1.Superconductors, materials that have no resistance to the flow of electricity, are
one of the last great frontiers of scientific discovery.
2.In 1911
superconductivity was first observed in mercury by Dutch physicist Heike
Kamerlingh Onnes of Leiden University.
3.The Type 1 category of
superconductors is mainly comprised of metals and metalloids that show some conductivity
at room temperature.
4.Type 1 superconductors
- characterized as the "soft" superconductors - were discovered first
and require the coldest temperatures to become superconductive.
5.Copper, silver and
gold, three of the best metallic conductors, do not rank among the superconductive elements.
6.Many additional
elements can be coaxed into a superconductive state with the application of high pressure.
7.The periodic table
below for all known elemental superconductors (including Niobium, Technetium
and Vanadium which are technically Type 2).
8.When he cooled it to
the temperature of liquid helium, 4 degrees Kelvin (-452F, -269C), its resistance suddenly disappeared.
9.In a superconductor
the induced currents exactly mirror the field that would have otherwise
penetrated the superconducting material - causing the magnet to be repulsed.
10.This
phenomenon is known as strong diamagnetism and is today often referred to as
the "Meissner effect"
11.In
1941 niobium-nitride was found to superconduct at 16 K.
12.In
1953 vanadium-silicon displayed superconductive properties at 17.5 K.
13.in
1962 scientists at Westinghouse developed the first commercial superconducting
wire, an alloy of niobium and titanium (NbTi).
14.High-energy,
particle-accelerator electromagnets made of copper-clad niobium-titanium were
then developed in the 1960s
15.first
employed in a superconducting accelerator at the Fermilab Tevatron in the US in 1987.
16.Theories ofSuperconductivity became know as the BCS theory
17.BCS
stands for Bardeen, Cooper, Schrieffer who won Nobel prize.
18.electrical
current would flow between 2 superconducting materials - even when they are
separated by a non-superconductor or insulator.
19.Brian
D. Josephson (above), a graduate student at Cambridge University, predicted that
20.electrical
current would flow between 2 superconducting materials - even when they are
separated by a non-superconductor or insulator.
21.The 1980's were a decade of unrivaled discovery in the field of
superconductivity.
22.In 1964 Bill Little of Stanford University had suggested the
possibility of organic (carbon-based) superconductors.
23.Magnetic
Resonance Imaging (MRI) was actually discovered in the mid 1940's.
24.The
first company to capitalize on high-temperature superconductors was Illinois
Superconductor, formed in 1989.
25.As if
ceramic superconductors were not strange enough, even more mysterious superconducting
systems have been discovered. One is based on compounds centered around the "Fullerene".
28.In
April of 2001, Chinese researchers at Hong Kong University found 1-dimensional
superconductivity in single-walled carbon
nanotubes at around 15
Kelvin.
29.In
February 2006, Physicists in Japan showed non-aligned, multi-walled carbon
nanotubes were superconductive at temperatures as high as 12 K.
30.Silicon-based
fullerides like Na2Ba6Si46 will also
superconduct.
31.However,
they are structured as infinite networks, rather than discrete molecules.
32.Organic
superconductors are composed of an electron donor and an electron acceptor
46."Organic" superconductors are part of the organic conductor family which
includes: molecular salts, polymers and pure carbon systems (including carbon
nanotubes and C60 compounds).
47."Borocarbides"
are one of the least-understood superconductor
systems of all.
48.It has always been assumed
that superconductors cannot be formed from ferromagnetic transition metals -
like iron, cobalt or nickel.
49.To date only one
superconductor has been found that has zero resistance at a single temperature
- the opposite of reentrant superconductivity.
51."Ruthenates" shortly after that SrRuO and SrYRuO6 were also found
to superconduct at similarly low temperatures.
52.Fluoroargentates bear
a strong similarity to oxocuprates, compounds that currently have the highest
transition temperatures of all known superconductors.
Anneal: To heat and
then slowly cool a material to reduce brittleness. Annealing of ceramic
superconductors usually follows sintering and is done in an oxygen-rich atmosphere
to restore oxygen lost during calcination. The oxygen content of a ceramic
superconductor is critical. For example, YBCO with 6.4 atoms
of oxygen will not superconduct. But YBCO with 6.5 atoms will.
Click here to see a graphical representation of this.
