Crystal, electronic and luminescence properties of Eu-doped Sr2Al2−xSi1+xO7−xNx

The crystal and electronic structures, as well as the luminescence properties of Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ are reported. First-principles calculations energetically confirm that the Al and Si atoms are in partial ordering in the 2a and 4e sites in Sr₂Al₂SiO₇. In addition, the band structure calculation shows that Sr₂Al₂SiO₇ has an indirect band gap with an energy gap of about 4.07 eV, which is in good agreement with the experimental data (5.3 eV) obtained from the diffuse reflection spectrum. The crystal structure of Sr₂Al₂SiO₇ can be modified by Si–N substitution for Al–O in the lattice with a maximum solubility of about x=0.6. The average bond length of Eu_Sr^-(O,N) slightly increases although the lattice parameters decrease with the incorporation of Si–N in Sr₂Al₂SiO₇:Eu²⁺. Under excitation in the visible spectral region, Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ emits blue to yellow light with a broad emission band in the range of 480–570 nm, varying with both the Eu concentration and the x value. The red shift of the emission band of Eu²⁺ is associated with an increase in the crystal-field splitting and the covalency, which arise from the incorporation of nitrogen as well as the energy transfer between the Eu ions at high Eu concentrations. Moreover, the Eu ions have a strong effect on both the concentration quenching and the thermal quenching in Sr₂Al_2−xSi_1+xO_7−xN_x. The temperature dependence of photoluminescence indicates that Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ shows strong thermal quenching due to the dominant nonradiative process at room temperature.     

Effect of pressure onTc in La2-xBaxCuO4 withx=0.125

The superconducting transition temperature,T _c , of La_2–x Ba _x CuO₄ has been measured under high pressure up to 8 GPa.T _c is found to change drastically at the pressure where the structural phase transition takes place. This finding clearly indicates that there exists an intimate relation between the crystal structure and superconductivity.     

High-Temperature Thermoelectric Power of The Metal Oxides: La2?xSrxCuO4 and Bi1?xSrxMnO3

We have investigated the high-temperature thermoelectric power (TEP) of La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35) and Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8) up to 700 K. Based on the TEP results we have discussed the phase transitions on each case. In the case of high-T _C cuprates, La_{2− x} Sr _x CuO₄ (0.05 ≤ x ≤ 0.35), the TEP shows different temperature dependences in three temperature regions. At low temperature, the positive TEP rises showing a broad peak at temperature T _P, which shifts to lower temperature upon Sr doping. Right above T _P, the TEP decreases linearly as temperature increases. At high temperature, TEP deviates from the linear-T dependence at a certain temperature, T _H, showing a saturation behavior. The systematic change of the TEP behavior is discussed in terms of the two-fluids model, which is an intrinsically inhomogeneous state, consisted of bound pairs and independent carriers in the normal state of the high-T _C superconductors. For Bi_{1− x} Sr _x MnO₃ (0.5 ≤ x ≤ 0.8), the negative TEP is almost temperature-independent in the high temperature regime (T _CO < T < 700 K). Near the charge ordering temperature (T _CO), however, TEP suddenly decreases with decrease of temperature, indicating the suppression of carrier mobility with charge ordering transition. As Bi concentration decreases, T _CO shifts to lower temperature from T _CO ∼ 520 K for x = 0.5 to T _CO ∼ 435 K for x = 0.8, which suggests that charge ordering is related to the local lattice distortion due to highly polarizable 6s² character of Bi³⁺ ion. In comparison with the resistivity data, the TEP results have been discussed in terms of the carrier localization accompanied by local lattice distortion.     

Anomalous Hall Effect in Sn1?x?yMnxEuyTe and Sn1?x?yMnxEryTe Mixed Crystals

The Anomalous Hall Effect (AHE) was investigated in IV–VI ferromagnetic semimagnetic semiconductors of Sn_1–x Mn _x Te codoped with either Eu or Er. The analysis of experimental data is as follows. Hall resistivity and magnetization showed that AHE coefficient R _s depends on temperature and its value decreases with thetemperature increase. We observe that above ferromagnet–paramagnet transition temperature R _s changes sign. We discuss the possible physical mechanisms responsible for observed temperature dependence of R _s , particularly change of the sign.     

