Incommensurate spin ordering and fluctuations in underdoped La_(2-x)Ba_xCuO_4

摘要

By using neutron scattering techniques, we have studied the incommensurate spin ordering as well as the low energy spin dynamics in single crystal underdoped La_(2-x)Ba_xCuO_4 with x ～0.095 and 0.08, which are high temperature superconductors with T_C～21 and 29 K, respectively. Static two dimensional incommensurate magnetic order appears below T_N=39.5 + 0.3 K in La_(2-x)Ba_xCuO_4 (x=0.095) and a similar temperature for x=0.08 within the low temperature tetragonal phase. The spin order is unaffected by either the onset of superconductivity or the application of magnetic fields of up to 7 T applied along the c axis in the x=0.095 sample. Such a magnetic field independent behavior is in marked contrast to the field induced enhancement of the staggered magnetization observed in the related La_(2-x)Sr_xCuO_4 system, which indicates that this phenomenon is not a universal property of cuprate superconductors. Surprisingly, we find that the incommensurability δ is only weakly dependent on doping relative to La_(2-x)Sr_xCuO_4. Dispersive excitations in La_(2-x)Ba_xCuO_4 (x =0.095) at the same incommensurate wave vector persist up to at least 60 K. The dynamical spin susceptibility of the low energy spin excitations saturates below T_C in a similar manner to that seen in the superconducting state of La_2CuO_(4+y).