Search

Multiferroic materials, like MnWO4, exhibit improper ferroelectricity driven by an inversion symmetry breaking magnetic order and strong spin lattice interactions. We show that the three magnetic phases in MnWO4, including one ferroelectric phase, can be controlled by substitution of copper for manganese and by application of external magnetic fields. Upon increasing Cu substitution, the paraelectric low-temperature phase is replaced by the ferroelectric phase which becomes the ground state at higher doping levels. We conclude that a mixed (para-/ferroelectric) ground state exists below 10% of Cu substitution.

The article presents the design of a compact ultrawideband (UWB) monopole antenna with microstrip-fed. The antenna has a simple elliptical-shaped radiator. The very compact size of 11 × 34 = 374 mm2 and wide bandwidth of 3.1–12.6 GHz are achieved by simply using a three-step staircase on both sides of the feed line and a slot on the ground plane. The antenna is studied and designed on a Rogers substrate using computer simulation and measured using Satimo Starlab system. The effects of feeding cable used in measurement are also studied. Simulated and measured results agree very well. Results show that the antenna has a wide bandwidth, stable realized gain and efficiency, and very stable radiation patterns across the UWB from 3 to 12 GHz. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2232–2236, 2013

We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe 2 As 2 via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at 49 K, which has a low critical field 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe 2 As 2 (or Ae122), where Ae = Ca, Sr, or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature ( T c ) only up to 38 K and pressurization has been reported to produce superconductivity with a T c up to 30 K. The unusual 49 K phase detected will be discussed.

MnWO4 is a bulk multiferroic showing three different magnetic phases at low temperature. The intermediate AF2 phase (7.8 K < T < 12.6 K) is also ferroelectric (multiferroic) due to the inversion symmetry breaking helical Mn-spin order and the strong coupling of the spins to the lattice. The complex phase sequence and the multiferroic properties are a consequence of a delicate balance of competing magnetic exchange interactions, anisotropy, and the resulting frustration. It is therefore of interest to tune and control these interactions by substituting the magnetic Mn ions with other magnetic or nonmagnetic ions. We have investigated the effects of chemical substitutions (Co, Zn, and Fe for Mn) on the multiferroic phases. In Mn1-xFexWO4 the helical (multiferroic) phase is quickly suppressed by less than 5% Fe. In contrast, Co substitution in MnWO4 of only 2% is sufficient to stabilize the helical (multiferroic) phase as the ground state. The substitution of nonmagnetic Zn for Mn also stabilizes the multife...

Many efforts have been made to improve the current carrying capability of Y1Ba2Cu3O7 (YBCO) films by introducing pinning centers using a variety of techniques. In this study, we have succeeded in producing Y2O3, BaZrO3 and Sm doped YBCO films for the development of coated conductors by a metal-organic deposition method using trifluoroacetates (TFA-MOD). Jc exceeding 5 MA /cm2 at 77 K and self field was obtained in Y2O3 doped YBCO film. Also it was found that YBCO films with Sm substituting for Y have shown an improved critical current density characteristic over a wide range of magnetic fields. A Jc of 0.25 MA/cm2 at 3 T and 77 K was obtained which is more than 10% of the zero-field Jc. Similar results were obtained in BaZrO3 doped YBCO films. Additionally, TEM analysis was performed to study the microstructures and the pinning mechanisms. The results indicate that significant enhancement of self-field and in-field Jc of Y2O3, BaZrO3 and Sm doped YBCO films may be due to introducing nano-scaled Y2O3 particles, nano-scaled 90deg rotated Y1/3Sm2/3Ba2Cu3O7 domains and BaZrO3 nanodots by different chemical doping, respectively. This indicates that chemical doping is a promising approach for enhancement of current carrying capability of YBCO coated conductors.