Superconducting Transition Temperatures in the Ru-1232 System, RuSr2(Gd1?xLnxCe1.8Sr0.2)Cu2Oz (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperature T _c in the Ru-1232 system, RuSr₂(Gd_{1− x} Ln _x Ce_1.8Sr_0.2)Cu₂O _z (Ln = Sm, Dy, and Ho). At first, in the case of the samples with Ln = Sm among almost the single 1232 phase samples, the values of the superconducting onset temperature T _co are almost the same for x = 0.00–0.15, and each of the lattice parameters a and c is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample with x = 0.05 shows the maximum values for both the superconducting onset temperature T _co and the zero resistivity temperature T _cz. Especially for the sample with Ln = Dy, the values of T _co and T _cz are 18.5 and 6.5 K, respectively. These are higher than those of the mother sample of RuSr₂(GdCe_1.8Sr_0.2)Cu₂O _z . Moreover, from variations of T _co, lattice parameters of a and c in the RuSr₂(Gd_{1− x} Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy content x, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

Magnetic properties of Y2Fe17?x Gax and Sm2Fe17?xGax compounds

The magnetic properties of Y₂Fe_17−x Ga_x and Sm₂Fe_17−xGa_x for 3 ≤ x ≤ 7 have been investigated using the ⁵⁷Fe M?ssbauer spectroscopy at room temperature. These compounds have the rhombohedral Th₂Zn₁₇ structure. X-ray diffraction analyses of aligned powders show that the easy direction of magnetization is parallel to the c-axis in Y₂Fe₁₀Ga₇ and Sm₂Fe₁₄Ga₃ and is perpendicular to the c-axis in Y₂Fe₁₄Ga₃, Y₂Fe₁₂Ga₅, Sm₂Fe₁₂Ga₅ and Sm₂Fe₁₀Ga₇. M?ssbauer studies indicate that all the samples studied are ferromagnetically ordered. The ⁵⁷Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for Y₂Fe_17−xGa_x and Sm₂Fe_17−xGa_x at room temperature is positive and the magnitude of δ increases with increasing Ga content.     

Suppression of Anti-ferromagnetism by K Doping in?(Cu0.5Tl0.5?xKx)Ba2Ca2Cu3O10?δ Superconductors

In the inner CuO₂ planes (IP) of (Cu_0.5Tl_0.5)Ba₂ Ca₂Cu₃O_10−δ superconductors the density of the carriers is in the under-doped state which promotes enhancement in the anti-ferromagnetism. Consequently, the critical temperature of the final compound is suppressed. In the present studies, we have enhanced the density of mobile carriers in the inner CuO₂ planes by doping K at the charge reservoir layer by preparing (Cu_0.5Tl_0.5−x K _x )Ba₂Ca₂Cu₃O_10−δ superconductors. The higher density of mobile carriers increases the Fermi velocity V _F, and hence enhances the superconductivity parameters such as T _c, H _c, and J _c. The main objective of these experiments was to suppress the anti-ferromagnetism in the final compound. We have enhanced the inter-plane coupling by partially substituting Be and Mg at the Ca sites. The melting point of the final compounds is significantly reduced with the incorporation of Be and Mg. The incorporation of these elements has been found to facilitate the formation superconductors with higher numbers of CuO₂ planes, i.e., (Cu_0.5Tl_0.5−x K _x )Ba₂Ca_3−y M _y Cu₄O_12−δ .     

