New particle charges cuprate superconductor theory

New fundamental particles aren't found only in particle accelerators, but can also be found hiding in plain pieces of ceramic, according to scientists at the University of Illinois in the US.

The newly formulated particle is a boson and has a charge of 2e, but does not consist of two electrons, the scientists say. The particle arises from the strong interactions between electrons and provides another piece of the high-temperature superconductivity puzzle.

"Hidden in the copper-oxide materials is a new particle, a boson with a charge of 2e," said Philip Phillips, a professor of physics at Illinois.

Surprisingly, this boson is not formed from excitations in electrons and ions. Instead, the particle emerges as a remnant of the strong interactions between electrons in the normal state.

"High- and low-energy scales are inextricably coupled in the cuprates," Phillips said. "Normally, when you remove a single electron from most systems, one empty state is created. In the cuprates, however, when you remove an electron, you create two empty states — both of which occur at low energy, but paradoxically, one of the states comes from the high-energy scale."

Experimental evidence of this 'one to two' phenomenon was first reported in 1990 by University of Groningen physicist George A. Sawatzky (now at the University of British Columbia) and colleagues. What was missing was a low-energy theory that explained how a high-energy state could live at low energy.

Phillips, with physics professor Robert G. Leigh and graduate student Ting-Pong Choy, have constructed a theory, and have shown that a charged 2e boson makes this all possible.

"When this 2e boson binds with a hole, the result is a new electronic state that has a charge of e," Phillips said. "In this case, the electron is a combination of this new state and the standard, low-energy state. Electrons are not as simple as we thought."

The new boson is an example of an emergent phenomenon — something that can't be seen in any of the constituents, but is present as the constituents interact with one another.

By constructing a low-energy theory of the cuprates, the researchers have moved a step closer to unravelling the mystery of high-temperature superconductivity.

"Until we understand how these materials behave in their normal state, we cannot understand the mechanism behind their high-temperature superconductivity," Phillips said.