Symbols

Terms

Units

Definitions

[µ₀]H_c

Critical magnetic field

Magnetic field below which Type I superconductors exhibit superconductivity.

[µ₀]H_c1

Lower critical magnetic field

T

Magnetic field below which Type II superconductors exhibit perfect diamagnetism (exclusion of an applied magnetic field from the superconductor). Lower than H_c.

[µ₀]H_c2

Upper critical magnetic field.

T

Magnetic field below which Type II superconductors exhibit stable superconductivity.

B

Magnetic induction

T

G

Gauss

Unit of magnetic flux often used for low fields, the flux density over the earth's surface is ~ 0.5 G.

HTS

High Temperature Superconductors

Rare earth cuprate based superconductors with T_c values in excess of 30 K. First discovered by Bednorz and Müller in 1986.

I_c

Critical Current

A

The electrical current below which a conductor exhibits superconductivity. The value is sensitive to the voltage criterion used.

J_c

Critical Current Density

A/m² but A/mm² reported

The electrical current density below which a conductor exhibits superconductivity. The value decreases with increasing temperature and applied field. The value is sensitive to the voltage criterion used. Commercial Nb-Ti strand can be purchased in kilometer lengths with J_c  in excess of 3000 A/mm² at 5 T.

K

Kelvin

Temperature scale with zero at absolute zero and unit size the same as centigrade. 0 K = -273 ºC.

LTS

Low Temperature Superconductors

All superconductors in use prior to the discovery of superconductivity in rare earth cuprates in 1986. The highest T_c in this class is for Nb₃Ge (23 K at 0 T)

T

Tesla

Unit preferred for high fields. 1 T = 10 kG

T_c

Critical Temperature

K

The temperature below which a material exhibits superconductivity. Typically given for zero current and applied field. The value decreases with increasing current and applied field.

Type I

Type I superconductors

Most elemental superconductors are of this type. They exhibit perfect diamagnetism.

Type II

Type II superconductors

Alloy and HTS superconductors as well as Nb, V and Tc. Retain superconductivity beyond initial flux penetration at H_c1 up to a much higher upper critical field, H_c2

k

Ginzburg-Landau parameter, "kappa"

None

l/x

l(T)

Magnetic penetration depth

Nm

Depth to which an external field penetrates a superconductor. As low as 30 nm for Nb and as high as 1000 nm for YBa₂Cu₃O₇ with field parallel to the a-b plane. Temperature (T) sensitive.

x(T)

Coherence Length

Nm

The minimum distance over which the density of superconducting electrons may change significantly. Temperature (T) sensitive. Ranges from ~2 nm for YBa₂Cu₃O₇ (with field parallel to the a-b plane) to 83 nm for Pb.

F

Magnetic flux

Wb

weber

t

Reduced temperature

T/T_c