Thermoelectric Properties of (Bi2Te3)1 ? x ? y(Sb2Te3)x(Sb2Se3)y Single Crystals

Homogeneous and graded n- and p-type (Bi₂Te₃)_{1 − x − y} (Sb₂Te₃) _x (Sb₂Se₃) _y crystals are grown by the Czochralski technique with melt supply through a floating crucible. The dimensionless thermoelectric figure of merit of the n- and p-type crystals is ZT = 1.1 (350 K) and 1.0 (375 K), respectively. It is shown that (Bi₂Te₃)_{1 − x − y} (Sb₂Te₃) _x (Sb₂Se₃) _y pseudoternary solid solutions can be used to produce monolithic n- and p- type graded and segmented thermoelectric materials by Czochralski growth. The thermoelectric power distribution across the seed-crystal interface is studied using scanning hot microprobe measurements. __________ Translated from Neorganicheskie Materialy, Vol. 41, No. 10, 2005, pp. 1186–1193. Original Russian Text Copyright ? 2005 by Svechnikova, Shelimova, Konstantinov, Kretova, Avilov, Zemskov, Stiewe, Zuber, Muller.     

Influence of Sm2Ba4CuBiOy phase content on Jc of SmBa2Cu3O7/Sm2Ba4CuBiOy nano-composites

Presence of various nano-scale Y₂Ba₄CuMO_y phase inclusions in YBa₂Cu₃O_7−δ phase matrix are shown to improve magnetic flux pinning over wide range of magnetic fields. We first fabricate single phase of Sm₂Ba₄CuBiO_y (Sm-2411) using a solid-state reaction and then introduced them into SmBCO single grains. Top seeded melt growth (TSMG) has been used to grow SmBCO single grains in air atmosphere. A significant improvement in J_c is observed, over wide magnetic fields, for single grains containing Sm-2411nano-phase inclusions when compared to that of SmBa₂Cu₃O_7−δ/Sm₂BaCuO₅ composites. When compared to YBCO nano-composites, SmBCO composites are shown to exhibit high J_c at medium range of magnetic fields (1–2 T) at 77 K.     

Nature of the Pseudogap in High-Tc Cuprates: Analysis?of?the?Bulk Magnetic Susceptibility of?La2?xSrxCu1?yZnyO4

The nature of the pseudogap in the quasiparticle spectral density in high-T _c cuprates is a matter of intense debate. In this study we have investigated the effects of Zn substitution on the uniform (q=0\boldsymbol{q}=0, where q\boldsymbol{q} is the wave vector) magnetic susceptibility, χ(T), of La_2−x Sr _x Cu_1−y Zn _y O₄ sintered samples with different hole concentrations, p(≡x), over a wide range of Zn contents (y). Non-magnetic Zn suppressed T _c most effectively and enhanced χ(T) systematically at low temperatures. We have extracted the characteristic pseudogap energy scale, ε _g, from the analysis of χ(T) data. Unlike T _c,ε _g was found to be fairly insensitive to the level of Zn substitution, even when T _c was completely suppressed by Zn. The Zn-induced Curie-like enhancement of the χ(T) was also found to be closely related to the PG energy scale. We discuss the possible implications of these findings in this paper.     

Crystal structure and anisotropy of iodine-intercalated Bi2Sr2Can?1CunOx

The electronic state and structural configuration of the intercalated iodine species in stage-1, I-Bi₂Sr₂Ca _n–1Cu _n O _x (n = l, 2), have been studied through polarization-resolved Raman and¹²⁹I Mössbauer spectroscopy. The polarization dependence of the Raman spectra and the Mössbauer measurement confirmed the dominant species to be triiodide ions, I ₃ ^– , with alignment of these linear molecules either along thea- orb-axis in the host crystals. Transport measurements such as thermoelectric power and Hall coefficient clearly indicated that hole carriers are doped into the CuO₂ planes upon intercalation, by whichT _c of the host superconductor is changed. Furthermore, based on resistivity measurements in a magnetic field, we suggest that the iodine intercalation leads to a decrease of the anisotropy both in normal and superconducting states, suppressing the extremely two-dimensional character of the Bi₂Sr₂Ca _n–1Cu _n O _x systems.     

