Hall and Nernst Coefficients of Underdoped HgBa$_2$CuO$_{4+\delta}$: Fermi-Surface Reconstruction in an Archetypal Cuprate Superconductor

The Hall coefficient $R_{\rm H}$ of underdoped HgBa$_2$CuO$_{4+\delta}$ (Hg1201) was measured at low temperature in magnetic fields large enough to suppress superconductivity. The normal-state $R_{\rm H}(T)$ is found to drop with decreasing temperature and become negative below 10~K, as also observed in the orthorhombic bi-layer cuprate YBa$_2$Cu$_3$O$_{y}$ (YBCO) at comparable doping. In YBCO, the negative $R_{\rm H}$ is the signature of a Fermi-surface reconstruction that produces a small electron pocket, attributed to the onset of charge-density wave order at low temperature. We infer that a similar Fermi-surface reconstruction occurs in the tetragonal single-layer material Hg1201. A striking similarity is also found in the normal-state Nernst coefficient $\nu(T)$, which drops below the pseudogap temperature $T^\star$, to reach a large negative value at low temperature, in both Hg1201 and YBCO. Our results are compelling evidence that the mechanisms responsible for Fermi-surface reconstruction and pseudogap formation in hole-doped cuprates are universal. Preprint reference: arXiv:1210.8411.