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New state of matter discovered by superconductivity gurus

The 'pseudogap' state could be key to high-temperature superconductors

Physicists may be one step closer to cracking the mystery behind high-temperature superconductivity, as they confirm that a new distinct state of matter forms just before a material enters its superconductive state.

Superconductors conduct electricity with zero resistance, a desirable property that is severely hindered by the large amounts of energy needed to force the material into its superconducting mode.

They need to be chilled below -135oC by using liquid helium or liquid nitrogen, which is expensive. If its electrical energy could be harnessed at high temperatures, it could radically change the way power is consumed by making it 100 per cent efficient.

The race is on to make a room-temperature superconductor. Now and again, physicists reach a new record temperature for a superconductor, but no one really knows how it works.

There isn’t a complete theory that describes the mechanism that causes a material to magically switch from non-superconducting to a superconducting state. Physicists call this a phase change. A common example of a phase change is water freezing to ice.

A paper published in Nature Physics [paywalled] shows that there is another phase transition in between the non-superconducting and superconducting states. Physicists working at the California Institute of Technology call it the “pseudogap.” It’s a distinct state of matter and has properties very different from the superconducting state.

Before the electrons order themselves in a highly structured state in superconductivity, they form a “highly unusual pattern that breaks nearly all of the symmetries of space,” David Hsieh, coauthor of the paper and assistant professor at the California Institute of Technology said.

“A peculiar property of all these high-temperature superconductors is that just before they enter the superconducting state, they invariably first enter the pseudogap state, whose origins are equally if not more mysterious than the superconducting state itself," Hsieh said.

The phase change was confirmed after examining the properties of electrons in YBa2Cu3Oy – a type of copper oxide – that cooled before the material reached its critical temperature. A phase change occurs when the old phase is “broken.”

Physicists know the old phase has been broken because the new phase enters a completely different structure that breaks time-reversal, and inversion and rotational symmetry – both spatial symmetries.

Water is a random jumble of H2O particles and has no symmetries, but as soon as it turns to ice the molecules arrange themselves in hexagonal structure. It no longer looks the same in every direction – its rotational symmetry has broken.

“The discovery of broken inversion and rotational symmetries in the pseudogap drastically narrows down the set of possibilities for how the electrons are self-organizing in this phase,” says Hsieh. “In some ways, this unusual phase may turn out to be the most interesting aspect of these superconducting materials.”

By understanding how the electrons behave in the pseudogap, it could shed light on the next phase change, where they pair up and overcome electromagnetic repulsion in superconductivity. ®

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