Planckian metals have the potential to power high-temperature
superconductors, quantum computers and a host of other next-generation
technologies. However, these "strange" metals—in which electrical resistance
increases linearly with temperature—are notoriously difficult to study, let
alone comprehend.
In the last decade, physicists have attempted to explore the inner workings
of these quantum materials with cold atom experiments, whereby the behavior
of electrons is simulated with neutral atoms, light beams and ultra-cold
temperatures. These 2D models provide an analog system that allows
experimentalists to see the interactions at more scrutable length and time
scales—microns and milliseconds, rather than angstroms and
femtoseconds—bringing them ever closer to understanding the materials'
unusual electrical functions.
Now, Cornell researchers led by Erich Mueller, professor of physics in the
College of Arts and Sciences, have found this experimental model doesn't
capture what's really happening inside strange metals at all.
Their paper, "Transport in the Two-Dimensional Fermi-Hubbard Model: Lessons
from Weak Coupling," published Oct. 25 in Physical Review B. The lead author
is doctoral student Thomas Kiely.
"These cold atom experiments are a really awesome way to try and learn about
this strange metal behavior, this crazy unusual resistivity, which we
believe is the key to understanding how to make higher-temperature
superconductors and all sorts of other things," Mueller said. "We found
there's actually a simple explanation for what happens in this experiment."
Kiely and Mueller spent two years trying a variety of approaches to model
the cold atom experiment. To visualize the experiment, imagine a Go board.
The atoms are the black and white stones that can be moved, via quantum
tunneling, from square to square, dissipating energy based on the strength
of their interactions—or couplings—with other atoms.
The researchers found the most illuminating approach was to change the
strength of the interactions between the atoms.
"This gives us a very clear picture of how to describe that system," Kiely
said. "When the atoms interact with each other very weakly, we can kind of
build in the effective interactions based on the fact that we know what's
going on when they're not interacting."
By locating the limit at which these interactions were the weakest, the
researchers were able to observe the exotic behavior of strange metals, but,
surprisingly, in a context that wasn't strange enough to warrant it. And the
behavior still could be quantitatively explained.
"The interpretation of the cold atom experiment was that the same physics
that was responsible for these high-temperature superconductors was
occurring in these analog experiments, that they had found a strange metal,"
Mueller said. "What Thomas showed was that although they saw the same thing
as in the materials, it's quite likely from a different source. This weakly
attracting limit that we modeled is certainly not what's going on in the
material."
While the Cornell researchers were able to explain with confidence what is
happening in cold atom experiments, they are still not certain what is
occurring inside strange metals themselves.
"It's a hard problem," Mueller said. "We're hoping to have a more controlled
setting to investigate the same physics, because the models that are being
explored with these cold atom experiments probably aren't sophisticated
enough to explain what's going on. But I think you can build a lot of great
stuff off this, actually looking at this weak coupling limit and how things
cross over into strong coupling."
Reference:
Thomas G. Kiely et al, Transport in the two-dimensional Fermi-Hubbard model:
Lessons from weak coupling, Physical Review B (2021).
DOI: 10.1103/PhysRevB.104.165143
Tags:
Physics