| Expression |
Explanation |
| absolute zero |
The lowest temperature theoretically possible; the temperature at which the thermal energy of random
motion of the particles of a system in thermal equilibrium is zero. 0 K = -273.15°C = -459.69°F |
| ampere |
The unit of electrical current in the MKS system of units. Abbreviated 'a'; 'A'; 'amp'. |
| ampere meter squared |
The SI unit of electromagnetic moment. Abbreviated Am2. |
| angstrom |
A linear dimensional unit, equal to one-ten thousandth micron or 10-10 meters. |
| BCS theory |
A theory advanced in 1957 by three researchers, John Bardeen, Leon Cooper, and J.R. Schrieffer,
that explained how low-temperature superconductors work. |
| ceramic |
Any product made from earth derived materials such as clays, silicates, or sand, usually
requiring the application of high temperature in a kiln or oven at some stage of the process. |
| conductor |
A substance or body that offers a relatively small resistance to the passage of an electrical
current. |
| Cooper Pairs |
The term given to pairs of bound electrons which occur in superconducting material according
to the Bardeen-Cooper-Schrieffer (BCS) theory. |
| coulomb |
The SI unit of electrical charge, defined as the charge transported in one second by an
electric current of one ampere. Symbol: C |
| critical current |
Cooled material experiences superconductivity properties up to a critical current, above
which the material exhibits resistance. |
| critical current density Jc |
The maximum value of electrical current per unit of cross-sectional area that a superconductor
can carry without resistance. For practical applications, Jc values
in excess of 1000 amperes per square millimeter (A/mm2, are
desirable in both bulk conductors and thin film superconductors. |
| critical magnetic field (Hc) |
Above this value of an externally applied magnetic field a superconductor becomes
nonsuperconducting. When an external magnetic field is applied to a Type I superconductor
the transition from superconducting to normal is sharp. Type II superconductors do not possess
perfect diamagnetism (flux penetration of the material is possible). When an external magnetic
field is applied to Type II superconductors the transition to the normal state is over a much
broader region between a lower critical field, Hc1
, and an upper critical field, Hc2. |
| critical temperature (Tc) |
The highest temperature at which superconductivity occurs in a material. Below this transition
temperature Tc the resistivity of the material is equal to zero. |
| cryogenics |
A branch of physics dealing with the properties of matter at extremely low temperatures. |
| current |
The rate of flow of electrons, measured in amperes, in a conductor. The conduction of current
of 1 ampere is equal to the flow of about 1018
electrons per second. |
| dewar |
A double-walled flask with a vacuum between the walls that are silvered on the inside,
used specifically for the storage of liquified gases. |
| electric field |
The space surrounding an electric charge. The area in which it is capable of exerting a
perceptible force on another charge. |
| electromagnet |
An iron core encircled by coils of wire that become magnetic when current flows through
the wire. |
| fluxoid |
Circulating vortices of current and flux contained in the vortices. |
| flux |
To "flow". Referring to the rate of flow of radiation from a given source. |
| flux pinning |
Superconducting material properties are altered locally by the presence of defects in the
material. A fluxoid adjacent to such a defect in the material has its energy altered and
its free motion through the superconductor is inhibited. Flux pining causes a field gradient
in the superconductor and gives rise to a net current in the material. |
| gauss |
The CGS-electromagnetic unit of magnetic flux density. 1 G = 10-4
tesla. Symbol: G |
| helium |
The element helium, (Greek, meaning the sun) was named so because it was first discovered
in the sun by spectrographics in 1868. Helium gas does not become liquid until the temperature
reaches 4.2 K, about -269°C. |
| high temperature superconductor (HTS) |
Refers to materials with much higher transition temperatures than previously known
superconductors. |
| insulator |
A substance that does not conduct electricity. |
| Josephson effect |
Electrons crossing an insulating barrier in a Josephson Junction,a process called tunneling,
that creates a "supercurrent" |
| Josephson Junction |
Consists of two superconductors separated by a thin insulating barrier. Is in fast electronic
switches or sensitive magnetometers. |
| Kelvin |
A scale of temperature measured form absolute zero. 1 K = 1 deg C. 0 K = -273°. |
| lattice |
A three dimensional grid-like pattern of the arrangement of atoms in a solid. |
| lower critical field |
Above this value of an externally applied magnetic field a superconductor is in a mixed state.
Below this value it is in the Meissner state. |
| magnetic flux |
A measure of the total size of a magnetic field, defined as the scaler product of the flux
density and area. |
| magnetic flux density |
The magnetic flux passing through unit area of a magnetic field in a direction at right
angles to the magnetic force. The vector product of the magnetic flux density and the current
in a conductor gives the force per unit length of the conductor. It is measured in teslas. |
| MRI |
Acronym for Magnetic Resonance Imaging. MRI is a diagnostic imaging technique that produces
cross-sectional images. Its primary use is for organic materials and soft body tissue. |
| magnetic field strength |
The magnitude of a magnetic field, measured in A/m |
| magnetic flux |
A measure of the total size of a magnetic field, defined as the scaler product of the flux
density and the area. It is measured in webers. |
| magnetometer |
An instrument for detecting magnetic fields. |
| Meissner effect |
The expulsion of magnetic fields from a superconductor. |
| Meissner state |
The regime of temperature and magnetic fields where the Meissner effect can be observed. |
| mixed state |
The regime of magnetic fields between the lower critical field,
Hc1, and the upper critical field, Hc2. Diamagnetism is less than
perfect because supercurrent vortices confine magnetic field within quantized filaments of
normal-state material that pass through the superconductor.
|
| ohm |
The SI unit of electrical resistance, defined as the resistance between two points on a
conductor through which a current of one ampere flows as a result of a potential difference
of one volt applied between the points. |
| Ohm's law |
The electrical current in any conductor is proportional to the potential difference between
its ends. Ohm's law is often expressed as I=E/R, where I is the current, E is the potential
difference, and R the resistance. |
| perovskite |
A type of crystal. Referring to the crystal structure shared by the 1-2-3 and other
high-temperature superconductors. |
| phonon |
Quantized atomic lattice vibration. It is the mechanism causing electron pairing in the
BCS theory. |
| quantum mechanics |
The modern theory of action. It applies primarily to atomic motion. |
| resistance |
The opposition to the flow of electrons in a conductor. Measured in ohms. |
| semiconductor |
An element or compound whose electrical properties are midway between a conductor and
insulator. A substance with relatively high resistance and corresponding low conductivity. |
| superconductor, Type-I |
Material with perfect electrical conductivity for direct current that also possesses perfect
diamagnetism. When an external magnetic field is applied on this superconductor, the transition
temperature from superconducting to normal is sharp. |
| superconductor, Type-II |
Material with perfect electrical conductivity for direct current that possesses moderate
diamagnetism at high field. When an external magnetic field is increased, the transition from
superconducting to normal state occurs after going through a broad "mixed state" region. |
| SQUID |
Acronym for Superconducting Quantum Interference Device. |
| tesla |
A unit used to describe the strength of magnetic fields in the MKS system. |
| upper critical field |
Above this value of an externally applied magnetic field, a type-II superconductor is in the
normal state. Below this value it is in the mixed state. |
| volt |
The unit of potential difference or electromotive force in the MKS system. |
