Measurements of the electrical resistivities ρ, Hall coefficients R H , thermoelectric powers S, electronic specific heat coefficients 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 M atom dopings to the superconducting LnFeAsO 1-x F x (Ln1111) system. The NMR longitudinal relaxation rates 1/T 1 have also been measured for samples of LaFe 1-y Co y AsO 1-x F x with various x values. Co atoms doped to the superconducting LnFeAsO 1-x F x are nonmagnetic, and the T c -suppression rate ⎪dT c /dx⎪ by the Co atoms has been found to be too small to understand by the pair breaking effect expected for the S ± superconducting symmetry proposed as the most probable one for the system. It throws a serious doubt whether the symmetry is realized in the system. Instead of the pair breaking, two mechanisms of the T c -suppression by the doped impurities have been found: One is the electron localization, which appears when the sheet resistance R exceeds h/4e 2 =6.45 kΩ, and another is the disappearance (or reduction) of the hole-Fermi-surfaces around the Γ point in the reciprocal space. The latter mechanism has been observed, when the electron number increases with increasing Co-doping level and the system changes from an "anomalous metal" to an ordinary one. On the two distinct T dependences of the NMR longitudinal relaxation rate 1/T 1 of LaFeAsO 1-x F x , 1/T 1 ∝T 6 reported by our group in the T region from T c to ~0.4 T c for samples with the highest T c values with varying x, and 1/T 1 ∝T 2.5-3.0 observed by many groups in the almost entire T region studied below T c , we discuss what the origin of the difference is, and show that, at least, the T 2.5-3.0 -like dependence of 1/T 1 cannot be considered as the experimental evidence for the S ± symmetry of ∆.KEYWORDS: LaFe 1-y M y AsO 1-x F x (M=Co, Mn), NMR relaxation rate, 1/T 1 , transport properties, specific heats

No abstract

No abstract

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.

Results of transport, magnetic, thermal, and 75 As-NMR measurements are presented for superconducting Sr2VFeAsO3 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 Sr2VO3-block layers have localized magnetic moments and that V-electrons do not contribute to the Fermi surface. The electronic characteristics of Sr2VFeAsO3 are considered to be common to those of other superconducting systems with Fe-pnictogen layers. 2−4) This implies that Sr 2 VFeAsO 3 may give us an opportunity to examine whether Fermi-surface nesting is important for the occurrence of superconductivity of Fe pnictides. On this point, Mazin has reported that, because the FSs constructed by only the Fe orbitals of Sr 2 VFeAsO 3 are similar to those expected in other Fe pnictides, the nesting condition is also satisfied in the present system. 5) However, it seems important to experimentally ensure whether or not the electrons of V ions are itinerant and really contributing to FSs, before studying the relation between Fermi-surface nesting and superconductivity. KEYWORDSWe have carried out transport, magnetic, thermal, and 75 As-NMR measurements, and found that although apparent anomalies in the temperature (T ) dependences of the magnetic and thermal properties of Sr 2 VFeAsO 3 exist at ∼150 K, no anomalies in the transport properties have been observed. On the basis of these results, we argue the electronic state of Sr 2 VFeAsO 3 and answer the question regarding the contribution of V ions to FSs.Polycrystalline samples of Sr 2 VFeAsO 3 were prepared as described in refs. 1 and 6. SrAs powder was first obtained by annealing mixtures of Sr and As in an evacuated quartz tube at 850• C. Mixtures with proper ratios of SrAs, FeAs, SrO, Fe, and V 2 O 3 were pressed into pellets, sealed in an evacuated quartz tube with Ti powder (Ti/Fe = 1/4), which probably acts as the reducing agent, and then fired for 10 h at 900• C and for 30 h 1050• C, successively. The X-ray powder diffraction pattern of one of the * corresponding author(i45323a@cc.nagoya-u.ac.jp) obtained pellets with CuKα radiation at a step of 0.01• of the scattering angle 2θ is shown in Fig.1 (a), where the reflection indices are attached to the corresponding peaks of Sr 2 VFeAsO 3 , and the asterisks indicate the peaks from impurity phases of Sr 2 VO 4 or Sr 3 V 2 O 7 .7,8) The lattice parameters a and c were estimated to be 3.9329(1) and 15.6703(18)Å, respectively.The magnetic moments M were measured at magnetic fields H of 10 Oe and 1 T using a Quantum Design MPMS. The electrical resistivity ρ was measured by the four-terminal method with increasing temperature T at H = 0. The thermoelectric power S was measured by a dc method, where the typical temperature range between two ends of the sample was 0.2−2 K, depending on the temperature region. Details of the me...

