An exotic phase of matter is obtained in graphene

An exotic phase of matter is obtained in graphene

The electron is a fundamental particle, so it is indivisible. Indeed, in particle physics experiments, detectors record the passage of ones that always have a charge equal to (or an integer multiple of) the charge of an electron. Quarks are an exception, their charge is a fraction (1/3 or 2/3) of the charge of an electron, but we can never observe a quark alone. What is even more surprising is that in matter, when we have a lot of electrons, we can observe “particles” with a charge that is less than the electron ratio! In some cases the electrons behave collectively, representing the properties of a particle, and we then speak of “quasiparticles”. Sometimes these particles exhibit fractional electric charges: instead of the usual charge – 1 of an electron, we can observe charges such as – 1/3. To observe this phenomenon, it is often necessary to use a strong magnetic field. Longju's team from the Massachusetts Institute of Technology (MIT) has just observed this in graphene without a magnetic field.

The history of partially charged phases is closely related to the history of the quantum Hall effect. This phenomenon appears in its classic version in a two-dimensional device exposed to a strong magnetic field perpendicular to the plane. When a current flows in a certain direction in the material, the magnetic field deflects the electrons, these charges accumulate on one side of the sample and a voltage perpendicular to the initial current is obtained.

When a material is cooled to temperatures near absolute zero, the conductivity (the reciprocal of resistance), which is defined as the ratio of current and Hall potential, changes in steps and equals an integer ν Multiplied by the square of the electric charge of the electron divided by Planck's constant (a quantum property of quantum phenomena). This quantum version of the Hall effect was discovered in 1980 by Claus von Klitzing.

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But under certain conditions using very pure materials, ν It also takes fractional values ​​(1/3, 2/5, 3/7, etc.), and then we talk about the “fractional quantum Hall effect”. This phenomenon was first observed in 1982 by Daniel Tsui and Horst Stürmer. Robert Laughlin gave a theoretical description: Thanks to the very low temperature and high purity of the substance, the scattered electrons interact more and form a “liquid” in which quasiparticles with a fractional charge appear. The latter is what explains levels with fractional values. This fractional charge was demonstrated in 1995.

The discovery of the partial quantum Hall effect has highlighted the richness of the physics of electrons, which cannot always be considered as individual particles. Collective behaviors are interesting because they give rise to strange properties that could potentially lead to technological applications. A particular type of partially charged excitation, Majorana quasiparticles (or Majorana fermions, or Majorana zero modes), are attractive scaffolds for fabricating noise-robust qubits (qubits) for quantum computers. Some teams have announced that they have produced Majorana quasiparticles, but these results are debated and demonstrate the difficulty of creating these strange behaviors of matter.

Other phases with partial charges have already been implemented in the laboratory, but there is a major obstacle standing in the way of exploiting them: they have only been formed in devices exposed to a strong external magnetic field.

In 2012, a team from Tsinghua University in Beijing observed the quantum Hall effect at the right charges in magnetic thin films without applying an external magnetic field. The idea was to use the magnetic moments of the atoms in the material to replace the external magnetic field. Hence, the obvious question becomes whether it is possible to create partial charges without using a magnetic field. Theorists have suggested that this is possible. In 2023, physicists at the University of Washington in Seattle obtained the same behavior by stacking two layers of molybdenum ditelluride (MoTe).2) by applying a 4 degree rotation between the two layers. This finding was confirmed by two other teams, one at Cornell University and the other at Jiao Tong University in Shanghai. Applying an angle creates a moiré effect that produces a regular pattern with a different periodicity than the MoTe layer2. These moiré systems display many interesting behaviors.

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Recently, Long Ju and his colleagues created partial charges in a stack of graphene layers. They stacked five layers of graphene spaced slightly apart like rungs of a ladder and sandwiched this assembly between two layers of boron nitride. Thus they get a moiré effect without rotation. Experimentally, the fractal phase in MoTe appears2 At a temperature of a few Kelvin, it is necessary to go down to 0.1 K for graphene. But graphene has the big advantage of being easy to work with.

The internal structure of these two systems will replace the role of the external magnetic field. But if it is in the case of MoTe2However, theorists expected that an anomalous partial quantum Hall effect could be seen (i.e. without a magnetic field) for a magic angle of 1.4 degrees (even if it was at 4 degrees). This was not the case for graphene. This result remains to be confirmed experimentally by other teams. For some theorists, the mechanism of action in graphene may be very different from that of MoTe2. Moiré probably plays a limited role.

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