The polarized Raman spectra of layered ruthenates of the Srn+1RunO3n+1 (n=1,2,3) Ruddlesden-Popper series were measured between 10 and 300 K. The phonon spectra of Sr3Ru2O7 and Sr4Ru3O10 confirmed earlier reports for correlated rotations of neighboring RuO6 octahedra within double or triple perovskite blocks. The observed Raman lines of Ag or B1g symmetry were assigned to particular atomic vibrations by considering the Raman modes in simplified structures with only one double or triple RuO6 layer per unit cell and by comparison to the predictions of lattice dynamical calculations for the real Pban and Pbam structures. Along with discrete phonon lines, a continuum scattering, presumably of electronic origin, is present in the zz, xx and xy, but not in the x′y′ and zx spectra. Its interference with phonons results in Fano shape for some of the lines in the xx and xy spectra. The temperature dependences of phonon parameters of Sr3Ru2O7 exhibit no anomaly between 10 and 300 K where no magnetic transition occur. In contrast, two B1g lines in the spectra of Sr4Ru3O10, corresponding to oxygen vibrations modulating the Ru-O-Ru bond angle, show noticeable hardening with ferromagnetic ordering at 105 K, thus indicating strong spin-phonon interaction.

The small intergrain effect of MgB/sub 2/ on supercurrent makes it one of the most promising candidates for superconducting conductors due to its easier processing and the associated lower manufacturing cost. Unfortunately, the superconducting transition temperature T/sub c/ is only 40 K. However, band-structure calculations predict that a higher T/sub c/ than that of MgB/sub 2/ is possible for isostructural and isovalent intermetallic compounds with greater lattice parameters or greater unit cell volumes. The prediction appears to be consistent with the negative pressure effect on T/sub c/ observed. The substitution of the larger Ca-ions for the smaller Mg-ions has thus been suggested to raise T/sub c/, but not yet realized. Alternatively, we have synthesized and studied a series of binary and pseudobinary intermetallic compounds, AGa/sub 2/, AGa/sub 2-x/Si/sub x/, and AAl/sub 2-x/Si/sub x/, where A=Ca, Sr, or Ba, which are isostructural to MgB/sub 2/ and have greater lattice parameters than MgB/sub 2/. In spite of the greater lattice parameters, AGa/sub 2/ are not superconducting. However, all pseudobinary compounds AGa/sub 2-x/Si/sub x/ and AAl/sub 2-x/Si/sub x/ for 0.6

We have synthesized and studied the binary intermetallic compounds, AGa2 and A[GaxSi2−x] (A=Sr, Ba, Ca), which are isostructural to the newly discovered 40 K superconductor MgB2. The binary compounds AGa2 were found to be non-superconducting above 2 K, even though they have the same valence electron number per formula as MgB2 and greater lattice parameters than MgB2. However, by varying the valence electron number, they become superconducting. This result suggests that the electronic structures of this class of compounds depend on more than just the valence electron density and the lattice parameters. The theoretical prediction that the Tc of MgB2 could be enhanced with the proper dopant to increase its carrier density or to expand the lattice could not be verified.

The magnetic and ferroelectric properties of the multiferroic system Mn$_{1-x}$Co$_x$WO$_4$ (x=0.135, 0.15, and 0.17) are studied in magnetic fields $H_c$ oriented along the monoclinic $c$-axis. Mn$_{0.85}$Co$_{0.15}$WO$_4$, which is right at the phase boundary between two helical spin structures, exhibits a spontaneous sign change of the ferroelectric polarization when cooled in fields $H_c>$ 25 kOe. The origin of the ferroelectric polarization is studied and two magnetic exchange interactions contributing to the polarization are identified. In Mn$_{0.85}$Co$_{0.15}$WO$_4$ domains of the characteristic helical spin structures, known for x$ $0.15, coexist and form domain boundaries. The contributions of the different domains to the global polarization are determined. The polarization reversal in Mn$_{0.85}$Co$_{0.15}$WO$_4$ can be explained by a combination of various contributions to the polarization and a strong correlation between magnetic domains of different helical spin orders resulting in a smooth transition across the domain walls which preserves the chirality of the spin spiral.

A simple spray/compress technique has been developed for the fabrication of thick film tapes of HBCCO, BSCCO, and YBCO on metallic substrates with high J C . The technique has successfully incorporated crucial processing factors to achieve composition uniformity, phase purity, preferred microstructure, grain alignment, and high density for high temperature superconducting tapes of high performance. We have obtained a J C of 8.8×10 4 A/cm 2 in YBa 2 Cu 3 O 6+δ thick films on Ag/Pd substrates at 77K and self-field; 7×10 4 A/cm 2 in HgBa 2 Ca 2 Cu 3 O 8+δ thick films on Ni substrates with a buffer layer at 77K and self-field; and 5×10 5 A/cm 2 and 3×10 5 A/cm 2 in Bi 2 Sr 2 CaCu 2 O x thick films on Ni substrates with a buffer layer at 4.2K in self-field and 8T, respectively.

