APPENDIX E

GLOSSARY

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 Am .

angstrom
A linear dimensional unit, equal to one-ten thousandth micron or 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 J The maximum value of electrical current per unit of cross-sectional area that a superconductor can carry without resistance. For practical applications, J values in excess of 1000 amperes per square millimeter (A/mm ), are desirable in both bulk conductors and thin film superconductors.

critical magnetic field (H )
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, H  , and an upper critical field, H  .

critical temperature (T )
The highest temperature at which superconductivity occurs in a material. Below this transition temperature T 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 10  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  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, H  , and the upper critical field, H  . 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.