel
Prize
In 1987, J. Georg Bednorz and K. Alexander Müller
received the Nobel Prize in Physics "for their
important breakthrough in the discovery of superconductivity
in ceramic materials."
In
1986, Bednorz and Müller synthesized a ceramic
material consisting of lanthanum-barium-copper-oxide
in carefully determined ratios which underwent an
abrupt transition to "zero" resistance at
a temperature near 35K, which was subsequently shown
to exhibit all the properties of a superconductor
- including the Meissner Effect. This transition temperature
was about 50% higher than the then highest known value
of 23K found in a Nb3Ge film that was first synthesized
by John Gavaler in 1973.
The
Bednorz and Müller collaboration had started
in 1983 when they initiated a program to search for
superconductivity in oxides. They began by investigating
oxide systems which exhibited a large Jahn-Teller
effect, La-Ni-O, then replacing some Ni by aluminum
and later by copper. Although these oxides showed
promising behavior they did not exhibit superconductivity.
In late 1985 stimulated by the work of French scientists
on the catalytic properties of Ba-La-Cu oxides with
a Perovskite structure which exhibited metallic properties,
they synthesized a series of solid state solutions
- varying the Ba/La ratio and then measuring their
temperature dependent resistance down to liquid helium
temperatures.
By
this careful work imbued with innate insight, they
finally managed to prepare the superconducting ceramic
mentioned above with a Tc of 35K. Stimulated by their
published results of superconductivity at 35K, scientists
world wide began
investigating these and other oxide systems which
soon achieved Tc's exceeding 100K.
1996
Nobel
Prize
In 1996, David M. Lee, Douglas D. Osheroff and Robert
C. Richardson received the Nobel Prize in Physics
"for their discovery of superfluidity in helium-3."
In
1972, Lee, Osheroff and Richardson discovered superfluid
3He. Both 3He and superconductors are composed of
Fermi particles. The ground state of both may be described
as a Bose-Einstein condensed pair. The bound Cooper
pairs of anti parallel spins in a superconductor have
a net spin of zero. Lee, Osheroff and Richardson were
able to show that the ground state of the paired 3He
nuclei exhibits a net spin of 1 and, in addition,
the pair has an orbital angular momentum of 1.
2003
Nobel
Prize
In 2003, Alexei A. Abrikosov, Vitaly L. Ginsburg and
Anthony J. Leggett received the Nobel Prize in Physics
"for pioneering contributions to the theory of
superconductors and superfluids.
In 1950, Ginzburg and Lev Landau published a phenomelogical
theory for superconductivity, wherein the order parameter
introduced by Landau to describe phase transitions
is identified as a scalar complex wave function.
According
to this theory, the properties of superconductors
depend on a dimensionless material constant - now
known as the Ginzburg-Landau constant, ?, which is
proportional to the ratio of the London penetration
depth ? to the coherence length ?, ?/?. Here, ? is
the distance that a magnetic field penetrates a superconductor,
and ? is a length within which the order parameter
cannot change appreciably. If ? < , the surface
energy of the interface between a normal material
and its superconducting phase is positive; and, if
? > , then it is negative.
In
1954, Alexei Abrikosov studied the magnetic properties
of superconducting films in these two limits, and
came to the conclusion that there existed two types
of superconductors, Type I with positive surface energy
and Type II with negative surface energy. In 1957,
Abrikosov investigated theoretically the properties
of bulk Type II superconductors. He found that in
a Type II superconductor the transition to the normal
state in a magnetic field is a second order phase
transition, as opposed to a Type I superconductor,
which is perfectly diamagnetic in the superconducting
state and where the magnetic transition is first order.
The threshold or critical value of the magnetic field
that will destroy superconductivity is called Hc in
a Type I superconductor.
In
a Type II superconductor, the magnetic field starts
to penetrate a bulk superconductor at a lower critical
field, Hc1, which is proportional to Hc/?. Superconductivity
disappears at the upper critical field, Hc2, which
is proportional to ?Hc. Superconductivity and magnetism
coexist in the mixed phase between Hc1 and Hc2. The
magnetic field penetrates the superconductor in the
form of flux quanta first introduced by Fritz London
in 1950.
Initially,
Abrikosov postulated that the flux lines in a superconductor
arrange themselves in a square lattice. It was later
determined that a lower energy state is achieved if
they are ordered in a triangular lattice. Most of
the superconductors discovered or synthesized since
1960 have been Type II superconductors. The work of
Abrikosov contributed significantly to the study of
these novel superconducting materials.
In
1972, Leggett proposed a theory for superfluid 3He
which "succeeded in explaining the relationship
between its properties and the many types of order
associated with an order parameter that has eighteen
components" because it is composed both of spin
angular momentum and orbital angular momentum. "His
theory helped experimentalist to interpret their measurements
and provided a framework for systematic investigations.
Leggett's theory has also been used in other fields,
as divers as liquid crystal physics and cosmology."
Meissner
Effect
1933
Meissner
Effect
In 1933, Fritz Walther Meissner and Robert Ochsenfeld
discovered that when a superconductor is cooled below
its superconducting temperature in a magnetic field,
the magnetic field lines are expelled from the interior
of the superconductor. Furthermore, it was found that
this effect is reversible. In the absence of a magnetic
field, the superconducting transition is a second
order thermodynamic phase transition, but, in the
presence of a magnetic field, it is a first order
transition.
By
1957, superconductors exhibiting this type of diamagnetic
behavior became known as type I superconductors. The
expulsion of the magnetic field in the superconducting
state is known as the Meissner Effect. The Meissner
Effect is a unique property of superconductivity,
which distinguishes superconductors from perfect conductors
and demonstrated that superconductivity is a reversible
thermodynamic state. Although the Meissner Effect
is one of the cornerstones of superconductivity, it
was never recognized by the Nobel Prize Committee.
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