# Physics Group - II (3110018)

BE | Semester-1   Winter-2019 | 02-01-2020

## Q5) (b)

#### Explain London's penetration depth.

• Maxwell’s equations could not explain the zero resistance and perfect diamagnetism observed as main characteristics of superconductivity.
• F. London and H. London (London brothers) suggested that motion of superconducting electrons in the presence of applied electric field E.
• If a magnetic field is applied to a superconductor which is initially in zero field, the magnetic field is a function of time.
• The potential decay of magnetic field at the interior surface of superconductor.
• Mathematical expression for the Hall voltage is given by:

Where, ${\mathrm{B}}_{0}$ = magnetic field at the surface
Where, $\mathrm{x}$ = distance inside the superconductor from surface
Where, ${\mathrm{\lambda }}_{\mathrm{L}}$ = Penetration depth

• The potential decay of magnetic field at the interior surface of superconductor.
• ${\mathrm{\lambda }}_{\mathrm{L}}$ is the distance across which the magnetic field becomes  ' e '  times weaker.
• ${\mathrm{\lambda }}_{\mathrm{L}}$ can be formulated as:

Where, $\mathrm{m}$ = charge carrier of mass
Where, $\mathrm{n}$ = number of density
Where, ${\mathrm{\mu }}_{0}$ = absolute permeability