  One of the most important outstanding problems in condensed matter physics is
determination the ground state of strongly correlated electron system, in
particular high-temperature superconductors (HTSC). For these materials,
theoretical1-6 and experimental7-13 evidence has been mounting in support of
the possibility that their ground state exhibits spin and charge density waves
(SDW and CDW), which are primarily one-dimensional (i.e. stripes) and which
either compete with or promote superconductivity. Coexistence of CDW or SDW and
superconductivity has previously been reported in the lower Tc
materials7-9,12,13 and in the presence of large magnetic fields10,11. The
absence of data showing stripes in the higher Tc materials in zero field has
lent support to the idea that stripes are competing with high temperature
superconductivity. However, here we show, using scanning tunneling spectroscopy
(STS), the existence of static striped density of electronic states in nearly
optimally doped Bi2Sr2CaCu2O8+d in zero field. This modulation is aligned with
the Cu-O bonds, with a periodicity of four lattice constants, and exhibits
features characteristic of a two-dimensional system of line objects. We further
show that the density of states modulation manifests itself as a shift of
states from above to below the superconducting gap. The fact that a single
energy scale (i. e., the gap) appears for both superconductivity and stripes
suggests that these two effects have the same origin.
