A recent study published in Nature reveals a remarkable phenomenon: atoms within ultra-thin materials dance in response to pulses of light, twisting and untwisting like synchronized dancers. This intricate choreography unfolds at lightning speed – on the scale of a trillionth of a second – making it invisible to the naked eye and too fast for conventional scientific instruments to capture.
To unveil this atomic ballet, researchers from Cornell University and Stanford University turned to ultrafast electron diffraction. This cutting-edge technique utilizes incredibly brief bursts of electrons fired at a sample immediately after a laser pulse hits it. Think of it like an ultra-high-speed camera frozen on specific moments during the dance. By analyzing how these atomic layers scatter the electrons, scientists can reconstruct their movement over time.
The subject of this atomic performance is a special type of material known as moiré materials. These materials are constructed by stacking extremely thin sheets – just a few atoms thick – on top of each other with slight misalignments. This seemingly minor offset creates unique properties that can be tuned by further adjusting the angle between the layers.
“Imagine stacking two pieces of paper with a slight twist,” explains Jared Maxson, professor of physics at Cornell and co-corresponding author on the study. “The way they overlap creates interesting patterns – moiré patterns – and these patterns influence how the material behaves.”
These properties can be manipulated to make materials act like superconductors – allowing electricity to flow with zero resistance – or create unusual electronic behaviors, opening doors for innovations in quantum electronics and other cutting-edge technologies.
Previously, scientists believed that once stacked at a fixed angle, the structure of these moiré materials remained static. However, this groundbreaking research shows that the atoms within these layered structures are far from rigid. Instead, they exhibit dynamic movement, briefly twisting together more tightly upon exposure to light before springing back, much like a compressed spring releasing its energy.
“This finding challenges previous assumptions,” adds Fang Liu, project lead at Stanford and co-corresponding author. “We’ve seen that the atoms within these moiré unit cells perform almost like a circle dance.”
The success of this experiment hinged on both the development of specialized materials by Liu’s team at Stanford and Cornell’s home-built ultrafast electron diffraction instrument, equipped with a highly sensitive detector called EMPAD.
The EMPAD, originally designed for capturing still images, was repurposed in this study to act as an incredibly fast camera capable of capturing these fleeting atomic movements. “Most detectors would have blurred the signal,” says Maxson. “The EMPAD allowed us to see incredibly subtle features that could easily have been lost.”
This collaborative effort marks a significant milestone in our understanding of moiré materials. It demonstrates the power of ultrafast electron diffraction for visualizing nanoscale phenomena and opens exciting avenues for manipulating quantum behavior in real time using light. Future experiments will explore how different materials and twist angles respond to light pulses, paving the way for potentially revolutionary advances in fields ranging from superconductivity to quantum computing.
