The cryogenic quantum twisting microscope and the phason mode of moiré systems

In the realm of modern physics, few materials have captured the imagination quite like graphene, a single layer of carbon atoms arranged in a honeycomb lattice. Its remarkable properties, such as exceptional electrical conductivity and strength, have made it a cornerstone of research into quantum materials. A recent study published in Nature ¹ takes this fascination a step further by exploring how electrons interact with vibrations in a special configuration called twisted bilayer graphene (TBG). This research introduces a groundbreaking technique called quantum twisting microscopy, offering new insights into the fundamental interactions that govern the behaviour of quantum materials.

The magic of twisted bilayer graphene

Graphene’s allure lies in its ability to conduct electricity with minimal resistance, thanks to the way its electrons move almost like particles of light. When two graphene sheets are stacked and twisted relative to each other, something extraordinary happens. The twist creates a moiré pattern, a kind of interference pattern that alters the electronic properties of the combined system. At certain “magic” twist angles, TBG can even become a superconductor, conducting electricity with zero resistance, or exhibit other exotic behaviors like “strange-metal” properties, where electrical resistance behaves unusually.

These phenomena are driven by how electrons interact with phonons, which are quantized vibrations of the atomic lattice, much like sound waves in a solid. Understanding this electron-phonon coupling (EPC) is crucial because it influences properties like electrical conductivity, heat transfer, and superconductivity. However, measuring these interactions directly, especially for specific phonon modes, has been a significant challenge—until now.

Cryogenic quantum twisting microscope

The researchers developed a novel tool called the cryogenic quantum twisting microscope, which allows them to probe these interactions with unprecedented precision. Imagine a microscope that not only sees atoms but can also twist two graphene layers relative to each other at very low temperatures, around 4 Kelvin (-269°C). This setup creates a tuneable interface between the layers, enabling the team to study how electrons tunnel from one layer to another while interacting with phonons.

The quantum twisting microscope works by measuring the electrical current and conductance as the twist angle between the graphene layers is adjusted. When electrons tunnel elastically (without losing energy), they reveal the electronic structure of the material. However, when they tunnel inelastically, losing energy to create a phonon, the quantum twisting microscope can map the phonon’s energy and momentum. This is a big deal because it provides a direct way to measure the strength of EPC for specific phonon modes, something previous techniques struggled to achieve, especially at low temperatures where phonon emission becomes significant.

The unique phason mode

One of the most exciting findings of this study is the identification of a special phonon mode in TBG called the phason mode. Unlike typical acoustic phonons, where atoms vibrate in sync and their coupling to electrons weakens as their momentum decreases, the phason mode behaves differently. This mode involves the two graphene layers vibrating out of sync, creating a layer-antisymmetric motion. The researchers found that the phason’s coupling to electrons actually increases as the twist angle gets smaller, amplifying its effect on the material’s electronic properties.

This unusual behaviour arises because the moiré pattern in TBG acts like a magnifying glass. Small atomic shifts in the layers cause significant distortions in the moiré pattern, which strongly influence the electrons’ behaviour. The study shows that this phason mode could play a key role in the exotic properties of TBG, such as superconductivity, especially at smaller twist angles close to the “magic” angle of about 1.1 degrees, where these effects are most pronounced.

A game-changer

The quantum twisting microscope’s ability to map phonon dispersions and EPC is a game-changer. Traditional methods, like Raman spectroscopy or electron energy loss spectroscopy, provide indirect or less precise measurements of these interactions. The quantum twisting microscope, however, directly measures the conductance changes caused by inelastic tunnelling, revealing the energy and momentum of phonons with high resolution. For instance, the study found that optical phonons (where atoms vibrate out of phase within a layer) have a coupling strength of about 300–350 meV, stronger than previously estimated by other methods. This suggests that TBG’s interactions are more complex than those in single-layer graphene.

The researchers also explored how the tunnelling depends on the contact area between the graphene layers and the density of electrons, controlled by an external voltage. By scanning the microscope’s tip over defects in a nearby material, they could even visualize the contact area, ensuring accurate measurements. These experiments confirmed that the phason mode’s coupling grows stronger at smaller twist angles, following a specific mathematical relationship, while the energy of the mode decreases linearly.

Unlocking the potential of moiré systems

The findings have profound implications for both fundamental physics and future technologies. By understanding how phonons and electrons interact in TBG, scientists can better predict and control properties like superconductivity, which could lead to more efficient electronic devices or quantum computers. The phason mode’s strong coupling at small twist angles suggests it could be a key player in achieving room-temperature superconductivity, a holy grail in materials science.

Moreover, the quantum twisting microscope’s versatility opens the door to studying other collective excitations in quantum materials, such as plasmons (oscillations of electron density) or magnons (magnetic excitations). This could lead to discoveries in a wide range of materials beyond graphene, from topological insulators to quantum spin liquids.

The discovery of the phason mode’s unique coupling in TBG highlights the complexity and potential of moiré systems, bringing us closer to unlocking their full technological promise. As we continue to twist and probe these remarkable materials, the future of quantum physics looks ever more exciting.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.