  The existence of charge density wave (CDW) correlations in cuprate
superconductors has now been established. However, the nature of the ground
state order has remained uncertain because disorder and the presence of
superconductivity typically limit the CDW correlation lengths to a dozen unit
cells or less. Here we explore the CDW correlations in YBa2Cu3Ox (YBCO)
ortho-II and ortho-VIII crystals, which belong to the cleanest available
cuprate family, at magnetic fields in excess of the resistive upper critical
field (Hc2) where the superconductivity is heavily suppressed. We find an
incommensurate, unidirectional CDW with a well-defined onset at a critical
field strength that is proportional to Hc2. It is related to but distinct from
the short-range bidirectional CDW that exists at zero magnetic field. The
unidirectional CDW possesses a long inplane correlation length as well as
significant correlations between neighboring CuO2 planes, yielding a
correlation volume that is at least 2 - 3 orders of magnitude larger than that
of the zero-field CDW. This is by far the largest CDW correlation volume
observed in any cuprate crystal and so is presumably representative of the
high-field ground-state of an "ideal" disorder-free cuprate.
