Quantum frustration leads to a new state of matter: chiral Bose-liquid state

Sedrakyan is especially interested in the phenomena of moat bands, kinetic frustration, and band degeneracy in strongly interacting quantum matter, which governs the alignment of localized states, hindered particle motion, and energy levels. This research helps us gain insights into the exotic states of matter that exist beyond our everyday experiences.

The research team consisted of a mixture of theoretical and experimental physicists to discover this novel state of matter, including Rui Wang, Lingjie Du, and Baigeng Wang from Nanjing University and Rui-Rui Du from Peking University.

Frustrated quantum systems

In most systems, particles interact with each other in a predictable manner, similar to billiard balls colliding and flying off in predictable directions. However, this is not true for frustrated quantum systems. 

In quantum systems, there are billions of particles and billions of parameters governing their interactions. This means that not all quantum systems can be well-described and understood.

Some physical systems have competing interactions between particles or components that prevent the system from achieving its lowest energy state. This leads to a state of frustration, where the system cannot fully optimize its energy or configuration due to conflicting influences.

In these systems, the arrangement and behavior of particles or spins become highly complex and can give rise to emergent phenomena and novel states of matter. Sedrakyan and his team exploited this phenomenon.

Engineering a frustration machine

The team developed a bilayer semiconductor device or a frustration. The top layer of the device was made to be electron-rich, in which the electrons can freely move about. The bottom layer only had holes, which are slots an electron can occupy. 

By bringing an electron-rich top layer and a bottom layer filled with holes close together, they introduced a local imbalance that frustrated the electrons, forcing them to occupy multiple possibilities. If the number of electrons and holes were equal, the particles would display correlated behavior, but the imbalance causes frustration. 

In a press release, Sedrakyan compares this situation to the game of musical chairs. "It is designed to frustrate the electrons. Instead of each electron having one chair to go to, they must now scramble and have many possibilities to sit," he said.

The frustration leads to a unique chiral edge state (chiral Bose-liquid state) with remarkable properties. When the quantum matter in this state is cooled to absolute zero, the electrons form a predictable pattern, and the emergent charge-neutral quasiparticles all spin in the same direction.

Their spins remain unchanged even when subjected to collisions or magnetic fields, offering robustness and potential for fault-tolerant data encoding, which is especially useful for quantum computing. 

Interestingly, in the chiral Bose-liquid state, the interaction of particles becomes highly correlated. Even when an outside particle collides, all particles react in unison due to long-range entanglement.

The key to observing this new state of matter depended upon the use of an extremely strong magnetic field capable of measuring electron movements as they race for the holes. 

These experiments reveal the first evidence of the chiral Bose-liquid state or the excitonic topological order.

The findings of the study are published in the journal Nature.