Taketo Moyoshi

Superconductivity of LaFe1-yCoyAsO1-xFx

We have prepared the superconducting system LaFe 1- y Co y AsO 1- x F x ( x =0.11) and carried out measurements of their electrical resistivity ρ and superconducting diamagnetism. 75 As- and 135 La-NMR studies have also been carried out. The Knight shift observed for 75 As has been found to be suppressed by the superconductivity; for 135 La, the shift is almost insensitive to the superconductivity. This result presents rather strong experimental evidence for singlet pairing. The Co doping effect on the superconducting transition temperature T c is not so significant as expected for superconductors with nodes, suggesting that the potential scattering does not seem to primarily suppress the superconductivity. Even for superconductors without nodes, it may not be so trivial to expect this small effect, if there are two different (disconnected) Fermi surfaces whose order parameters have opposite signs. As a possible explanation of the observed T c suppression, which is found to be small, seems to be related w...

Magnetic Correlation of NaxCoO2 and Successive Phase Transitions of Na0:5CoO2 —NMR and Neutron Diffraction Studies—

Data taken by various methods including NMR/NQR and neutron scattering are presented for Na x CoO 2 , particularly Na 0.5 CoO 2 . Attention has also been paid to the x dependence of the electronic nature of this system. The pseudo-gap-like behavior observed in the region of x < x c ∼0.6 is emphasized. For samples with x ≤0.6, signals from two kinds of Co sites with different quadrupole frequencies ν Q of ∼4.1 and ∼2.8 MHz were observed, and the former ν Q , which is nearly equal to that of the superconducting system Na 0.3 CoO 2 · y H 2 O, becomes dominant with decreasing x to 0.3. For Na 0.5 CoO 2 , the Co sites with the larger ν Q have larger magnetic moments. They align antiferromagnetically at T c1 ∼87 K with their direction within the plane, while the Co sites with the smaller ν Q have smaller moments. They align in the direction parallel to the c axis. The ordered moments at the two distinct sites exhibit the same T -dependence, indicating that the existence of these sites is not due to macroscopic ...

Distinct Transport Behaviors of LaFe1-yCoyAsO1-xFx (x=0.11) between the Superconducting and Nonsuperconducting Metallic y Regions Divided by y ∼0.05

Electrical resistivities, Hall coefficients, and thermoelectric powers have been measured for polycrystalline samples of LaFe 1- y Co y AsO 1- x F x ( x =0.11) with various values of y . The results show that there exists a clear distinction of these transport behaviors between the superconducting and nonsuperconducting metallic regions of y divided by the boundary value y c ∼0.05. We have found that the behaviors in both regions are very similar to those of high- T c Cu oxides in the corresponding phases. If they reflect, as in the case of Cu oxides, the effects of strong magnetic fluctuations, the energy scale of the fluctuations is considered to be smaller than that of the high- T c Cu oxides by a factor of ∼1/2. Arguments on the electronic nature and superconducting symmetry are presented on the basis of the observed small rate of the T c suppression caused by Co doping.

Relationship between Magnetic Structure and Ferroelectricity of LiVCuO4

Neutron scattering studies and measurements of the dielectric susceptibility e and ferroelectric polarization P have been carried out under various magnetic fields H for single-crystal samples of the multiferroic system LiVCuO 4 with quasi one-dimensional spin 1/2 Cu 2+ chains formed of edge-sharing CuO 4 square planes, and the relationship between the magnetic structure and ferroelectricity has been studied. The ferroelectric polarization is significantly suppressed by the magnetic field H above 2 T applied along the a and b axes. When H = 0, a helical magnetic structure with the helical axis parallel to c has been confirmed. The magnetic structure under the field along a has been determined, where the a b -plane structure changes to the helical one with the helical axis parallel to H with increasing the field through ∼2 T. The ferroelectric polarization along a at H = 0 is found to be proportional to the neutron magnetic scattering intensity, indicating that the magnetic order is closely related to the ...

Magnetic Structure of YBaCo4O7 with Kagome and Triangular Lattices

Neutron diffraction studies have been carried out in the temperature(T) range between 10 K and 130 K on a single crystal of YBaCo4O7, which has the stacking of kagome and triangular lattices of CoO4 tetrahedra along the c-axis. Structural transitions have been found at two temperatures Tc1~70 K and Tc2~105 K. With decreasing T, magnetic order appears along with the transition at Tc2, but it does not grow to an ideal long-range order even at the lowest T studied (~10 K). Two groups of magnetic diffuse reflections, which originate from the Co-moments of the kagome and triangular lattices, are observed separately in the reciprocal space. With further decreasing T, the growth rates of the intensities of these two are enhanced by another transition at Tc1. These magnetic behaviors seem to be related to the release of the geometrical frustration. At 10 K, the patterns of the magnetic short-range order have been determined for both sites of the kagome and triangular lattices.

