Stacking and twisting graphene unlocks a rare form of magnetism
Twisting a monolayer and a bilayer sheet of graphene into a three-layer structure leads to new quantum mechanical states.

Date:
October 12, 2020
Source:
Columbia University
Summary:
A team of researchers has discovered that a variety of exotic
electronic states, including a rare form of magnetism, can arise
in a three-layer graphene structure.



FULL STORY ========================================================================== Since the discovery of graphene more than 15 years ago, researchers
have been in a global race to unlock its unique properties. Not only
is graphene -- a one-atom-thick sheet of carbon arranged in a hexagonal
lattice -- the strongest, thinnest material known to man, it is also an excellent conductor of heat and electricity.


==========================================================================
Now, a team of researchers at Columbia University and the University of Washington has discovered that a variety of exotic electronic states,
including a rare form of magnetism, can arise in a three-layer graphene structure.

The findings appear in an article published Oct. 12 in Nature Physics.

The work was inspired by recent studies of twisted monolayers or twisted bilayers of graphene, comprising either two or four total sheets. These materials were found to host an array of unusual electronic states driven
by strong interactions between electrons.

"We wondered what would happen if we combined graphene monolayers
and bilayers into a twisted three-layer system," said Cory Dean,
a professor of physics at Columbia University and one of the paper's
senior authors. "We found that varying the number of graphene layers
endows these composite materials with some exciting new properties that
had not been seen before." In addition to Dean, Assistant Professor
Matthew Yankowitz and Professor Xiaodong Xu, both in the departments of
physics and materials science and engineering at University of Washington,
are senior authors on the work.

Columbia graduate student Shaowen Chen, and University of Washington
graduate student Minhao He are the paper's co-lead authors.



==========================================================================
To conduct their experiment, the researchers stacked a monolayer sheet
of graphene onto a bilayer sheet and twisted them by about 1 degree. At temperatures a few degrees over absolute zero, the team observed an
array of insulating states -- which do not conduct electricity --
driven by strong interactions between electrons. They also found that
these states could be controlled by applying an electric field across
the graphene sheets.

"We learned that the direction of an applied electric field matters a
lot," said Yankowitz, who is also a former postdoctoral researcher in
Dean's group.

When the researchers pointed the electric field toward the monolayer
graphene sheet, the system resembled twisted bilayer graphene. But when
they flipped the direction of the electric field and pointed it toward
the bilayer graphene sheet, it mimicked twisted double bilayer graphene --
the four-layer structure.

The team also discovered new magnetic states in the system. Unlike
conventional magnets, which are driven by a quantum mechanical property of electrons called "spin," a collective swirling motion of the electrons in
the team's three-layer structure underlies the magnetism, they observed.

This form of magnetism was discovered recently by other researchers in
various structures of graphene resting on crystals of boron nitride. The
team has now demonstrated that it can also be observed in a simpler
system constructed entirely with graphene.

"Pure carbon is not magnetic," said Yankowitz. "Remarkably, we can
engineer this property by arranging our three graphene sheets at just the
right twist angles." In addition to the magnetism, the study uncovered
signs of topology in the structure. Akin to tying different types of
knots in a rope, the topological properties of the material may lead to
new forms of information storage, which "may be a platform for quantum computation or new types of energy-efficient data storage applications,"
Xu said.

For now, they are working on experiments to further understand the
fundamental properties of the new states they discovered in this
platform. "This is really just the beginning," said Yankowitz.


========================================================================== Story Source: Materials provided by Columbia_University. Original written
by Carla Cantor.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Shaowen Chen, Minhao He, Ya-Hui Zhang, Valerie Hsieh, Zaiyao Fei, K.

Watanabe, T. Taniguchi, David H. Cobden, Xiaodong Xu, Cory R. Dean,
Matthew Yankowitz. Electrically tunable correlated and topological
states in twisted monolayer-bilayer graphene. Nature Physics,
2020; DOI: 10.1038/s41567-020-01062-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201012115949.htm

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