Exotic quantum particles -- less magnetic field required
Research paves the way for future quantum devices and applications

Date:
December 15, 2021
Source:
Harvard John A. Paulson School of Engineering and Applied Sciences
Summary:
Researchers have observed exotic fractional states at low magnetic
field in twisted bilayer graphene for the first time.



FULL STORY ========================================================================== Exotic quantum particles and phenomena are like the world's most daring
elite athletes. Like the free solo climbers who scale impossibly steep
cliff faces without a rope or harness, only the most extreme conditions
will entice them to show up. For exotic phenomena like superconductivity
or particles that carry a fraction of the charge of an electron, that
means extremely low temperatures or extremely high magnetic fields.


==========================================================================
But what if you could get these particles and phenomena to show up
under less extreme conditions? Much has been made of the potential of room-temperature superconductivity, but generating exotic fractionally
charged particles at low- to-zero magnetic field is equally important
to the future of quantum materials and applications, including new types
of quantum computing.

Now, a team of researchers from Harvard University led by Amir Yacoby, Professor of Physics and of Applied Physics at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and Ashvin Vishwanath, Professor of Physics in the Department of Physics, in collaboration
with Pablo Jarillo-Herrero at the Massachusetts Institute of Technology,
have observed exotic fractional states at low magnetic field in twisted
bilayer graphene for the first time.

The research is published in Nature.

"One of the holy grails in the field of condensed matter physics is
getting exotic particles with low to zero magnetic field," said Yacoby,
senior author of the study. "There have been theoretical predictions that
we should be able to see these bizarre particles with low to zero magnetic field, but no one has been able to observe it until now." The researchers
were interested in a specific exotic quantum state known as fractional
Chern insulators. Chern insulators are topological insulators, meaning
they conduct electricity on their surface or edge, but not in the middle.



==========================================================================
In a fractional Chern insulator, electron interactions form what's known
as quasiparticles, a particle that emerges from complex interactions
between large numbers of other particles. Sound, for example, can
be described as a quasiparticle because it emerges from the complex interactions of particles in a material. Like fundamental particles, quasiparticles have well defined properties like mass and charge.

In fractional Chern insulators, electron interactions are so strong
within the material that quasiparticles are forced to carry a fraction of
the charge of normal electrons. These fractional particles have bizarre
quantum properties that could be used to create robust quantum bits that
are extremely resilient to outside interference.

To build their insulator, the researchers used two sheets of graphene
twisted together at the so-called magic angle. Twisting unlocks new and different properties in graphene, including superconductivity, as first discovered by Jarillo-Herrero's group at MIT, and states known as Chern
bands, which hold great potential to generate fractional quantum states,
as shown theoretically by Vishwanath's group at Harvard.

Think of these Chern bands like buckets that fill up with electrons.

"In previous studies, you needed a large magnetic field in order to
generate these buckets, which are the topological building blocks you
need to get these exotic fractional particles," said Andrew T. Pierce, a graduate student in Yacoby's group and co-first author of the paper. "But magic-angle twist bilayer graphene already has these useful topological
units built in at zero magnetic field." To generate fractional states,
the researchers need to fill the buckets a fraction of the way with
electrons. But here's the hitch: for this to work, all the electrons
in a bucket must have nearly the same properties. In twisted bilayer
graphene, they don't. In this system, electrons have different levels
of a property known as the Berry curvature, which causes each electron
to experience a magnetic field tied to its particular momentum. (It's
more complicated than that, but what isn't in quantum physics?)


==========================================================================
When filling up the buckets, the electrons' Berry curvature needs to be
evened out for the fractional Chern insulator state to appear.

That's where a small applied magnetic field comes in.

"We showed that we can apply a very small magnetic field to evenly
distribute Berry curvature among electrons in the system, allowing us to observe a fractional Chern insulator in the twisted bilayer graphene,"
said Yonglong Xie, a postdoctoral fellow at SEAS and co-first author of
the paper. "This research sheds light on the importance of the Berry
curvature to realize fractionalized exotic states and could point to
alterative platforms where Berry curvature isn't as heterogeneous as it is
in twisted graphene." "Twisted bilayer graphene is the gift that keeps
on giving and this discovery of fractional Chern insulators is arguably
one of the most significant advances in the field," said Vishwanath,
senior author of the study. "It is astonishing to think that this wonder material is ultimately made of the same stuff as your pencil tip. "
"The discovery of low magnetic field fractional Chern insulators in
magic angle twisted bilayer graphene opens a new chapter in the field of topological quantum matter," said Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT and senior author of the study. "It offers the realistic prospect of coupling these exotic states with superconductivity, possibly enabling the creation and control of even more exotic topological quasiparticles known as anyons." The research was co-authored by Jeong
Min Park, Daniel E. Parker, Eslam Khalaf, Patrick Ledwith, Yuan Cao,
Seung Hwan Lee, Shaowen Chen, Patrick R. Forrester, Kenji Watanabe,
Takashi Taniguchi.

It was supported in part by the U.S. Department of Energy, Basic Energy Sciences Office, Division of Materials Sciences and Engineering under
award DE- SC0001819, Gordon and Betty Moore Foundation, National Science Foundation, and the Simons Foundation.

========================================================================== Story Source: Materials provided by Harvard_John_A._Paulson_School_of_Engineering_and_Applied
Sciences. Original written by Leah Burrows. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Xie, Y., Pierce, A.T., Park, J.M. et al. Fractional Chern
insulators in
magic-angle twisted bilayer graphene. Nature, 2021 DOI:
10.1038/s41586- 021-04002-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211215113319.htm

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