More indirect evidence for the pseudogap

Signatures of the pseudogap in various transport properties for the underdoped compound YBa₂Cu₄O₈. At high temperatures the resistivity (red solid line) decreases linearly with temperature, consistent with a non-Fermi-liquid behaviour (red dashed line). However, in the pseudogap region (i.e. below T_p) it drops faster with temperature, before falling to zero at the superconducting transition temperature (about 85 K). Similarly the NMR relaxation time (green squares) displays non-Fermi-liquid behaviour (green dashed line) before deviating at around 200 K. The NMR shift (blue squares) also deviates from the non-Fermi-liquid behaviour (not shown) below T_p. Note that the pseudogap expresses itself as a gradual change in behaviour as the sample is cooled and that the onset temperature is different for the different properties. (B Bucher et al. 1993 Phys. Rev. Lett. 70 2021; H Yasuoka 1997 Hyperfine Interactions 105 27; H Alloul et al. 1989 Phys. Rev. Lett. 63 689).

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