Absolute Hero: Heike Onnes's Discovery of Superconductors Turns 100 [Slide Show]
A century after the discovery of materials that conduct electricity without resistance, the applications remain disappointingly limited. That may be about to change
Credits: Leiden Institute of Physics
THE IRON AGE OF SUPERCONDUCTORS In 2006 Hideo Hosono (pictured) and his team at the Tokyo Institute of Technology were trying to create new materials for flat-panel displays when they stumbled on an entirely new kind of high-temperature superconductor. Their material, made of iron, oxygen, phosphorus and the rare element lanthanum, created a buzz in physics circles and sparked an explosion in research. No one expected an iron compound could be superconducting, because iron is magnetic and magnetic fields were thought to be incompatible with superconductivity.
THE WOODSTOCK OF PHYSICS: Physicists crammed into a hastily arranged session at a 1987 meeting of the American Physical Society—a session that went on until 3:15 A.M. the next morning and became known as the "Woodstock of physics". Outside, hundreds of others watched on closed-circuit TV screens.
The excitement, rather unusual for a physics meeting, was justifiable. Just a few months before, IBM researchers Karl Müller and Johannes Bednorz had unexpectedly discovered that family of ceramic materials, called cuprates, became superconducting at higher temperatures than the 30 K that physicists believed was the limit for superconductivity. All of a sudden it seemed that the sky was the limit. Whereas the BCS seemed to require low temperatures, perhaps this new phenomenon could extend even to room temperature.
Researchers soon realized that the cuprates did not obey BCS theory. To this day, there is no conclusive explanation for how high-temperature superconductors work: This question remains one of the holy grails of theoretical physics. Müller and Bednorz received the Nobel Prize in Physics in 1987.
American Institute of Physics
CRACKING THE PUZZLE: In 1957 the U.S. physicists John Bardeen (who had already shared a Nobel Prize for the discovery of the transistor), Leon Neil Cooper and John Robert Schrieffer finally cracked the puzzle of superconductivity—a goal that had eluded minds of the caliber of Albert Einstein, Werner Heisenberg and Richard Feynman. Their theory became known by the first letters of their last names—BCS theory—and earned them a Nobel Prize in Physics 1972.
In metals atoms donate one of more of their electrons to a common pool of "conduction electrons". The positively charged ions left behind form a regular crystal lattice. When electrons collide with the ions, they may lose energy, which is why ordinary conductors dissipate heat.
Graham P. Collins described BCS theory in the August 2009 issue of
Scientific American. When an electron moves inside the metal its negative charge, Collins wrote, "tugs on the metal's ions" and "leaves in its wake a temporary region of distorted lattice"; this distortion travels along the lattice and is, in essence, a quantum of mechanical vibration. Because the distorted region has a slightly increased density of positive charge, a second electron may experience a small attractive force toward it.
In a superconducting state, when the material is cold enough, two electrons can then travel close together—despite having like charges—and form a common quantum state of twice the charge, called a Cooper pair. These double particles in turn form wavelike quantum states that can travel along the material unimpeded. National High Magnetic Field Laboratory
ONNES'S LOST NOTEBOOK: "Mercury practically zero." Onnes wrote those words in his notebook at 4 P.M. on April 8, 1911, when his assistant Gilles Holst reported that at the temperature of 3 K the resistance in mercury dropped so low it could no longer be measured. The historic remark is on the right-hand side, seventh line from the bottom, and reads in Dutch: "
Kwik nagenoeg nul."
This notebook, numbered 56, was mislabeled as being from 1910 instead of 1911, and was scribbled in to the point that the handwriting is almost incomprehensible. So, for many decades, historians thought that the 1911 notebook recording the great discovery had been lost. But in October 2009 recently retired physicist Peter Kes of the University of Leiden began a painstaking study of Onnes's notebooks and realized notebook 56 ended by recording the visit of Marie Curie to the lab. Curie had visited in July 1911, which means that the notebook could not have been from the year before. "The deciphering was quite a job," Kes recalls.
The discovery cleared various misconceptions concerning superconductivity's discovery and, in particular, that it had happened by accident because a lab technician had fallen asleep. Boerhaave Museum Advertisement
WHEN RESISTANCE DISAPPEARED: This graph of how electrical resistance (vertical axis) depends on temperature (horizontal axis) shows the resistance of mercury suddenly dropping to zero at around 3 K.
Courtesy of Dirk van Delft
THE CRYOGENIC APPARATUS: This diagram details the system of cryogenic pumps and flasks built by Onnes and his assistant Gerrit Jan Flim; the capillaries holding mercury were made by the lab's master glassblower Oskar Kesselring.
Leiden Institute of Physics
HEIKE KAMERLINGH ONNES: Onnes (1853–1926) was awarded the Nobel Prize in Physics in 1913—not for discovering superconductivity but for liquefying helium. But it was his discovery of the former that made him famous.
Onnes observed the phenomenon while trying to answer an open question about electrons.
At the temperatures of liquid helium, ordinary thermometers would freeze. Physicists could still measure temperature by measuring the electrical resistance in metals, based on the empirical observation that electrical resistance was directly proportional to temperature. That idea, however, had never been tested close to absolute zero.
Two competing theories existed for what would happen: Lord Kelvin had postulated that electrons in a metal would "freeze," and thus stop moving. The conductor would therefore become an insulator. German theorist Paul Drude instead maintained that electrons inside a metal acted like a gas, so that resistance would gradually and linearly approach zero.
As Onnes showed, neither theory was correct.
Kamerlingh Onnes Laboratory
THE LEIDEN CRYOGENIC LABORATORY: Built by Heike Kamerlingh Onnes and pictured here in 1895, the Leiden Cryogenic Laboratory was, in the words of science historian Dirk van Delft, "a profusion of tubes, taps, gas flasks, gas holders, liquefiers, Dewar flasks, cryostats, clattering pumps and droning engines, glassblowing and other workshops, instruments and appliances for scientific research."
It was here that on July 10, 1908, Onnes managed to liquefy helium—at 5 kelvins (about –268 degrees Celsius)—for the first time in history. That was the lowest temperature ever achieved, and no other lab would be able to match it until 1923. (The noble gas itself had only been isolated in 1895, by Scottish chemist William Ramsay; James Dewar's vacuum flask, aka the thermos bottle, had been invented in 1892.)
The building that hosted the lab is now the Leiden Law School.
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