Anti-ferromagnetism: A state of matter where adjacent ions in a material are aligned
in opposite or "anti-parallel" arrays. Such materials display almost
no response to an external magnetic field at low temperatures and only a weak
attaction at higher temperatures. There is evidence that anti-ferromagnetism in
the copper oxides plays a role in the formation of Cooper pairs and, thus, in
facilitating a superconductive state in some compounds.
BCS Theory: The first widely-accepted theory to explain superconductivity
put forth in 1957 by John Bardeen, Leon Cooper, and John Schreiffer. The theory
asserts that, as electrons pass through a crystal lattice, the lattice deforms
inward towards the electrons generating sound packets known as
"phonons". These phonons produce a trough of positive charge in the
area of deformation that assists subsequent electrons in passing through the
same region in a process known as phonon-mediated coupling. This is analogous
to rolling a bowling ball up the middle of a bed. 2 people, one lying on each
side of the bed, will tend to roll toward the center of the bed, once the ball
has created a depression in the mattress. And, a 2nd bowling ball, placed at
the foot of the bed, will now, quite easily, roll toward the middle. For a more
technical explanation click here.
Borocarbides: Superconducting borocarbides are compounds containing both
boron and carbon in combination with rare-earth and transition elements; some
of which exhibit the unusual ability to return to a normal, non-superconductive
state at temperatures below Tc. For more on
this, click here.
BSCCO: An acronym for
a ceramic superconductor system containing the elements Bismuth, Strontium,
Calcium, Copper and Oxygen. Typically, a small amount of lead is also included
in these compounds to promote the highest possible Tc. BSCCO has probably found
the widest acceptance among high-Tc superconductor applications due to its
unique properties. BSCCO compounds exhibit both an intrinsic Josephson effect and anisotropic (directional) behavior. You can view a list of
BSCCO compounds on the "Type 2" page.
Ceramics: Ceramic superconductors are inorganic compounds formed by
reacting a metal with oxygen, nitrogen, carbon or silicon. The best-known of
these are the copper-perovskites. Ceramics are typically hard, brittle,
heat-resistant materials formed by a process known as solid-state reaction.
Charge Reservoirs: In superconductors, charge reservoirs are the layers that may
control the oxidation state of adjacent superconducting planes (even though
they themselves are not superconducting). In the layered cuprates, these
consist of copper-oxide chains.
Chevrel (phases): A class of molybdenum chalcogenides (compounds containing Group
VI elements S, Se or Te along with molybdenum and a positively charged metal
ion) - named for Roger Chevrel of the University of Rennes, whose research
brought them to the attention of the scientific community in the early 1970's.
Recently, the Superconductivity Group at the University of Durham (UK) reported
a novel fabrication technique that increases Bc2 (upper critical field) in the
chevrel PbMo6S8 from 50 T in bulk materials up to >
100 T. Click HERE to read a [technical] writeup on this (as a PDF file). Or click
HERE to see a short list of some of these compounds alongside their
Tc's.
Coherence Length: The size of a cooper pair - representing the shortest distance
over which superconductivity can be established in a material. This is
typically on the order of 1000Å; although it can be as small as 30Å in the
copper oxides.
Cooper Pair: Two electrons that appear to "team up" in accordance
with theory - BCS or other - despite the fact that they both have a negative
charge and normally repel each other. (Named for Leon Cooper.) Below the
superconducting transition temperature, paired electrons form a condensate - a
macroscopically occupied single quantum state - which flows without resistance.
However, since only a small fraction of the electrons are paired, the bulk does
not qualify as being a "bose-einstein condensate". Click here to see an animation of a cooper pair.
DAC: An acronym for "diamond anvil cell". Often the Tc of a
superconductor can be coaxed upward with the application of high pressure. The
DAC is used to accomplish this in the laboratory. A DAC is composed of 2
specially-cut diamonds and a stainless steel gasket. The gasket goes between
the diamonds and seals a small chamber in which a fluid is placed. Since
neither the diamonds nor the liquid will compress, hydrostatic forces in excess
of a million atmospheres can be brought to bear on a sample suspended within
the fluid. Click here to see a graphic of a DAC.
