The physics of raising your hand
In our daily lives, many things have two sides or polarities—such as left and right, up and down, positive and negative, “To be, or not to be”. Often, there’s no preference for which side to choose. Imagine, for example, a classroom where the teacher asks students to raise their hands. Some students will raise their right hands, while others will raise their left. Overall, it makes no difference whether hands are raised left or right.
Now, if the teacher asks everyone to switch hands—putting down the raised hand and raising the other—again, it doesn’t change the overall situation: students randomly raise either their left or right hands.
In physics, this property is known as “symmetry,” which describes a state that remains unchanged under certain operations. In this scenario, the hand-raising process demonstrates symmetry, as switching hands does not affect the overall state. Symmetry plays an essential role in modern physics and is related to one of the most fascinating phenomena in physics: spontaneous symmetry breaking.
Our study establishes semiconducting moiré superlattices as an intriguing platform to realize exotic states of excitons, which will also open up novel device concepts in photonics and quantum information science.
Richen Xiong
A fridge magnet is a spontaneous symmetry-breaking
Imagine a teacher asking all students to raise their hands, and suddenly, everyone raises their right hand without being instructed. It seems as if they communicated secretly, resulting in a unanimous decision. In this case, the symmetry between left and right hands is broken, with all students choosing the same side. This is an example of “spontaneous symmetry breaking”.
While the classroom scenario is just a thought experiment, spontaneous symmetry breaking happens frequently in the microscopic physical world and has significant applications around us.
Take fridge magnets, for example—common objects in our daily lives. The reason they can attract or repel each other stems from a phenomenon known as “ferromagnetism.” Like your left and right hands, ferromagnet electrons have “spins” with spin-up and spin-down polarities. (Here, spin refers to an intrinsic degree of freedom of electrons.)
When you hold a non-magnet, the electron spins inside it points in random directions, resulting in no net magnetic effect. But in a magnet, spontaneous symmetry breaking happens, and all the spins align in the same direction (say, spin-up). This alignment involves an immense number of electrons (about 1024 spins), far greater than the number of people on Earth.
The behaviour of these electron spins is dictated by quantum mechanics. Their ability to align and “communicate” stems from the Coulomb interaction between electrons. This showcases how a small change at the microscopic level can have a large-scale effect, resulting in a magnet with a unified spin state. They attract or repel when placed near another magnet based on this collective spin alignment.
Credit. Author
From fermions to bosons
As seen in common materials, spontaneous symmetry breaking in electron spins leads to magnetism. These electrons belong to a group of particles known as fermions. Another category, bosons, follows different quantum rules. While fermions include electrons, protons, and neutrons, bosons encompass particles like photons. Together, they form the building blocks of the universe.
Everyday magnetism is largely a result of fermions, such as electrons. However, bosons can theoretically exhibit magnetic phases and are predicted to display unusual behaviours. Despite this, such magnetic phases in bosons have mainly been demonstrated in quantum simulations using a few cold atoms and remain difficult to observe in condensed matter systems involving large numbers of atoms, like everyday objects.
Recent findings reveal evidence of magnetism in bosons within two layers of semiconductor materials. The research focused on excitons, composite bosons formed by an electron-hole pair. Using polarisation-resolved pump-probe techniques, magnetic phases were observed in exciton spins referred to as valley pseudospins in a bosonic correlated insulator made from angle-aligned semiconductor WSe2/WS2 moiré superlattices. Notably, the ferromagnetic phase of excitons spontaneously increased spin polarisation, with the circular helicity of the emission light exceeding that of the excitation. These magnetic phases were also shown to be highly tunable through exciton doping and external magnetic fields.
Credit. Author
Perspective
This study establishes semiconducting moiré superlattices as an intriguing platform for realising exotic excitons (bosons) states. Correlated excitons with valley pseudospins represent a rare experimental realisation of the two-component Bose–Hubbard model, potentially supporting a plethora of exotic phases even beyond ferromagnetic orders, such as superfluid states, supersolid states, and Wigner crystals.
Correlated excitons in semiconducting moiré systems also hold promise for novel applications in photonics and valleytronics, extending beyond what is achievable within a single-particle framework. The discovered magnetic phases of exciton spin, along with their sensitivity to magnetic fields and exciton doping, are consequences of strong correlations. They could enable efficient light source control and optical gates akin to electronic phase-change transistors. The ability to amplify valley polarisation through ferromagnetic order may also serve as a cornerstone for memory and error correction codes, paving the way for photonic quantum computation.
In essence, the research uncovered new spontaneous symmetry breaking phases in correlated bosons through advanced optical techniques. As a result of strong correlations, these magnetic phases lay the groundwork for future applications and may reveal more of nature’s untapped phenomena.
🔬🧫🧪🔍🤓👩🔬🦠🔭📚
Journal reference
Xiong, R., Brantly, S. L., Su, K., Nie, J. H., Zhang, Z., Banerjee, R., … & Jin, C. (2024). Tunable exciton valley-pseudospin orders in moiré superlattices. Nature Communications, 15(1), 4254. https://doi.org/10.1038/s41467-024-48725-z
