Meissner Effect and Magnetic
Levitation - Photo by Keletso
Mmalane. Discussion contributed by Keletso Mmalane and Mayuri
placed a disk of YBa2Cu3O7 the
bottom third of a Styrofoam cup and cooled it down by pouring liquid
nitrogen until it was almost submerged. On top of the YBa2Cu3O7
we placed a miniature magnet. The magnet began to levitate as soon as
had cooled down to its transition temperature. At this
temperature, near that of liquid nitrogen, the YBa2Cu3O7
becomes a superconductor.
The reason the magnet began to levitate is that once the super
conductor was cooled to its transition state, external magnetic fields
could not penetrate it. Therefore the exclusion of magnetic fields from
the superconductor created a force to levitate the small magnet. The
magnet stays in the same place because its magnetic fields are pinned
in the microscopic voids in the superconductor which consists of a
sintered ceramic of many tiny YBa2Cu3O7
Conventional superconductors usually have critical temperatures ranging
from around 20 K to less than 1 K. Cuprate superconductors, such
as the YBa2Cu3O7
demonstrated here, can have much higher critical
is one of the first cuprate superconductors that was
discovered, and it has a critical temperature of 92 K.
Mercury-based cuprates have been found with critical temperatures in
excess of 130 K. The explanation for these high critical temperatures
remains unknown. Electron pairing due to phonon exchanges explains
superconductivity in conventional superconductors, but it does not
explain superconductivity in the newer superconductors that have a very
high critical temperature.
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