Hiding in the Pseudogap

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Science  03 Dec 2010:
Vol. 330, Issue 6009, pp. 1291
DOI: 10.1126/science.330.6009.1291-a

When cuprate superconductors, which can conduct electricity without resistance up to temperatures on the order of 150 K, were first discovered almost 25 years ago, it was expected that the explanation for their extraordinary properties was around the corner and would lead to even higher transition temperatures Tc. Today this mechanism remains a mystery; however, most researchers agree that the so-called pseudogap region, which lies above Tc but is characterized by a non-zero excitation gap that disappears at a higher temperature T*, is key to understanding the entire phase diagram. Along the doping axis, the pseudogap region borders an antiferromagnetic phase, the remnants of which can be detected by neutron scattering as a resonance centered around the corner of the Brillouin zone. Li et al. use polarized neutron scattering to detect another magnetic excitation that is present at all wave vectors and, surprisingly, has an integrated spectral weight an order of magnitude higher than that of the resonance. Because the studied compound, HgBa2CuO4+δ, is thought to be representative of the cuprates, and the new magnetic excitation appears at the same T* determined from prior transport and neutron diffraction measurements, it is tempting to interpret this result as a corroborating piece of evidence that a true phase transition, rather than a crossover, takes place at T*.

Nature 468, 283 (2010).

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