[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

Magnetic flux per unit area.

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 (A/mm² for LTS, A/cm² for HTS typically 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 °C.

LTS

low temperature superconductors

Typically refers to the 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). More recently discovered superconductors such as MgB₂ with higher T_cs but not as high as HTS are sometimes referred to as intermediate temperature superconductors.

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.

κ

Ginzburg-Landau parameter, "kappa"

None

λ/ξ where λ = penetration depth, ξ = coherence length. Type I superconductors have κ < 1/√2 whereas type II have κ > 1/√2.

λ(T)

magnetic penetration depth

nm

Depth to which an external field penetrates a superconductor in the Meissner state. 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.

ξ(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.

Φ

magnetic flux

Wb

The product of magnetic induction and area. Note: a flux density of one Wb/m² = 1 T

t

reduced temperature

T/T_c

ℓ

electron mean free path

m (typically nm)

The average distance an electron travels between collisions.