The story of superconductivity

Below is a talk I delivered at our local Toastmaster Club meeting.

Today I wish to tell you a few things about superconductivity. This is a very old area of physics, which began almost 100 years ago, yet new things were found as late as last month. It is a very wonderful example of scientific drama, in which you have accidental discoveries, exciting moments, and frustration in searching for answers to the puzzles.

So first, what is superconductivity?
           Superconductivity refers to the complete loss of electrical resistance in a material. For example, lead or mercury will become superconducting if you cool them down to very low temperatures. Because the material has no resistance, if you inject an electric current into a loop made of such a material, the current would circulate inside the loop for a million years --- never disappears. This is a truly wonderful phenomenon indeed.

Superconducting only happens at very low temperatures, - 270 ºC in the case of mercury. We know Calgary’s coldest winter could reach -40 ºC or so, therefore you’d need more than 6 Calgary winters to freeze that drop of mercury in the thermometer into superconducting.

How it was discovered?

Superconductivity was discovered in 1911. At that time, people just developed new techniques to gain low temperatures. They achieved liquidizing oxygen, nitrogen, and then finally the “last gas” helium was liquidized at – 269 ºC (4.2 K) by a Dutch physicist named Kammerlingh Onnes. With liquid helium, Onnes somehow came up with the idea of measuring mercury’s resistance at that low temperature.

This move opened a new gate to the world of science. What he found was completely unexpected: the mercury’s resistance suddenly dropped to so low that his apparatus could not even measure! After checking his experiment, he confirmed the result was real, and coined the term superconductivity. Now the search began -----

Why superconducting?

The search for an explanation to this phenomenon lasted nearly half a century. During that period, quantum theory was established from scratch and fully developed into a very powerful tool. And it was only after applying that theory to superconductors had people finally solved the mystery.

Three American physicists proposed the theory for superconductivity. It was called BSC theory after the physicists’ names. This term may sounds familiar to you, because it also represented mad cow disease found here in Alberta.  The key of the theory is this: in superconductors, electrons are paired together, so that they can overcome the “road blocks” of positively charged ions in a metal, whereas in the normal case, single electrons flow past the ions and get attracted, thus causing resistance.

One of the trio, John Bardeen, was truly a genius, who also participated in the invention the transistor. The S in the name stands for Schrieffer. As we are talking, the professor may still be in jail: he likes driving fast. In 2004, the professor crashed his Mercedes at a speed of more than 100 miles, and killed two people. He was sentenced 2 years in prison --- he would have got longer jail time had he not won the Noble Prize for BSC theory.

The game is not over yet

With BSC theory, a chapter was turned. However, science is a never- ending game. People were curious about how high the transition temperature could go, because no theory ever predicted the upper limit.

A few people kept searching for the so-called high-temperature superconductors. And sure enough, near the end of 1986, an explosive news came from Switzerland: two physicists found some new superconductor! I still remember the moment when I first heard about this: it was early in the morning, I was lying in bed, somewhere in the neighborhood a radio was broadcasting news. I couldn’t hear it clearly but I caught the word “superconductivity”. I immediately jumped from bed: it was like an old friend, you haven’t heard from him for thirty years, and now there is news coming. That must be something. And my guess was right. The news triggered a huge storm in physics.

Three months later, the annual meeting of American Physical Society was held in New York City. And you can imagine the extreme excitement among the people. It was later called the Woodstock of physics.

This time again, the frustration of solving the puzzle returned: the old BSC theory didn’t apply to this type of superconductors. A new theory is needed. However, twenty years pasted now, the answer still isn’t there yet.

When I was preparing for this talk, I searched the Internet and found that a new superconductor with record high transition temperature was reported last month.

 So it seems this subject is like a volcano, you never know when it would erupt again. And when it does, it would send shocking news.

      Thank you.