59Co-NMR Knight Shift of Aligned Crystals and Polycrystalline Samples of Superconducting Na0.3CoO2·1.3H2O

The temperature ( T ) dependence of the 59 Co-NMR Knight shifts K of Na 0.3 CoO 2 ·1.3H 2 O has been studied, where samples of randomly oriented powder and aligned crystals have been used for the applied magnetic fields H ∥ a b plane and H ∥ c -axis, respectively. For both directions of H , the shift K decreases below the superconducting transition temperature T c ( H ) with decreasing T , indicating that the superconducting electron pairs are in the singlet state. The upper critical fields H c2 ( T ) determined from the K ( H )– T curves are found to be consistent with the values reported from the resistivity measurements for both directions of H .

Studies of Multiferroic System of LiCu2O2: II. Magnetic Structures of Two Ordered Phases with Incommensurate Modulations

Neutron diffraction and 7 Li-NMR have been applied to determine the three dimensional magnetic structures of the multiferroic system LiCu 2 O 2 , which has four chains (ribbon chains) of edge-sharing CuO 4 square planes in a unit cell. First, we have confirmed that there are successive magnetic transitions at T N1 = 24.5 K and T N2 = 22.8 K. In the T region between T N1 and T N2 , the quasi one-dimensional spins ( S = 1/2) of Cu 2+ ions within a chain have a collinear and sinusoidally modulated structure with Cu-moments parallel to the c -axis and with the modulation vector along the b -axis. At T < T N2 , an ellipsoidal helical spin structure with the incommensurate modulation has been found. We present detailed parameters, describing the modulation amplitudes, helical axis vectors as well as the relative phases of the modulations of four ribbon chains, which can well reproduce both the NMR and neutron results, in the two magnetically ordered phases. This finding of the rather precise magnetic structures...

Impurity Effects on the Superconducting Transition Temperature of NazCoO2·yH2O

Superconducting transition temperature T c of Na z Co 1- x M x O 2 · y H 2 O has been studied for M = Ir and Ga, which can substitute for Co. It has been found that T c is suppressed by the M-atom doping. The decreasing rates |d T c /d x | are of the order of 1 K/% and too small to be explained by the pair breaking mechanism for the anisotropic order parameters by non-magnetic impurities. Brief arguments are given on possible origins of the T c suppression.

Studies on Superconducting LaFe1-yCoyAsO1-xFx (

NMR longitudinal relaxation rates 1=T1 and Knight shifts K have been measured for superconducting samples of LaFe1� yCoyAsO1� xFx with y ¼ 0:0 and 0.0075 and for a nonsuperconducting metallic sample with y ¼ 0:1, where the x values are fixed at 0.11. The temperature (T) dependence of 1=T1 below the superconducting transition temperature Tc has been found to be markedly different from the behavior 1=T1 / T 2:5{3:0 reported by many groups in the entire T range measured [(0:1{ 0:2) Tc < T < Tc]. The nonexistence of the coherence peak has also been found. On the basis of the results of the measurements and other kinds of existing data, arguments are presented on the superconducting symmetry of the Fe pnictide systems, where the several points that cannot be easily explained by existing theories, are pointed out. Results of the measurements on the nonsuperconducting metallic samples are also presented.

x=0.11

Studies of the NMR Knight shift K of Na 0.3 CoO 2 · y H 2 O have been carried out in detail. The suppression of K by the occurrence of the superconductivity reported previously by the present authors in both magnetic field directions perpendicular and parallel to the c -axis has been confirmed, indicating that the Cooper pairs are in the singlet state. The anisotropy of the suppression amplitudes is consistent with the anisotropy of the hyperfine coupling constant A spin estimated from the K –χ plot. It has also been found that even samples that do not exhibit a significant the Curie–Weiss-like increase of the uniform magnetic susceptibility χ with decreasing temperature T , exhibit superconducting transition, which indicates that the superconducting Na 0.3 CoO 2 ·1.3H 2 O is not necessarily in the region of proximity of the ferromagnetic phase. It has also been confirmed that the superconducting transition temperature T c of the samples prepared by mixing Na 0.7 CoO 2 , H 2 O (or D 2 O) and bromine for 4...