An artistic representation of the data showing the breaking of spatial inversion and rotational symmetries in the pseudogap region of superconducting materials – evidence that the pseudogap is a distinct phase of matter. Rings of light reflected from a superconductor reveal the broken symmetries. Image: Hsieh Lab/Caltech
One of the greatest mysteries of experimental physics is how so-called high-temperature superconducting materials work. Despite their name, high-temperature superconductors – materials that carry electrical current with no resistance – operate at chilly temperatures less than minus 135 degrees Celsius. They can be used to make superefficient power cables, medical MRIs, particle accelerators, and other devices. Cracking the mystery of how these materials actually work could lead to superconducting devices that operate at room temperatures--and could revolutionize electrical devices, including laptops and phones.
In a new paper in the journal Nature Physics, researchers at Caltech have at last solved one piece of this enduring puzzle. They have confirmed that a transitional phase of matter called the pseudogap – one that occurs before these materials are cooled down to become superconducting – represents a distinct state of matter, with properties very different from those of the superconducting state itself.
When matter transitions from one state, or phase, to another--say, water freezing into ice – there is a change in the ordering pattern of the materials' particles. Physicists previously had detected hints of some type of ordering of electrons inside the pseudogap state. But exactly how they were ordering – and whether that ordering constituted a new state of matter – was unclear until now.
"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," says David Hsieh, professor of physics at Caltech and principal investigator of the new research. "We have discovered that in the pseudogap state, electrons form a highly unusual pattern that breaks nearly all of the symmetries of space. This provides a very compelling clue to the actual origin of the pseudogap state and could lead to a new understanding of how high-temperature superconductors work."
The phenomenon of superconductivity was first discovered in 1911. When certain materials are chilled to super-cold temperatures, as low as a few degrees above absolute zero (a few degrees Kelvin), they carry electrical current with no resistance, so that no heat or energy is lost. In contrast, our laptops are not made of superconducting materials and therefore experience electrical resistance and heat up.
Chilling materials to such extremely low temperatures requires liquid helium. However, because liquid helium is rare and expensive, physicists have been searching for materials that can function as superconductors at ever-higher temperatures. The so-called high-temperature superconductors, discovered in 1986, are now known to operate at temperatures up to 138 Kelvin (minus 135 degrees Celsius) and thus can be cooled with liquid nitrogen, which is more affordable than liquid helium. The question that has eluded physicists, however – despite three Nobel Prizes to date awarded in the field of superconductivity--is exactly how high-temperatures superconductors work.