We have applied a microanalysis technique on micrograin samples in order to directly identify the 23 K superconducting phase in the Y–Pd–B–C system. Through the physical and structural characterization of such micrograins we have found the tetragonal YPd 2 B 2 C x phase with a space group I4/mmm and lattice parameters a =0.371 nm and c =1.08 nm to be the responsible phase for the 23 K superconducting transition in this system.

Thermoelectric power (S) and lattice parameter $(a,$ b, $c)$ measurements are used to determine the change in the carrier concentration (n) in both isotope exchanged and oxygen deficient samples of ${\mathrm{La}}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{MnO}}_{3\ensuremath{-}\ensuremath{\delta}}.$ It is observed upon substitution of the ${}^{18}\mathrm{O}$ isotope for ${}^{16}\mathrm{O}$ that the Curie temperature ${T}_{C}$ decreases but no significant changes in S at 290 K and a, b, or c are seen. In contrast, for samples that are oxygen deficient, significant changes in S at 290 K and a, b, and c are observed, in addition to decreases in ${T}_{C}.$ It is found that the estimated $\ensuremath{\Delta}n$ correlating to the observed $\ensuremath{\Delta}S$ cannot account for the isotope-induced change in ${T}_{C}.$ Therefore it is concluded that any oxygen deficiencies introduced by the isotope exchange are negligible and do not account for the ${T}_{C}$ change observed.

Sr is doped into Ba 2 Ca 2+ x Cu 3+ y O z ( T c =126 K) to improve its chemical stability. A solubility limit of Sr/(Ba+Sr+Ca)∼0.3 was observed under our synthesis conditions. Below this limit, Sr is equally distributed among the Ca and Ba sites, and does not affect the superconductivity significantly. Unfortunately, improvement in the chemical stability of the compound is limited. Above the solubility limit, however, a new compound with an as-synthesized T c of 106 K becomes the major phase. This phase decomposes into another new compound with T c ∼80 K after exposure to humid air. A 0212-type model is proposed, with both Ba and Sr located in the rock-salt layers. The model can accommodate both the local cation stoichiometry and the XRD pattern for this new 106 K-phase.

A new family of C-free high-temperature superconducting oxides in the Ba–Ca–Cu–O system showing a T c up to 107 K has been synthesized under high pressure. Structural analyses, based on X-ray and electron diffractions and high resolution electron microscopy, have revealed that these oxides can be represented by CaBa 2 Ca 2 Cu 3 O 9− δ [Ca-1223-Ba] with a tetragonal subcell that has lattice constants a ∼3.85 A and c ∼16.5 A and is isostructural to Tl-1223-Ba with the Tl sites occupied mainly by Ca. These oxides have the same active block of [(CuO 2 )(Ca)(CuO 2 )(Ca)(CuO 2 )] but with cation mixing and substitution of Ca by Cu within the charge reservoir block [(BaO)(CaO)(BaO)]. This results in the observed variations in composition and superstructure but has only minor influence on T c .

MnWO 4 has attracted attention because of its ferroelectric property induced by frustrated helical spin order. Strong spin–lattice interaction is necessary to explain ferroelectricity associated with this type of magnetic order. We have conducted thermal expansion measurements along the a, b, c axes revealing the existence of strong anisotropic lattice anomalies at T 1 = 7.8 K , the temperature of the magnetic lock-in transition into a commensurate low-temperature (reentrant paraelectric) phase. The effect of hydrostatic pressure up to 1.8 GPa on the FE phase is investigated by measuring the dielectric constant and the FE polarization. The low-temperature commensurate and paraelectric phase is stabilized and the stability range of the ferroelectric phase is diminished under pressure.

