Preparation of Tl2Ba2Ca1 − y CeyCu2O8 − x/2Fx Ce-and F-substituted superconductors

High-T _c superconducting ceramics with the compositions Tl₂Ba₂CaCu₂O_{8 ? x/2} (0 ≤ x ≤ 0.3), Tl₂Ba₂(Ca_{1 ? y} Ce_y)Cu₂O_x (0 ≤ y ≤ 0.2), and Tl₂Ba₂(Ca_0.9Ce_0.1)(Cu_1.98Fe_0.02)F_0.2O_8.01 are synthesized. Partial fluorine substitution for oxygen in Tl₂Ba₂CaCu₂O_{8 ? x/2} (0 ≤ x ≤ 0.1) alters the carrier density in the Cu-O planes of the material, raising its superconducting transition temperature from 106 to 110 K. Partial cerium substitution for calcium in Tl₂Ba₂(Ca_{1 ? y} Ce_y)Cu₂O_x (0 ≤ y ≤ 0.2) reduces T _c. The combined substitution in Tl₂Ba₂(Ca_0.9Ce_0.1)(Cu_1.98Fe_0.02)F_0.2O_8.01 has an insignificant effect on T _c.     

Effect of Ca Substitution on the Superconductivity of La2.5Y0.5CaBa3(Cu0.88Fe0.12)7O z

The structural and superconducting properties of single-phase La_2.5?y Y_0.5Ca_1+y Ba₃ (Cu_0.88Fe_0.12)₇O _z (LYCaBCuFe) (y= 0.0?1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. T _c ^R=0 shows a marginal increase from 31 K to 37 K for y = 0.0?0.21 and thereafter it decreases with increasing y leading to zero T _c ^R=0 at y ≥ 0.84. This shows that the suppression of T _c from 80 K to 31 K by Fe doping at x = 0.12 La_2.5Y_0.5CaBa₃(Cu_1?x Fe _x )₇O _z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.     

Phase equilibria and crystal structures of phases in the La-Fe-Ni-O system at 1370 K in air

The phase equilibria in the La-Fe-Ni-O system have been studied at 1370 K in air, and the La-Fe-Ni-O phase diagram at constant temperature and pressure has been constructed. Based on x-ray diffraction results for samples prepared by standard solid-state reactions and via citrate and nitrate routes, the following solid solutions have been shown to exist at 1370 K in air: LaFe_{1 ? x} Ni _x O_{3 ? δ} (0 < x ≤ 0.4, sp. gr. Pbnm; 0.6 ≤ x ≤ 0.8, sp. gr. R-3 c), La₄(Ni_{1 ? y} Fe _y )₃O_{10 ? δ} (0 < y ≤ 0.3), La₃(Ni_{1 ? z} Fe _z )₂O_{7 ? δ} (0 < z ≤ 0.05), La₂Ni_{1 ? v} Fe _v O_{4 + δ} (0 < v ≤ 0.05), Ni_kFe_{3 ? k} O₄ (0.81 ≤ k ≤ 1.05), Ni_{1 ? m} Fe _m O (0 < m ≤ 0.05), and Fe_{2 ? p} Ni _p O₃ (0 < p ≤ 0.04). The lattice constants and structural parameters of single-phase samples have been refined by the Rietveld profile analysis method.     

Effect of Te Doping on the Structure and Superconductivity of K x Fe2−y Se2−z Te z Single Crystals

We report the structure, transport, and magnetic properties of K _x Fe_2?y Se_2?z Te _z single crystals grown by optical floating zone technique. The phase separation phenomena were observed in the Te-doped samples. With increasing Te doping level, the c-axis lattice parameter expands for both insulating/semiconducting and superconducting phases while the superconducting transition temperature (T _c) decreases and eventually vanishes at z = 0.51. The critical current density was estimated to be 10³–10⁴ A/cm² for the all doped samples. The upper critical field and anisotropic superconducting ratio increase with Te doping. We compared the results of critical current density J _c, upper critical field μ ₀ H _c2 and apparent thermally activated energy U ₀ for the samples with z = 0, 0.09, and 0.16. The influence of Te doping on the vortex pinning and the implication of U ₀ versus μ ₀ H is discussed.     

