2. MEISSNER EFFECT
• When a material makes the transition from the normal to
superconducting state, it actively excludes magnetic
fields from its interior; this is called the Meissner
effect.
• The German physicists Walther Meissner and Robert
Ochsenfeld discovered this phenomenon in 1933 by
measuring the magnetic field distribution outside
superconducting tin and lead samples. The samples, in the
presence of an applied magnetic field, were cooled below
their superconducting transition temperature(Tc),
whereupon the samples cancelled nearly all interior
magnetic fields.
3. • This Meissner effect happens when electric current loops spontaneously appear
on the surface of a material that becomes superconducting in the presence of
a magnetic field. These currents create a magnetic field, similar to that of
an electromagnet.
• Superconductors in the Meissner state exhibit perfect diamagnetism,
or superdiamagnetism, meaning that the total magnetic field is very close to
zero deep inside them (many penetration depths from the surface).
• A superconductor with little or no magnetic field within it is said to be in
the Meissner state. The Meissner state breaks down when the applied magnetic
field is too strong. Superconductors can be divided into two classes
according to how this breakdown occurs
• In type-I superconductors, superconductivity is abruptly destroyed when the
strength of the applied field rises above a critical value Hc.
• Type II superconductors have a different magnetic behavior. It has two
critical magnetic fields Hc1 and Hc2 below Hc1 it behaves as type I, and
above Hc2 it becomes normal.
4. LONDON'S EQUATION :
The Meissner effect was given a phenomenological explanation by the brothers
Fritz and Heinz London, who showed that the electromagnetic free energy in a
superconductor is minimized.
∇2 H = λ-2H ,this is known as London's equation where H is the magnetic field
and λ is the London's penetration depth.
This equation predicts that the magnetic field in a superconductor decays
exponentially from whatever value it possesses at the surface. This
exclusion of magnetic field is a manifestation
of the superdiamagnetism emerged during the phase transition from conductor
to superconductor, for example by reducing the temperature below critical
temperature.
5. APPLICATIONS OF MEISSNER EFFECT :
This effect of superconductivity, is used in magnetic
levitation which is the base of modern high-speed
bullet trains. In superconducting state (phase), due
to expulsion of external magnetic field, the sample
of superconducting material levitates above magnet or
vise-versa. Modern high-speed bullet trains use the
phenomenon of magnetic levitation.
Standard test – Proof for a superconductor
A separation technique, based on the Meissner effect,
to purify and classify superconducting powders was
developed.
