Unmasking the magic of superconductivity in twisted graphene
Magic-angle graphene and high-temperature superconductivity linked

Date:
October 20, 2021
Source:
Princeton University
Summary:
Researchers report an uncanny resemblance between the
superconductivity of magic graphene and that of high temperature
superconductors. Magic graphene may hold the key to unlocking
new mechanisms of superconductivity, including high temperature
superconductivity.



FULL STORY ==========================================================================
The discovery in 2018 of superconductivity in two single-atom-thick
layers of graphene stacked at a precise angle of 1.1 degrees (called 'magic'-angle twisted bilayer graphene) came as a big surprise to the scientific community.

Since the discovery, physicists have asked whether magic graphene's superconductivity can be understood using existing theory, or
whether fundamentally new approaches are required -- such as those
being marshalled to understand the mysterious ceramic compound that superconducts at high temperatures. Now, as reported in the journal
Nature, Princeton researchers have settled this debate by showing an
uncanny resemblance between the superconductivity of magic graphene and
that of high temperature superconductors. Magic graphene may hold the
key to unlocking new mechanisms of superconductivity, including high temperature superconductivity.


==========================================================================
Ali Yazdani, the Class of 1909 Professor of Physics and Director of the
Center for Complex Materials at Princeton University led the research. He
and his team have studied many different types of superconductors over the years and have recently turned their attention to magic bilayer graphene.

"Some have argued that magic bilayer graphene is actually an ordinary superconductor disguised in an extraordinary material," said Yazdani, "but
when we examined it microscopically it has many of the characteristics
of high temperature cuprate superconductors. It is a de'ja` vu moment." Superconductivity is one of nature's most intriguing phenomena. It is a
state in which electrons flow freely without any resistance. Electrons are subatomic particles that carry negative electric charges; they are vital
to our way of life because they power our everyday electronics. In normal circumstances, electrons behave erratically, jumping and jostling against
each other in a manner that is ultimately inefficient and wastes energy.

But under superconductivity, electrons suddenly pair up and start
to flow in unison, like a wave. In this state the electrons not
only do not lose energy, but they also display many novel quantum
properties. These properties have allowed for a number of practical applications, including magnets for MRIs and particle accelerators
as well as in the making of quantum bits that are being used to build
quantum computers. Superconductivity was first discovered at extremely low temperatures in elements such as aluminum and niobium. In recent years,
it has been found close to room temperatures under extraordinarily high pressure, and also at temperatures just above the boiling point of liquid nitrogen (77 degrees Kelvin) in ceramic compounds.

But not all superconductors are created equal.



========================================================================== Superconductors made of pure elements like aluminum are what researchers
call conventional. The superconductive state -- where the electrons pair together - - is explained by what is called the Bardeen-Cooper-Schrieffer
(BCS) theory.

This has been the standard description of superconductivity that has
been around since the late 1950s. But starting in the late 1980s new superconductors were discovered that did not fit the BCS theory. Most
notable among these "unconventional" superconductors are the ceramic
copper oxides (called cuprates) that have remained an enigma for the
past thirty years.

The original discovery of superconductivity in magic bilayer graphene
by Pablo Jarillo-Herrero and his team at the Massachusetts Institute
of Technology (MIT) showed that the material starts out first as an
insulator but, with small addition of charge carriers, it becomes superconducting. The emergence of superconductivity from an insulator,
rather than a metal, is one of the hallmarks of many unconventional superconductors, including most famously the cuprates.

"They suspected that superconductivity could be unconventional, like the cuprates, but they unfortunately did not have any specific experimental measurements of the superconducting state to support this conclusion,"
said Myungchul Oh, a postdoctoral research associate and one of the lead co-authors of the paper.

To investigate the superconductive properties of magic bilayer graphene,
Oh and his colleagues used a scanning tunneling microscope (STM) to
view the infinitesimally small and complex world of electrons. This
device relies on a novel phenomenon called "quantum tunneling," where
electrons are funneled between the sharp metallic tip of the microscope
and the sample. The microscope uses this tunneling current rather than
light to view the world of electrons on the atomic scale.

"STM is a perfect tool for doing these types of experiments," said
Kevin Nuckolls, a graduate student in physics and one of the paper's
lead co-authors.

"There are many different measurements that STM can do. It can access
physical variables that are typically inaccessible to other [experimental techniques]." When the team analyzed the data, they noticed two major characteristics, or "signatures," that stood out, tipping them off
that the magic bilayer graphene sample was exhibiting unconventional superconductivity. The first signature was that the paired electrons
that superconduct have a finite angular momentum, a behavior analogous
to that found in the high-temperature cuprates twenty years ago. When
pairs form in a conventional superconductor, they do not have a net
angular momentum, in a manner analogous to an electron bound to the
hydrogen atom in the hydrogen's s-orbital.



==========================================================================
STM operates by tunneling electrons in and out the sample. In a
superconductor, where all the electrons are paired, the current between
the sample and the STM tip is only possible when the superconductor's
pairs are broken apart. "It takes energy to break the pair apart,
and the energy dependence of this current depends on the nature
of the pairing. In magic graphene we found the energy dependence
that is expected for finite momentum pairing," Yazdani said. "This
finding strongly constrains the microscopic mechanism of pairing
in magic graphene." The Princeton team also discovered how magic
bilayer graphene behaves when the superconducting state is quenched by increasing the temperature or applying a magnetic field. In conventional superconductors, the material behavior is the same as that of a normal
metal when superconductivity is killed -- the electrons unpair. However,
in unconventional superconductors, the electrons appear to retain some correlation even when not superconducting, a situation that manifests
when there is roughly a threshold energy for removing electrons from
the sample. Physicists refer to this threshold energy as a "pseudogap,"
a behavior found in the non-superconducting state of many unconventional superconductors. Its origin has been a mystery for more than twenty years.

"One possibility is that electrons are still somewhat paired together
even though the sample is not superconducting," said Nuckolls. "Such a pseudogap state is like a failed superconductor." The other possibility,
noted in the Nature paper, is that some other form of collective
electronic state, which is responsible for the pseudogap, must first
form before superconductivity can occur.

"Either way, the resemblance of an experimental signature of a peusdogap
with the cuprates as well as finite momentum pairing can't be all a coincidence," Yazdani said. "These problems look very much related."
Future research, Oh said, will involve trying to understand what causes electrons to pair in unconventional superconductivity -- a phenomenon
that continues to vex physicists. BCS theory relies on weak interaction
among electrons with their pairing made possible because of their mutual interaction with the underlying vibration of the ions. The pairing of
electrons in unconventional superconductors, however, is often much
stronger than in simple metals, but its cause -- the "glue" that bonds
them together -- is currently not known.

"I hope our research will help the physics community to better
understand the mechanics of unconventional superconductivity," Oh
said. "We further hope that our research will motivate experimental
physicists to work together to uncover the nature of this phenomenon." ========================================================================== Story Source: Materials provided by Princeton_University. Original written
by Tom Garlinghouse. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Oh, M., Nuckolls, K.P., Wong, D. et al. Evidence for unconventional
superconductivity in twisted bilayer graphene. Nature, 2021 DOI:
10.1038/ s41586-021-04121-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211020135912.htm

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