Resistance Zero: A Century of Chasing Superconductors

Discovery by accident of a cold experiment

In 1911, the Dutch physicist Heike Kamerlingh Onnes was doing something only his laboratory in Leiden could do: working with liquid helium, which he had been the first to produce three years earlier, at temperatures within a few degrees of absolute zero. He was measuring how the electrical resistance of metals behaved as they got extremely cold. The expectation was that resistance would fall smoothly and level off. Instead, when he cooled mercury to about 4.2 kelvin, its resistance did not just fall. It vanished, abruptly and completely, dropping to something indistinguishable from zero. A current set flowing in a loop of such material would circulate, in principle, forever, without a battery and without fading. Onnes called the new state superconductivity, and it won him a Nobel Prize in 1913.

More than just zero resistance

For two decades superconductivity was understood as simply the disappearance of resistance, which turned out to be only half the story. In 1933 Walther Meissner and Robert Ochsenfeld found that a superconductor does something stranger. It actively expels magnetic fields from its interior. Cool a superconductor in a magnetic field, and at the critical temperature the field is pushed out, as if the material refuses to let magnetism inside. This is the Meissner effect, and it is why a magnet will hover in midair above a piece of superconductor, held up by the field it cannot penetrate. The expulsion is a genuinely separate property from zero resistance, and any real theory had to explain both.

Why it happens

That theory took until 1957, almost half a century after the discovery. John Bardeen, Leon Cooper, and Robert Schrieffer worked out what is now called BCS theory, and the heart of it is counterintuitive. Electrons normally repel each other, being negative charges, and they scatter off the vibrating atoms of a metal, which is what ordinary resistance is. But at low temperature, in the right material, an electron moving through the lattice tugs the positive ions slightly toward it, creating a faint trail of positive charge that a second electron is drawn into. The upshot is a weak attraction between electrons, mediated by the vibrations of the lattice, and it binds them into loose couples called Cooper pairs. These pairs behave completely differently from single electrons. They lock into a single coordinated quantum state that flows through the material without scattering, because to stop one pair you would have to disrupt all of them at once. Resistance disappears because there is no longer any way to lose energy in small amounts. BCS theory earned its authors the Nobel Prize in 1972, and for Bardeen it was a second one, having already shared the prize for inventing the transistor.

The race to warm it up

The practical catch was always temperature. Cooling things to a few degrees above absolute zero with liquid helium is expensive and finicky, which kept superconductors confined to specialized uses. So the history of the field became a race to raise the critical temperature, and for a long time it crept upward slowly.

 Critical temperature of selected superconductors (kelvin)

 250 +                                              * LaH10 (2019, 170 GPa)
     |                                          * H3S (2015, 150 GPa)
 200 +
     |
 150 +
     |              (liquid nitrogen, 77 K) - - - - - - - - - -
  92 +                            * YBCO (1987)
     |                       * cuprate (1986)
  23 +              * Nb3Ge (1973)
   4 +  * Hg (1911)
     +--+------+------+------+------+------+------+------+----
       1911   1941   1973   1986  1987   1993   2015   2019

The slow climb broke open in 1986, when Georg Bednorz and Alex Müller, working at IBM in Zurich, found superconductivity in a copper-oxide ceramic at a temperature higher than BCS materials had reached. Within a year other groups pushed the same family of cuprates above 92 kelvin. That number mattered enormously, because it is above 77 kelvin, the temperature of liquid nitrogen, which is cheap and easy to handle, unlike liquid helium. Suddenly superconductors could be cooled with a coolant you can buy by the tankful. Bednorz and Müller had their Nobel in 1987, astonishingly fast. The awkward part, still not fully resolved, is that nobody is certain why the cuprates superconduct at such high temperatures. The original BCS mechanism does not seem sufficient, and the high-temperature cuprate problem remains one of the open questions in condensed matter physics.

Where things stand

The most recent surprises have come from squeezing hydrogen-rich compounds under enormous pressure. In 2015 a sulfur hydride was found to superconduct at around 203 kelvin, and in 2019 a lanthanum hydride reached roughly 250 kelvin, only about 23 degrees below the freezing point of water. The catch is the pressure: these materials only work when crushed to well over a million times atmospheric pressure, between two diamond anvils, which makes them laboratory curiosities rather than useful devices. The dream of a material that superconducts at room temperature and ordinary pressure has not been achieved, and the field has learned to be cautious about claims that it has.

Even without that final prize, superconductors are already woven into modern technology. The powerful magnets inside every hospital MRI scanner are superconducting coils. So are the magnets that steer particles around the Large Hadron Collider, and the ones being developed to confine plasma in experimental fusion reactors. A current that flows forever, a magnet that floats, electrons that have learned to move in perfect step: a hundred years on, superconductivity remains one of the clearest places where quantum behavior breaks the surface and shows up in the everyday world, provided you keep things cold enough.