  An outstanding problem in the field of high-transition-temperature (high Tc)
superconductivity is the identification of the normal state out of which
superconductivity emerges in the mysterious underdoped regime. The normal state
uncomplicated by thermal fluctuations is effectively accessed by the use of
applied magnetic fields sufficiently strong to suppress long-range
superconductivity at low temperatures. Proposals in which the normal ground
state is characterised by small Fermi surface pockets that exist in the absence
of symmetry breaking have been superseded by models based on the existence of a
superlattice that breaks the translational symmetry of the underlying lattice.
Recently, a charge superlattice model that positions a small electron-like
Fermi pocket in the vicinity of the nodes (where the superconducting gap is
minimum) has been proposed a replacement for the prevalent superlattice models
that position the Fermi pocket in the vicinity of the pseudogap at the
antinodes (where the superconducting gap is maximum). Although some ingredients
of symmetry breaking have been recently revealed by crystallographic studies,
their relevance to the electronic structure remains unresolved. Here we report
angle-resolved quantum oscillation measurements in the underdoped copper oxide
YBa2Cu3O6+x. These measurements reveal a normal ground state comprising
electron-like Fermi surface pockets located in the vicinity of the
superconducting gap minima (or nodes), and further point to an underlying
superlattice structure of low frequency and long wavelength with features in
common with the charge order identified recently by complementary spectroscopic
techniques.
