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Scientists Uncover a New Phase of Matter – The Chiral Bose-Liquid State

Abstract physics illustration showing a texture of blue waves dotted with yellow particles
Credit: Gerd Altmann / Pixabay.
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Researchers at Nanjing University and the University of Massachusetts Amherst have discovered a new phase of matter. Published in Nature, the observation of this new “chiral bose-liquid state” is a significant step forward for quantum physicists in describing how charged particles interact at the quantum level.

Exploring unusual quantum states

There are three classical states of matter that describe everything that we encounter in daily life – solid, liquid and gas. But at the atomic scale, and at extremely low temperatures approaching absolute zero, strange new quantum states can emerge.

“You find quantum states of matter way out on these fringes,”
said study co-author Tigran Sedrakyan, assistant professor of physics at the University of Massachusetts Amherst. “And they are much wilder than the three classical states we encounter in our everyday lives.”

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In a classical system, actions and reactions are highly predictable. If you roll a bowling ball toward a set of pins, then the bowling ball will predictably knock over the pins. But in a so-called frustrated quantum system, there are infinite possibilities for what might happen next.

Frustration arises when there are many different interactions among particles that are fighting to outcompete the others, leading to difficulties in establishing the true ground state of the system. With the system unable to come to rest, almost anything can happen. The bowling ball might shoot off in a new direction, for example, or inexplicably start levitating.

Theoretical and experimental physicists, such as Sedrakyan and his colleagues, are interested in frustrated systems because studying how particles interact in these fringe scenarios can tell us more about how the sub-atomic world works. Ultimately, quantum physicists want to characterize all possible phases of matter, building a grand picture of how matter behaves in our world.

Researchers engineer a semiconductor “frustration machine” to study quantum states

In their new paper, Sedrakyan and colleagues describe the engineering of a new bilayer semiconducting device that can act as a “frustration machine” for quantum experiments.

The bilayer device consists of an upper layer that is very electron-rich and a lower layer that is filled with a larger number of “holes” for these electrons to occupy.

The imbalance between the roaming electrons and the many holes in the bottom later induces a state of frustration in the system.

“It’s like a game of musical chairs,” Sedrakyan said, “designed to frustrate the electrons. Instead of each electron having one chair to go to, they must now scramble and have many possibilities in where they ‘sit’.”

“On the edge of the semiconductor bilayer, electrons and holes move with the same velocities,”
added co-author Lingjie Du, a professor at the Nanjing University School of Physics. “This leads to helical-like transport, which can be further modulated by external magnetic fields as the electron and hole channels are gradually separated under higher fields.”

As they describe in the new paper, when the researchers applied a strong external magnetic field across the bilayer device, they were able to observe a new chiral edge state – the chiral bose-liquid state – for the first time.

The chiral bose-liquid state

This newly-discovered quantum state has a few particularly notable oddities. When quantum matter in the chiral bose-liquid state is cooled down to absolute zero, all electrons present freeze in a very predictable way, and any emergent neutral quasiparticles will adopt the same quantum spin number. Even when another particle is smashed into one of the electrons, or if another magnetic field is introduced, these spin values do not change.

The researchers also observed some long-range quantum entanglement in this system. This means that if an outside particle collides with a particle kept in this chiral bose-liquid state, all of the other particles in the state will react together in the same way. Picture that bowling ball striking the lead pin and all of the other pins simultaneously falling in the same way, even though they themselves were not struck by the ball or another pin.

Discovering new states of matter

The resilience of particles in the chiral bose-liquid state is of particular interest to the researchers. For example, they suggest that the fixed spin values could one day be used to encode digital data for quantum computers in a fault-tolerant way. The researchers also say that their work could enable new ways of researching similar exotic states of matter.

Reference: Wang R, Sedrakyan TA, Wang B, Du L, Du RR. Excitonic topological order in imbalanced electron–hole bilayers. Nature. 2023:1-6. doi: 10.1038/s41586-023-06065-w

This article is a rework of a press release issued by the University of Massachusetts Amherst. Material has been edited for length and content.