Guiding a superconducting future with graphene quantum magic
Date:
April 19, 2022
Source:
Nagoya University
Summary:
Superconductors are materials that conduct electrical current
with practically no electrical resistance at all. This ability
makes them extremely interesting and attractive for a plethora
of applications such as loss-less power cables, electric motors
and generators, as well as powerful electromagnets that can be
used for MRI imaging and for magnetic levitating trains. Now,
researchers have detailed the superconducting nature of a new class
of superconducting material, magic-angle twisted bilayer graphene.
FULL STORY ========================================================================== Superconductors are materials that conduct electrical current with
practically no electrical resistance at all. This ability makes them
extremely interesting and attractive for a plethora of applications such
as loss-less power cables, electric motors and generators, as well as
powerful electromagnets that can be used for MRI imaging and for magnetic levitating trains. Now, researchers from Nagoya University have detailed
the superconducting nature of a new class of superconducting material, magic-angle twisted bilayer graphene.
==========================================================================
For a material to behave as a superconductor, low temperatures are
required.
Most materials only enter the superconducting phase at extremely low temperatures, such as -270DEGC, lower than those measured in outer
space! This severely limits their practical applications because such
extensive cooling requires very expensive and specialized liquid helium
cooling equipment. This is the main reason superconducting technologies
are still in their infancy.
High temperature superconductors (HTS), such as some iron and copper-based ones, enter the superconducting phase above -200DEGC, a temperature that
is more readily achievable using liquid nitrogen which cools down a system
to ?195.8DEGC. However, the industrial and commercial applications of HTS
have been thus far limited. Currently known and available HTS materials
are brittle ceramic materials that are not malleable into useful shapes
like wires. In addition, they are notoriously difficult and expensive
to manufacture. This makes the search for new superconducting materials critical, and a strong focus of research for physicists like Prof. Hiroshi Kontani and Dr. Seiichiro Onari from the Department of Physics, Nagoya University.
Recently, a new material has been proposed as a potential superconductor
called magic-angle twisted bilayer graphene (MATBG). In MATBG, two layers
of graphene, essentially single two-dimensional layers of carbon arranged
in a honeycomb lattice, are offset by a magic angle (about 1.1 degrees)
that leads to the breakage of rotational symmetry and the formation of a high-order symmetry known as SU(4). As temperature changes, the system experiences quantum fluctuations, like water ripples in the atomic
structure, that lead to a novel spontaneous change in the electronic
structure and a reduction in symmetry.
This rotational symmetry breaking is known as the nematic state and
has been closely associated with superconducting properties in other
materials.
In their work published recently in Physical Review Letters, Prof. Kontani
and Dr. Onari use theoretical methods to better understand and shine light
on the source of this nematic state in MATBG. "Since we know that high temperature superconductivity can be induced by nematic fluctuations in strongly correlated electron systems such as iron-based superconductors, clarifying the mechanism and origin of this nematic order can lead
to the design and emergence of higher temperature superconductors,"
explains Dr. Onari.
The researchers found that nematic order in MATBG originates from the interference between the fluctuations of a novel degree-of-freedom that combines the valley degrees of freedom and the spin degrees of freedom, something that has not been reported from conventional strongly correlated electron systems. The superconducting transition temperature of twisted
bilayer graphene is very low, at 1K (-272DEGC), but the nematic state
manages to increase it by several degrees. Their results also show that although MATBG behaves in some ways like an iron-based high temperature superconductor, it also has some distinct properties that are quite
exciting, such as a net charge loop current giving rise to a magnetic
field in a valley polarized state, while the loop current is canceled
out by each valley in the nematic state. Besides, the malleability of
graphene can also play an important role in increasing the practical applications of these superconductors. With a better understanding of
the underlying mechanisms of superconductivity, science and technology
inch closer to a conducting future that is indeed super.
========================================================================== Story Source: Materials provided by Nagoya_University. Note: Content
may be edited for style and length.
========================================================================== Journal Reference:
1. Seiichiro Onari, Hiroshi Kontani. SU(4) Valley+Spin Fluctuation
Interference Mechanism for Nematic Order in Magic-Angle Twisted
Bilayer Graphene: The Impact of Vertex Corrections. Physical Review
Letters, 2022; 128 (6) DOI: 10.1103/PhysRevLett.128.066401 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/04/220419092330.htm
--- up 7 weeks, 1 day, 10 hours, 51 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)