Solving a superconducting mystery with more precise computations

Date:
January 28, 2022
Source:
University of Texas at Austin, Texas Advanced Computing Center
Summary:
A new, more precise method of simulating quantum materials has
revealed the basis for superconductivity in copper-based oxides
known as cuprates.

Researchers, using powerful supercomputers, found that phonons,
vibrational energy from crystal structure, contribute to a key
feature observed in cuprates, which may indicate their indispensable
contribution to superconductivity. If true, the finding may pave
the way for tunable superconductivity in materials.



FULL STORY ========================================================================== Researchers have known about high-temperature superconducting copper-based materials, or cuprates, since the 1980s. Below a certain temperature (approximately -130 degree Celsius), electrical resistance vanishes
from these materials and magnetic flux fields are expelled. However,
the basis for that superconductivity continues to be debated and explored.


==========================================================================
"It has been widely accepted that traditional superconductors result from electrons interacting with phonons, where the phonons pair two electrons
as an entity and the latter can run in a material without resistance,"
said Yao Wang, assistant professor of physics and astronomy at Clemson University.

However, in cuprates, strong repulsions known as the Coulomb force were
found between electrons and were believed to be the cause of this special
and high- temperature superconductivity.

Phonons are the vibrational energy that arise from oscillating atoms
within a crystal. The behavior and dynamics of phonons are very different
from those of electrons, and putting these two interacting pieces of
the puzzle together has been a challenge.

In November 2021, writing in the journal Physical Review Letters, Wang,
along with researchers from Stanford University, presented compelling
evidence that phonons are in fact contributing to a key feature observed
in cuprates, which may indicate their indispensable contribution to superconductivity.

The study innovatively accounted for the forces of both electrons and
phonons together. They showed that phonons impact not only electrons in
their immediate vicinity, but act on electrons several neighbors away.



==========================================================================
"An important discovery in this work is that electron-phonon coupling
generates non-local attractive interactions between neighboring electrons
in space," Wang said. When they used only local coupling, they calculated
an attractive force an order of magnitude smaller than the experimental results. "This tells us that the longer-range part is dominant and
extends up to four unit cells," or neighboring electrons.

Wang, who led the computational side of the project, used the National
Science Foundation (NSF)-funded Frontera supercomputer at the Texas
Advanced Computing Center (TACC) -- the fastest academic system in
the world -- to replicate experiments carried out at the Stanford
Synchrotron Radiation Lightsource and presented in Sciencein Sept. 2021
in a simulation.

The results relied not only on Frontera's super-fast parallel computing capabilities, but on a new mathematical and algorithmic method that
allowed for far greater accuracy than ever before.

The method, called variational non-Gaussian exact diagonalization,
can perform matrix multiplications on billions of elements. "It's a
hybrid method," Wang explained. "It treats the electron and phonon by
two different approaches that can adjust with each other. This method
performs well and can describe strong coupling with high precision." The
method development was also supported by a grant from NSF.

The demonstration of phonon-mediated attraction has a significant impact
even beyond the scope of superconductors. "Practically, the results
mean we've found a way to manipulate Coulomb interactions," Wang said, referring to the attraction or repulsion of particles or objects because
of their electric charge.



==========================================================================
"If superconductivity comes from Coulomb forces only, we cannot easily manipulate this parameter," he said. "But if part of the reason comes from
the phonon, then we can do something, for instance, putting the sample
on some substrate that will change the electron-phonon interaction. That
gives us a direction to design a better superconductor." "This research
gives new insights into the mystery of cuprate superconductivity that may
lead to higher temperature superconducting materials and devices," said
Daryl Hess, a program director in Division of Materials Research at NSF.

"They may find their way into future cell phones and quantum computers. A journey started by human creativity, clever algorithms, and Frontera."
Wang and collaborator Cheng-Chien Chen, from the University of
Alabama, Birmingham, also applied this new approach and powerful TACC supercomputers to study laser-induced superconductivity. They reported
these findings in Physical Review X in November 2021. And working with a
team from Harvard, Wang used TACC supercomputers to study the formation
of Wigner crystals in work published in Nature in June 2021.

As is the case in many fields of science, supercomputers are the only
tool that can probe the quantum behavior and explain the underlying
phenomena at play.

"In physics, we have very beautiful frameworks to describe an electron
or an atom, but when we're talking about real materials with 1023 atoms,
we don't know how to use these beautiful frameworks," Wang said.

For quantum or correlated materials in particular, physicists have
had a hard time applying 'beautiful' theory. "So instead, we use ugly
theory -- numerical simulation of the materials. Although we don't
have a well-established quantum computer for now, using classical high performance computers, we can push the problem forward a lot. Ultimately,
this will guide experiment." Wang is currently working with IBM
and IonQ to develop quantum algorithms to test on current and future
quantum computers. "Supercomputing is our first step." When it comes
to big future developments in technology, Wang believes computational
studies, in conjunction with experiment, observation and theory, will
help untangle mysteries and achieve practical goals, like tunable superconducting materials.

"A new algorithm can make a difference. More numerical precision
can make a difference," he said. "Sometimes we don't understand
the nature of a phenomenon because we didn't look closely enough
at the details. Only when you push the simulation and zoom in
to the nth digit will some important aspect of nature show up." ========================================================================== Story Source: Materials provided by University_of_Texas_at_Austin,_Texas_Advanced_Computing Center. Original written by Aaron Dubrow. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yao Wang, Zhuoyu Chen, Tao Shi, Brian Moritz, Zhi-Xun Shen,
Thomas P.

Devereaux. Phonon-Mediated Long-Range Attractive Interaction in
One- Dimensional Cuprates. Physical Review Letters, 2021; 127 (19)
DOI: 10.1103/PhysRevLett.127.197003 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220128100759.htm

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