The Raman-active phonons in orthorhombic perovskitelike ${\mathrm{YMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ were studied by measuring Raman spectra in various scattering configurations. The experimental Raman line wave numbers and the expected shapes for the phonon modes were compared to those reported for other perovskitelike compounds with the $\mathrm{Pnma}$ structure and to the results of lattice dynamical calculations. The observed Raman lines in the spectra of ${\mathrm{YMnO}}_{3}$ and ${\mathrm{LaMnO}}_{3}$ were assigned to definite atomic motions. The much larger linewidths and strong variation of the Raman spectra with increasing laser power in the case of ${\mathrm{LaMnO}}_{3}$ provide evidence for a structural instability that may result in a phase transition under laser annealing.

Anomalies in the dielectric constant and loss tangent have been observed in the ferroelectromagnet YMnO{sub 3} near its N{acute e}el temperature of {approximately}80 K and below its ferroelectric Curie temperature of {approximately}914 K. These anomalies are indicative of coupling between the ferroelectric and antiferromagnetic orders in this compound. A small but distinct magnetoelectric effect and a magnetoresistive effect up to {approximately}15{percent} were also detected in a magnetic field at 5 T. The results will be contrasted with previous theoretical predictions. {copyright} {ital 1997} {ital The American Physical Society}

The stability of Hg 1− x Ba 2 Ca n −1 Cu n O 2 n +2+δ [Hg12( n − 1) n ] and its competing phase, HgCaO 2 , at synthesis temperatures were studied in terms of their equilibrium Hg vapor pressure P Hg . It was found that the stability of Hg12( n − 1) n varies with n in a complex way. It was also observed that traces of moisture and/or carbon dioxide reduce the P Hg of HgCaO 2 , while partially substituting Re, Pb and Mo for Hg suppresses the P Hg of Hg12( n − 1) n . Possible relationships between the beneficial substitution effect, the change of cation valence, and the defect structures in Hg12( n − 1) n are proposed.

Layered ZrTe${}_{3}$ crystallizes in an unusual structure that consists of both quasi-one-dimensional and quasi-two-dimensional features, conducive to the formation of charge-density wave and superconductivity, respectively. Bulk superconductivity up to 4 K in single crystalline samples of layered ZrTe${}_{3}$ has been successfully induced through high growth temperature. This procedure induces atomic disorders at both the Zr and the Te1 sites, as evident from the x-ray-diffraction study. As a result, the charge-density wave is partially suppressed without chemical doping or pressurization. The observation helps to understand the peculiar superconductivity of ZrTe${}_{3}$ and suggests a new path for the induction of superconductivity in complicated compounds with competitive orderings.

Gas effusion spectra have been obtained in the temperature range 30{endash}1000{degree}C for underdoped, optimized, and overdoped HgBa{sub 2}CuO{sub 4+{delta}} samples. We observe a large oxygen multipeak structure at high temperatures and a wide low-intensity oxygen {open_quote}{open_quote}tail{close_quote}{close_quote} at lower temperatures. The two lowest-temperature oxygen peaks are believed to originate from bonding sites for excess oxygen. The {delta} values derived from these two peaks are consistent with {delta} values derived from neutron powder diffraction (NPD). The wide low-temperature oxygen tail in the effusion spectra suggests the existence of loosely bound disordered oxygen in overdoped samples and accounts for the discrepancy between {delta} values determined by NPD and thermogravimetry analysis. Our results provide further evidence that each doping oxygen atom contributes, on average, only about one carrier to the CuO{sub 2} plane in HgBa{sub 2}CuO{sub 4+{delta}}. {copyright} {ital 1996 The American Physical Society.}

HgBa2CuO4+δ (Hg-1201) samples synthesized with various initial compositions and at different Hg vapor pressures were examined by SEM, TEM, EDS and X-ray diffraction. The results suggest that Hg-1201 is intrinsically a Hg non-stoichiometry phase. The allowed Hg vapor pressure, which is limited by the competitions from HgBaO2, required an equilibrium defect density in the Hg site at synthesis temperatures. These defects correspond to a 30% Hg deficiency at ∼800°C with a balance Hg vapor pressure at the order of 10−1 atm. However, the equilibrium defect density should decrease with temperature, and is approximately zero at room temperature based on our EDS data. Therefore, cation diffusion, defect coarsening, and possible phase segragation occur during the cooling, which create various impurities and inclusions in the samples obtained. Further investigations on the related phase diagrams are needed to design a better synthesis procedure.

