Superconductivity & DC Resistivity

Measurements on a Superconductor

Resistance Results for YBCO:

The above graph shows the resistance vs. temperature results for the superconducting sample. The sample that I use is Yitrium(1)Barium(2)Copper(3)Oxygen(6.95). Unlike Copper, the YBCO pellet experiences a phase change on the quantum mechanical level at approximately 90 Kelvin. At this temperature the electrons begin to form pairs due to a weak attraction. The attraction between the electrons is a rather amazing feat considering the fact that the particles involved have like charges. The attraction is actually a result of the electrons' interaction with the crystal lattice of the YBCO atoms. As a result of the pairing, the electrons share the same quantum mechanical wave function. Now if one electron runs into an impurity in the sample it will only stop if all of the paired electrons come to a stop. Since a single impurity is incapable of stopping all of the electrons, the electron in question will pass the impurity without being scattered. The ramification of this purely quantum mechanical phenominon is that the sample will have absolutely ZERO resistance - a perfect conductor. This can be clearly seen on the graph where the resistance dives downward as the sample is cooled to below its transition temperature.

Perfect Conductivity

So now that we have a sample that is capable of displaying perfect conductivity, we are in an interesting position with respect to our previous induction demonstration. If the magnet finally fell onto the copper after several seconds due to the fact that the copper had some resistance, what will happen if a magnet is lowered onto a superconducting sample? As the magnet is brought close, induced currents on the surface of the superconductor create an opposed magnetic field that attempts to repel the magnet. When the magnet is close enough, equilibrium is reached and voila!:

The magnet never makes it to the surface, IT LEVITATES! It never decends below the height in the photo because the currents that are responsible for creating the magnetic field that suspends the magnet never decay away.

And with a little twist of the wrist...

The magnet does eventually stop SPINNING due to air friction, but the LEVITATION goes on forever and ever as long as the sample is kept cold enough for the electrons to stay paired.
Actually, to be correct, supercondutors are more interesting than perfect conductors. In fact, superconductors are really perfect diamagnets. To see what this entails you have to check out the last demo on the Meissner Effect.

Links To and Fro..

  • Back to the Physics Outreach Home Page
  • Back to the Superconductor Demo Menu
  • Onward to the Meissner Effect