Researchers from the Universities of Brown and Columbia have identified previously unknown states of matter that occur in bilayer stacks of graphene, a two-dimensional nanomaterial. These new states, known as the fractional quantum Hall effect, arise from the complex interactions of electrons both within and between graphene layers.
"The results show that stacking 2-D materials in close proximity leads to a completely new physics," said Jia Li, assistant professor of physics at Brown, who co-authored this work during a post-doctoral research at Columbia initiated with Cory Dean. Professor of Physics and Jim Hone, Professor of Mechanical Engineering. "In terms of materials science, this work shows that these layer systems could be able to develop new types of electronic devices that use these new quantum Hall states."
The study was published in the journal Nature Physics .
First and foremost, says Hone, Wang Fong-Jen of Mechanical Engineering at Columbia Engineering, several of these new quantum Hall states could be "useful for making fault-tolerant quantum computers."
The Hall effect occurs when a magnetic field is applied perpendicular to a current flow to a conductive material. Due to the magnetic field, the current is deflected and creates a voltage in the transverse direction, which is referred to as Hall voltage. The strength of the Hall voltage increases with the strength of the magnetic field. The quantum version of the Hall effect was first discovered in 1980 in experiments at low temperatures and strong magnetic fields. The experiments showed that the Hall voltage does not increase uniformly with the magnetic field strength, but increases stepwise (or quantized). These steps are integer multiples of the basic constants of nature and completely independent of the physical composition of the material used in the experiments. The discovery was awarded the Nobel Prize for Physics in 1985.
Some years later, researchers working at temperatures near absolute zero and with very strong magnetic fields found new types of quantum Hall states in which the quantum steps in the Hall voltage fractions correspond to numbers, hence the name fraction quantum Hall effect. The discovery of the fractional quantum Hall effect won another Nobel Prize in 1998. Later, the theorists found that the fractional quantum Hall effect is related to the formation of quasiparticles, which are called composite fermions. In this state, each electron combines with a quantum magnetic flux to form a composite fermion, which carries a fraction of an electron charge and causes the rupture values of the Hall voltage.
The theory of compound fermions has successfully explained a variety of factors phenomena that are observed in single quantum well systems. This new study used a two-layer graph to investigate what happens when two quantum wells are close together. The theory had suggested that the interaction between two layers would lead to a new type of composite ferric, but this had never been observed in the experiment.
For the experiments, the team built on years of work in Columbia to improve the quality of graphene devices that produce ultra-clean devices made entirely of atomically flat 2D materials. The core of the structure consists of two graphene layers separated by a thin layer of hexagonal boron nitride as an isolation barrier. The bilayer structure is enveloped by hexagonal boron nitride as a protective insulator and graphite as a conductive gate to alter the charge carrier density in the channel.
"Once again, the incredible versatility of graphene has allowed us to push the boundaries of device structures beyond what we've been able to do," says Dean, a professor of physics at Columbia University. "The precision and tunability with which we can manufacture these devices now enables us to explore a whole area of physics that was recently considered completely inaccessible."
The graphene structures were then exposed to strong magnetic fields – millions of times stronger than the Earth's magnetic field. The investigations revealed a series of fractional quantum Hall states, some of which show excellent agreement with the composite fermion model, and some have never been predicted or seen.
"Apart from the compound interlayer vacancies, we could observe other features that could not be observed to be explained in the composite Fermion model," said Qianhui Shi, first co-author and postdoctoral fellow of the Columbia newspaper. "A more careful study has shown that these new states are, to our surprise, a result of the mating between compound fermions, and the mating of adjacent layers within the same layer creates a multitude of new quantum phenomena that make bilayer graphene an exciting platform to study."  "Of particular interest," says Hone, "are several new states that have the potential to host non-Abelian wavefunctions – states that do not quite match the traditional composite fermion model." In non-Abelian states, electrons retain a kind "Memory" of their past positions relative to each other. This has the potential to enable quantum computers that do not require error correction, which is currently a major obstacle in this area.
"These are the first new candidates for non-Abelian states in 30 years," said Dean. "It's really exciting to see new physics emerge from our experiments."
The study is titled "Mating states of compound fermions in bilayer graphene".
Graphite offers a new quantum surprise
J.I.A.Li et al., Mating States of Composite Fermions in Bilayer Graphene, Nature Physics (2019). DOI: 10.1038 / s41567-019-0547-z
Examinations show exotic quantum states in two-layer graphene (2019, June 26)
retrieved on June 26, 2019
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