Metal-insulator transition near the superconducting phase of two-dimensional cobalt oxides

我们提出了三个主要结果，认为库珀对形成在Na_xCoO_2·yH_2O超级导体中的机制的重要性是什么。（1）我们已经揭示了在超导电状态中Na_xCoO_2·yH_2O存在于自旋单状状态中的超导电对，通过对^<59>Co-NMR Knight Shift（H）方向并行和相对于CoO_2计划中的超导电状态使用单个相位单晶体的超导电状态的测量。（2） The superconducting transition temperatures T_c of Na_xCoO_2·yH_2 O have been found to exhibit the systematic dependence on the <59>^Connuclear electric quadrupole resonance v_Q.在T-3 ν_Q相位图中，3ν_Q的超传导区域，在~ 12.0和~ 12.9 MHz之间传播，在~ 12.5和~ 12.6 MHz之间将非超传导相位划分为两个区域。我们根据详细的实验测量测量了非超传导相位的起源。 ... More 在^<59>Co-NQR和^2D-核磁共振中发现的非超导电阶段是在电荷密度波（CDW）或电荷分散状态中发现的，是在磁性时刻诱导的原始情况下，由于这种转变在低温度下冻结。（3） We have revealed that the x dependence of the electronic nature of Na_x，CoO_2 which is the parent compound of Na_<0.3>CoO_2·1. 3 H_2 O superconductors。在<0.5>87 K （T_<c1>）和53 K （T_<c2>）上观察到的成功相位转换在Na_CoO_2上已完成研究。Na_CoO_2<0.5>指示T_以下的抗磁排序<c1>，我们在哪里通过^<59>Co-NMR和中子差分研究确定了不寻常的磁结构。它从X依赖性的χ-T和1/T_1-T曲线中发现了Na_xCoO_2电子的两个维度性质增强，并且在这个系统中增加了抗磁相关性，即Na集中x分解。因此，超级行为对是预期出现在单一的状态中的。这与Knight shift studies in Na_CoO_2 <0.3>- 1.3 H_2 O相一致。Less（低）

We have produced principally three results, which are of considering importance to the mechanism of the formation of the Cooper pairs in Na_xCoO_2・yH_2O superconductors.(1) We have revealed that the superconducting pairs in the superconducting state of Na_xCoO_2・yH_2O are in the spin-singlet state, by the measurements of the ^<59>Co-NMR Knight shift for both magnetic field (H) directions parallel and perpendicular to the CoO_2planes in the superconducting state using single phase single crystals.(2) The superconducting transition temperatures T_c of Na_xCoO_2・yH_2O have been found to exhibit the systematic dependence on the ^<59>Co nuclear electric quadrupole resonance frequency ν_Q. In the T-3ν_Q phase diagram, the superconducting region of 3ν_Q, spreading over the region between 〜12.0 and 〜12.9 MHz, is divided into two regions by the non-superconducting phase between 〜12.5 and 〜12.6 MHz.We have investigated the origin of the non-superconducting phase by detailed experimental measurem … More ents of ^<59>Co-NQR and ^2D-NMR and found that the non-superconducting phase is in the charge-density-wave (CDW) or the charge disproportionate state, in which the magnetic moments induced inhomogeneously by this transition become frozen at low temperatures.(3) We have revealed that the x dependence of the electronic nature of Na_x,CoO_2 which is the parent compound of Na_<0.3>CoO_2・1.3H_2O superconductors. In particular, the successive phase transitions observed in Na_<0.5>CoO_2 at 87 K (T_<c1>) and 53 K (T_<c2>) have been studies. Na_<0.5>CoO_2 indicates the antiferromagnetic ordering below T_<c1>, where we have determined the unusual magnetic structure by ^<59>Co-NMR and neutron diffraction studies.It has been found from the X dependence of χ-T and 1/T_1-T curves that the two-dimensional nature of the electrons of Na_xCoO_2 is enhanced and the antiferromagnetic correlation increases in this system as the Na concentration x decreases. Therefore, the superconducting pairs are expected to be in the singlet state. This is consistent with the results of the Knight shift studies in Na_<0.3>CoO_2-1.3H_2O. Less