Yttrium doped La1−xYxO0.9F0.1FeAs superconductors: Hall and thermopower studies

The effect of yttrium substitution at the lanthanum site on the superconducting properties of La_1?xY_xO_0.9F_0.1FeAs (‘x’ = 0, 0.10, 0.20, 0.30, 0.50 and 0.60) oxypnictides has been studied. Powder X-ray diffraction studies confirm single phases till x = 0.1 beyond which minor amount of Y₂O₃ is observed. The temperature dependence of resistivity measurements confirm the superconducting transition temperature (T_c) of 34.8 (±0.05) K and corresponding Meissner transition at 34.3 K in the ‘x’ = 0.3 composition which is higher than that reported for the parent phase (LaO_0.9F_0.1FeAs (T_c = 28 K)). Further increase in the concentration of yttrium leads to broadening and suppression of the superconducting transition. The value of H_c2 at zero temperature is estimated to be about 60.5 T. The Seebeck coefficient (S) shows a negative sign indicating that the major contribution to the conductivity is by electrons. The Hall coefficient (R_H) also remains negative throughout the temperature range supporting the thermopower results. The lattice parameters (a and c) decreases and the charge-carrier density increases with yttrium doping.     

Effects of simultaneous carrier doping in the charge reservoir and conducting layers of superconducting CeO0.9F0.1Fe1−xCoxAs

Electron-doping of the semimetal (CeOFeAs) by either fluorine (max T_c ∼ 43 K) or cobalt (max T_c ∼ 11 K) leads to superconductivity. Here we show the effect of transition metal (Co) substitution at the iron site on the superconducting properties of CeO_0.9F_0.1FeAs (T_c ∼ 38 K) to understand the interplay of charge carriers in both the rare earth-oxygen and Fe–As layers. Simultaneous doping of equivalent number of charge carriers in both layers leads to a T_c of 9.8 K which is lower than the T_c obtained when either the conducting layer (FeAs) or charge reservoir layer (CeO) is individually doped. This suggests a clear interplay between the two layers to control the superconductivity. Resistivity upturn and negative magnetoresistance are observed with Co doping that is interpreted in the gamut of Kondo effect. Hall coefficient and thermoelectric power indicate increased carrier concentration with cobalt doping in CeO_0.9F_0.1FeAs. The rf penetration depth both for CeO_0.9F_0.1Fe_0.95Co_0.05As and CeO_0.9F_0.1FeAs show an exponential temperature dependence with gap values of ∼1.6 and 1.9 meV respectively.     

Short-range magnetic order,irreversibility and giant magnetoresistance near the triple points in the (x,T) magnetic phase diagram of ZrMn<Subscript>6</Subscript>Sn<Subscript>6-x</Subscript>Ga<Subscript>x</Subscript>

We have studied pseudo-layered ZrMn₆Sn_6-xGa_x intermetallics (0.55 ≤ x ≤ 0.81) using magnetic, magnetoresistivity and powder neutron diffraction measurements. All the alloys studied have magnetic ordering temperatures in the 450-490 K temperature range. They present complex temperature-dependent partially disordered magnetic structures whose ferromagnetic component develops upon increasing the Ga content. ZrMn₆Sn_6-xGa_x alloys with x ≤ 0.69 are essentially collinear antiferromagnets at high-temperature and adopt antifan-like arrangements at low temperature. For x ≥ 0.75, the alloys order ferromagnetically and evolve to a fan-like structure upon cooling. The intermediate compositions (x = 0.71 and 0.73) present a canted fan-like order at high temperature and another kind of antifan-like arrangement at low temperature. The degree of short-range order tends to increase upon approaching the intermediate compositions. The (x, T) phase diagram contains two triple points (x ~ 0.70; T ~ 460 K and x ~ 0.74; T ~ 455 K), where the paramagnetic, an incommensurate and a commensurate phases meet, which possess some of the features of Lifshitz point. Irreversibilities manifest in the low-temperature magnetization curves at the antifan-fan or fan-ferromagnetic boundaries as well as inside the fan region. Giant magnetoresistance is observed, even above room temperature.     