Effects of Zn Doping on Superconductivity in Tl0.4K0.4Fe2−δ−x Zn x Se2

We have studied the transport and magnetic properties of the Tl_0.4K_0.4Fe_2?δ?x Zn _x Se₂ (nominal ratio 0.01≤x≤0.15) single crystal samples. It is found that although Zn doping leads to a decrease of charge carrier density, it does not affect the superconductivity significantly when x≤0.1. The doped superconductors exhibit properties similar to the parent compound (Tl_0.4K_0.4Fe_2?δ Se₂), e.g., the almost unchanged superconducting transition temperature (T _c) and the “hump”-like behavior in the resistivity curves, which denotes the transition from insulating phase to metallic phase. As the doping density rises, the hump obviously shifts to low temperature region, implying the quick enlargement of the ratio of insulating phase. The most plausible explanation for the above phenomena is that the Zn impurities prefer entering into the insulating phase only, therefore, merely hindering the formation of superconducting phase without directly changing its properties. The ESR measurement gives low signal-to-noise ratio and just weak paramagnetic signals, revealing strong itinerancy of the Zn doped superconductor.     

Effect of M and M-Ca Substitution on the Structure and Superconductivity of GdBa2Cu3O7−δ [M = Mo, Hf ]

The structural and superconducting properties of (Gd_1?x?y Ca _y M _x )Ba₂Cu₃O _z with M = Mo, Hf are investigated using X-ray diffraction, electrical resistivity, and oxygen content measurements. The effect of increasing the Mo concentration in (Gd_1?x Mo _x )Ba₂Cu₃O _z changes the structure from orthorhombic to tetragonal accompanied by a large increase in resistivity and a fast decrease in T _c at the rate of 1.9 K per at.% of Mo, unlike that of Hf substitution in (Gd_1?x Hf _x )Ba₂Cu₃O _z , which maintains the orthorhombic structure and decreases T _c very slowly at the rate of 0.6 K per atm.% of Hf with nearly no change in resistivity. The suppression of T _c by M = Mo, Hf can be counterbalanced by hole doping by Ca which increases T _c with increasing Ca content showing maximum compensation for Mo. A comparative study of M = Mo, Hf doped samples in (Gd_1?x?y Ca _y M _x )Ba₂Cu₃O _z indicates that the valence of the dopant M = Mo^4+,6+, Hf⁴⁺ and its ionic radius play an important role in controlling the structural and superconducting properties of the systems.     

Preparation and Superconducting Properties of (Tl,Pb)-Based Compounds with 1234-Type Structure

(Tl_1?x Pb _x )(Sr_2?y Ba _y )Ca₃Cu₄O _z samples were synthesized by using a two-step solid state reaction method and examined with respect to the (Tl,Pb)-1234 phase formation and superconducting properties. Samples were prepared from various starting compositions of 0≤x≤0.5 and 0.4≤y≤2.0. Nearly single-phased samples were obtained for (Tl_0.8Pb_0.2)(Sr_2?y Ba _y )Ca₃Cu₄O _z (1.0≤y≤1.25) and (Tl_1?x Pb _x )(SrBa)Ca₃Cu₄O _z (0.15≤y≤0.35). We have found that both the partial substitution of Ba for Sr and the partial substitution of Pb for Tl are effective for the formation of the Tl-1234 phase. Bulk superconductivity with a T _c value of 106 K is observed in the (Tl_0.7Pb_0.3)(SrBa)Ca₃Cu₄O _z sample.     

Comparison of Superconductivity and Structure for YBa2Cu3Oy with Potassium and Sodium Doping

The nominal composition of YBa_2?x M _x Cu₃O _y (M = K, Na) cuprates with x ≤ 0.30 were synthesized by the standard solid-state reaction technique. X-ray diffraction (XRD) and the resistivity measurements were used to characterize the structure and the superconductivity of the doped cuprates. There was no impurity phase detected within the whole doping range. The structure of the main phase (“123”) has the orthorhombic with P _mmm symmetry. With increasing the content of dopants, the lattice constants and some other structural parameters are almost unchanged for M = K, whereas they changed for M = Na. The refined contents of dopants are consistent with that of the nominal ones. The zero resistance temperature T _c0 decreases sharply with the increase of the content of potassium in potassium-doped samples as x ≤ 0.20. For sodium-doped YBa_2?z Na _x Cu₃O _y cuprates, T _c0 varies very little. The difference in superconductivity depression may result from the shift oxygen, which transfers conducting carriers from Cu-O chains to Cu-O₂ sheets or the structural stress effect.     

