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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