A RADICAL new theory about superconductors claims that protons, and not just electrons, can travel unobstructed through metals. And intriguingly, proton superconductivity should work at room temperature.
Up to now, researchers have mainly studied the superconductivity that occurs in certain metals at low temperatures. In these conditions, electrons form so-called Cooper pairs 鈥� two electrons that are linked together by vibrations in the lattice. Each pair ends up in a peculiar quantum state that causes it to be 鈥渄elocalised鈥�, so that it can exist throughout the lattice rather than at any one fixed position, and can thus move smoothly through the lattice. It鈥檚 a cascade of these Cooper pairs that gives rise to superconductivity.
Now physicist Julian Brown of the University of Oxford is arguing that protons can also form pairs and sneak through the metallic lattice in a similar manner. In theory, the protons should superconduct, he says.
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Brown predicts some important advantages over electron superconductivity (). As protons are much heavier than electrons, the pairing of protons is less likely to be destroyed by high temperatures than the pairing of electrons. 鈥淲e should see this phenomenon at room temperature,鈥� he says.
鈥淧rotons are much heavier than electrons, so they should act as superconductors even at room temperature鈥�
But the higher mass of protons in such pairs also means that they would interact more easily with impurities in the metal than their electron counterparts, which would destroy their pairing. As a result, proton superconductivity can only occur over the tiny distances between impurities, says Brown.
That could be one reason why no one has ever observed the phenomenon. Another could be that proton superconductivity can only occur when the metal lattice is packed with hydrogen at a ratio approaching one hydrogen atom for each metal atom 鈥� to supply sufficient protons. 鈥淭hat鈥檚 very difficult to achieve,鈥� says Brown.
But there are some experimental clues that seem to back up his idea. In 2003, researchers at the Rutherford Appleton Laboratory in Oxfordshire, UK, fired neutrons at samples of niobium and palladium packed with hydrogen and deuterium atoms. Instead of scattering off the hydrogen and deuterium nuclei as expected, the neutrons appeared to sail straight through them. Brown says his theory offers an explanation: hydrogen nuclei (which have one proton each) or deuterium nuclei (which have one proton and one neutron each) pair up and become delocalised, making collisions between the projectile neutrons and the nuclei less likely.
Brown鈥檚 theory is gaining support. 鈥淭he idea is new, interesting and attractive,鈥� says Sasha Alexandrov, an expert on superconductivity at Loughborough University in the UK.
But Alexandrov cautions that superconductivity can be decisively proved only by the observation of both zero resistance and the Meissner effect, in which a superconductor excludes an external magnetic field. These effects are yet to be seen where protons are concerned.
