Quantum oscillations have revealed the presence of a small pocket in the
Fermi surface of the cuprate superconductor YBCO, whose nature and origin are
the subject of much debate. Interpretations include electron and hole pockets;
scenarios include Fermi-surface reconstruction by antiferromagnetism,
d-density-wave order, and stripe order. Here we report quantum oscillations in
the Seebeck and Nernst coefficients of YBCO and show, from the magnitude and
sign of the Seebeck coefficient, that they come from an electron pocket. Using
measurements of the Seebeck coefficient as a function of hole doping p, we show
that the evolution of the Fermi surface in YBCO is the same as in Eu-LSCO, a
cuprate where stripe order (a modulation of spin and charge densities) is well
established. The electron pocket is most prominent where stripe order is
strongest, at p = 1/8. This shows that Fermi-surface reconstruction is a
generic mechanism of underdoped cuprates, intimately related to stripe order.Comment: 15 pages, 5 figures, Supplementary information now integrated into
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