Amazing stuff! First, and for a long time humans believed the atom was the smallest possible unit of matter ... What lies beneath?
"... But in very special states of matter, electrons can splinter into fractions of their whole. This phenomenon, known as "fractional charge," is exceedingly rare, and if it can be corralled and controlled, the exotic electronic state could help to build resilient, fault-tolerant quantum computers. ...
Now, MIT physicists have observed the elusive fractional charge effect, this time in a simpler material: five layers of graphene—an atom-thin layer of carbon that stems from graphite and common pencil lead. They report their results in Nature.
To date, this effect, known to physicists as the "fractional quantum Hall effect," has been observed a handful of times, and mostly under very high, carefully maintained magnetic fields. Only recently have scientists seen the effect in a material that did not require such powerful magnetic manipulation.
They found that when five sheets of graphene are stacked like steps on a staircase, the resulting structure inherently provides just the right conditions for electrons to pass through as fractions of their total charge, with no need for any external magnetic field.
The results are the first evidence of the "fractional quantum anomalous Hall effect" (the term "anomalous" refers to the absence of a magnetic field) in crystalline graphene, a material that physicists did not expect to exhibit this effect. ...
In their new work, the researchers did some calculations and found that electrons might interact with each other even more strongly if the pentalayer structure were aligned with hexagonal boron nitride (hBN) — a material that has a similar atomic structure to that of graphene, but with slightly different dimensions.
In combination, the two materials should produce a moiré superlattice—an intricate, scaffold-like atomic structure that could slow electrons down in ways that mimic a magnetic field. ..."
From the abstract:
"The fractional quantum anomalous Hall effect (FQAHE), the analogue of the fractional quantum Hall effect at zero magnetic field, is predicted to exist in topological flat bands under spontaneous time-reversal-symmetry breaking. The demonstration of FQAHE could lead to non-Abelian anyons that form the basis of topological quantum computation. So far, FQAHE has been observed only in twisted MoTe2 at a moiré filling factor v > 1/2. Graphene-based moiré superlattices are believed to host FQAHE with the potential advantage of superior material quality and higher electron mobility. Here we report the observation of integer and fractional QAH effects in a rhombohedral pentalayer graphene–hBN moiré superlattice. At zero magnetic field, we observed plateaus of quantized Hall resistance
at v = 1, 2/3, 3/5, 4/7, 4/9, 3/7 and 2/5 of the moiré superlattice, respectively, accompanied by clear dips in the longitudinal resistance Rxx. Rxy equals
at v = 1/2 and varies linearly with v, similar to the composite Fermi liquid in the half-filled lowest Landau level at high magnetic fields. By tuning the gate-displacement field D and v, we observed phase transitions from composite Fermi liquid and FQAH states to other correlated electron states. Our system provides an ideal platform for exploring charge fractionalization and (non-Abelian) anyonic braiding at zero magnetic field, especially considering a lateral junction between FQAHE and superconducting regions in the same device"
Electrons become fractions of themselves in graphene, study finds (MIT News) An exotic electronic state observed by MIT physicists could enable more robust forms of quantum computing.
Fractional quantum anomalous Hall effect in multilayer graphene (no public access)
No comments:
Post a Comment