Contest between superconductivity and insulating states in Magic Angle Graphene

Date:
July 7, 2020
Source:
ICFO-The Institute of Photonic Sciences
Summary:
A team of researchers develop a set of entirely novel knobs to
control correlated electrons and demonstrate that superconductivity
can exist without insulating phases in Magic Angle Twisted Bi-layer
Graphene.



FULL STORY ==========================================================================
If you stack two layers of graphene one on top of the other, and rotate
them at an angle of 1.1-o (no more and no less) from each other --
the so-called magic- angle, experiments have proven that the material
can behave like an insulator, where no electrical current can flow,
and at the same can also behave like a superconductor, where electrical currents can flow without resistance.


==========================================================================
This major finding took place in 2018. Last year, in 2019, while ICFO researchers were improving the quality of the device used to replicate
such breakthrough, they stumbled upon something even bigger and totally unexpected.

They were able to observe a zoo of previously unobserved superconducting
and correlated states, in addition to an entirely new set of magnetic
and topological states, opening a completely new realm of richer physics.

So far, there is no theory that has been able to explain superconductivity
in magic angle graphene at the microscopic level. However, this
finding has triggered many studies, which are trying to understand
and unveil the physics behind all these phenomena that occur in this
material. In particular, scientists drew analogies to unconventional
high temperature superconductors - - the cuprates, which hold the
record highest superconducting temperatures, only 2 times lower than
room temperature. Their microscopic mechanism of the superconducting
phase is still not understood, 30 years after its discovery.

However, similarly to Magic Angle Twisted Bi-layer Graphene (MATBG), it is believed that an insulating phase is responsible for the superconducting
phase in proximity to it. Understanding the relationship between the superconducting and insulating phases is at the centre of researcher's interest, and could lead to a big breakthrough in superconductivity
research.

In such pursuit, in a study recently published in Nature, ICFO researchers
Petr Stepanov, Ipsita Das, Xiaobo Lu, Frank H. L. Koppens, led by
ICFO Prof. Dmitri Efetov, in collaboration with an interdisciplinary
group of scientists from MIT, National Institute for Materials Science
in Japan, and Imperial College London, have delved deeper into the
physical behaviour of this system and report on the detailed testing and screening-controlled of Magic-Angle Twisted Bi-layer Graphene (MATBG)
devices with several near-magic-angle twist angles, to find a possible explanation for the mentioned states.

In their experiment, they were able to simultaneously control the speed
and interaction energies of the electrons, and so turn the insulating
phases into superconducting ones. Normally, at the magic angle, an
insulating state is formed, since electrons have very small velocities,
and in addition they strongly repel each other through the Coulomb
force. In this study Stepanov and team used devices with twist-angles
slightly away from the magic-angle of 1.1DEG by +/- 0.05DEG, and placed
these very close to metallic screening layers, separating these by only
few nano-meters by insulating hexagonal boron nitride layers. This allowed
to reduce the repulsive force between the electrons and to speed these
up, so allowing them to move freely, escaping the insulating state.

By doing so, Stepanov and colleagues observed something quite
unexpected. By changing the voltage (carrier density) in the different
device configurations, the superconductivity phase remained while the correlated insulator phase disappeared. In fact, the superconducting
phase spanned over larger regions of densities even when the carrier
density varied. Such observations suggest that rather than having the same common origin, the insulating and superconducting phase actually could
compete with each other, which puts into question the simple analogy with
the cuprates, that was believed previously. However, the scientist soon realized, that the superconducting phase could be even more interesting,
as it lies in close proximity to topological states, which are activated
by recurring electronic interaction by applying a magnetic field.

Superconductivity with Magic-Angle Graphene Room temperature
superconductivity is the key to many technological goals such as efficient power transmission, frictionless trains, or even quantum computers, among others. When discovered more than 100 years ago, superconductivity was
only plausible in materials cooled down to temperatures close to absolute
zero. Then, in the late 80's, scientists discovered high temperature superconductors by using ceramic materials called cuprates. In spite
of the difficulty of building superconductors and the need to apply
extreme conditions (very strong magnetic fields) to study the material,
the field took off as something of a holy grail among scientists based
on this advance. Since last year, the excitement around this field has increased. The double mono-layers of carbon have captivated researchers because, in contrast to cuprates, their structural simplicity has become
an excellent platform to explore the complex physics of superconductivity.


========================================================================== Story Source: Materials provided by
ICFO-The_Institute_of_Photonic_Sciences. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Petr Stepanov, Ipsita Das, Xiaobo Lu, Ali Fahimniya, Kenji Watanabe,
Takashi Taniguchi, Frank H. L. Koppens, Johannes Lischner,
Leonid Levitov, Dmitri K. Efetov. Untying the insulating and
superconducting orders in magic-angle graphene. Nature, 2020; DOI:
10.1038/s41586-020- 2459-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200707113236.htm

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