UBC Researchers Find Flaws in
Current Theoretical HTS Models
Researchers with
the University of British Columbia (UBC) claim to have raised
significant doubts concerning single-band Hubbard physics, a model
that has been used to predict and calculate HTS behavior for 20
years. The findings are reportedly the first evidence challenging
the model under certain conditions, and could necessitate entirely
new theoretical approaches to explaining superconductivity in
cuprates.
Cuprates
normally act as insulators but become superconductors when electrons
are removed, a process known as doping holes into the material.
Physicists consider a material optimally doped when it achieves
superconductivity at the highest, most accessible temperature.
However, UBC researchers where able to break the single-band Hubbard
model by overdoping a crystal cuprate superconductor past its optimal
range, a level of doping that is difficult to achieve and rarely
studied. While the model explains the material's electron behavior
during doping, the model falls apart as even more electrons are
removed.
Theory Not
Confirmed By Empirical Studies
"Single-band
Hubbard physics has been used for 20 years to predict how
superconducting cuprate materials accommodate the holes left by
electron removal," said Darren Peets, Researcher at Kyoto
University and lead author of the study while a UBC doctoral student.
"But now it looks like the approaches that underpin a large
fraction of the theoretical work in the field just don't work across
all the ranges of superconductivity we can study. The part of the
cuprates' superconducting phase diagram we looked at could exhibit
less-bizarre behavior, or we could be seeing completely new physics,
but in either case the usual theoretical approaches do not work here.
“By probing
the electronic states using tunable-energy X-rays, we were able to
show that this region accommodates electron holes in a fundamentally
different manner, and that the interactions among the holes already
in the material change completely."
The research was
supported by the Natural Sciences and Engineering Research Council of
Canada, the Canada Research Chairs program, the British Columbia
Synchrotron Institute, and the Canadian Institute for Advanced
Research.
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