Spatially modulated 'Mottness' in La _2-x Ba _x CuO ₄

Competition between magnetism and the kinetic energy of mobile carriers (typically holes) in doped antiferromagnets may lead to 'stripe' phases ^1-4 , which are charged rivers separating regions of oppositely phased antiferromagnetism. In copper oxides the main experimental evidence for such coexisting static spin and charge order comes from neutron scattering in La _1.48 Nd _0.4 Sr _0.12 CuO ₄ (LNSCO; ref. 5) and La _1.875 Ba _0.125 CuO ₄ (LBCO; ref. 6). However, as a neutron is neutral, it does not detect charge but rather its associated lattice distortion ⁷ , so it is not known whether the stripes involve 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 near the Fermi level as well as near the correlated gap ^8,9 , demonstrating significant modulation in both the doped-hole density and the 'Mottness', or the degree to which the system resembles a Mott insulator ¹⁰ . The peak-to-trough amplitude of the valence modulation is estimated to be 0.063 holes, which suggests ¹¹ an integrated area of 0.59 holes under a single stripe, close to the expected 0.5 for half-filled stripes.