That hasn’t stopped a team led by Professor Eli Zeldov at the Weizmann Institute of Science from making some pretty awe-inspiring headway. Now they’re starting to reveal the magnetic properties of rhombohedral graphene. Utilizing an innovative nano-superconducting quantum interference device (SQUID) on a tip probe, researchers collected crucial data on this exotic material at extremely low temperatures. These unexpected results, published in Nature Physics, shed light into the nature of magnetic anisotropy and isospin-related textures in multilayer graphene.
In this work, the research team measured at a base temperature of around 20 millikelvin, close to absolute zero. This low temperature provided a perfect setting to start to notice the distinct quantum stages of rhombohedral graphene. This special kind of multilayer graphene exhibits unusual long-range magnetic order. These qualities are key to making them future quantum computing and memory storage applications.
Innovative Measurement Techniques
The nano-SQUID-on-tip probe, which is a highly sensitive superconducting sensor mounted at the tip of a sharp pipette, allowed the researchers to measure magnetic field strengths as low as 10 nanotesla. Such sensitivity is unprecedented and is essential for uncovering the subtle magnetic textures inherent to rhombohedral graphene.
Dr. Surajit Dutta, co-first author of the study, described how they were able to obtain so much data.
“We scanned a few hundreds of nanometers above the dual-gated rhombohedral tetralayer graphene devices inside a vector magnetic field,” – Dr. Surajit Dutta.
To modulate the electron density, the team employed small alternating current (a.c.) voltages on the gates. This tricky method caused the sample to go through very different changes in its magnetization. This form of modulation produced a local oscillating magnetic field. To do this, the team focused their SQUID-on-tip probe onto the mangular magnetic material, thus tracing out the minigroove patterns inside.
Unveiling Exotic Quantum Phases
Rhombohedral graphene hosts a wide range of exotic quantum phases, such as spin-polarized half metal and spin-valley polarized quarter metal phases. These phases exhibit a little understood directionally dependent magnetism, or magnetic anisotropy.
The main results of these calculations show that in the half-metal phase, spins have strong anisotropy. Because of this, just a few tens of millitesla can deflect them away from each other. In contrast, in the quarter-metal phase, spins are rigidly locked along a valley-polarized out-of-plane direction.
“Our paper began with a simple question: in rhombohedral multilayer graphene, how do the four isospin flavors (two spins, two valleys) magnetically order in the absence of an external magnetic field at low temperature?” – Prof. Eli Zeldov.
This study provides interesting and important directions for future research. For example, it probes how the magnetic properties that arise in multilayer graphene may influence charge transport, optoelectronic, and thermoelectric phenomena.
Indeed, the research team’s ultimate goal is to further understand rhombohedral graphene. To do this, they’ll increase the temperature in their cooling apparatus slowly over time. Then they will be able to watch how the magnetic texture changes with increasing temperature.
“This clear contrast in the anisotropy allows us to set a lower bound on an electron-electron interaction energy scale, Hund’s exchange coupling. This energy scale had not been extracted through any prior experiment in the rhombohedral multilayer graphene systems despite its key role in setting the energetics hierarchy among competing symmetry-broken states.” – Dr. Dutta.
This research has major implications. The innovative work therein holds the promise of non-volatile memory technology and deepens our knowledge of correlated physics in two-dimensional materials.
Future Directions and Implications
The research team plans to further enhance their understanding of rhombohedral graphene by gradually increasing the temperature in their cooling device. This will allow them to observe how the magnetic texture evolves as temperature rises.
Professor Zeldov expressed his aspirations for future studies:
“This will let us pinpoint the Curie temperature—the temperature at which the magnetism finally switches off—and track how the corresponding magnetic anisotropy evolves. Beyond that, our bigger future goal is to see how the magnetic ordering of isospins in the symmetry-broken states shapes the integer and fractional quantum anomalous Hall states and whether it can spark unconventional superconductivity across the multilayer rhombohedral graphene family.” – Prof. Eli Zeldov.
The implications of this research are profound, offering potential advancements in non-volatile memory technology and a deeper understanding of correlated physics within two-dimensional materials.