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Spectroscopic techniques, including photoelectron spectroscopy, diffuse reflectance, and x-ray absorption, are used to investigate the electronic structure and the local structure of LaOBiS 2 − x Se x thermoelectric material. It is found that Se substitution effectively suppresses local distortion that can be responsible for the increased carrier mobility together with a change in the electronic structure. The results suggest a possible way to control thermoelectric properties by tuning of the local crystal structure of these materials. [ABSTRACT FROM AUTHOR]

We report crystallographic and magnetic properties of layered iron oxypnictide CeFe1-xCrxPO (x=0.000-0.692). Interlayer distances between Ce2O2 and (Fe1-xCrx)2P2 layers increase as a function of x, suggesting suppression of Kondo coupling among hybridized conducting orbitals and localized Ce 4f orbitals. CeFe1-xCrxPO (x=0.100-0.384) exhibits finite ferromagnetic transition temperatures (Tcurie) obtained by Arrott plots, although 57Fe Mössbauer spectra reveal paramagnetic Fe sublattice at T⩾4.2 K. These results indicate that the ferromagnetic phase transitions of samples are mainly due to Ce sublattice. For the samples with x⩾0.500, no ferromagnetic order is observed down to 2K. These results verify that ferromagnetic quantum critical points of CeFe1-xCrxPO appear at 0.045⩽x⩽0.100 and 0.384⩽x⩽0.500. [ABSTRACT FROM AUTHOR]

The electronic nature and the layered oxysulfide [Sr.sub.2-x][Ca.sub.x][Cu.sub.2]Co[O.sub.2][S.sub.2] with Co[O.sub.2] which has strongly correlated Co[O.sub.2] square planes is studied. A remarkable decrease in the electrical resistivity in the Sr defect system, and the chemical shifts of S 2s and S 2p orbitals suggest that carriers are doped into [Cu.sub.2][S.sub.2] layers.

The synthesis, magnetic and electronic properties of layered manganese oxysulfide Sr2CuMnO3S is examined. Low-dimensional antiferromagnetic ordering of Mn(super +3) in pyramidal MnO5 layers was observed in magnetic susceptibility.

The magnetic properties of layered oxysulfide Sr2Cu2(Co,Cu)O2S2 with the (Co,Cu)O2 planes are examined. Long time decay is observed at 10 and 75 K and the divergence between zero-field cooled and field cooled magnetizations is observed below 80 K.

The effect of Arsenic substitution on the properties of Bi-2212 superconductors was studied. Solid-state reaction technique was used for the synthesis of Bi2−xAsxSr2Ca1.1Cu2.1O8 samples, x = 0, 0.1, 0.2 and 0.4. The X-ray diffraction confirmed the formation of the tetragonal phase of Bi-2212 superconductors. The decrement in the c parameter confirmed that As insertions were introduced into the Bi sites of the system. The grain morphology of the surface was analysed by scanning electron microscopy, where the amount of arsenic increases with As substitution. The superconducting transition temperature (Tc) was determined from the DC-electrical resistivity by the standard four-probe technique. Tc was enhanced up to x = 0.1 above which it was suppressed. For magnetism, half magnetic-hysteresis loops were obtained by SQUID magnetometer at different temperatures (5 and 30 K). The hysteresis loops sizes increased with increasing As-concentrations. The critical current density (Jc) and the flux pinning mechanisms were investigated via different models. [ABSTRACT FROM AUTHOR]

A series of samples with Co substitution ([Fe.sub.1-x][Co.sub.x][Cr.sub.2][S.sub.4], 0(less than or equal)x(less than or equal)1) is designed and the Co doping effect in Fe[Cr.sub.2][S.sub.4] is studied. It is concluded that because of the randomness of the electron spin, magnetic polaron increased by Co doping, which caused a decrease in the magnetic susceptibility and MR increase.

We have been investigating the effect of (CoFe2O4)x and (ZnFe2O4)x (x is the wt%) nano-sized ferrites on the superconducting (SC) properties of the high-Tc SC compound GdBa2Cu3O7-δ (Gd123) prepared by the conventional solid-state reaction technique. In this work, the magnetic hysteresis for pure Gd123 samples and samples added with nano-sized ferrites was measured using a superconducting quantum interference device (SQUID) magnetometer using a dc field up to 30,000 Oe. The magnetic Jc was estimated using the extended Bean model. While a clear increase in Jc values with the addition of (CoFe2O4)x nano-sized ferrites is observed, a less clear positive effect on the Jc values is observed with the addition of (ZnFe2O4)x nano-sized ferrites. These results indicate an improvement of grain connectivity, decrease of voids between the grains, and decrease of porosity in Gd123 compound resulting from CoFe2O4 addition consistent with the mechanical properties measured before. (CoFe2O4)x nano-sized ferrite addition has thus resulted in an improvement in the SC properties of Gd123 high-Tc SC compound making it more convenient for applications. [ABSTRACT FROM AUTHOR]