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7−y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa₂(Cu_1?x Zn _x )₃O_7?y withy～0.1 has been measured forx≤0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa₂Cu₃O_7?y causes a rapid and nearly linear depression of the superconducting transition temperature,T _c , withT _c going to 0 K forx≥ 0.10. YBa₂(Cu_1?x Zn _x )₃O_7?y retains the YBa₂Cu₃O_7-y orthorhombic structure forx≤0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,ρ(T), is metallic-like (dρ/dT>0) andρ increases gradually with increasing Zn content. However, forx≥ 0.10,ρ(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.     

Magnetic excitations in iron chalcogenide superconductors

Nuclear magnetic resonance and neutron scattering experiments in iron chalcogenide superconductors are reviewed to make a survey of the magnetic excitations in FeSe, FeSe_1−xTe_x and alkali-metal-doped A_xFe_2−ySe₂ (A = K, Rb, Cs, etc). In FeSe, the intimate relationship between the spin fluctuations and superconductivity can be seen universally for the variations in the off-stoichiometry, the Co-substitution and applied pressure. The isovalent compound FeTe has a magnetic ordering with different wave vector from that of other Fe-based magnetic materials. The transition temperature T_c of FeSe increases with Te substitution in FeSe_1−xTe_x with small x, and decreases in the vicinity of the end member FeTe. The spin fluctuations are drastically modified by the Te substitution. In the vicinity of the end member FeTe, the low-energy part of the spin fluctuation is dominated by the wave vector of the ordered phase of FeTe; however, the reduction of T_c shows that it does not support superconductivity. The presence of same wave vector as that of other Fe-based superconductors in FeSe_1−xTe_x and the observation of the resonance mode demonstrate that FeSe_1−xTe_x belongs to the same group as most of other Fe-based superconductors in the entire range of x, where superconductivity is mediated by the spin fluctuations whose wave vector is the same as the nesting vector between the hole pockets and the electron pockets. On the other hand, the spin fluctuations differ for alkali-metal-doped A_xFe_2−ySe₂ and FeSe or other Fe-based superconductors in their wave vector and strength in the low-energy part, most likely because of the different Fermi surfaces. The resonance mode with different wave vector suggests that A_xFe_2−ySe₂ has an exceptional superconducting symmetry among Fe-based superconductors.     

Phase equilibria and crystal structures of solid solutions in the Sr-Fe-Ni-O system at 1100°C in air

The phase equilibria in the Sr-Fe-Ni-O system at 1100°C in air have been studied by x-ray diffraction, and the corresponding phase diagram at constant temperature and pressure has been constructed. The system has been shown to contain two solid-solution series at 1100°C in air: SrFe_{1 ? x} Ni _x O_{3 ? δ} (0 < x ≤ 0.075, sp. gr. Cmmm) and Sr₃(Fe_{1 ? y} Ni _y )₂O_{7 ? δ} (0 < y ≤ 0.15, sp. gr. I4/mmm). Neither Sr₄(Fe_{1 ? z} Ni _z )₆O₁₃ nor Sr(Fe_{1 ? z} Ni _z )₁₂O₁₉ solid solutions have been identified. The lattice constants and structural parameters of single-phase samples have been refined by the full profile Rietveld analysis method.     

High-pressure studies on Tc and crystal structure of iron chalcogenide superconductors

Abstract

The superconducting transition temperature, T_c, in iron-based solids can be enhanced by applied pressure: T_c increases from 8 to 37 K for the 11-type FeSe when the pressure is raised from 0 to 4 GPa. High-pressure studies can elucidate the mechanism of superconductivity in such novel materials. In this paper, we present a high-pressure study of Fe(Se_1?xTe_x) and Fe(Se_1?xS_x). In the case of Fe(Se_1?xTe_x), the maximum T_c under high pressure did not exceed the T_c of FeSe, which can be attributed to the structural transition to the monoclinic phase. For Fe(Se_1?xS_x) (0 < x < 0.3), T_c exhibited a significant increase with pressure; however, the maximum T_c under high pressure did not exceed the T_c of FeSe. This may be due to the disorder induced by substituting S for Se, which is similar to the pressure effect on T_c for the 1111-type superconductor Ca(Fe_1?xCo_x)AsF. The T_c of Fe(Se_1?xS_x) showed a complex behavior below 1 GPa, first decreasing and then increasing with increasing pressure. From high-pressure x-ray diffraction measurements, the T_c (P) curve was correlated with the local structural parameter.     