Diamagnetism: The ability of a material to repel a magnetic field. Many
naturally-occurring substances (like water, wood and paraffin, and many of the elements) exhibit weak diamagnetism. Superconductors exhibit strong
diamagnetism below Tc. In a few rare
compounds, a material may become superconductive at a higher temperature
than the point at which diamagnetism appears. But, as a rule, the onset of
strong diamagnetism is one of the most reliable ways to ascertain when a
material has become superconductive. To see a short movie of a magnet being
levitated by a superconductor, click here.
D-Wave: A form of
electron pairing in which the electrons travel together in orbits resembling a
four-leaf clover. Wave functions help theoreticians describe (and predict)
electron behavior. The d-wave models have gained substantial support recently
over s-wave pairing as the mechanism by which high-temperature
superconductivity might be explained. Click here to see a graphic.
Energy Gap: This is the energy required to break up a pair of electrons.
According to BCS theory, the formula for determining the energy gap (in meV) is
Eg=7/2 KTc. Where K = Boltzmann's constant (8.62e-5
eV/K). And where Tc is the critical transition temperature in
Kelvin. Since electron-pairing is universally agreed to be the method by which
superconductivity occurs, this is the amount of energy required to disrupt the
superconducting state.
ESR: An acronym for
"Electron Spin Resonance" (also EPR: Electron Paramagnetic
Resonance). This is another mechanism by which superconductivity might be explained
in some materials. Simply put, ESR is the response of electrons to electromagnetic
radiation or magnetic fields at discrete frequencies. Electrons, as they move,
create tiny magnetic moments. Nearby electrons are influenced either beneficially
or adversely. When the moments are complementary, the electrons become paired
and can help each other move through a crystal lattice.
Ferrite: Ferrites are
ceramics with magnetic properties. They are included on this page because many
of the same elements used in ferrites (e.g. Ba, Sr, Tm, O) are also key
constituents in ceramic superconductors. This may be an important clue in understanding
high-temperature superconductivity.
Ferromagnetism: A state wherein a material exhibits magnetization through the
alignment of internal ions (neighboring magnetic moments). This contrasts with
paramagnetism, which is temporary, much weaker and results from unpaired
electrons.
Flux-Lattice: A configuration created when flux lines from a strong magnetic
field try to penetrate the surface of a Type 2 superconductor. The tiny
magnetic moments within each resulting vortex repel each other and a periodic
lattice results as they array themselves in an orderly fashion.
Fluxon: The smallest
magnetic flux (flux quantum) that exists in nature. Just as electrons are
quantized charge, fluxons are a quantized flux. The term is used in association
with vortices, which result from magnetic fields penetrating Type 2 superconductors
in single fluxon quanta. Click here to view a
hollographic depiction of waves of fluxons on the surface of superconducting
Niobium.
Flux-Pinning: The phenomenon where a magnet's lines of force (called flux)
become trapped or "pinned" inside a superconducting material. This
pinning binds the superconductor to the magnet at a fixed distance. Flux-pinning
is only possible when there are defects in the crystalline structure of the
superconductor (usually resulting from grain boundaries or impurities).
Flux-pinning is desirable in high-temperature ceramic superconductors in order
to prevent "flux-creep", which can create a pseudo-resistance and
depress Jc and Hc. Click here to see a
superconductor suspended in air by flux-pinning.
Four-point Probe: The most common method of determining the Tc of a superconductor.
Wires are attached to a material at four points with a conductive adhesive.
Through two of these points a voltage is applied and, if the material is
conductive, a current will flow. Then, if any resistance exists in the material, a voltage will appear across the other
two points in accordance with Ohm's law (voltage equals current times
resistance). When the material enters a superconductive state, its resistance
drops to zero and no voltage appears across the second set of points. By using
the four-point method, instead of just two points, resistance in the adhesive
and wires can be ignored; as the second set of points do not themselves conduct
any current and can, therefore, only reflect what voltage exists across the
body of the material.
Hall Effect: When a magnetic field is applied perpendicularly to a thin metal
film or semiconductor film that is conducting an electric current, a small
voltage will appear perpendicular to the axis of both the film and the magnetic
field. This voltage is proportional to the strength of the applied field.
However, the output is typically not linear. The Hall resistance (the ratio of
the Hall voltage to the current) changes in steps, pursuant to the laws of
quantum mechanics. This is known as the Integral Quantum Hall Effect or just
Quantum Hall Effect. Discovered in 1879, the Hall effect was named for its
discoverer Edwin H. Hall, a graduate student at Johns Hopkins University.