We designed a series of samples with Co substitution (Fe1-xCoxCr2S4, 0<=x<=1) and studied the Co doping effect in FeCr2S4. Spin glass transitions are observed in lower temperature regions and ferrimagnetic transitions are observed in higher temperature regions. The paramagnetic-ferrimagnetic transition temperature (TC) increases with the amount of Co substitution. In order to clarify the magnetic character at the transition point, a critical exponent was obtained at about TC. The exponent changes from Ising to Heisenberg as x increases. [ABSTRACT FROM AUTHOR]

We have studied the electronic nature of the layered oxysulfide Sr2-xCaxCu2CoO2S2 which has strongly correlated CoO2 square planes. These materials exhibit narrow-gap semiconducting behavior and the conduction mechanism depends on the temperature. Valence band x-ray photoemission spectroscopy spectra show that Ca doping and the production of Sr defects cause the generation of a quasiparticle band near the Fermi level. These phenomena suggest that the materials are changing from correlated semiconductors to bad metals. [ABSTRACT FROM AUTHOR]

We designed a series of samples with Co substitution (Fe1-xCoxCr2S4, 0<=x<=1) and studied the Co doping effect in FeCr2S4. Spin glass transitions are observed in lower temperature regions and ferrimagnetic transitions are observed in higher temperature regions. The paramagnetic-ferrimagnetic transition temperature (TC) increases with the amount of Co substitution. In order to clarify the magnetic character at the transition point, a critical exponent was obtained at about TC. The exponent changes from Ising to Heisenberg as x increases. [ABSTRACT FROM AUTHOR]

We have studied the magnetic properties of layered oxysulfide Sr[sub 2]Cu[sub 2](Co,Cu)O[sub 2]S[sub 2] with the (Co,Cu)O[sub 2] planes. Magnetic susceptibility shows a broad maximum indicative of two-dimensional antiferromagnetic nature above 140 K. Below 80 K, the divergence between zero-field cooled and field cooled magnetizations is seen; long time decay is observed at 10 and 75 K, while it is not observed at 45 K. These feature is discussed from the viewpoint of the strongly and frustrated spin system which is different from the traditional spin glass. [ABSTRACT FROM AUTHOR]

In iron-based pnictides, one of the interesting topics is homogeneous coexistence or phase segregation at the boundary between antiferromagnetic (AF) and superconducting (SC) phases. We addressed this problem on a microscopic level by means of As NMR measurements in LaFeAsOF ( x=0.026) (La1111), and CaFeCoAsF ( x=0.06) (Ca1111) having an intermediate electronic phase diagram between Ba(FeCo)As and the La1111 series. NMR spectra for 6 % Co-doped Ca1111 samples were very similar to those for the undoped samples even below T, suggesting homogeneous coexistence of the AF and SC states. For 2.6 % F-doped La1111 samples, AF and paramagnetic (PM) domains coexist at ambient pressure, and AF and SC domains coexist at 3.0 GPa. The coexistence of domains is explained by considering a SC dome separated from the AF phase in the phase diagram. The homogeneous coexistence support S-wave superconductivity, whereas separation of the AF and SC phases gives credence to S-wave superconductivity. [ABSTRACT FROM AUTHOR]

CeRuPO, a ferromagnetic Kondo lattice, undergoes two novel pressure-induced structural phase transitions. CeRuPO undergoes an isostructural transition to a collapsed tetragonal structure near 7.5 GPa. Since pressure suppresses the initial ferromagnetic ordering of Ce3+, our observation suggests a quantum critical point for CeRuPO near this pressure. The collapsed tetragonal phase transforms into an orthorhombic Cmma structure. This phase is isostructural with the low temperature phase of LaFeAsO, which is superconducting under high pressure. Since CeRuPO exhibits a similar relationship between the two phases of LaFeAsO, the collapsed tetragonal and orthorhombic phases of CeRuPO may adopt a novel magnetic ground state. [ABSTRACT FROM AUTHOR]

Ac susceptibility measurements have been carried out on superconducting LaO1-xFxFeAs for x = 0.07 and 0.14 under He-gas pressures to ∼0.8 GPa. Not only do the measured values of dTc/dP differ substantially from those obtained in previous studies using other pressure media, but also the Tc(P) dependences observed depend on the detailed pressure/temperature history of the sample. A sizeable sensitivity of Tc(P) to shear stresses provides a possible explanation. [ABSTRACT FROM AUTHOR]

The pressure dependence on the superconducting transition temperature ( T c) was investigated for the iron-based superconductors LaFeAsO1− xF x and SmFeAsO1− xF x. The T c’s increase largely for LaFeAsO1− xF x with a small increase of pressure, while a sharp decrease of T c was observed for SmFeAsO1− xF x. The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO1− xF x system. [ABSTRACT FROM AUTHOR]

