CHARGE DENSITY WAVE STATES

Description

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Context

A group of researchers from Indian Institute of Science (IISc), Bengaluru, has discovered that Graphene may possess a new phase called charge density wave states.

About Graphene

Graphene is an allotrope of carbon consisting of a single layer of atoms arranged in a two-dimensional honeycomb lattice
It is the building-block of Graphite.It is the thinnest, most electrically and thermally conductive material in the world, while also being flexible, transparent and incredibly strong.
Graphene is also known as a wonder material due to its vast potential in the energy and medical world.

Phases of Graphene

Graphene is a material in which a wide variety of phases such as insulator, superconductor, and even phases that imitate liquid crystals.

Twisted Bilayer Graphene

Graphene when combined with other materials to form useful devices show unique properties.
One such material is twisted bilayer graphene. This is made by placing two layers of graphene on top of each other such that they are aligned well, and then twisting one layer. When such a twist is given, the lattice shows what are called moire patterns.
Moire patterns are wavy fringelike patterns that we see, for example, when two layers of synthetic fabric overlap and move with respect to each other. It is the geometrical design that results when a set of straight or curved lines is superposed onto another set.

Moire Patterns

Twisted Bilayer Graphene with Moire Pattern

Recent Study

Twisted bilayer graphene has been studied for several years. Researchers in IISc produced the material for study. The second layer was twisted with respect to the first by a small angle between one and two degrees. Then they made a third layer of tungsten diselenide to complete the device.
Tungsten diselenide shows a property called spin-orbit coupling, which is a link between the intrinsic spin and the orbital angular momentum of the electrons.
Adding this layer, they endowed the material with unique properties and created the charge density wave states.

Characteristics of Charge Density Wave States

Normally in a conductor, the charge carriers, for example electrons, are free to move anywhere in the conductor.
But in charge density wave state, some areas are restricted and the electrons can move only in the allowed regions.
Just like there are variations in particle density in a sound wave, these states show a wave-like variation in the charge density.

Charge Density Waves

Charge density wave (CDW) is an intriguing physical phenomenon especially found in two-dimensional (2D) layered systems. A charge density wave (CDW) is an ordered quantum fluid of electrons in a linear chain compound or layered crystal. The electrons within a CDW form a standing wave pattern and sometimes collectively carry an electric current.

Charge Density Waves form due to the wave-like nature of electrons – a manifestation of quantum mechanical wave-particle duality – causing the electronic charge density to become spatially modulated, i.e., to form periodic "bumps" in charge.

CDW is vital for understanding lattice modification, strongly correlated electronic behaviors, and other related physical properties.

Application of Charge Density Wave States

These devices may reveal when studied deeper, excitations like the Majorana modes, or exotic magnetic phases, which will be useful in quantum computation.

Elementary Particles

Elementary particles are the most fundamental components of matter and cannot be further subdivided into smaller constituents. Elementary particles include quarks (the constituents of protons and neutrons), leptons (electrons, muons, taus, and neutrinos), gauge bosons (photons, gluons, and W and Z bosons) and the Higgs boson.

Fermions

Fermions include particles in the class of leptons (e.g., electrons, muons), baryons (e.g., neutrons, protons, lambda particles), and nuclei of odd mass number (e.g., tritium, helium-3, uranium-233). A fermion is any particle that has an odd half-integer (like 1/2, 3/2, and so forth) spin. Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions. Bosons are those particles which have an integer spin (0, 1, 2…). All the force carrier particles are bosons.

Majorana Fermion

A Majorana fermion, is a fermion that is its own antiparticle. They were hypothesized by Ettore Majorana in 1937. The term is sometimes used in opposition to a Dirac fermion, which describes fermions that are not their own antiparticles. With the exception of the neutrino, all of the Standard Model fermions are known to behave as Dirac fermions at low energy and none are Majorana fermions. The nature of the neutrinos is not settled—they may turn out to be either Dirac or Majorana fermions.

https://www.thehindu.com/sci-tech/science/wavelike-phase-of-electrons-seen-in-fabricated-device/article65326139.ece

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