Superconductors, a Magnet, and Some Luck

The National High Magnetic Field Laboratory in Tallahassee, FL
The National High Magnetic Field Laboratory in Tallahassee, FL

At -343 degrees F in complete darkness, a tiny black crystal was at the center of one of the strongest magnets on earth, powered by a quarter of Tallahassee’s electricity.  The crystal was suspended by gold wires thinner than hair inside a stainless steel tube immersed in liquid helium.  The helium was in a container surrounded by another filled with liquid nitrogen.  Near perfect vacuums insulated the ultra-cold liquids from the lab, preventing them from boiling away too quickly by the room temperature air surrounding the experiment–temperatures that would evaporate them like a hot-poker would water.

Millimeters away from these extreme temperatures was a big 33 Tesla magnet, capable of generating fields 600,000 times stronger than the earth’s.  When energized a short circuit in the magnet could kill people nearby, but smart design and careful monitoring prevented that from happening.  Large helical plates like compressed DNA strands with slots drilled in them carried the electricity through the magnet.  The slots were for water: lots of it, pumped continuously from super-sized chillers in a noisy room down the hall.  Without the water, the plates would melt like a blown fuse and destroy the magnet.  With the water, the heat generated from the huge electrical currents was dissipated and pumped to one of four cooling towers outside the National High Magnetic Field Laboratory in Tallahassee, FL.

Magnet cell 9 at the National High Magnetic Field Laboratory being prepared for an experiment.
Magnet cell 9 at the National High Magnetic Field Laboratory being prepared for an experiment.

I was across town at the third ‘Physical Phenomena at High Magnetic Field’ conference listening to professor Stephen Hill describe his novel method of probing similar crystals.  He transmitted radio waves down a thin metallic tube called a wave guide and into an effective echo chamber.  Like certain sounds in an enclosed space, specific radio frequencies resonated in the small chamber where his crystal was placed.  A second wave guide carried the reflected waves back to a very sensitive receiver.  Significant and repeatable changes in the waves could be correlated to changes in the electronic and magnetic properties of the crystal being studied.

It was a fascinating talk, but my mind was preoccupied with the liquid helium level in my experiment back at the lab.  If it ran dry, my crystal would warm up and the physical structure of the material would change.  It took days to get the molecules to align properly in a process not well understood at the time, but one that I was studying.  The temperature at which this material became a superconductor was directly related to this alignment process and I couldn’t let this multi-day experiment come to an end because the helium ran out.

Against the will of one of my instructors who wanted me at the conference to field questions on another topic, I rode off to refill the helium and nitrogen–a 15 minute round trip by motorcycle.  That gave me another seven hours of worry-free time to avail myself to the visiting scientists.  After a lunch break, we gathered for a group photo.

I ended up in the front near Nobel Laureate Bob Schriefer because people filling in from the back needed some of us to move forward.  Schriefer taught a magnetism course at the lab, where he was justifiably treated like royalty.  Years earlier his graduate research on superconductivity earned him the Nobel Prize in Physics.  Before BCS theory (the S standing for Schriefer) of superconductivity, few had any idea what the hell was going on.  The topic was a mystery, and it is still hard to fathom.  It’s like having zero friction on a surface, for example if a hockey puck kept sliding for ever without slowing down at all…ever.  Within a superconductor, free electrons joining together into pairs were the pucks in BCS theory and Shriefer’s model explained their perpetual motion mathematically.

After the conference I went back to the lab for my third consecutive day with one of the big magnets.  I’d never been granted that much continuous magnet time before, and it was complete luck that I got it this time.  A group from Japan was having trouble getting their experiment ready and I received days of their time to extend my own.  This provided critical extra hours for the molecules in my crystal to align between magnetic field sweeps, and the results were telling.

Electrical resistance of the organic superconductor known as Cu-Br versus magnetic field.
Electrical resistance of the organic superconductor known as Cu-Br versus magnetic field.

The quantum oscillations in the resistance of the material amplified and smoothed out the more the crystal molecules aligned.  The oscillations labeled ‘E’ in the above graph were recorded after approximately 36 hours of alignment.  Before each field sweep, the temperature was lowered to 0.45 Kelvin (less than half a degree from coldest anything can ever get).

These ‘Shubnikov de Hass’ oscillations were caused by masses of electrons in the material jumping from one quantum level to another.  The levels themselves were a manifestation of the increasing magnetic field.  As the field increased, the quantum levels rose right through the crystal’s conduction electrons, and these electrons had to choose only one of the allowed levels as they passed by.

This would be like going up an escalator and then told to stay on the tenth step.  Right before the tenth step became the eleventh, you’d have to step down to the ninth, which would immediately turn into the tenth.  You would end up walking in place and your stepping motion would be analogous to the oscillations above.

With these results we confirmed the effective mass of the conduction electrons, determined the average distance an electron would travel before colliding with an impurity, and extracted other properties of the material.  But most importantly, we were able to explain why other groups around the world were getting conflicting values for these same properties.

It turned out that like the superconducting transition temperature, these values too were directly related a peculiar molecular alignment process that required extensive amounts of time to complete–time that we were lucky enough to have been awarded when it was needed most.

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