Resolving the puzzles of graphene superconductivity
Discovery of superconductivity in trilayer graphene

Date:
December 10, 2021
Source:
Institute of Science and Technology Austria
Summary:
Since superconductivity in three-layered graphene was discovered
in September, the physics community has been left puzzled. Now,
three months later, physicists can successfully explain the results
by drawing from a theory of unconventional superconductivity.



FULL STORY ==========================================================================
A single layer of carbon atoms arranged in a honeycomb lattice makes
up the promising nanomaterial called graphene. Research on a setup of
three sheets of graphene stacked on top of one another so that their
lattices are aligned but shifted -- forming rhombohedral trilayer
graphene -- revealed an unexpected state of superconductivity. In this
state electrical resistance vanishes due to the quantum nature of the electrons. The discovery was published and debated in Nature, whilst the origins remained elusive. Now, Professor Maksym Serbyn and Postdoc Areg Ghazaryan from the Institute of Science and Technology (IST) Austria in collaboration with Professor Erez Berg and Postdoc Tobias Holder from the Weizmann Institute of Science, Israel, developed a theoretical framework
of unconventional superconductivity, which resolves the puzzles posed
by the experimental data.


==========================================================================
The Puzzles and their Resolution Superconductivity relies on the
pairing of free electrons in the material despite their repulsion
arising from their equal negative charges. This pairing happens
between electrons of opposite spin through vibrations of the crystal
lattice. Spin is a quantum property of particles comparable, but not
identical to rotation. The mentioned kind of pairing is the case at
least in conventional superconductors. "Applied to trilayer graphene," co-lead-author Ghazaryan points out, "we identified two puzzles that
seem difficult to reconcile with conventional superconductivity." First,
above a threshold temperature of roughly -260 DEGC electrical resistance
should rise in equal steps with increasing temperature. However, in
the experiments it remained constant up to -250 DEGC. Second, pairing
between electrons of opposite spin implies a coupling that contradicts
another experimentally observed feature, namely the presence of a nearby configuration with fully aligned spins, which we know as magnetism. "In
the paper, we show that both observations are explainable," group leader
Maksym Serbyn summarizes, "if one assumes that an interaction between
electrons provides the 'glue' that holds electrons together. This leads to unconventional superconductivity." When one draws all possible states,
which electrons can have, on a certain chart and then separates the
occupied ones from the unoccupied ones with a line, this separation line
is called a Fermi surface. Experimental data from graphene shows two Fermi surfaces, creating a ring-like shape. In their work, the researchers draw
from a theory from Kohn and Luttinger from the 1960's and demonstrate
that such circular Fermi surfaces favor a mechanism for superconductivity
based only on electron interactions. They also suggest experimental
setups to test their argument and offer routes towards raising the
critical temperature, where superconductivity starts appearing.

The Benefits of Graphene Superconductivity While superconductivity
has been observed in other trilayer and bilayer graphene, these known
materials must be specifically engineered and may be hard to control
because of their low stability. Rhombohedral trilayer graphene, although
rare, is naturally occurring. The proposed theoretical solution has
the potential of shedding light on long-standing problems in condensed
matter physics and opening the way to potential applications of both superconductivity and graphene.

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


========================================================================== Journal Reference:
1. Areg Ghazaryan, Tobias Holder, Maksym Serbyn, Erez
Berg. Unconventional
Superconductivity in Systems with Annular Fermi Surfaces:
Application to Rhombohedral Trilayer Graphene. Physical Review
Letters, 2021; 127 (24) DOI: 10.1103/PhysRevLett.127.247001 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211210103127.htm

--- up 6 days, 7 hours, 13 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)