Superconductivity at 36 K in gadolinium-arsenide oxides GdO<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>F<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FeAs

In this paper we report the fabrication and superconducting properties of GdO_1−x F _x FeAs. It was found that when x is equal to 0.17, GdO_0.83F_0.17FeAs is a superconductor with the onset transition temperature T _c^on ≈ 36.6 K. Resistivity anomaly near 130 K was observed for all samples up to x = 0.17, and such a phenomenon is similar to that of LaO_1−x F _x FeAs. Hall coefficient indicates that GdO_1−x F _x FeAs is conducted by electron-like charge carriers. Recommended by Prof. Nie Yuxin, Executive Editor of Science in China Series G-Physics, Mechanics & Astronomy Supported by the National Natural Science Foundation of China (Grant Nos. 10221002/A0402 and 10774170/A0402), the National Basic Research Program of China (973 Program) (Grant Nos. 2006CB601000, 2006CB921107 and 2006CB921802), and the Chinese Academy of Sciences (ITSNEM)     

High-pressure neutron diffraction study of BaFe<Subscript>2</Subscript>As<Subscript>2</Subscript>

The crystal structure of BaFe₂As₂ was studied by high-pressure neutron powder diffraction in the pressure range from ambient to 6.5 GPa as well as in the temperature range from 12 K to 293 K at 4.4 GPa and no pressure or temperature induced phase changes were observed. The compression mechanism of BaFe₂As₂ was found to be anisotropic as the a- and c-axes are reduced by 2.49 and 3.66%, respectively at 6.5 GPa. Within the FeAs layers the Fe-As and Fe-Fe bonds decrease by 2.49 and 3.66%, respectively. The Ba-As distance decreases by 3.70% while the As-As inter-atomic distance along the c-axis exhibits a complex pressure dependence. The bulk modulus B ₀ and its pressure derivative B ₀' were determined to be B ₀ = 59(2) GPa and B ₀' = 6.1(7) at ambient temperature.     

Physical properties,crystal and magnetic structure of layered Fe<Subscript>1.11</Subscript>Te<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> superconductors

The physical and structural properties of Fe_1.11Te and Fe_1.11Te_0.5Se_0.5 have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the Fe_1.11Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting Fe_1.11Te_0.5Se_0.5 compound, the superconducting transition with T _c = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field H _c2 is obtained by measuring the resistivity under different magnetic fields. The Kim’s critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that Fe_1.11Te_0.5Se_0.5 crystalizes in tetragonal structure at 300 K as in the parent compound Fe_1.11Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.     

Magnetic phase diagram and correlation between metamagnetism and superconductivity in Ru<Subscript>0.9</Subscript>Sr<Subscript>2</Subscript>YCu<Subscript>2.1</Subscript>O<Subscript>7.9</Subscript>

The magnetic superconductorRu_0.9Sr₂YCu_2.1O_7.9 (Ru-1212Y) has beeninvestigated using neutron diffraction under variable temperature and magnetic field. Withthe complementary information from magnetization measurements, we propose a magnetic phasediagram T-H for the Ru-1212 system. Uniaxialantiferromagnetic (AFM) order of 1.2μ _B /Ruatoms with moments parallel to the c-axis is found below the magnetictransition temperature at  ~140 K in the absence of magnetic field. In addition,ferromagnetism (FM) in the ab-plane develops below  ~120 K, butis suppressed at lower temperature by superconducting correlations. Externally appliedmagnetic fields cause Ru-moments to realign from the c-axis to theab-plane, i.e. along the ?1,1,0? direction, and induce ferromagnetismin the plane with  ~1μ _B at 60 kOe.These observations of the weak ferromagnetism suppressed by superconductivity and thefield-induced metamagnetic transition between AFM and FM demonstrate not only competingorders of superconductivity and magnetism, but also suggest a certain vortex dynamicscontributing to these magnetic transitions.     

Anisotropy of the heat capacity of a mixed-state superconducting Nd<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> single crystal for various magnetic field orientations with respect to the crystallographic axes

The anisotropy in the superconducting properties of single-crystal Nd_1.85Ce_0.15CuO₄ was studied from measurements of the heat capacity within the temperature interval 2–40 K in zero magnetic field and in a magnetic field of 8 T. We report on the first observation of heat capacity jumps occurring at the superconducting transition for various magnetic field orientations with respect to the crystallographic axes and on a strong anisotropy of the magnetic contribution to heat capacity in magnetic fields oriented in the a-b plane and perpendicular to it. These measurements yielded the anisotropy in the electronic heat capacity coefficient γ_n(H) and in the superconducting transition temperature T_c(H). The angular dependence of the Sommerfeld coefficient γ_n in the a-b plane observed in a magnetic field of 8 T exhibits four-lobe symmetry and zero gap direction of the order parameter. A comparison of the results obtained on the Nd_1.85Ce_0.15CuO₄ single crystal with the data available for La_1.85Sr_0.15CuO₄ permits one to conclude that the mechanisms of superconductivity in the electron-and hole-doped superconductors are similar.     

