Physics Photo of the Week

February 19, 2010


Meissner Effect and Magnetic Levitation - Photo by Keletso Mmalane.  Discussion contributed by Keletso Mmalane and Mayuri Patel.

We 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 the YBa2Cu3O7 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 crystals.

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 temperatures.  YBa2Cu3O7 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.




Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

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