After 20 years of trying, scientists succeed in doping a 1D chain of
cuprates

Date:
September 9, 2021
Source:
DOE/SLAC National Accelerator Laboratory
Summary:
After 20 years of trying, scientists doped a 1D copper oxide chain
and found a surprisingly strong attraction between electrons that
may factor into the material's superconducting powers.



FULL STORY ==========================================================================
When scientists study unconventional superconductors -- complex materials
that conduct electricity with zero loss at relatively high temperatures --
they often rely on simplified models to get an understanding of what's
going on.


========================================================================== Researchers know these quantum materials get their abilities from
electrons that join forces to form a sort of electron soup. But modeling
this process in all its complexity would take far more time and computing
power than anyone can imagine having today. So for understanding one key
class of unconventional superconductors -- copper oxides, or cuprates -- researchers created, for simplicity, a theoretical model in which the
material exists in just one dimension, as a string of atoms. They made
these one-dimensional cuprates in the lab and found that their behavior
agreed with the theory pretty well.

Unfortunately, these 1D atomic chains lacked one thing: They could not
be doped, a process where some atoms are replaced by others to change
the number of electrons that are free to move around. Doping is one of
several factors scientists can adjust to tweak the behavior of materials
like these, and it's a critical part of getting them to superconduct.

Now a study led by scientists at the Department of Energy's SLAC
National Accelerator Laboratory and Stanford and Clemson universities
has synthesized the first 1D cuprate material that can be doped. Their
analysis of the doped material suggests that the most prominent
proposed model of how cuprates achieve superconductivity is missing a
key ingredient: an unexpectedly strong attraction between neighboring
electrons in the material's atomic structure, or lattice. That attraction,
they said, may be the result of interactions with natural lattice
vibrations.

The team reported their findings today in Science.

"The inability to controllably dope one-dimensional cuprate systems has
been a significant barrier to understanding these materials for more than
two decades," said Zhi-Xun Shen, a Stanford professor and investigator
with the Stanford Institute for Materials and Energy Sciences (SIMES)
at SLAC.



==========================================================================
"Now that we've done it," he said, "our experiments show that our
current model misses a very important phenomenon that's present in the
real material." Zhuoyu Chen, a postdoctoral researcher in Shen's lab
who led the experimental part of the study, said the research was made
possible by a system the team developed for making 1D chains embedded in
a 3D material and moving them directly into a chamber at SLAC's Stanford Synchrotron Radiation Lightsource (SSRL) for analysis with a powerful
X-ray beam.

"It's a unique setup," he said, "and indispensable for achieving the high- quality data we needed to see these very subtle effects." From grids to chains, in theory The predominant model used to simulate these complex materials is known as the Hubbard model. In its 2D version, it is based
on a flat, evenly spaced grid of the simplest possible atoms.



==========================================================================
But this basic 2D grid is already too complicated for today's computers
and algorithms to handle, said Thomas Devereaux, a SLAC and Stanford
professor and SIMES investigator who supervised the theoretical part
of this work. There's no well-accepted way to make sure the model's calculations for the material's physical properties are correct, so if
they don't match experimental results it's impossible to tell whether
the calculations or the theoretical model went wrong.

To solve that problem, scientists have applied the Hubbard model to 1D
chains of the simplest possible cuprate lattice -- a string of copper and oxygen atoms. This 1D version of the model can accurately calculate and
capture the collective behavior of electrons in materials made of undoped
1D chains. But until now, there hasn't been a way to test the accuracy
of its predictions for the doped versions of the chains because no one
was able to make them in the lab, despite more than two decades of trying.

"Our major achievement was in synthesizing these doped chains,"
Chen said. "We were able to dope them over a very wide range and get
systematic data to pin down what we were observing." One atomic layer
at a time To make the doped 1D chains, Chen and his colleagues sprayed a
film of a cuprate material known as barium strontium copper oxide (BSCO),
just a few atomic layers thick, onto a supportive surface inside a sealed chamber at the specially designed SSRL beamline. The shape of the lattices
in the film and on the surface lined up in a way that created 1D chains
of copper and oxygen embedded in the 3D BSCO material.

They doped the chains by exposing them to ozone and heat, which added
oxygen atoms to their atomic lattices, Chen said. Each oxygen atom pulled
an electron out of the chain, and those freed-up electrons become more
mobile. When millions of these free-flowing electrons come together, they
can create the collective state that's the basis of superconductivity.

Next the researchers shuttled their chains into another part of the
beamline for analysis with angle-resolved photoemission spectroscopy,
or ARPES. This technique ejected electrons from the chains and measured
their direction and energy, giving scientists a detailed and sensitive
picture of how the electrons in the material behave.

Surprisingly strong attractions Their analysis showed that in the doped 1D material, the electrons' attraction to their counterparts in neighboring lattice sites is 10 times stronger than the Hubbard model predicts,
said Yao Wang, an assistant professor at Clemson University who worked
on the theory side of the study.

The research team suggested that this high level of "nearest-neighbor" attraction may stem from interactions with phonons -- natural vibrations
that jiggle the atomic latticework. Phonons are known to play a role in conventional superconductivity, and there are indications that they could
also be involved in a different way in unconventional superconductivity
that occurs at much warmer temperatures in materials like the cuprates, although that has not been definitively proven.

The scientists said it's likely that this strong nearest-neighbor
attraction between electrons exists in all the cuprates and could help
in understanding superconductivity in the 2D versions of the Hubbard
model and its kin, giving scientists a more complete picture of these
puzzling materials.

Researchers from DOE's Oak Ridge National Laboratory contributed to this
work, which was funded by the DOE Office of Science. SSRL is an Office
of Science user facility.

========================================================================== Story Source: Materials provided by
DOE/SLAC_National_Accelerator_Laboratory. Original written by Glennda
Chui. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Zhuoyu Chen, Yao Wang, Slavko N. Rebec, Tao Jia, Makoto Hashimoto,
Donghui Lu, Brian Moritz, Robert G. Moore, Thomas P. Devereaux,
Zhi-Xun Shen. Anomalously strong near-neighbor attraction in
doped 1D cuprate chains. Science, 2021; 373 (6560): 1235 DOI:
10.1126/science.abf5174 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210909162237.htm

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