Hc: The scientific
notation representing the "critical field" or maximum magnetic field
that a superconductor can endure before it is "quenched" and returns
to a non-superconducting state. Usually a higher Tc also brings a higher Hc.
Heavy Fermions: Compounds containing the elements cerium, ytterbium or uranium;
whose (inner shell) conduction electrons often have effective masses (called quasiparticle masses) several hundred times as great as that of a
"free" (normal) electron mass. This gives them what's known as a low
"Fermi energy" and makes them unlikely - and unusual -
superconductors. Research suggests cooper-pairing in heavy fermion systems
arises from the magnetic interactions of the electron spins.
Hole: A
positively-charged vacancy within a crystal lattice resulting from the shortage
of an electron in that region. Holes are typically induced by doping a material
with an impurity. However, they can also be synthesized electronically with
devices like the field-effect transistor (FET). Modern electronic devices rely
heavily on holes (as p-type semiconductors) to function. There is evidence that
the holes of hypocharged oxygen in charge-reservoirs are, in fact, what makes
possible high-temperature superconductivity in the layered cuprates.
HTS: An acronym for
"High-Temperature Superconductor" (or Superconductivity). There is no
widely-accepted temperature that separates HTS from LTS (Low-Temperature
Superconductors). However, all the superconductors known before the 1986
discovery of the superconducting oxocuprates would be classified LTS. The barium-lanthanum-cuprate
fabricated by Müller and Bednorz, with a Tc of 30K, is generally considered to
be the first HTS material. Certainly any compound that will superconduct above
the boiling point of liquid nitrogen (77K) would be HTS.
Hysteresis (loop): Hysteresis, as it applies to a superconductor, relates to the
dynamic response of a superconductor to a strong magnetic field impinged upon
it. As the strength of a nearby magnetic field (H) increases, the critical
transition temperature (Tc) of a superconductor will decrease. And, at some
point superconductivity will completely disappear, as it becomes
"quenched". However, as the magnetic field is gradually withdrawn,
the superconductor may NOT immediately return to a superconductive
state. Herein lies the hysteresis. The graph of H-vs-Tc is different retreating
than it is advancing (creating a "loop" shape). This fact must be
weighed carefully in high-current applications where the superconductor Hc may,
even briefly, be exceeded; as significant power losses can result.
Infinite layer: Infinite layer compounds have no clear separation between
molecules. Rather than electrostatic bonding between discrete molecules to form
a bulk crystalline aggregate, all the atoms are bound together by covalent or
co-ionic bonding to form the equivalent of one huge molecule. (Ba,Sr)CuO2
and Na2Ba6Si46 are examples of "infinite
layer" or "infinite network" superconductor compounds.
Isotope Effect: The influence atomic mass contributes to the critical
transition temperature of a superconductor. For example, 203.4Hg has
a Tc of 4.126K. While 198Hg has a Tc of 4.177K. Since both forms of
mercury have the same lattice structure, this difference in Tc can be
attributed solely to the difference in mass. To learn more, click here.
Jc: The scientific
notation representing the "critical current density" or maximum
current that a superconductor can carry. Also note that, as the current flowing
through a superconductor increases, the Tc will usually
decrease.
Josephson Effect (also DC Josephson Effect): A phenomenon named
for Cambridge graduate student Brian Josephson, who predicted that electrons
would "tunnel" through a narrow (<10 angstroms)
non-superconducting region, even in the absence of an external voltage. In a
normal conductor, electrical current only flows when there's a voltage
differential and contiguous electrical connection. It has been theorized that
the Josephson Effect arises from the incoherent phase relationships between
superconducting electrons in the two (separated) superconductors. The AC Josephson Effect is where the current
flow oscillates as an external magnetic field impinged upon it increases beyond
a critical value. [at a frequency of 2eV/h, where e is the electron charge, V
is the voltage that appears, and h is Planck's constant] Sidebar: This
oscillation frequency has, in fact, resulted in an upward revision of Planck's
constant from 6.62559e-34 to 6.626196e-34.
Josephson Junction: A thin layer of insulating material sandwiched between 2 superconducting
layers. Electrons "tunnel" through this non-superconducting region in
what is known as the "Josephson effect" (see above). Sidebar: The
standard volt is now defined as the voltage required to produce a frequency of
483,597.9 GHz in a Josephson Junction.
