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

在La_xt；2-x&gt；BaxCuO_4(0.1&lt；x&lt；0.14)中，在T_1-60K附近出现电子反常，晶体结构转变为低温四方(LTT)或PCCN斜方结构，超导电性受到严重抑制。最近报道了x=0.12的La_&lt；1.6-x&gt；ND_&lt；0.4&gt；Sr_xCuO_4(LNSCO)的电子反常起源于Cu-O面内的条状电荷(空穴)和自旋有序，而条状电荷(空穴)和自旋有序是由晶格势引起的空穴钉扎稳定的。然而，在La_&lt；1.8-x&gt；ND_&lt；0.2&gt；BaxCuO_4(LNBCO)系统中，条带有序的稳定性似乎与LTT(或PCCN)结构无关，因为在T_1提高到100K以上时，由条带有序引起的电子反常保持不变。得到的结果如下：L)在x-1/8的样品中，条纹电荷(空穴)和自旋关联在T^&lt；**&gt；(-60K)附近发展，与晶体结构无关。如果T^&lt；**&gt；处的晶体结构为LTT(或PCCN)，则由于晶格势的作用，通过钉扎空穴来稳定条纹关联。可公度能对x=1/8.2的晶体中的钉扎也有贡献)由于Nd^&lt；3+&gt；-位上的局域磁矩引起的随机磁势干扰了CuO平面内的条带自旋关联。

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.