Two-parameter fracture criterion (Kρ,c-Tef,c) based on notch fracture mechanics

A two-parameter fracture criterion has been proposed to predict fracture conditions of notched components. This criterion includes the critical notch stress intensity factor K _ρ,c , which represents fracture toughness of a material with a notch of radius ρ, and the effective T-stress. The effective T-stress T _ef has been estimated as the average value of the T-stress distribution in the region ahead of the notch tip at the effective distance X _ef . These parameters were derived from the volumetric method of notch fracture mechanics. The results of numerical T _ef,c -stress estimation are compared to the T _ef,c -stress results obtained from experimental analysis. The material failure curve or master curve K _ρ,c = f(T _ef,c ) has been established as a result of the notched specimen tests. A large T _ef,c range was covered from −0.80 σ _Y to +0.19 σ _Y using SENT, CT, RT (roman tile) and DCB specimens. It was shown that the notch fracture toughness is a linear decreasing function of the T _ef,c -stress. The use of the material failure curve to predict fracture conditions was demonstrated on gas pipes with the surface notch.     

Substitution Effect of Ba for Sr on Superconductivity in?the?(Pb0.75P0.25)(Sr2?xBax)(Y0.4Ca0.6)Cu2Oz System

The Pb-based 1212 compounds containing phosphorus were already discovered by us in the (Pb,P)Sr₂(Y,Ca)Cu₂O _z system. Among the almost-single phase samples, a sample with the nominal composition of (Pb_0.75P_0.25)Sr₂(Y_0.4Ca_0.6)Cu₂O _z was a superconductor with the highest T _c of 38 K in the system. Recently, we have been investigating on substitution effect of Ba for Sr on superconductivity in the (Pb_0.75P_0.25)(Sr_2−x Ba _x )(Y_0.4Ca_0.6)Cu₂O _z system. As a result, it is found that the almost–single 1212 phase samples are obtained in the composition area of 0.0≤x≤0.4. In this area, with increase of x, the lattice parameters a,c and the cell volume V are found to be gradually increasing. This finding can be considered to show that larger Ba²⁺ ions certainly occupied the Sr-sites in the sample. However, we obtain such interesting results as that the T _c value is not so enhanced by the substitution in comparison with that of the sample of x=0.0, and that the maximum value of T _c was 40 K for x=0.2 and 0.3 at the best.     

Structural and Physical Properties of Ca1?xSrxFe2As2 (0≤x≤1.0) and Ca0.5Sr0.5Fe2?yCoyAs2 (0≤y≤0.6)

The Ca_1−x Sr _x Fe₂As₂ (0≤x≤1) and Ca_0.5Sr_0.5 Fe_2−y Co _y As₂ (0≤y≤0.6) materials were prepared by a solid state reaction method. X-ray diffraction analysis shows that the lattice parameters of Ca_1−x Sr _x Fe₂As₂ decrease continuously with increasing Ca content. Resistivity measurements reveal that the Ca/Sr ratio has a visible influence on the resistivity anomaly associated with the spin-density-wave (SDW) instability. Experimental measurements on Ca_0.5Sr_0.5Fe_2−y Co _y As₂ show clear superconducting transitions for Co in range of 0.26≤y≤0.43; the highest critical transition temperature is found to be at about 17 K. Transmission electron microscopy (TEM) investigations on the superconducting Ca_0.5Sr_0.5Fe_2−y Co _y As₂ samples reveal visible inhomogeneous distribution of Sr and Ca elements, which could result in structural distortions and broaden superconducting transitions as observed in our experiments.     

Thermal conductivity of high-Tc superconductors

This paper reviews existing data on the thermal conductivity of high-T _c superconductors. Included are discussions of pristine polycrystalline high-T _c ceramics, single crystal specimens, and high-T _c materials structurally modified by substitution or by radiation damage. The thermal conductivity of high-T _c superconductors is compared with that of conventional superconductors, and dramatic differences are found between the two families. Mechanisms of thermal conductivity applicable to high-T _c perovskites are discussed and implications for theories of high-T _c superconductivity are noted.     

Microstructures of high-Jc melt-textured YBa2Cu3O7?x/Ag superconductors

Silver has been previously added to the melt-textured YBa₂Cu₃O_7–x in order to increase the critical current density (J _c ) of these materials. However, the effect of this addition on theJ _c is presently unclear. The purpose of this study is to investigate the effect of silver on both critical current density and the microstructure of the melt-textured YBa₂Cu₃O_7–x superconductors by means of X-ray diffraction, optical polarized microscopy, and transmission electron microscopy (TEM). TheJ _c of the MTG YBCO/Ag samples is more than 10⁴A/cm² under the 5 kOe magnetic field. It has been shown that as the concentration of silver increases, the fraction of the 211 phase dispersed within the 123 matrix decreases. Therefore, theJ _c slightly decreases. These results, together with the effect of the 211 phase, dislocations, and other structure defects on flux pinning, are described in this paper.     

