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Picture of a strontium ruthenate Sr2RuO4 SQUID
used to demonstrate that strontium ruthenate is an odd-parity
superconductor. A thin layer of a conventional superconductor is
deposited on the front and two sides of a black Sr2RuO4 crystal. Au wires
of 25 microns in diameter are attached to the SQUID for current
measurements.
Crystal structure of strontium ruthenate
Sr2RuO4: layers of RuO6 octahedrons separated by SrO2 layers.
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In addition to their scientific interest, superconductors
have a number of practical applications. These include superconducting magnets,
which have enabled the development of high-resolution magnetic-resonance imaging
in medicine, and superconducting wires, which transport electrical power without
loss due to heating of the cable by electrical resistance.
A material becomes superconducting because electrons in the
material form pairs, known as Cooper pairs. Liu likens the pairing process to
dancers on a dance floor: "The electrons, crowded together, form pairs and move
to the 'music' of phase coherence, a quantummechanical property that
synchronizes the steps of all the dancing pairs." These pairs, described
mathematically by a quantummechanical wave function, move tightly together
despite tendencies that would force them apart.
Physicists theorize that there are two categories of
electronic state in superconductors, based on the quantummechanical
characteristics of the Cooper pairs. Although their properties vary widely,
almost all superconductors found so far belong to the same category because they
share a fundamental property, known as even-parity symmetry. "Each Cooper pair
in a superconductor can be thought of as being born with a little one-handed
internal clock that indicates the 'time', or the phase, of the pair," explains
Liu. "When the hand points to midnight the phase of the Cooper pair is zero
degrees. When the hand points to three, the phase is 90 degrees, at six it is
180 degrees. Quantum mechanics demands that the phase of two pairs moving in
opposite directions be different by exactly zero or 180 degrees." If the clocks
of two Cooper pairs moving in opposite directions have the same time, the
symmetry of the pairs is designated as even parity.
In elemental superconductors - first discovered almost 100
years ago - the two electrons in a pair tend to be close together without any
relative motion. In so-called high-temperature superconductors--materials
discovered a couple of decades ago that still are poorly understood - the
electrons in a pair tend to be farther apart, with substantial relative motion.
Although these Cooper pairs behave very differently and the superconductors
exhibit rather different features, they share the same property of even-parity
symmetry.
On the other hand, if the clocks for two pairs moving in the
opposite directions are six hours apart--a phase difference of 180 degrees--the
symmetry of the Cooper pairs is designated as odd-parity symmetry. These
odd-parity Cooper pairs form a new electronic state in superconductors. "The
pairing symmetry is important because it dictates many physical properties of a
superconductor. An odd-parity superconductor behaves very differently from an
even-parity superconductor," says Liu. The article to appear in Science, "Odd-Parity
Superconductivity in Sr2RuO4," confirms unambiguously that strontium ruthenate,
Sr2RuO4, which is the only known superconducting ruthenium oxide material, is a
member of this category of odd-parity superconductors.
Although other experiments have indicated that odd-parity
pairing was involved, Liu's experiment provides the first definitive proof of
this new type of pairing. "Theorists had predicted that superconductivity in
strontium ruthenate could be associated with odd-parity pairing," says Liu "Earlier
experiments did provide plenty of evidence to support the prediction, but those
results also could be questioned by counter examples and attributed to something
else. Our experiment is a 'yes-or-no' test of the odd-parity pairing that
settles the issue."
The basic idea of the experiment is to measure the dependence
of the phase of the Cooper-pair wave function on the direction in which the
Cooper pair moves, using the phenomenon of wave interference. "Essentially, we
want to compare the clocks of the strontium ruthenate Cooper pairs moving in the
opposite directions. We connected a strontium ruthenate superconductor to an
even-parity, conventional superconductor through two parallel surfaces that are
oppositely faced, forming two so-called Josephson junctions. This procedure
makes a superconducting quantum-interference device, known as a SQUID. The
clocks of the strontium ruthenate pairs moving into the conventional
superconductor through the two junctions are then six hours apart, or 180
degrees different in phase. The Cooper-pair waves from the two junctions will
then interfere destructively," says Liu. This interference pattern was detected
by measuring a current going through the SQUID as a function of an applied
magnetic field. By confirming through the interference patterns that the
oppositely moving Cooper pairs naturally position themselves in their respective
time zones six hours apart - a 180 degree phase difference - Liu's team
demonstrated that strontium ruthenate does exhibit an odd-parity symmetry.
The discovery is of interest to physicists because it breaks
new scientific ground that also could have useful applications. "In nature,
particles can be paired in specific ways depending on the interactions that
create the attractive force," says Liu. "Odd-parity pairing has been found to
exist in unusual systems ranging from small and cold - such as atoms of helium-3
at very low temperatures, a couple of thousandths of a degree above absolute
zero, to large and hot - such as neutrons in neutron stars at hundreds of
millions of degrees."
The phenomenon of odd-parity superconductivity in strontium
ruthenates occurs only below a temperature of about one and a half degrees above
absolute zero, well below room temperature. However, Liu points out that now
that the odd-parity superconductor has been shown to exist, the unique features
of this type of superconductor can be studied for potential practical
applications. In addition to possible expansion of current superconductor uses,
odd-parity superconductors someday may be used for special purposes; for
instance, in the research effort to develop quantum computers. |
