Nav: Home

UBC research pokes holes in Hubbard model

August 19, 2009

New UBC research has literally and figuratively poked holes in single-band Hubbard physics--a model that has been used to predict and calculate the behavior of high-temperature superconductors for 20 years.

The findings are the first compelling evidence challenging the model under certain conditions, and could necessitate entirely new theoretical approaches to explaining superconductivity in cuprate materials, one of the outstanding mysteries in condensed-matter physics.

"Single-band Hubbard physics has been used for 20 years to predict how superconducting cuprate materials accommodate the 'holes' left by electron removal," says Darren Peets, lead author of the study who conducted the research 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 behaviour, or we could be seeing completely new physics, but in either case the usual theoretical approaches do not work here."

The findings were published today in the journal Physical Review Letters.

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.

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 very rarely studied. While the model explains the material's electron behaviour during doping, Peets and his team found the model falls apart as even more electrons are removed.

"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."

Special crystal samples grown at UBC enabled the team to overdope the superconductor to a degree rarely possible with most materials. "Few materials exist in this doping range, and they tend to be very difficult to grow crystals of," says Peets. "In the case of these crystals, thallium oxide--which is toxic--boils off near growth temperatures if you allow it. So a fair bit of work and care is required."

Discovered in 1986, high-temperature superconductors are cuprates--copper oxides. The materials, which exhibit superconducting properties at usually cold temperatures--often in excess of 90 kelvin--remain an enigma despite intense scrutiny. And because their superconducting state persists at more manageable temperatures, more commercial applications are feasible.
-end-
Peets, currently a post-doctoral researcher at Kyoto University, conducted the research at the Berkeley Advanced Light Source synchrotron under the supervision of UBC Physics and Astronomy Professor Douglas Bonn, and with UBC chemist and physicist Professor George Sawatzky.

The work 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.

University of British Columbia

Related Superconductivity Articles:

First report of superconductivity in a nickel oxide material
Scientists at SLAC and Stanford have made the first nickel oxide material that shows clear signs of superconductivity - the ability to transmit electrical current with no loss.
A hallmark of superconductivity, beyond superconductivity itself
Physicists have found 'electron pairing,' a hallmark feature of superconductivity, at temperatures and energies well above the critical threshold where superconductivity occurs.
Manipulating superconductivity using a 'mechanic' and an 'electrician'
Strongly correlated materials can change their resistivity from infinity to zero with minute changes in conditions.
Triplet superconductivity demonstrated under high pressure
Researchers in France and Japan have demonstrated a theoretical type of unconventional superconductivity in a uranium-based material, according to a study published in the journal Physical Review Letters.
The mechanism of high-temperature superconductivity is found
Russian physicist Viktor Lakhno from Keldysh Institute of Applied Mathematics, RAS considers symmetrical bipolarons as a basis of high-temperature superconductivity.
More Superconductivity News and Superconductivity Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Erasing The Stigma
Many of us either cope with mental illness or know someone who does. But we still have a hard time talking about it. This hour, TED speakers explore ways to push past — and even erase — the stigma. Guests include musician and comedian Jordan Raskopoulos, neuroscientist and psychiatrist Thomas Insel, psychiatrist Dixon Chibanda, anxiety and depression researcher Olivia Remes, and entrepreneur Sangu Delle.
Now Playing: Science for the People

#537 Science Journalism, Hold the Hype
Everyone's seen a piece of science getting over-exaggerated in the media. Most people would be quick to blame journalists and big media for getting in wrong. In many cases, you'd be right. But there's other sources of hype in science journalism. and one of them can be found in the humble, and little-known press release. We're talking with Chris Chambers about doing science about science journalism, and where the hype creeps in. Related links: The association between exaggeration in health related science news and academic press releases: retrospective observational study Claims of causality in health news: a randomised trial This...