The humble pencil contains graphite.
Graphite is made of carbon in an unusual configuration giving it some remarkable properties.
Graphite becomes graphene when you are able to remove just one layer.
This material is so remarkable that even today we are uncovering new properties that were
not expected and push way beyond the boundaries of what we understand.
Let's dive in and find out more.
In 1859, Benjamin Brodie noted the highly lamellar structure of thermally reduced graphite
oxide.
It would take until 1916 for Peter Debye and Paul Scherrer to determine the structure of
graphite.
Wind forward 30 years to 1947 and P.R.
Wallace was the first to explore the existence of graphene when he was examining the electronic
properties of 3D graphite.
The following year g.
Reuss and F. Vogt would capture images of thin graphite samples which consisted of a
few graphene layers.
1961 that Hanns-Peter Boehm published a study of extremely thin flakes of graphite and coined
the term graphene for the hypothetical single-layer structure.
At this point, it really wasn't clear if any of the images captured actually contained
a single layer of graphene rather than multiple stacked layers.
Attempts to make thin films of graphite by mechanical exfoliation started in 1990 but
it would take until 2004 for Andre Geim and Konstantin Novoselov to isolate a single layer
of graphene.
For this, they would eventually receive the Nobel Prize in 2010.
Graphene has some remarkable properties.
Three of the four outer-shell electrons of each atom occupy three sp2 hybrid orbitals.
The remaining outer shell electrons occupy a Pz orbital that is oriented perpendicularly
to the plane.
These orbitals hybridize together to form two half-filled bands of free-moving electrons
which are responsible for most of graphene's notable electronic properties.
It is not just the thinnest material known but also incredibly flexible, hundreds of
times stronger than steel, and more conductive than copper.
In the past researcher have been able to synthesize graphene superconductors by placing the material
in contact with other superconducting metals.
This allows graphene to inherit some of the superconducting behaviours.
In a new piece of work, the researchers wanted to take this one step further and make graphene
a superconductor on its own.
They accomplished this by creating a superlattice of two graphene sheets stacked together.
Not precisely on top of each other but rotated ever so slight at what they call the magic
angle of 1.1 degrees.
This creates a small offset in the hexagonal lattice creating a moire pattern that is predicted
to induce strange strongly correlated interactions between the electrons in the graphene sheets.
In any other stacked configuration, the graphene prefers to remain distinct and not interact
much with the other layer.
When the layers were rotated at the magic angle the two sheets of graphene exhibit nonconducting
behaviour.
When they then applied a voltage small amounts of electrons were added to the graphene superlattice.
At a certain point, the electrons broke out of the initial insulating state and flowed
without resistance, as if through a superconductor.
In a related study, scientists took this concept further by also altering the spin-orbit coupling.
Spin-orbit coupling is a state of electron behaviours in certain materials in which each
electron's spin becomes linked to its orbit around the atomic nucleus
Spin-orbit coupling is a relativistic interaction of a particle's spin with its motion inside
a potential.
This phenomenon can lead to shifts in an electron's atomic energy levels, due to the interaction
between the electron's magnetic dipole, its orbital motion, and the electrostatic field
of the positively charged nucleus.
This is detectable as a splitting of spectral lines, which can be thoughts of as a Zeeman
effect product of two relativistic effects.
The apparent magnetic field seen from the electron perspective and the magnetic moment
of the electron associated with its intrinsic spin.
Spin-orbit coupled can give rise to a wide range of interesting quantum phenomena but
it is not normally present in magic-angle graphene.
So in order to accomplish this, the researchers interfaced magic-angle graphene with a special
block of tungsten diselenide, which is a material that has strong spin-orbit coupling.
They discovered that when an electric current was flowing in one direction across the material
in the presence of an external magnetic field it produced a voltage in the direction perpendicular
to the current.
This voltage is known as the Hall effect and is a telltale sign of an intrinsic magnetic
field in a material.
Much to the surprise of the researchers, they were able to demonstrate that they could control
the magnetism of the sample turning it into ferromagnets.
They discovered that they could control the magic angle graphene with both an external
magnetic field and electric field.
This opens up new avenues for computer storage.
Another potential application is in quantum computing.
One of the barriers here is the interface between ferromagnets and superconductors as
magnets are generally destructive to superconductivity.
But a material that is capable of both ferromagnetism and superconductivity could
provide such an interface.
There are many other strange experiments that have been conducted using magic angle graphene.
In one, the scientists were able to create a 'strange metal' phase in which the resistivity
scales linearly with temperature down to very low temperatures.
This temperature dependence is unlike that of most metals and has proved difficult to
explain with traditional models of electron conduction.
The transport of electrical and heat currents in metals was once a well-understood part
of condensed matter physics.
This stems back to a formula developed by Paul Drude in 1900.
This treated the electrons as classical particles and largely ignored electron-electron interactions.
Remarkably this formula has survived and was still used to describe metallic transport
as late as the mid-1980s.
This all changed soon after the discovery of high-temperature superconductivity in the
cuprate compounds in 1986.
In this compound, researchers identified a strange metal phase where the resistivity
was also proportional to temperature.
This could not be explained using Drude's formula.
So finding another strange metal in magic-angle graphene puts new pressure on theorists to
come up with an explanation for this phenomenon.
Despite several efforts to explain this linear temperature dependence using the Drude model,
it has become clear that a quasiparticle description fails in this strange metal regime.
Instead, efforts are focussed on a transport in which charge and heat are not carried by
individual particles but collectively by a many-body state that connects large numbers
of particles through long-range quantum entanglement.