  Stripe phases were predicted to arise in doped antiferromagnets through
competition between magnetism and the kinetic energy of mobile carriers
(typically holes). In copper-oxides the main experimental evidence for stripes
is neutron scattering from La1.48Nd0.4Sr0.12CuO4 (LNSCO) and La1.875Ba0.125CuO4
(LBCO) which reveals coexisting static spin and charge order whose wavelengths
differ by a factor of two, reminiscent of charged rivers separating regions of
oppositely-phased antiferromagnetism. A neutron is an electrically neutral
object, however, so does not detect charge but rather its associated lattice
distortion ; it is not known if the "stripe" phase in LNSCO and LBCO actually
involves ordering of the doped holes. Here we present a study of the charge
order in LBCO with resonant soft x-ray scattering (RSXS). We observe giant
resonances at both the mobile carrier and upper-Hubbard band features in the OK
edge. These demonstrate a substantial modulation in the doped hole density as
well as the amount of spectral weight near the correlated gap, i.e. the degree
of "Mottness". The peak-to-trough amplitude of the valence modulation is
estimated to be 0.063 holes, which if interpreted with a model of the stripe
form factor suggests an integrated area of 0.59 holes under a single stripe.
While only an estimate, this number agrees with what is expected for
half-filled stripes.