Magnetic excitations in iron chalcogenide superconductors

Abstract

Nuclear magnetic resonance and neutron scattering experiments in iron chalcogenide superconductors are reviewed to make a survey of the magnetic excitations in FeSe, FeSe_1?xTe_x and alkali-metal-doped A_xFe_2?ySe₂ (A = K, Rb, Cs, etc). In FeSe, the intimate relationship between the spin fluctuations and superconductivity can be seen universally for the variations in the off-stoichiometry, the Co-substitution and applied pressure. The isovalent compound FeTe has a magnetic ordering with different wave vector from that of other Fe-based magnetic materials. The transition temperature T_c of FeSe increases with Te substitution in FeSe_1?xTe_x with small x, and decreases in the vicinity of the end member FeTe. The spin fluctuations are drastically modified by the Te substitution. In the vicinity of the end member FeTe, the low-energy part of the spin fluctuation is dominated by the wave vector of the ordered phase of FeTe; however, the reduction of T_c shows that it does not support superconductivity. The presence of same wave vector as that of other Fe-based superconductors in FeSe_1?xTe_x and the observation of the resonance mode demonstrate that FeSe_1?xTe_x belongs to the same group as most of other Fe-based superconductors in the entire range of x, where superconductivity is mediated by the spin fluctuations whose wave vector is the same as the nesting vector between the hole pockets and the electron pockets. On the other hand, the spin fluctuations differ for alkali-metal-doped A_xFe_2?ySe₂ and FeSe or other Fe-based superconductors in their wave vector and strength in the low-energy part, most likely because of the different Fermi surfaces. The resonance mode with different wave vector suggests that A_xFe_2?ySe₂ has an exceptional superconducting symmetry among Fe-based superconductors.     

Phase equilibria in the system LaMnO3 + δ-SrMnO3-LaFeO3-SrFeO3 − d

We have revealed the formation of a continuous series of orthorhombic LaMn_{1 ? y} Fe_yO₃ solid solutions (0<y<1); La_{1 ? x} Sr_xFeO₃ solid solutions in the composition range 0 < x ≤ 0.8, with an orthorhombic structure at 0 < x ≤ 0.6 and a cubic structure at 0.6 < x ≤ 0.8; and a tetragonal SrMn_{1 ? y} Fe_yO₃ phase in the range 0.6 ≤ y ≤ 1. The composition stability limits of the perovskite phase La_{1 ? x} Sr_xMn_{1 ? y} Fe_yO₃ have been determined, and the 1100°C isotherm of the La₂O₃-SrO-Mn₃O₄-Fe₂O₃ system in air has been constructed.     

High-pressure studies on Tc and crystal structure of iron chalcogenide superconductors

The superconducting transition temperature, T_c, in iron-based solids can be enhanced by applied pressure: T_c increases from 8 to 37 K for the 11-type FeSe when the pressure is raised from 0 to 4 GPa. High-pressure studies can elucidate the mechanism of superconductivity in such novel materials. In this paper, we present a high-pressure study of Fe(Se_1−xTe_x) and Fe(Se_1−xS_x). In the case of Fe(Se_1−xTe_x), the maximum T_c under high pressure did not exceed the T_c of FeSe, which can be attributed to the structural transition to the monoclinic phase. For Fe(Se_1−xS_x) (0 < x < 0.3), T_c exhibited a significant increase with pressure; however, the maximum T_c under high pressure did not exceed the T_c of FeSe. This may be due to the disorder induced by substituting S for Se, which is similar to the pressure effect on T_c for the 1111-type superconductor Ca(Fe_1−xCo_x)AsF. The T_c of Fe(Se_1−xS_x) showed a complex behavior below 1 GPa, first decreasing and then increasing with increasing pressure. From high-pressure x-ray diffraction measurements, the T_c (P) curve was correlated with the local structural parameter.     

