Successive Structural Transitions and Anomalous Superconductivity in La214 systems under High Pressures

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In La_<2-x>Ba_xCuO_4 (LBCO) with 0.1<x<0.14, an electronic anomaly appears around T_1-60K where the crystal structure is changed into a low-temperature tetragonal (LTT) or Pccn orthorhombic one, and the superconductivity is seriously suppressed. It was recently reported for La_<1.6-x>Nd_<0.4>Sr_xCuO_4 (LNSCO) with x=0.12 that the electronic anomaly around T_1 originates from a stripe charge (hole) and spin ordering within the Cu-O planes, which is stabilized by pinning of holes due to lattice potential. However, the stabilization of the stripe ordering seems to be irrelevant to the LTT (or Pccn) structure in La_<1.8-x>Nd_<0.2>Ba_xCuO_4 (LNBCO) system, because the electronic anomaly due to the stripe ordering remains unchanged while T_1 is enhanced up to above 100 K.In the present study, therefore, the relationship between the crystal structure and the stripe ordering was examined on LBCO and LNBCO systems under high pressures. The results obtained are followings :l) In samples with x-1/8, the stripe charge (hole) and spin correlation develops around T^<**>(-60K) independent of the crystal structure. If the crystal structure at T^<**> is LTT (or Pccn), the stripe correlation is stabilized through the pinning of holes due to the lattice potential. The commensurability energy also contributes to the pinning in crystals with x= 1/8.2) The random magnetic potential due to local magnetic moments on Nd^<3+>-sites disturbs the stripe spin correlation within the Cu-O plane.