Research pokes holes in Hubbard model: Could help solve enigma of high-temperature superconductors

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.

Source: University of British Columbia (news : web)

   

• Alizee - Aug 19, 2009
  • Rank: 2.3 / 5 (6)
  The problems of mainstream science with explanation of HT superconductivity aren't accidental and they're demonstrating the weakness of formal approach in physics. Mathematicians simply cannot think in wider consequences and they cannot separate problem into smaller logically consistent areas, which can be solved intuitively.
  
  In addition, because formal math is atemporal and based on consecutive logics, it doesn't handle well the parallel logics of multicomponent systems where many time arrows are involved, the complex interactions of many particles and atoms in particular.
  
  What's even worse, it purportedly ignores theories and experimental results, based on less formal, yet more effective approach. We have room temperature superconductors already for many years, but the selfish and proud mainstream theorists ignore these results, until they cannot be explained by their own theories.
  
  But the society needs HT superconductors first, the formal theory may come latter. In this way, the progress in science is effectively blocked by arrogant dumbness of formal approach in physics. We all are paying for this arrogance. The obstructions in cold fusion research and/or acceptation of Aether theory is the problem of the very same category.
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• vantomic - Aug 19, 2009
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  please stop
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• Alexa - Aug 19, 2009
  • Rank: 2.6 / 5 (8)
  Why these experiments weren't attempted to replicate by peer-reviewed press? Who is responsible for it?
  
  http://www.iop.or...8/3/319/
  http://www.lenr-c...Navy.htm
  
  The silencing of opponents is the only strategy of mainstream science here.
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• out7x - Aug 20, 2009
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  Excellent Scientific American article Aug.2009.
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• Nik_2213 - Aug 20, 2009
  • Rank: 5 / 5 (1)
  D'uh, why must folk invoke a new theory of *everything* to cope with something that workers in the field knew was probably only applicable to a limited range, anyway ? And couldn't account for the different varieties of SC ??
  
  And why scream 'conspiracy' over work that is too cutting-edge and toxic for easy replication ? Thallium is scary at best, but volatile thallium compounds are almost malevolent...
  
  My head hurts...
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• Alexa - Aug 20, 2009
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  ..that workers in the field knew was probably only applicable to a limited range..

  Can you show us some publication, which presents such stance? I'm expert in this field and I found none.
  

  ..why scream 'conspiracy' over work..

  Because here's no conspiracy at all.
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