Neutron scattering study on stripe correlations in La_2-xBa _xCuO₄

We have performed neutron scattering experiment on the single crystals of La_2-xBa_xCuO₄ (x = 0.10 and 0.125) in order to study the doping dependence of stripe correlations. At low temperatures, clear incommensurate (IC) peaks from spin-density-wave orders (SDW) were observed in both samples. The incommensurability (δ) of the SDW in the x = 0.10 sample was found to be 0.110, which slightly deviates from the simple relation of δ = x seen in La_2-xSr_xCuO₄, possibly due to the large stripe pinning effect in the low-temperature tetragonal (LTT) structure. The inelastic magnetic signal at the low-energy region below 6 meV drastically changes around the structural transition temperature between LTT and low-temperature orthorhombic (LTO) phases T_d2. Temperature dependences of the peak-width and the incommensurability also show an anomalous behavior at T_d2 rather than the spin ordering temperature, T _sp. These results suggest that spin correlations are well stabilized in the LTT phase as expected from the stripe model. With further increasing temperature in LTO phase, the incommensurability decreases and the peak-width grows due to the degradation of stripe correlations. However, the stiffness of stripe correlation seen in the low-energy IC spin fluctuations depends on the doping. In the x = 0.10 sample no well-defined magnetic excitation peak was observed below 6 meV at 200 K, while a flat top shaped single peak remains at 6 meV in the x = 0.125 sample. Deviation from the 1/8 doping more easily lose the spatial coherence of spin correlations above T_d2.