The effects of the precursor on the synthesis of HgBa 2 Ca 2 Cu 3 O 8+δ (Hg-1233) under high pressure are studied. It was observed that HgCaO 2 and Hg-1212 appear in the early stage of the formation of Hg-1223. Through intercalation of BaCuO and CaCuO, HgCaO 2 and Hg-1212 can be converted to Hg-1223. We found that the mixing level of Ca- and Cu-ions strongly affects the formation of Hg-1223. The existence of large CaO grains in the precursor will increase the needed synthesis temperature or synthesis time, and impurities may appear. A precursor in which Ca-ions are well reacted with other ions facilities the formation of Hg-1223. We also found that this well-reacted precursor is insensitive to moist air.

The chemical transformations of three barium cuprates, BaCuO 2 , Ba 2 Cu 3 O 5.9 , and Ba 2 CuO 3 , which are the precursors and decomposition products of the Ba-based family of superconducting cuprates, were studied through their Raman bands under various laser irradiation conditions in air. BaCuO 2 was stable up to 3 mW/μm 2 . Ba 2 Cu 3 O 5.9 transformed into BaCuO 2 under laser irradiation of 2 mW/μm 2 or higher. Ba 2 CuO 3 transformed to Ba 2 Cu 3 O 5.9 , first under laser irradiation of ∼1 mW/μm 2 , then to BaCuO 2 when the irradiation was increased to above 2 mW/μm 2 . At elevated temperatures in air, BaCuO 2 seems to be the most stable of these barium cuprates and Ba 2 CuO 3 the least stable.

The recent observation of superconductivity above 160 K under high pressures has prompted us to investigate the possibility of retaining such high-temperature superconductivity at ambient pressure by substituting Sr for Ba in HgBa 2 Ca 2 Cu 3 O 8 (Hg-1223). High-purity (≥90%) Hg-1223 phase samples were synthesized with a nominal composition of Hg z Sr x Ba 2− x Ca 2 Cu 3 O 8+δ (where z =0.75 and x =0, 0.5 or 1). By adding a second element, Pb or Mo, supposedly to the Hg site, (Hg,Pb/Mo)Sr 2 Ca 2 Cu 2 O 8+δ with purity up to 90% was achieved. Unfortunately, T c decreases with Sr substitution. The roles of Sr in the phase stability, and the synthesis conditions of the Sr based Hg-1223, and the Sr substitution effect on the T c of these samples will be discussed.

The magnetic, thermodynamic, and pyroelectric properties of LiFeP2O7 single crystals are investigated with emphasis on the magnetoelectric interaction of the electrical polarization with the magnetic order parameter. The magnetic order below TN ≃ 27 K is found to be a canted antiferromagnet with a weak ferromagnetic component along the b−axis. A sharp peak of the pyroelectric current at TN proves the strong internal magnetoelectric interaction resulting in a sizable polarization decrease at the onset of magnetic order. The magnetoelectric effect in external magnetic fields combines a linear and a quadratic field dependence below TN. Thermal expansion data show a large uniaxial magnetoelastic response and prove the existence of strong spin lattice coupling. LiFeP2O7 is a polar compound with a strong interaction of the magnetic order parameter with the electric polarization and the lattice.

We report the observation a continuous rotation of the polarization in Mn$_{0.95}$Co$_{0.05}$WO$_{4}$ under magnetic field. At zero field, this compound shows a transition into the spiral magnetic and ferroelectric phase at 12.2K, which is the ground state, with the polarization oriented along the b-axis. Increasing b-axis magnetic fields rotate the ferroelectric polarization continuously toward the a-axis, indicating a rotation of the spin spiral plane. This rotation extends over a large field and temperature range. At a constant magnetic field of 3 Tesla, the polarization also rotates from the a-axis at the ferroelectric transition toward the b-axis upon decreasing temperature., 3 pages, 2 figures

Multiferroic rare earth manganites attracted recent attention because of the coexistence of different types of magnetic and ferroelectric orders resulting in complex phase diagrams and a wealth of physical phenomena. The coupling and mutual interference of the different orders and the large magnetoelectric effect observed in several compounds are of fundamental interest and bear the potential for future applications in which the dielectric (magnetic) properties can be modified by the onset of a magnetic (dielectric) transition or the application of a magnetic (electric) field. The physical mechanisms of the magnetoelectric effect and the origin of ferroelectric order at magnetic transitions have yet to be explored. We discuss multiferroic phenomena in the hexagonal HoMnO 3 and show that the strong magneto-dielectric coupling is intimately related to the lattice strain induced by unusually large spin-phonon correlations.