Synthesis and physical properties of LaO1-xFxFeAs

We have prepared the newly discovered Fe-based superconducting material LaO_1-xF_xFeAs (0 ≤ x ≤ 0.2) in polycrystalline form and have investigated the samples by means of structural, thermodynamic and transport measurements. Our investigations reveal a non superconducting phase at 0 ≤ x 0.04 which for x = 0 is characterized by a structural transition towards an orthorhombic distortion at T_s ≈ 160 K and antiferromagnetic spin order at T_N ≈ 138 K. Both transitions lead to strong anomalies in various transport properties as well as in magnetization and in specific heat. Remarkably, the transition temperatures are only weakly doping dependent up to x ≈ 0.04. However, the transitions are abruptly suppressed at x ≥ 0.05 in favour of a superconducting phase with a critical temperature T_c 20 K. Upon further increasing the F-doping T_c increases up to a maximum of T_c = 26.8 K at x = 0.1 which is followed by a decrease down to T_c ≈ 10 K at x ≥ 0.15.     

Dependence of T<Subscript>c</Subscript> on hydrostatic pressure in a 123 superconductor

Hydrostatic pressure studies on the tetragonal 123 superconductor (La_1-xCa_x) (Ba_1.75-xLa_0.25+x)Cu₃O_y for x =0.1 and variable oxygen content y show that increases rapidly under pressure (+5 K/GPa) for underdoped, optimally doped and overdoped samples. This points to a common cause for the pressure-induced changes in the superconductivity at all levels of doping, with negligible effects from charge transfer. Weak relaxation behavior in is observed only for the most underdoped sample. Received 26 August 1999     

EuSr2Ru1-xTaxCu2O8中Ta替代效应

通过对EuSr₂Ru_1-xTa_xCu₂O₈ (x=0.0, 0.1, 0.2, 0.5和1.0)体系的结构、电阻和磁化强度的观测，发现EuSr₂RuCu₂O₈(x=0.0)样品在130.2K以下呈现铁磁有序，在35K时发生了超导转变，并呈现典型的欠掺杂高温超导体特征；随着Ta对Ru替代浓度x值的增加，铁磁相变温度和超导临界温度均下降 关键词： 高温超导电性 铁磁有序 Ru-Cu氧化物     

CO<Subscript>2</Subscript> activation on single crystal based ceria and magnesia/ceria model catalysts

Novel multifunctional ceria based materials may show an improved performance in catalytic processes involving CO₂ activation and reforming of hydrocarbons. Towards a more detailed understanding of the underlying surface chemistry, we have investigated CO₂ activation on single crystal based ceria and magnesia/ceria model catalysts. All model systems are prepared starting from well-ordered and fully stoichiometric CeO₂(111) films on a Cu(111) substrate. Samples with different structure, oxidation state and compositions are generated, including CeO_2-x/Cu(111) (reduced), MgO/CeO_2-x/Cu(111) (reduced), mixed MgO-CeO₂/Cu(111) (stoichiometric), and mixed MgO-CeO_2-x/Cu(111) (reduced). The morphology of the model surfaces is characterized by means of scanning tunneling microscopy (STM), whereas the electronic structure and reactivity is probed by X-ray photoelectron spectroscopy (XPS). The experimental approach allows us to compare the reactivity of samples containing different types of Ce³⁺, Ce⁴⁺, and Mg²⁺ ions towards CO₂ at a sample temperature of 300 K. Briefly, we detect the formation of two CO₂-derived species, namely carbonate (CO₃ ^2-) and carboxylate (CO₂ ^-) groups, on the surfaces of all investigated samples after exposure to CO₂ at 300 K. In parallel to formation of the carbonate species, slow partial reoxidation of reduced CeO_2-x/Cu(111) occurs at large doses of CO₂. The reoxidation of the reduced ceria is largely suppressed on MgO-containing samples. The tendency for reoxidation of Ce³⁺ to Ce⁴⁺ by CO₂ decreases with increasing degree of intermixing between MgO and CeO_2-x. Additionally, we have studied the stability of the formed carbonate species as a function of annealing temperature.     

EPR of Ce<Superscript>3+</Superscript> ions in mixed yttrium-lutecium orthoaluminates

The results of EPR studies of Ce³⁺ ions incorporated into single crystals of mixed yttrium-lutecium orthoaluminates Y_1?xLu_xAlO₃ (YLuAP, x=0.1, 0.3) are reported. In compositionally disordered YLuAP compounds, in comparison to YAlO₃, new paramagnetic Ce³⁺ centers are found. These centers are caused by the changes in symmetry and in the crystal field magnitude due to the isomorphic substitution of Y³⁺ ions by Lu³⁺ in the yttrium sublattice of orthoaluminates. It is shown that the formation of 27 different types of centers is possible in YLuAP with variation of the Lu content. The probabilities of formation of new paramagnetic centers are calculated.     

