Physicists have just observed in a real material a mysterious state of matter that was first predicted 40 years ago. And if you thought quantum mechanics couldn’t get any weirder, think again.
An international team has observed for the first time a quantum spin liquid, a state in which electrons break apart and behave in a very curious way. Electrons in typical magnetic materials are well aligned when the material is cooled down to absolute zero. But in a quantum spin liquid, electrons are not organized.
"This is a new quantum state of matter, which has been predicted but hasn't been seen before," Dr. Johannes Knolle of Cambridge's Cavendish Laboratory, one of the paper's co-authors, said in a statement. It should be noted, it is not really a "liquid" per se – rather, the term indicates that electrons are not lined up as they should be.
Electrons are thought of as fundamental indivisible particles, but they can also be mathematically described by two quasiparticles bound together, one representing the spin and one the charge. Quasiparticles are essentially the fundamental properties of the electron acting as individual particles, although they can’t move freely through space.
In a quantum spin liquid, the spin and charge quasiparticle can move independently from each other and the electron is broken. The free spin quasiparticle is also a Majorana fermion, a curious excitation that is its own antiparticle. The first Majorana fermion was only discovered last October.
"Until recently, we didn't even know what the experimental fingerprints of a quantum spin liquid would look like," said paper co-author Dr. Dmitry Kovrizhin. "One thing we've done in previous work is to ask, if I were performing experiments on a possible quantum spin liquid, what would I observe?"
The new state was observed in crystals of ruthenium chloride (RuCl3). The team at the Oak Ridge National Laboratory shot neutrons at the crystals and looked at the magnetic properties. The results are published in Nature Materials.
"This is a new addition to a short list of known quantum states of matter," said Knolle.
"It's an important step for our understanding of quantum matter," added Kovrizhin. "It's fun to have another new quantum state that we've never seen before – it presents us with new possibilities to try new things."
The understanding of quantum spin liquid could have consequences for room-temperature superconductors and quantum computers. Quantum spin liquid could even be used as memory storage for quantum computers.