Erika Satomi

Studies on Effects of Impurity Doping and NMR Measurements of La 1111 and/or Nd 1111 Fe-Pnictide Superconductors

Measurements of the electrical resistivity ρ, Hall coefficient R H , thermoelectric power S , and the electronic specific heat coefficient γ have been carried out for samples of LnFe 1- y M y AsO 1- x F x (Ln=La, Nd; M=Co, Mn; x =0.11) obtained by doping M atoms into the superconducting LnFeAsO 1- x F x (Ln 1111) system. The NMR longitudinal relaxation rate 1/ T 1 has also been measured for samples of LaFe 1- y Co y AsO 1- x F x with various x values. Co atoms doped into the superconducting LnFeAsO 1- x F x are nonmagnetic, and the rate of T c -suppression |d T c /d x | by Co atoms has been found to be too small to be explained by the pair-breaking effect expected for the S ± superconducting symmetry proposed as the most probable symmetry for the system. This result throws a serious doubt on whether the symmetry is realized in the system. Instead of the pair breaking, two mechanisms of T c suppression by the doped impurities have been found: One is the electron localization, which appears when the sheet r...

Effects of Ru Doping on the Transport Behavior and Superconducting Transition Temperature of NdFeAsO0.89F0.11

The transport behavior and superconducting transition temperature T c of NdFe 1- y Ru y AsO 0.89 F 0.11 have been studied for various y values. Because Ru impurities are isoelectronic to host Fe atoms, we basically expect that the number of electrons does not change with y , at least in the region of small y values. The results indicate that the rate of T c suppression by Ru atoms is too small to be explained by the pair breaking effect of nonmagnetic impurities expected for the S ± symmetry, confirming our previous results for Co doping.

Study of Ni-Doping Effect of Specific Heat and Transport Properties for LaFe1-yNiyAsO0.89F0.11

Specific heats and transport quantities of the LaFe1� yNiyAsO0:89F0:11 system have been measured, and the results are discussed together with those reported previously by our group mainly for LaFe1� yCoyAsO0:89F0:11 and LaFeAsO0:89� xF0:11þx systems. The y dependence of the electronic specific heat coefficientcan basically be understood by using the rigid-band picture, where Ni ions provide two electrons to the host conduction bands and behave as nonmagnetic impurities. The superconducting transition temperature Tc of LaFe1� yNiyAsO0:89F0:11 becomes zero as the carrier density p (¼ 2y þ 0:11) doped to LaFeAsO reaches its critical value, pc � 0:2. This pc value of � 0:2 is commonly observed for LaFe1� yCoyAsO0:89F0:11 and LaFeAsO0:89� xF0:11þx systems, in which the relations p ¼ x þ 0:11 and p ¼ y þ 0:11 hold, respectively. As we pointed out previously, the critical value corresponds to the disappearance of the hole-Fermi surface. These results indicate that the carrier number solely determines the Tc value. We have not observed appreciable effects of pair breaking, which originates from the nonmagnetic impurity scattering of conduction electrons and strongly suppresses Tc values of systems with sign reversal of the order parameter over the Fermi surface(s). On the basis of the results, the so-called ssymmetry of the order parameter with sign reversal is excluded.

Superconducting Transition Temperatures and Transport Properties of LaFe1-yRuyAsO0.89F0.11 and LaFeAsO0.89-xF0.11+x

The transport behaviors and superconducting transition temperatures T c of the title systems have been experimentally studied for various x and y values. Together with results reported previously by our group, data are now available for many sets of samples of RFe 1- y M y AsO 0.89 F 0.11 with R = La and Nd and M = Co, Mn and Ru and for RFeAsO 0.89- x F 0.11+ x . Dopings of Co and Mn atoms introduce electrons and holes into the FeAs planes, respectively. They also introduce, more or less, randomness effects. Ru atoms introduce scattering centers within the FeAs planes without changing the number of carrier electrons or holes. In contrast, F atoms introduce electron carriers, but not randomness effects or scattering centers into the FeAs planes. From these studies, we can conclude that there are two appreciable mechanisms of T c suppression by the dopings. The disappearance of or reduction in the area of the hole Fermi surfaces around the Γ point in the reciprocal space has been clearly shown as one of the...