Effect of Blocking and Superconducting Layer Doping on the Superconductivity and Magnetic Properties of Polycrystalline Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>6</Subscript>

The possible difference in the properties upon doping the Sr₂CaCu₂O₆ superconducting or blocking layers with Fe and Eu respectively was investigated in this work. The homogeneous Sr_2?yEu_yCaCu₂O_6+δ and Sr₂CaCu_2?xFe_xO_6+δ (y = 0, 0.1, 0.5, x = 0, 0.05) compounds were produced by a high-pressure synthesis route. Judging by the magnetic susceptibility measurements, all samples exhibit a superconductivity transition and the Eu/Fe concentration dependencies on the diamagnetic moment and average T_c have been constructed using the experimental data. As a result, an unusual behavior of the T_c value was observed for the samples with doped Eu: a fivefold reduction in the europium concentration in the sample does not give a noticeable effect on the transition temperature value while the diamagnetic signal becomes more stronger. Complex superconducting dome was found for Eu-doped material: 0.1 ≤ y ≤ 0.5 region T_c vs. concentration data were approximated by inverted parabola-like curve with a maximum at y = 0.3. Difference in properties of the Eu and Fe-doped samples was also found in the behavior of the hysteresis loops showed the opposite orientations.     

Mechanochemical synthesis and reactivity of La1 − x Sr x FeO3 − y perovskites (0 ≤ x ≤ 1)

Mechanochemical synthesis of La_{1 ? x} Sr _x Fe_{3 ? y} (0 ≤ x ≤ 1) perovskites was carried out from simple oxides. Undoped lanthanum ferrite samples calcined at 900 and 1100°C for 4 h are nearly single-phase systems, while some substituted ones (samples with 0.3 ≤ x ≤ 0.8 calcined at 900°C and samples with x = 0.3, x = 0.4 and x = 0.8 calcined at 1100°C) are two-phase systems consisting of orthorhombic perovskite La_{1 ? x} Sr _x FeO_{3 ? y} (A) and cubic or tetragonal perovskite Sr _z La_{1 ? z} FeO_{3 ? y} (B) phases. In CO and CH₄ oxidation processes, the specific catalytic activity (SCA) of samples calcined at 1100°C varies non-monotonously with the Sr content. In CO oxidation, SCA reaches a maximum in the range of La substitution by Sr where phase transition occurs and samples are comprised of two coexisting phases (x = 0.3 and 0.8). In CH₄ oxidation, SCA decreases with the Sr content.     

Co-doping effects of Ca and Ce on the Superconducting Properties in Y1?x Ca x (Ba1?y Ce y )2Cu3O7?δ

A series of Y_1?x Ca _x (Ba_1?y Ce _y )₂Cu₃O_7??? (0??x??0.3, 0??y??0.3) polycrystalline superconductor samples were prepared using the solid-state reaction technique. The phase identification, crystal structure, and superconducting transition temperature (T _c ) were studied by means of X-ray diffraction (XRD) and resistivity measurements. The results indicted that the phase of samples changed from orthorhombic phase to tetragonal phase with increasing Ca concentration x and Ce concentration?y, and Ce did not form the superconducting structure. The lattice constants had a little change. The a-axis and c-axis lattice parameters increased. The b-axis lattice parameter decreased. The T _c and resistance had an obvious dropping tendency with increasing Ca and Ce concentrations. The transition width became sharper with the increase of x (=y). We drew a conclusion that the Ce-doping had an effect for strengthening the intergrain connectivity, and it counteracted the weakening effect of Ca-doping which introduced the hole causing a reduction in the interlayer coupling strength.     

Superconductivity of barium-doped Bi2Sr2CaCu2O y

Sintered ceramic samples of Bi₂Sr_2?x Ba _x CaCu₂O_y with nominal barium fraction 0≤x≤0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

Superconducting gap in Pr x Y1−x Ba2Cu4O8 and YBa2−y Sr y Cu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1?x Ba₂Cu₄O₈ and YBa_2?y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration atω～300 cm^?1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.