Reconciliation of local and long-range tilt correlations in underdoped La2-xBaxCuO4 (0 ≤ x ≤ 0.155)

A long-standing puzzle regarding the disparity of local and long-range CuO6 octahedral tilt correlations in the underdoped regime of La2-xBaxCuO4 is addressed by utilizing complementary neutron powder diffraction and inelastic neutron scattering (INS) approaches. This system is of interest because of the strong depression of the bulk superconducting transition at x = 1/8 in association with charge and spin stripe order. The latter unidirectional order is tied to Cu-O bond-length anisotropy present in the so-called low-temperature tetragonal (LTT) phase. On warming, the lattice exhibits two sequential structural transitions, involving changes in the CuO6 tilt pattern, first to the low-temperature orthorhombic (LTO) and then the high-temperature tetragonal (HTT) phase. Despite the changes in static order, inspection of the instantaneous local atomic structure suggests that the LTT-type tilts persist through the transitions. Analysis of the INS spectra for the x = 1/8 composition reveals the dynamic nature of the LTT-like tilt fluctuations within the LTO and HTT phases. Within the low-temperature phase, the Cu-O bond-length splitting inferred from lattice symmetry and fitted atomic position parameters reaches a maximum of 0.3% at x = 1/8, suggesting that electron-phonon coupling may contribute to optimizing the structure to stabilize stripe order. This splitting is much too small to be resolved in the pair distribution function, and in fact we do not resolve any enhancement of the instantaneous bond-length distribution in association with stripe order. This study exemplifies the importance of a systematic approach using complementary techniques when investigating systems exhibiting a large degree of complexity and subtle structural responses.