Specific heats and transport quantities of the LaFe 1-y Ni y AsO 0.89 F 0.11 system have been measured, and the results are discussed together with those reported previously by our group mainly for LaFe 1-y Co y AsO 0.89 F 0.11 and LaFeAsO 0.89-x F 0.11+x systems. The y dependence of the electronic specific heat coefficient γ can basically be understood by using the rigid-band picture, where Ni ions provide 2 electrons to the host conduction bands and behave as nonmagnetic impurities. The superconducting transition temperature T c of LaFe 1-y Ni y AsO 0.89 F 0.11 becomes zero, as the carrier density p (=2y+0.11) doped to LaFeAsO reaches its critical value p c ∼0.2. This p c value of ∼0.2 is commonly observed for LaFe 1-y Co y AsO 0.89 F 0.11 and LaFeAsO 0.89-x F 0.11+x systems, in which the relations p = x+0.11 and p = y+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 T c value. We have not observed appreciable effects of pair breaking, which originates from the nonmagnetic impurity scattering of conduction electrons and strongly suppresses T c values of systems with sign-reversing of the order parameter over the Fermi surface(s). On the basis of the results, the so-called s ± symmetry of the order parameter with the sign-reversing is excluded.

No abstract

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. , various studies have been carried out to identify the symmetry of their superconducting order parameter ∆. In many of these studies, much effort has been made to find experimental evidence for the S ± symmetry proposed theoretically at the early stage of the study.2, 3) For such a symmetry, reflecting the sign difference between the order parameters on disconnected Fermi surfaces around the Γ [= (0, 0)] and M [= (π, 0)] points in the reciprocal space, important features can be expected in several observable physical quantities: Neutron inelastic scattering measurements have been carried out to find the so-called "resonance peak" in the magnetic excitation spectra χ"(Q, ω)4-6) expected in the superconducting phase around a point in the scattering vector(Q)-energy(ω) space, 7,8) and an observed peak has been discussed in relation to the "resonance peak". After the confirmation of the singlet state of Cooper pairs by Knight-shift measurements, 9, 10) the temperature (T) dependence of the NMR longitudinal relaxation rate 1/T 1 has been extensively discussed, and the absence of the coherence peak has been pointed out by many research groups.11-15) The T dependence described by the relation 1/T 1 ∝T n with n∼3 has been reported below T c in almost the entire T region studied for LaFeAsO 1-x F x 11, 13,14) and LaFeAsO 1-δ .12) These results have been discussed to be favorable for the S ± symmetry.The effects of impurity doping can provide information on the relative signs of the order parameters on Fermi surfaces around the Γ and M points, and on the basis of Co doping studies, we have emphasized that the observed rate of T c decrease due to the doping of nonmagnetic impurities is too small to be explained by the pair breaking effect expected for the S ± symmetry of the order parameter. 9,[15][16][17][18][19] The result seems to be consistent with those of studies carried out by neutron 20) and α-particle 21) irradiations. Onari and Kontani have pointed out from the theoretical side that the above data of doping effects cannot be explained by the pair breaking of nonmagnetic impurities. 22) Regarding the magnetic excitation spectra χ"(Q, ω), it has been pointed 23) out that a "peak" in χ"(Q, ω) is also expected for the S symmetry of the order parameter, which has no sign difference between the Fermi surfaces around the Γ and M points, suggesting that we have to be careful in arguing whether the observed data really indicate the existence of the "resonance peak". On the T dependence of...