CuCr2S4 shows a p-type metallic conductivity and ferromagnetism with Curie temperatures Tc of 375K, and exhibit magnetic anomaly at 84K (=Tm). CuCr2S4 exhibits the coexistence of both the ferromagnetism and spin-glass with the irreversible magnetic effect below Tm. The partial 20% Mg-substitution for Cu in CuCr2S4 slightly reduces the Tc and Tm. Magnetorsistance of MgxCu1-xCr2S4 can be expressed as Δρ=aH2-bHn below Tm. a, b, and n are temperature-dependent adjustable parameters. These unique MR behavior would be attributed to the coexistence of the ferromagnetism and spin-glass-like order below Tm. Mg-substitution decreased the conducting carrier, which forms the conduction band whose orbital motion gave the positive MR contribution. Therefore, Mg-substitution gave smaller positive MR contribution and consequently gave larger negative MR (maximum 7% at 10K) in Mg0.2Cu0.8Cr2S4. These results suggest that Cr 3d band filling control can be expected to give functional magnetoelectric transport properties in CuCr2S4. [Copyright &y& Elsevier]

We studied the electric and magnetic behavior near the phase boundary between antiferromagnetic (AF) and superconducting phases for a prototype of high-7, pnictides LaFeAsO1-xFx by using nuclear magnetic resonance, and found that paramagnetic-metal (PM) domains segregate from AF domains. PM domains grow in size with increasing electron doping level and are accompanied by the onset of superconductivity, and thus the application of pressure or increasing the doping level causes superconductivity. The existence of PM domains cannot be explained by the existing paradigm that focuses only on the relationship between superconductivity and antiferromagnetism. Based on orbital fluctuation theory, the existence of PM domains is evidence of the ferroquadrupole state [ABSTRACT FROM AUTHOR]

We have performed 31P-NMR measurements on Ce(Ru1-xFex)PO in order to investigate ferromagnetic (FM) quantum criticality, since a heavy-fermion (HF) ferromagnet CeRuPO with a two-dimensional structure turns into a HF paramagnet by an isovalent Fe substitution for Ru. We found that Ce(Ru0.15Fe0.85)PO shows critical fluctuations down to ˜0.3 K, as well as the continuous suppression of Curie temperature and the ordered moments by the Fe substitution. These experimental results suggest the presence of a FM quantum critical point (QCP) at x˜0.86, which is a rare example among itinerant ferromagnets. In addition, we point out that the critical behaviors in Ce(Ru0.15Fe0.85)PO share a similarity with those in YbRh2Si2, where the local criticality of f electrons has been discussed. We reveal that Ce(Ru1-xFex)PO is a new system to study FM quantum criticality in HF compounds. [ABSTRACT FROM AUTHOR]

We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with two-dimensional XY-type anisotropy shows a metamagnetic behavior at the metamagnetic field HM ≃ 4 T perpendicular to the c axis and that a critical behavior is observed around HM. Although the magnetic character is entirely different from that in other Ce-based HF metamagnets, HM in these metamagnets is linearly proportional to the inverse of the effective mass, or to the temperature where the susceptibility shows a peak. This finding suggests that HM is a magnetic field breaking the local Kondo singlet, and the critical behavior around HM is driven by the Kondo breakdown accompanied by the Fermi-surface instability. [ABSTRACT FROM AUTHOR]

Abstract: We report experimental results of 75As and 139La nuclear magnetic resonance (NMR) in the layered oxypnictide system ) (, 0.04, 0.07 and 0.11) where superconductivity occurs in x greater than 0.04. In the undoped LaFeAsO, of 139La exhibits a distinct peak at below which the La NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the sample, of 75As exhibits a Curie–Weiss temperature dependence down to 30K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the sample, in contrast, pseudogap behavior is observed in at the 75As site with a gap value of . The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure by the electron-doping. As for the superconducting properties for , 0.07 and 0.11, of 75As in all compounds does not exhibit a coherence peak just below and follows a dependence at low temperatures. These results seemingly suggests that unconventional superconductivity with zero gap along lines, whereas the lack of the residual density of states at the low temperatures is incompatible with the presence of the line-nodes. We discuss similarity and difference between and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of and . [Copyright &y& Elsevier]

The pressure dependence on the superconducting transition temperature ( T c) was investigated for the iron-based superconductors LaFeAsO1− xF x and SmFeAsO1− xF x. The T c’s increase largely for LaFeAsO1− xF x with a small increase of pressure, while a sharp decrease of T c was observed for SmFeAsO1− xF x. The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO1− xF x system. [ABSTRACT FROM AUTHOR]

Abstract: Pressure dependence on superconducting transition temperature (T c) was examined for iron-based superconductor LaFeAsO1− x F x , SmFeAsO1− x F x and LaFePO. The T c’s increase largely for LaFePO and LaFeAsO1− x F x with a small increase of pressure. The electrical resistivity measurements reveal the pressure-induced superconductivity in undoped LaFeAsO and SmFeAsO. X-ray diffraction measurements were also performed under high pressure up to 10GPa for LaFePO and LaFeAsO1− x F x system, where the anisotropic decrease of the lattice constants was observed with applying pressure. [Copyright &y& Elsevier]