Physicists have discovered a new phase of matter, a “chiral Bose-liquid state.” This state, discovered by studying kinetic frustration in quantum systems, exhibits stable properties such as invariant electron spin and long-range entanglement. The discovery, which requires strong magnetic fields to observe, expands our understanding of the physical world and may have applications in error-tolerant digital data encoding.
For experimental physicists, quantum frustration leads to a fundamental discovery
The “chiral boose-liquid state” is a new phase of matter, according to the UMass Amherst professor.
A team of physicists, including assistant professor Tigran Sedrakian of the University of Massachusetts, recently announced in the journal Nature that they discovered a new phase of matter. Dubbed the “chiral Bose-liquid state,” the discovery opens a new path in the centuries-old effort to understand the nature of the physical world.
In everyday conditions, matter can be solid, liquid, or gaseous. But once you dare to go beyond the everyday – in temperatures approaching absolute zerothings smaller than part of an atom or which have extremely low energy states – the world looks very different. “At these limits you find quantum states of matter,” says Sedrakian, “and they are much wilder than the three classical states that we encounter in our everyday lives.”
Sedrakian has spent years studying these wild quantum states, and he is particularly interested in the possibility of what physicists call “band degeneracy,” “trench stripes” or “kinetic frustration” in strongly interacting quantum matter.
Depiction of the trench band that thwarts particles and leads to a chiral Bose-liquid state. Credit: Tigran Sedrakian
Normally, the particles in any system bump into each other and thus produce predictable effects, like billiard balls hitting each other and then reacting in a predictable pattern. In other words, effects and particles are related. But in a frustrated quantum system, there are infinite possibilities that arise from the interaction of particles—perhaps the billiard ball levitates or moves away at an impossible angle—and some of these infinite possibilities can lead to new quantum states.
What Sedrakian and his colleagues have done is create a frustrating machine: a double-layer semiconductor device. The topmost layer is rich in electrons and these electrons can move freely. The bottom layer is full of “holes” or places that a wandering electron can occupy. The two layers are then brought extremely close to each other – interatomic close.
If the number of electrons in the upper layer and holes in the lower layer were the same, then you would expect to see the particles acting in a correlated manner, but Sedrakian and his colleagues designed the lower layer so that there is a local imbalance between the number of electrons and holes in the lower layer . “It’s like a game of musical chairs,” says Sedrakian, “designed to thwart electrons. “Instead of each electron having one chair to go to, it now has to climb and have many options for where to ‘sit’.”
This frustration gives rise to the new chiral end state, which has a number of surprising features. For example, if you cool quantum matter in a chiral state to absolute zero, the electrons freeze in a predictable pattern, and the resulting charge-neutral particles in that state will spin either clockwise or counterclockwise. Even if you smash another particle into one of these electrons or introduce a magnetic field, you can’t change its spin – it’s surprisingly robust and can even be used to encode digital data in an error-resistant way.
Even more surprising is what happens when an external particle crashes into one of the particles in the chiral final state. To go back to the pool ball metaphor, you would expect to send the eighth ball flying when the cue ball hits it. But if the pool balls were in a chiral Bose-fluid state, all 15 of them would react in exactly the same way when the eight balls were hit. This effect is due to the long-range entanglement present in this quantum system.
The chiral Bose-liquid state is difficult to observe, which is why it has remained hidden for so long. To do this, the team of scientists, including theoretical physicists Rui Wang and Baigeng Wang (both from Nanjing University), as well as experimental physicists Lingjie Du (Nanjing University) and Rui-Rui Du (Beijing University), designed a theory and an experiment that uses an extremely strong magnetic field that is able to measure the motions of electrons as they compete for chairs.
“At the edge of a double-layer semiconductor, electrons and holes move at the same speed,” says Lingzhi Du. “This results in a helical transport that can be further modulated by external magnetic fields as the electron and hole channels gradually separate at higher fields.” The magneto-transport experiments therefore successfully reveal the first evidence of the chiral Bose liquid , which the authors also call “excitonic topological order” in the published paper.
Reference: “Excitonic Topological Order in Unbalanced Electron-Hole Double Layers” by Rui Wang, Tigran A. Sedrakyan, Baigeng Wang, Lingjie Du, and Rui-Rui Du, 14 Jun 2023, Nature.
DOI: 10.1038/s41586-023-06065-w
This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Jiangsu Innovative Talents and Entrepreneurs Program, the Xiaomi Foundation, the Chinese Academy of Sciences, and the National Science Foundation.