Amazing stuff!
"... physicists report the unexpected discovery of electrons forming crystalline structures in a material only billionths of a meter thick. The work adds to a gold mine of discoveries originating from the material, which the same team discovered about three years ago.
In a paper published [2022], the team describes how electrons in devices made, in part, of the material can become solid, or form crystals, by changing the voltage applied to the devices when they are kept at a temperature similar to that of outer space. Under the same conditions, they also showed the emergence of two new electronic states that add to work they reported [2024] showing that electrons can split into fractions of themselves. ...
the material, one composed of five layers of atomically thin carbon; the other composed of four layers. This indicates “that there’s a family of materials ...
rhombohedral pentalayer graphene ...
For example, in 2023 they created a sandwich of rhombohedral pentalayer graphene with “buns” made of hexagonal boron nitride. By applying different voltages, or amounts of electricity, to the sandwich, they discovered three important properties never before seen in natural graphite.
[2024] ... reported yet another important and even more surprising phenomenon: Electrons became fractions of themselves upon applying a current to a new device composed of rhombohedral pentalayer graphene and hexagonal boron nitride. This is important because this “fractional quantum Hall effect” has only been seen in a few systems, usually under very high magnetic fields. The Ju work showed that the phenomenon could occur in a fairly simple material without a magnetic field. As a result, it is called the “fractional quantum anomalous Hall effect” (anomalous indicates that no magnetic field is necessary). ...
In the current work, ... reports yet more unexpected phenomena from the general rhombohedral graphene/boron nitride system when it is cooled to 30 millikelvins ... discovering two more of these fractional states.
They also found another unusual electronic phenomenon: the integer quantum anomalous Hall effect in a wide range of electron densities. The fractional quantum anomalous Hall effect was understood to emerge in an electron “liquid” phase, analogous to water. In contrast, the new state that the team has now observed can be interpreted as an electron “solid” phase — resembling the formation of electronic “ice” — that can also coexist with the fractional quantum anomalous Hall states when the system’s voltage is carefully tuned at ultra-low temperatures.
One way to think about the relation between the integer and fractional states is to imagine a map created by tuning electric voltages: By tuning the system with different voltages, you can create a “landscape” similar to a river (which represents the liquid-like fractional states) cutting through glaciers (which represent the solid-like integer effect) ...
observed all of these phenomena not only in pentalayer rhombohedral graphene, but also in rhombohedral graphene composed of four layers. This creates a family of materials, and indicates that other “relatives” may exist.
“This work shows how rich this material is in exhibiting exotic phenomena. We’ve just added more flavor to this already very interesting material,” ..."
From the abstract:
"Electrons in topological flat bands can form new topological states driven by correlation effects. The pentalayer rhombohedral graphene/hexagonal boron nitride (hBN) moiré superlattice was shown to host fractional quantum anomalous Hall effect (FQAHE) at approximately 400 mK, triggering discussions around the underlying mechanism and role of moiré effects.
In particular, new electron crystal states with non-trivial topology have been proposed.
Here we report electrical transport measurements in rhombohedral pentalayer and tetralayer graphene/hBN moiré superlattices at electronic temperatures down to below 40 mK. We observed two more fractional quantum anomalous Hall (FQAH) states and smaller Rxx values in pentalayer devices than those previously reported.
In the new tetralayer device, we observed FQAHE at moiré filling factors v = 3/5 and 2/3. With a small current at the base temperature, we observed a new extended quantum anomalous Hall (EQAH) state and magnetic hysteresis, where Rxy = h/e2 and vanishing Rxx spans a wide range of v from 0.5 to 1.3.
At increased temperature or current, EQAH states disappear and partially transition into the FQAH liquid. Furthermore, we observed displacement field-induced quantum phase transitions from the EQAH states to the Fermi liquid, FQAH liquid and the likely composite Fermi liquid. Our observations established a new topological phase of electrons with quantized Hall resistance at zero magnetic field and enriched the emergent quantum phenomena in materials with topological flat bands. "
Extended quantum anomalous Hall states in graphene/hBN moiré superlattices (no public access, but here is the link to the preprint version)
No comments:
Post a Comment