We have studied the growth of disks of single-grain YBa 2 Cu 3 O 7 (3BCO) with diameter to 5 cm using a top-seeded melt-texture technique. The quality of these YBCO disks was found to depend critically on the quality and size of the seed used, the temperature schedule of the growth adopted, the amount of Y 2 BaCuO 5 and Pt added, and the size of the final disks processed. Samples of a diameter of 2.5 cm so-prepared have been observed to shield a magnetic field greater than 20 T at 4.2 K and to trap a field of 1.3 T at 77 K. However, no significant increase in the trapped field has been achieved by increasing the diameter of the disk to beyond ∼ 3 cm.

We have studied (LuC${)}_{2}$(${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$) and (LuC)(${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$) at ambient and high pressures. Superconductivity was discovered in (LuC${)}_{2}$(${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$) at 2.9 K. Pressure was found to suppress both the electrical resistivity and the superconducting transition temperature rapidly for (LuC${)}_{2}$(${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$) but only slightly for (LuC)(${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$), in spite of their similar chemical-bonding behavior. The observations can be understood in terms of the smaller density of states at the Fermi surface and the greater pressure-induced band broadening of (LuC${)}_{2}$(${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$) than (LuC)(${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$).

Several well-characterized HgBa2CuO4+δ samples were annealed under various conditions. Above 300°C and at various oxygen partial pressures, both reversible oxygen intake/release and irreversible Hg-loss take place with different time constants. By choosing the proper annealing temperature Ta, oxygen pressure P(O2) and annealing time t, the Hg-loss effects were negligible and the bulk superconducting transition temperature Tc was reversibly changed with δ from 0 K on the underdoped side (an insulator) through a peak of 97 K, then down to ∼ 20 K on the overdoped side. The measured 3D variation of Tc vs. (1/Ta, ln P(O2)) appears as a parabolic surface with iso-Tc segments projected on a 2D 1/Ta-ln P(O2) plane as straight lines. The slope [∂ ln P(O2)/∂(1/Ta)]Tc of the projections corresponds to the oxidation enthalpy ΔH, and the derivative (∂ ln P(O2)/∂Tc)Ta which is a measure of the Tc variation with the oxygen partial pressure used in the annealing, is related to the defect-interactions and the Tc−δ correlation. The data for YBa2Cu3O7−δ, Bi2Sr2CaCu2O8+δ, and HgBa2CuO4+δ are compared.

We have carried out a systematic structural, electric, and magnetic study on YPdBC samples with different compositions with emphasis on the as-cast and annealed YPd5B3C0.3 which was first reported to superconduct at ∼ 23 K by Cava et al. We found that the tetragonal body-centered YPd2B2C with lattice parameters a=3.71 A and c=10.81 A is the phase responsible for the 23 K superconductivity and that YPd2B2C is metastable, which is consistent with the suggestion made by Cava et al. [1]: it is not stable at high temperatures nor stabilizable by Ni doping, although its isostructural compound, YNi2B2C, exists. Two new phases with Y:Pd ratios of 1:7 and 2:3, respectively, have also bee detected.

The Raman spectra of several as-synthesized barium cuprates, BaCuO 2 , Ba 2 CuO 3 and Ba 2 Cu 3 O 5.9 , and annealed Ba 2 Cu 3 O 5.9 were studied in this work. Characteristic Raman bands were found at 633, 563 and 592 cm -1 , respectively. They were polarized. These three strong bands can be used to identify these cuprates. The 563 cm -1 band of Ba 2 CuO 3 could be assigned to the vibration of vibrational mode of the apical oxygen along the c -axis. The 633 cm -1 band of Ba 2 Cu 3 O 5.9 could be assigned to the vibration of O(VI). Other Raman bands were not assigned due to the complicated structures. The Raman spectra of annealed Ba 2 Cu 3 O 5.9 under different conditions indicated that the Ba 2 Cu 3 O 5.9 decomposed to BaCuO 2 ; this way initiated by a high temperature and completed over a long-period treatment.

