Theoretical physicists reveal how twisting layers of material creates mysterious electron path deflection effects

In 2018, discoveries in materials science shocked the entire community. Ritesh Agarwal of the University of Pennsylvania said the research team has shown that two layers of graphene (honeycombed layers of carbon extracted from graphite) become superconductors when stacked at a precise “magic angle”. It is said that

This ignited the field of ‘twistronics’, revealing that layered materials can be twisted to unlock extraordinary material properties.

Building on this concept, Agarwal, Penn theoretical physicist Eugene Mele, and collaborators took twistronics into new territory.

In a study published in Nature, they investigated spirally stacked tungsten disulfide (WS2) crystals and discovered that by twisting these layers, they could use light to manipulate electrons. The result is similar to the Coriolis force, which curves the path of objects in a rotating frame, similar to the behavior of winds and ocean currents on Earth.

“What we discovered is that we can control the movement of electrons simply by twisting the material,” says Agarwal, the Srinivasa Ramanujan Distinguished Scholar in the School of Engineering and Applied Sciences. This phenomenon was especially noticeable when the researchers shined circularly polarized light on the WS2 spiral, causing the electrons to deflect in different directions based on the material’s internal twist.

The research team’s latest findings date back to the early days of the coronavirus pandemic lockdown, when labs were closed and first author Zurun (Judy) Gee was wrapping up her PhD.

Unable to conduct physics experiments in space, she shifted her focus to more theoretical research and collaborated with Mele, the Christopher H. Brown Distinguished Professor of Physics in the College of Arts and Sciences.

Together, they developed a theoretical model for the behavior of electrons in twisted environments, based on the conjecture that continuously twisted lattices create strange and complex landscapes in which electrons can exhibit novel quantum behaviors. .

“The structure of these materials is reminiscent of DNA or a spiral staircase. This means that the normal laws of periodicity in crystals, where atoms exist in a neat repeating pattern, no longer apply,” Ji said. says.

As 2021 arrives and pandemic restrictions are lifted, Agarwal explains during a scientific conference that his former colleague Song Jin at the University of Wisconsin-Madison is growing crystals by continuously twisting them into spirals. I learned. Realizing that Jin’s spirally twisted WS2 crystals were the perfect material to test Ji and Mele’s theory, Agarwal arranged for Jin to send a batch. The experimental results were interesting.

Mele said this effect reflects the Coriolis force, which is usually associated with the mysterious lateral deflections seen in rotating systems. Mathematically, this force is very similar to magnetic deflection, and explains why electrons behave as if there is no magnetic field. This insight was very important because it linked the interaction of crystal twisting with circularly polarized light.

Agarwal and Mele compare the electron response to the classical Hall effect, where a current flowing through a conductor is deflected laterally by a magnetic field. But whereas the Hall effect is driven by a magnetic field, here “twisted structures and Coriolis-like forces were guiding the electrons,” Mele said.

“This discovery was not just about discovering this force, but about understanding when and why it appears and, more importantly, when it should not appear.”

One of the big challenges was that once researchers realized that this Coriolis deflection could occur in twisted crystals, the idea seemed to work too well, Mele said. adds. This effect appeared so naturally in theory that it seemed difficult to switch off even in scenarios where the effect should not exist. It took nearly a year to establish the exact conditions under which this phenomenon would be observed or suppressed.

Agarwal describes the behavior of electrons in these materials as being “like sliding down a slide at a water park. If the electrons were to slide down a straight slide like in a traditional material lattice, everything would be smooth. But , when you send an electron down a spiral slide, it’s a completely different experience, the electron feels a force pushing it in a different direction, and it’s like it feels a little “vertigo” when it comes out the other end . ”

This “vertigo” is particularly exciting for the team because it introduces a new level of control over the movement of electrons, achieved purely through geometric twisting of the material. Additionally, this study also revealed strong optical nonlinearities. This means that the material’s response to light has been greatly amplified.

“Optical nonlinearity is weak in common materials, but it is significantly stronger in our twisted system, suggesting potential applications in photonic devices and sensors,” Agarwal said.

Another aspect of the study was the moiré pattern, which is the result of small angular misalignments between layers that play a key role in the effect. In this system, the length scale of the moiré created by twisting is comparable to the wavelength of light, allowing light to interact strongly with the structure of the material.

“This interaction between light and Moiré patterns adds a layer of complexity that enhances the effects we are observing. And this coupling allows light to very effectively control the behavior of electrons. ” says Agarwal.

When light interacted with the twisted structure, the researchers observed complex wave functions and behaviors not seen in normal two-dimensional materials. This result, coupled with the concept of “higher-order quantum geometries” such as Berry curvature multipoles, provides insight into the quantum state and behavior of materials.

These findings suggest that twisting fundamentally changes the electronic structure, creating new pathways to control electron flow in ways not possible with conventional materials.

And finally, this study shows that the strength of the optical Hall effect can be fine-tuned by slightly adjusting the thickness and handedness of the WS2 spiral. This tunability suggests that these twisted structures could be powerful tools for designing new quantum materials with highly tunable properties.

“There have always been limits to how we can manipulate the behavior of electrons in materials,” Agarwal says. “What we’ve shown here is that by controlling the twist, you can introduce entirely new properties.” .

“We’ve only scratched the surface of what’s actually possible. With spiral structures providing new ways for photons and electrons to interact, we’re about to step into something entirely new.” What more can the system reveal?”