Today's KNOWLEDGE Share
In new research published in Nature, Weizmann Institute scientists introduce a powerful tool to explore quantum phenomena—the cryogenic Quantum Twisting Microscope (QTM).
Using this pioneering instrument, researchers have observed—for the first time—the interactions between electrons and an exotic atomic vibration in twisted sheets of graphene, called a phason. These findings shed new light on the mysterious superconductivity and strange metallicity that emerge when graphene sheets are rotated to the magic angle.
The fundamental properties of materials depend critically on their underlying particles—the flow of electrons governs electrical resistance, and atomic lattice vibrations, termed phonons, drive heat conductivity. However, when electrons and phonons are coupled, remarkable new phenomena can emerge.
Perhaps the most intriguing phenomenon occurs when the coupling enables phonons to effectively bind electrons into pairs, resulting in superconductivity—a state where electrical current flows without resistance. Despite its crucial role, measurements of electron-phonon coupling for individual phonon modes have remained an outstanding challenge.
Two years ago, a team of researchers from the Weizmann Institute of Science, led by Prof. Shahal Ilani, developed the Quantum Twisting Microscope. This microscope uses an atomically-thin van der Waals material at its tips as a quantum interferometer, enabling direct measurement of the electronic wave functions within a quantum material. With their original QTM, operating at room temperature, they were able to image the electronic spectrum of various materials.
Now, creating a QTM that works at cryogenic temperatures, the team discovered that it can also image phonons with unprecedented precision. The new QTM employs an inelastic process, where electrons tunneling between two atomically-thin layers emit a phonon whose energy and momentum are controlled by adjusting the voltage bias and twist angle between the layers. By systematically tuning these parameters, they could map the complete phonon energy spectrum of the material under investigation.
"Our technique not only measures the phonon spectrum but also quantifies how strongly electrons couple to each phonon mode," says Dr. John Birkbeck, a lead author of this study. "Materials host numerous phonon modes, each can have a wide range of momenta. Our microscope quantitatively reveals how electrons interact with each mode individually, providing unprecedented insight into electron–phonon dynamics."
Applying this novel technique to twisted bilayer graphene yielded a surprising discovery: a unique low-energy vibration known as a phason, whose coupling to electrons grows stronger as the graphene layers approach the magic angle. This behavior had never been observed before and suggests that phasons may play a key role in the strange metal behavior and superconductivity observed in this system.
Our method extends far beyond phonons," adds Jiewen Xiao, another lead author on the study. "It can detect any excitation coupled to tunneling electrons, opening exciting avenues to explore other collective modes such as plasmons, magnons, spinons and other goldstone modes across a diverse range of quantum materials."
"This study makes us feel optimistic about future discoveries," says Alon Inbar, a fellow lead author. "Significant progress in our understanding of these fundamental modes in quantum materials will come shortly."
With this significant expansion in its capabilities, the QTM is poised to become a transformative instrument for quantum materials research. Its unique ability to probe both electronic states and collective excitations paves the way for discoveries relevant to quantum computing, sensing technologies, and future quantum electronic devices.
source: Weizmann Institute of Science /phys.org
