At temperatures within a few degrees of absolute zero, the ratio of a material’s thermal conductivity to its electrical conductivity should be proportional to its temperature. This principle, known as the Wiedemann-Franz law, was first formulated in 1853, but as our understanding of condensed-matter physics grew, its scope was amended so that it only applies if the same quasiparticles are responsible for carrying both heat and charge. In quantum materials where electrons interact very strongly, it should not hold.
Or so it was thought. Theorists led by Wen Wang of the US Department of Energy’s SLAC National Accelerator Laboratory and Stanford University have now found that the law should continue to be obeyed within one type of quantum material: the copper oxide (cuprate) superconductors. These materials are known as unconventional superconductors, and they conduct electricity without resistance at relatively high temperatures compared to their conventional counterparts. The finding means that physicists will not have to resort to oversimplified and conceptually problematic assumptions involving quasiparticles or Boltzmann equations when predicting how electrons in these so-called strongly correlated materials should behave.
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