Transport and Magnetic Properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)y Superconductors

We investigated the superconducting critical temperature, the intra- and intergranular critical current density, and the thermopower properties of Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiCl) _y samples. All these properties have been compared with those of Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiF) _y specimens. It was found that the critical temperature determined from resistive and AC complex susceptibility measurements show a maximum and the transition width shows a minimum for the intermediate values of y. Powder X-ray diffraction studies and the AC complex susceptibility measurements reveal that in our samples the amount of Bi₂Sr₂Ca₂Cu₃O_{10 +} high-temperature superconducting phase is maximum for y 0.02. The amount of LiCl in Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiCl) _y changes the superconducting properties of the grains as well as of the intergrain matrix. The splitting of the peak in the temperature dependence of the imaginary part of the complex susceptibility, corresponding to the dissipation inside the grains, was also observed.     

Optical study of charge dynamics along thec-axis in high-Tc superconductors

Comparing the optical spectra withEc for typical high-T _c superconducting cuprates, we discuss the charge dynamics along thec-axis. The plasma energy or the mass anisotropic factorm _* ^c //m _* ^b is one of the key parameters determining the spectrum forEc. In Bi₂Sr₂CaCu₂O₈ and La_2–x Sr _x CuO₄ with a largem _* ^c /m _* ^b , the plasma energy is smaller than the superconducting gap energy, and thus the supercurrent along thec-axis is a kind of Josephson current flowing through insulating layers such as the BiO layers. On the other hand, in YBa₂Cu₃O₇ with a smallm _* ^c /m _* ^a , it seems that the coherent supercurrent flows along thec-axis. Although the spectrum forEc strongly depends on samples, presumably due to the difference in the hole concentration, a clear anisotropy between theab- and thec-directions is observed in the characteristics energy scales such as the plasma energy as well as the reflectivity knee energy.     

A simple approach of fabricating thermoelectric γ-NaxCoO2 and superconductive Nax(H2O)yCoO2-δ films using the sol–gel spin-coating method

We report a simple approach of fabricating thermoelectric γ-Na_xCoO₂ film with the c-axis orientation using the sol–gel spin-coating method. The inferred sodium content is x = 0.65 according to the correlation between the c-axis lattice constant and x. Temperature dependence of both the resistivity and thermopower resembles that of the γ-Na_0.68CoO₂ film grown by the reactive solid-phase epitaxy. The fitted thermopower data show that the bandwidth of γ-Na_xCoO₂ is found to be ~ 101 meV, being close to the quasi-particle band (70–100 meV) derived from an angle-resolved photoemission study of γ-Na_0.7CoO₂. These results enable the possibility of low-cost fabrication of γ-Na_xCoO₂-based thermoelectric film devices. Furthermore, we have also topotactically transformed the of γ-Na_xCoO₂ film to a superconducting Na_x(H₂O)_yCoO_2-δ film with T_{c, onset} = 4.12 K.     

Tc trend in high-Tc superconductors

In our previous works, we have shown that most existing ceramic superconductors can be considered to be built of superconductor-semiconductor composite and we have estimated the change in phonon spectrum of the intrinsic superconductor unit if a semiconductor unit is attached to it. Moreover, the proximity effect under the size quantization condition has been examined in the superconductor-semiconductor composite. Each of the stated effects by itself could causeT _c enhancement in general as more semiconductor blocks are added to the system. We extend our study in this paper to analyze the combined actions of phonon spectral change and proximity effect without size quantization condition onT _c variation in members of the Tl₁ series of high-T _c superconductors. Our results indicate that an optimumT _c is obtained if the stated effects are included in the idealized unit cells of the superconductors made up of a superconductor-semiconductor array.