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.
Reference:
Anomalously strong near-neighbor attraction in doped 1D cuprate chains,
Science (2021).
DOI: 10.1126/science.abf5174
No comments:
Post a Comment