High-Temperature Multiferroic State of RBaCuFeO5 (R = Y, Lu, and Tm)

Magnetic and/or dielectric/ferroelectric behaviors have been studied for YBaCuFeO 5 , LuBaCuFeO 5 , and TmBaCuFeO 5 , which are members of oxygen-deficient ordered perovskite systems RBaCuFeO 5 (R = lanthanide Ln and other trivalent elements) and have two magnetic transitions. The magnetic structure of the high-temperature ( T ) ordered phase is basically antiferromagnetic, and in the low- T ordered phase, a modulated magnetic component is superposed on the antiferromagnetic moments. The results of the pyrocurrent measurements indicate that electric polarization is induced in all these systems by the ordering to the modulated magnetic structure. For TmBaCuFeO 5 , the transition to this low- T phase is found at a temperature as high as the melting point of ice.

75As-NMR Studies of LaFeAsO1-xFx for Various x Values

Results of 75 As NMR and NQR measurements are reported for superconducting samples of LaFeAsO 1- x F x with various x values. The x dependent widths of the superconducting transition and those of the NQR spectra indicate that the optimally doped LaFeAsO 1- x F x sample with the superconducting transition temperature T c ∼28 K has the smallest width or smallest inhomogeneity of the superconducting order parameter Δ. In the analyses of the temperature ( T ) dependence of the NMR longitudinal relaxation rate 1/ T 1 , we have tried to use a simple relation 1/ T 1 ∼ T n to see how fast 1/ T 1 decreases when the superconducting order parameter grows. The relation holds rather well in the T region of ∼0.3 T c < T ≤ T c , and n varies from 2.5–6 with varying x , having the maximum value 5–6 at the optimal x . These results indicate that the x dependence of n originates from the spatial distribution of Δ, and that the T 5–6 behavior is considered to be intrinsic to this system.

Crystal Structure of New Pt-Doped Fe Pnictide Superconductors

Single crystals of the recently found superconducting system Ca–Fe–Pt–As have been prepared and used in X-ray Rietveld analyses, where we have identified two types of structure; SrZnBi 2 type (type A; I 4/ m m m ) and HfCuSi 2 type (type B; P 4/ n m m ). Both structures consist of Fe 1- x Pt x As conducting layers with As and Ca atoms between them. The unit cell of the system of type A has two Fe 1- x Pt x As layers, while that of type B has only one layer. The superconducting transition appears in both systems, even though the fraction of Pt atoms, which act as strong electron scatterers, is as large as ∼0.2.

Magnetic Ordering in V-Layers of the Superconducting System of Sr2VFeAsO3

Results of transport, magnetic, thermal, and 75 As-NMR measurements are presented for superconducting Sr 2 VFeAsO 3 with an alternating stack of FeAs and perovskite-like block layers. Although apparent anomalies in magnetic and thermal properties have been observed at ∼150 K, no anomaly in transport behaviors has been observed at around the same temperature. These results indicate that V ions in the Sr 2 VO 3 -block layers have localized magnetic moments and that V-electrons do not contribute to the Fermi surface. The electronic characteristics of Sr 2 VFeAsO 3 are considered to be common to those of other superconducting systems with Fe-pnictogen layers.

NMR Studies on Iron Pnictide Superconductors of LaFeAsO0.89F0.11 and Ca-Fe-Pt-As

We first show results of (NMR-1/T1)-T curves of LaFeAsO1−xFx samples with various x values. It does not have the coherence peak and absolute values of dlog(1/T1)/d log(T/Tc) shows the dome shaped x dependence with a maximum at the optimum x value. Considering the T dependent energy broadening of quasi-particles and magnetic susceptibility below Tc, we show that the nonexistence of the coherence peak cannot necessarily be regarded as the evidence for the S± symmetry of the order parameter. We also present NMR data on the superconducting Ca-Fe-Pt-As system recently found by Kakiya et al. 75As-NMR spectra taken for single crystals of the system indicate clear evidence that Pt atoms are doped into the FeAs conducting layers consistently with the results of the structure analyses, indicating that the superconductivity survives even in the existence of strong impurity scattering of the conduction electrons.

Absence of Magnetic Order in Ising Honeycomb-Lattice Ba3Co2O6(CO3)0.7

Ba3Co2O6(CO3)0.7 is a rare example of Ising spin chains on a honeycomb lattice. We have found the quasi-one-dimensional electric conduction and its localization at low temperatures below 100 K The magnetic susceptibility exhibits the highly anisotropic Curie-Weiss behavior with the Weiss constant of 69 K for the magnetic field parallel to the chain and −104 K for perpendicular to the chain, which indicates the intrachain ferromagnetic interaction and the interchain antiferromagnetic one. No indication of magnetic order was observed down to 2 K Based on these experimental results, we suggest the spin liquid state.