Kelvin: A scale of
temperature measurement that starts at "absolute zero", the coldest
theoretical temperature attainable. (Named for Lord William Thomson Kelvin.)
Meissner Effect: Exhibiting diamagnetic properties to the total exclusion
of all magnetic fields. (Named for Walther Meissner.) This is a classic
hallmark of superconductivity and can actually be used to levitate a strong
rare-earth magnet. To see a movie of a magnet being levitated by a
superconductor, click here.
Mott Transition: The Mott transition is the shift from an insulating to a
metallic state in a material. The high- temperature copper oxides are composed of
CuO2 planes that are separated from each other by ionic "blocking
layers". Although it has one conduction electron (or hole) per Cu site,
each CuO2 plane is originally insulating because of the large electron
correlation. That behavior is typical of the Mott insulator state, in which all
the conduction electrons are tied to the atomic sites. The superconducting
state emerges when holes from the blocking layers dope the CuO2 layers in a way
that alters the number of conduction electrons and triggers the Mott transition.
Researchers believe that the strong antiferromagnetic correlation, which originates
in the Mott-insulating CuO2 sheets and persists into the metallic state, could
be a possible mechanism of high-temperature superconductivity. (courtesy Science
Week)
Organics: Organic
superconductors are a sub-class of organic conductors that include molecular
salts, polymers and pure carbon systems (including carbon nanotubes and C60
compounds). They may also be referred to as "molecular" superconductors.
They are typically large, carbon-based molecules of 20 or more atoms, consisting
of a planar organic molecule and a non-organic anion. For a non-technical
write-up on organics, click here. Or, for a more technical paper on this subject, click here.
Penetration Depth (also London Penetration
Depth): This term relates to how deeply a magnetic
field will penetrate the surface of a superconductor. An external magnetic
field impinged upon a Type 2 superconductor will decay exponentially into the
surface based on the paired electron density within the superconductor (only a
small fraction of the electrons are in a superconductive state). The
"London" name comes from brothers F. London and H. London, who in
1935 created a theoretical model of superconductivity. For a more technical
explanation and the actual formula to calculate penetration depth, click here.
Perovskites: A large family of crystalline ceramics that derive their name
from a mineral known as a perovskite. They are the most abundant minerals on
earth and have a metal-to-oxygen ratio of approximately 2-to-3. Copper-oxide
superconductors are layered perovskites. The perovskite name comes from Russian
mineralogist Count Lev Aleksevich von Perovski.
Phase-Slip (also Quantum Phase-Slip): A point where a material in a superconductive state
spontaneously changes from one state to another, generating a topological
"defect". This defect causes paired electrons to become "out of
step" with each other, producing a voltage and, ergo, non-zero electrical
resistance. This phenomenon has been observed in ultra-thin wires less than a
few tens-of-nanometers in diameter. Though bulk superconductivity may persist
(T<Tc), one consequence of phase slip is a lower current-carrying
state. A similar phenomenon occurs in Josephson Junctions.
Planar Weight Disparity (PWD): A term referring to the method by which Tc can often be
increased by adjusting the relative weights of alternating layers in
copper-oxide superconductors. The greatest improvements usually occur when
making the insulating layers heavy/light OR the Cu-O2 planes heavy/light - but
not both. Click HERE to read more about this discovery.
Proximity Effect: The phenomenon where a thin film of non-superconductive material
in close proximity with a superconductor takes on superconductive properties.
The Josephson junction is a device that takes advantage of this phenomenon. The
Inverse Proximity Effect is where just the opposite occurs. A non-superconductive metal
can enhance the Tc of an adjacent superconductor. This inverse effect has been
observed with silver and lead.
P-Wave: A rare form of
electron pairing in which two electrons travel together in spherical orbits;
with both having the same direction of rotation. (See "D-Wave" explanation
above.)
Quasiparticle: A bare particle that is "dressed" or
"clothed" by a cloud of other surrounding particles. Quasiparticles
behave similarly to bare (normal) particles, but usually have a larger
effective mass due to this cloud moderating interactions with other particles.
Quench: The phenomenon
where superconductivity in a material is suppressed; usually by exceeding the
maximum current the material can conduct (Jc) or the maximum magnetic field it
can withstand (Hc).
