At Lawrence Berkeley National Laboratory in California, scientists have created a strange new form of matter — and it works at temperatures millions of times warmer than anything tried before.
The researchers built a tiny device made from two ultra-thin semiconductor layers, each just a few atoms thick. The top layer, called molybdenum diselenide, holds electrons. The bottom layer, tungsten diselenide, holds the opposite — places where electrons are missing, called holes. A razor-thin barrier of hexagonal boron nitride sits between them, close enough for electrons and holes to feel each other's pull and bind together into particles called excitons.
These excitons don't act like ordinary particles. When cooled to about 2 kelvins — that's minus 456 degrees Fahrenheit, close to the coldest temperature possible — the excitons stop behaving as separate individuals and start acting as one collective blob, all moving together. This state is called a Bose-Einstein condensate, often described as a "fifth state of matter." It's the same kind of quantum state scientists have pursued for over 60 years, but usually, it requires extreme lab conditions and ultra-cold gases.
The breakthrough, published in the journal Nature, is that the Berkeley team made this work in a solid semiconductor device at temperatures far less freezing than before. "What is unusual here is that the excitons are not just short-lived particles created by light," said Ruishi Qi, a co-first author who now works elsewhere but conducted the research at Berkeley Lab. "They form an equilibrium quantum fluid in a device that we can tune electrically and magnetically."
Even stranger: the condensate has hidden internal structure. The electrons and holes carry not just electrical charge, but also two quantum properties called spin and valley. These give the excitons different "flavors" — patterns like up-up, down-down, up-down, or down-up. The researchers discovered their BEC isn't one simple blob. It has two parts, each with its own flavor mix, and those parts can be switched around just by applying a small magnetic field.
"The exciting part is that this is not just a simple condensate," Qi said. "It has internal structures that we can control."
Principal investigator Feng Wang, who leads the project at Berkeley Lab and teaches at UC Berkeley, said earlier work couldn't easily tell whether excitons had formed a true condensate or what internal order they might have. "Our work provides a way to access that hidden structure directly," he said.
The discovery could someday help build better quantum computers and communications devices, the team said, because quantum fluids in solid materials are easier to work with than gases in vacuums. For now, the researchers hope to demonstrate even more useful behaviors from these switchable quantum states.