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

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^&lt；59&gt；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^&lt；59&gt；Co nuclear electric quadrupole resonance frequencyν_Q.In the T-3ν_Q phase diagram，the superconducting region of 3ν_Q，spreading over the region between~12.0and~12.9 MHz，is divided into two regions by the non-superconducting phase between~12.5and~12.6MH.We have investigated the origin of the non-superconducting phase by detailed experimental measurem…More ents of^&lt；59&gt；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 reveathat the x dependence of the electronic nature of Na_x，CoO_2which the parent compound of Na_lt；03.In particular，the successive phase transitions observed in Na_&lt；0.5&gt；CoO_2at87K(T_&lt；c1&gt；)and 53K(T_&lt；c2&gt；)have been studies。Na_&lt；0.5&gt；CoO_2indicates the antiferromagnetic ordering below T_&lt；c1&gt；，where we have determined the unusual magnetic structure by^&lt；59&gt；Co-NMR and neutron diffraction studies.It has been found from the X dependence ofχ-T and1/T_1-T curves that the two-dimensional nature of the electrons of Na_xCoO_2is enhanced and the antifromagnetic correlation increases in the electrons of Na_xCoO_2is enhanced and the antiferromagnetic correlation increases the two-dimensional nature of the electrons of Na_xCoO_2is enhanced and the antiferromagnetic correlation increases in thireas。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_&lt；0.3&gt；CoO_2-1.3H_2O。Less：Less

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