Re-entrant (behavior): A condition where a material retreats from its superconductive
state and then re-enters it. This can be caused by a strong external magnetic
field that dynamically exceeds the Hc of the material and/or is mis-aligned (in
the case of some organic
superconductors), a discordant
temperature below Tc (in the case of some borocarbides), or by Jc hysteresis (momentarily exceeding the critical
current density, causing the Tc to shift downward).
Resistance: The opposition of a material to the flow of electrical current
through it. Energy lost due to resistance is a result of vibrations at the
molecular level and manifests itself as heat in proportion to the square of the
current flow. In a superconductor all resistance disappears below a certain
temperature. However, this applies only to direct current (DC) electricity.
Other types of losses result when transporting alternating current (AC).
Examples of this include hysteresis, reactive-coupling and radiational losses.
In the new high-temperature ceramic superconductors, the power loss in
applications like transmission lines is inversely proportional to the critical
current density for low magnetic field applications. This limitation can be
compensated for to some degree by increasing the ratio of voltage to current.
In Type 2 superconductors carrying high-frequency alternating current,
"skin effect" losses also result as the energy tends to migrate to
the surface where the conductive medium is incontiguous, producing a
pseudo-resistance. In some materials the amount of resistance may also depend
on the direction of current flow (anisotropic resistivity) and/or presence of
an external magnetic field (hall effect).
Room-temperature Superconductor: There are NO confirmed room-temperature superconductors (as was
once reported for lithium-beryllium-hydride and for lead-silver-carbonate).
However, it has been theorized that a metallic form of hydrogen might be
a room-temperature superconductor. In 1996 physicists at Lawrence Livermore
Laboratory were able to briefly create metallic hydrogen. But, its existence was fleeting and no measurement of the
Meissner effect was possible. Zero resistance has been observed at room
temperatures in ballistic quantum wire. However, having one-dimensional geometry, this wire does not
exhibit the Meissner effect, except when configured as a closed loop.
Sinter: The process of
heating a material to just below its melting point. An extended period of
sintering is the method by which the constituent components of a ceramic
superconductor are combined in a solid-state reaction. Since ceramic superconductors
are inherently brittle, sintering helps promote intergranular bonding and hardness.
SQUID: A
superconducting loop interrupted in 2 places by Josephson junctions. When
sufficient electrical current is conducted across the squid body, a voltage is
generated proportional to the strength of any nearby magnetic field. The SQUID,
an acronym for Superconducting QUantum Interference Device, is the most
sensitive detector known to science. Click here to see a graphic.
Stripes: Stripes are
microscopic rivers of charge that flow across the surface of a Type 2
superconductor. It is theorized that stripes encourage "holes" to
pair up and, as such, may play a role in facilitating charge transfer.
Recently, at the Stripes 2000 conference in Rome, Italy, it was shown that
there exists a critical value of micro-strain that must be exerted upon the CuO2
planes for stripes to form. Click here to learn more.
Superconductor: An element, inter-metallic alloy, or compound that will conduct
electricity without resistance below a certain temperature. However, this applies only to
direct current (DC) electricity and to finite amounts of current. All known
superconductors are solids. None are gases or liquids. And all require extreme
cold to enter a superconductive state. Once set in motion, current will flow
forever in a closed loop of superconducting material - making it the closest
thing to perpetual motion in nature. Scientists refer to superconductivity as a
"macroscopic quantum phenomenon". In addition to being classified Type 1 and Type 2,
superconductors can be categorized further by their dimensionality. Most are
3-D. But some compounds, like surface-doped NaWO3 and some organic
superconductors are 2-D. Li2CuO2 and single-walled carbon
nano-tubes have shown rare 1-D superconductivity. In addition to repelling
magnetic fields, enhanced thermal conductivity, higher optical reflectivity and
reduced surface friction are also properties of superconductors. The term
"superconductor" is also used in some instances to refer to materials
that have near infinite thermal conductivity - such as carbon nanotubes.
However, on this website it is used in the context of electrical conductivity
only.
Susceptibility: A measure of the relative amount of induced magnetism in a
material. Magnetic susceptibility is often used in lieu of resistance
measurements to determine the transition temperature of a superconductor.
