Hall, seebeck, and nernst coefficients of underdoped HgBa₂CuO_4+δ: Fermi-Surface reconstruction in an archetypal cuprate superconductor

Charge-density-wave order has been observed in cuprate superconductors whose crystal structure breaks the square symmetry of the CuO₂ planes, such as orthorhombic YBa₂Cu₃O_y (YBCO), but not so far in cuprates that preserve that symmetry, such as tetragonal HgBa₂CuO_4+δ (Hg1201). We have measured the Hall (R_H), Seebeck (S), and Nernst (υ) coefficients of underdoped Hg1201 in magnetic fields large enough to suppress superconductivity. The high-field R_H (T) and S(T) are found to drop with decreasing temperature and become negative, as also observed in YBCO at comparable doping. In YBCO, the negative R_H and S are signatures of a small electron pocket caused by Fermi-surface reconstruction, attributed to charge-density-wave modulations observed in the same range of doping and temperature. We deduce that a similar Fermi-surface reconstruction takes place in Hg1201, evidence that density-wave order exists in this material. A striking similarity is also found in the normal-state Nernst coefficient υ(T), further supporting this interpretation. Given the model nature of Hg1201, Fermi-surface reconstruction appears to be common to all hole-doped cuprates, suggesting that density-wave order is a fundamental property of these materials.