Nature of low-energy excitations in La<Subscript>1.87</Subscript>Sr<Subscript>0.13</Subscript>CuO<Subscript>4</Subscript> superconducting cuprate

The spectra of the conductivity and dielectric constant of La_1.87Sr_0.13CuO₄ cuprate have been directly measured in the frequency range of 0.3 to 1.2 THz (10–40 cm⁻¹) and the temperature range of 5 to 300 K in the E | c polarization (the electric field vector of radiation is perpendicular to the copper-oxygen planes). Excitation has been observed in the superconducting phase, and its nature has been attributed to the transverse optical excitation of the condensate of Cooper pairs, which appears because Josephson junctions between CuO planes are modulated due to in-plane magnetic and charge stripes. Additional quasiparticle absorption of unknown origin has been detected at frequencies below ≈15 cm⁻¹ at liquid helium temperatures.     

Superhyperfine structure of the EPR spectra of Ce<Superscript>3+</Superscript> ions in Li<Emphasis Type="Italic">R</Emphasis>F<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Y,Lu, Tm) double fluorides

The EPR spectra of Ce³⁺ impurity ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals have been investigated at a frequency of ∼9.3 GHz in the temperature range 5–25 K. The effective g factors of the ground Kramers doublet of the cerium ions in three crystals are close to each other (g _‖ = 2.737, g _⊥ = 1.475 for LiYF₄:Ce³⁺). A superhyperfine structure of the EPR spectrum of Ce³⁺ ions in the LiTmF₄ Van Vleck paramagnet has been observed in the external magnetic field B oriented along the crystallographic axis c (B ‖ c). The superhyperfine structure of the EPR soectra of the Ce³⁺ ions in the LiYF₄ and LiLuF₄ diamagnetic matrices is resolved for B ⊥ c. Possible factors responsible for this pronounced difference in the properties of the systems studied have been discussed.     

Low-energy Ce spin excitations in CeFeAsO and CeFeAsO<Subscript>0.84</Subscript>F<Subscript>0.16</Subscript>

We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO_0.84F_0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of T _N ^Ce ? 4 K. On warming to above T _N ^Ce ? 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T _N ^Fe ? 140 K. For superconducting CeFeAsO_0.84F_0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.     

Temperature dependence of phase separation and magnetic anisotropy by electron spin resonance in Pr<Subscript>0.6</Subscript> Ca<Subscript>0.4</Subscript> Mn<Subscript>0.9</Subscript> Ru<Subscript>0.1</Subscript>O<Subscript>3</Subscript>

Electron spin resonance (ESR) measurements have been performed on polycrystalline samples of Pr_0.6Ca_0.4Mn_1-xRu_xO₃ (x = 0, 0.1). The substitution of Ru in the Mn-site strengthens ferromagnetic interactions due to the double exchange between the Mn³⁺ and Mn⁴⁺ species and super-exchange between the Ru⁵⁺ and Mn³⁺ species. The temperature dependence of the ESR spectra indicates development of magnetic phase separation in Pr_0.6Ca_0.4Mn_0.9Ru_0.1O₃ in contrast with the un-doped sample.     

Investigation of luminescence spectra of the Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> and Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce single-crystal films

The luminescence spectra of single-crystal films and bulk crystals of yttrium-aluminum garnet Y₃Al₅O₁₂ and Ce³⁺-activated Y₃Al₅O₁₂ were investigated. It was shown that the room-temperature luminescence intensity of the Ce³⁺-free single-crystal Y₃Al₅O₁₂ film was considerably lower than that of the bulk crystals, while the luminescence intensity of the Ce³⁺ ions in the Y₃Al₅O₁₂:Ce films was considerably higher than that one for the corresponding bulk crystal.     

Far-infrared optical excitations in multiferroic TbMnO<Subscript>3</Subscript>

We provide a detailed study of the reflectivity of multiferroic TbMnO₃ for wave numbers from 40 cm^-1 to 1000 cm^-1 and temperatures 5 K < T < 300 K. Excitations are studied for polarization directions E || a, the polarization where electromagnons are observed, and for E || c, the direction of the spontaneous polarization in this material. The temperature dependencies of eigenfrequencies, damping constants and polar strengths of all modes are studied and analyzed. For E || a and below the spiral ordering temperature of about 27 K we observe a transfer of optical weight from phonon excitations to electromagnons, which mainly involves low-frequency phonons. For E || c an unusual increase of the total polar strength and hence of the dielectric constant is observed indicating significant transfer of dynamic charge probably within manganese-oxygen bonds on decreasing temperatures.     

Insulator-metal transition shift related to magnetic polarons in La<Subscript>0.67-x</Subscript>Y<Subscript>x</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

The magnetic transport properties have been measured for La_0.67-xY_xCa_0.33MnO₃ ( 0 ⩽ x ⩽ 0.14) system. It was found that the transition temperature T _p almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above T _p , there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of T _p vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ∼ 15.4 ? which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than T _p conform to the variable-range hopping mechanism. Received 27 August 2002 / Received in final form 2 December 2002 Published online 14 March 2003