Although, on occasion, the two techniques produce very different Tc's. In a
typical superconductor, the (arbitrary) value of susceptibility will change
from zero to a negative number as the temperature drops through Tc. However, in
some materials it changes from positive to negative, as paramagnetism yields to
diamagnetism.
S-Wave: A form of
electron pairing in which the electrons travel together in spherical orbits,
but in opposite directions. (See "D-Wave" explanation above.)
Tc: The scientific
notation representing the critical transition temperature below which a
material begins to superconduct. The sudden loss of resistance in a superconductive
medium may occur across a range as small as 20 millionths of a degree or, in
the case of some stoichiometrically imperfect compounds, tens of degrees. Click
here to see a graphic example. ("Tc" is not to be confused
with the atomic symbol for Technetium.)
Thin Film (Deposition): A method of fabricating ceramic superconductors to more
precisely control the growth of the crystalline structure to eliminate grain
boundaries and achieve a desired Tc. This can involve Pulsed-Laser Deposition
(PLD) or Pulsed-Electron Deposition (PED) of the material. A variation of this
technique can be used to increase the Tc of a superconductor by growing
it on a supporting material with a smaller interatomic spacing. The supporting
material acts as a molecular "girdle" to compress the atomic lattice
of the superconductor, thereby raising its transition temperature.
Superconductive tape is made using thin film deposition technology.
Translational
Symmetry: As it applies to superconductivity,
translational symmetry is where the process of charge transfer is repeated
exactly as the charge carriers (paired electrons) traverse the solid. In a
normal conductor, latent heat continuously vibrates the atomic lattice,
deflecting mobile free electrons and preventing "perfect"
translational symmetry. In a superconductor this scattering tendency is
overcome.
Tungsten-bronze: A nebulous term used to describe alkali metal tungstenates, vanadates,
molybdates, titanates and niobates. The term was originally coined to describe
NaxWO3 compounds; the crystals of which look much like
the copper-tin alloy known as bronze. There have been reports of
superconductivity as high as 91K for a surface-doped sodium tungsten-bronze. This material was the first high-temperature superconductor discovered that
does not contain any copper.
Ultraconductor: Materials known as ultraconductors™ display room-temperature
resistance many orders of magnitude lower than the best metallic
conductors. Examples of these materials include oxidized atactic polypropylene
(OAPP) and other polymers. Since ultraconductor™ is a colloquial term, these
materials might better be described as "hyperconductors". The
Meissner effect cannot be confirmed in them, but strong (giant) diamagnetism is
in evidence. Some of them may actually find acceptance in high-current
applications ahead of superconductors as a result of their low losses at
ambient temperatures and pressures. (A primer on ultraconductors is available
by clicking HERE.)
Undressing: The process by which a quasiparticle becomes more like a bare (normal) particle. It is theorized this
may be a driving force behind superconductivity, as undressed electrons are
significantly lighter and can, thus, conduct current more readily. To learn
more, click here.
Unit Cell: A unit cell is the smallest assemblage of atoms, ions, or
molecules in a solid, beyond which the structure repeats to form the
3-dimensional crystal lattice.
Vortices (plural of vortex): Swirling tubes of electrical current induced by an external
magnetic field into the surface of a superconducting material that represent a
topological singularity in the wavefunction. These are particularly evident in
Type 2 superconductors during "mixed-state" behavior when the surface
is just partially superconducting. Superconductivity is completely suppressed
within these volcano-shaped structures. Recent research suggests that flux
vortices may NOT possess quantum values (equal to multiples of Planck's
constant divided by 2 times electron charge). But may instead have but a tiny
fraction of the basic unit of magnetism. The movement of vortices can produce a
pseudo-resistance and, as such, is undesirable. While superconductivity is a
"macroscopic" phenomenon, vortices are a "mesoscopic"
phenomenon. (See the graphic at the top of this page.)
YBCO: An acronym for
a well-known ceramic superconductor composed of Yttrium, Barium, Copper and
Oxygen. This was the first truly "high temperature" ceramic
superconductor discovered; having a transition temperature well above the
boiling point of liquid nitrogen - a commonly available coolant. Its actual
molecular formula is YBa2Cu3O7, making it a
"1-2-3" superconductor. YBCO compounds exhibit d-wave electron pairing. The patent for YBCO is held by Lucent
Technologies. (You can view a list of the best-performing 1-2-3 compounds on
the "Type 2" page.)