A giant magnetoelectric polarization is found in HoAl${}_{3}$(BO${}_{3}$)${}_{4}$. The polarization in transverse field geometry at 70 kOe reaches 3600 $\ensuremath{\mu} \mathrm{C}/{\mathrm{m}}^{2}$, which is significantly higher than reported values of linear magnetoelectric or even multiferroic compounds. The magnetostrictive effect is also measured and compared with the magnetoelectricity. The results show that spin-lattice coupling in HoAl${}_{3}$(BO${}_{3}$)${}_{4}$ is extremely strong and that the magnetic field causes a large polar distortion of the ionic positions in the unit cell.

We report on the remarkably robust ferroelectric state in the multiferroic compound ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{WO}}_{4}$. Substitution of the magnetic ${\mathrm{Mn}}^{2+}$ with nonmagnetic Zn${}^{2+}$ reduces the magnetic exchange and provides control of the various magnetic and multiferroic states of ${\mathrm{MnWO}}_{4}$. Only $5%$ of Zn substitution results in complete suppression of the frustrated collinear (paraelectric) low-temperature phase. The helical magnetic and ferroelectric phase develops as the ground state. The multiferroic state is stable up to a high level of substitution of more than $50%$. The magnetic, thermodynamic, and dielectric properties, as well as the ferroelectric polarization of single crystals of ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{WO}}_{4}$, are studied for different substitutions up to $x=0.5$. The magnetic phases have been identified in single-crystal neutron-scattering experiments. The ferroelectric polarization scales with the neutron intensity of the incommensurate peak of the helical phase.

The fabrication of high-temperature superconducting YBa 2 Cu 3 O 7−δ −Ag thin films has been investigated using a high-pressure Ag coevaporation-DC sputtering technique. Various analytical techniques including X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), energy dispersive X-ray analysis (EDX), and T c measurement have been utilized for characterization of the films. Highly reproducible c -oriented (00 l ) films have been prepared in-situ at a relatively low growth temperature (≃670°C) by this technique. The transition temperatures T c gradually decreased with added Ag from 90 K (0 at.% Ag) to 72 K (22.3 at.% Ag) for the films deposited on MgO (100) substrates. X-ray diffraction indicated that the Ag is in the metallic state as a separate phase with respect to the YBCO and that the c -lattice parameter of the grown films remained unchanged with Ag addition. Surface morphology of the YBCO-Ag films exhibited a particulate-type structure with the grain size increasing as the Ag content increased. The temperature coefficient of the normalized resistance, d R ( T )/ R (280 K)/d T , was also noticed to decrease for the Ag- containing samples as compared to Ag-free YBCO films. AES depth profiling identified Ag residing mainly at the YBCO/MgO interface. The effect of Ag addition to the films has been described by a particle growth model.

The superconducting transition temperatures (T c 's) of HgBa 2 CaCu 2 O 6+δ with different dopings and different ambient T c 's were examined magnetically under hydrostatic pressures. We observed that T c increases with pressure linearly at a rate of 0.22±0.02 K/kbar up to 18 kbar for as-synthesized samples with an ambient T c ∼111 K, but at a similar initial rate only below ∼4 kbar, and at a reduced rate of 0.11±0.02 K/kbar above, for the oxygenated samples with an ambient T c ∼117 K. The observation suggests the possible existence of a Van Hove singularity in the electronic energy spectrum of HgBa 2 CaCu 2 O 6+δ and a possible pressure-induced Fermi-surface-topology change in the oxygenated compound

Samples of HgBa 2 Ca 2 Cu 3 O 8+δ were successfully prepared and investigated. By examining the X-ray diffraction, transmission electron microscopy, magnetization and resistivity, we found that HgBa 2 Ca 2 Cu 3 O 8+δ possesses an orthorhombic structure with lattice parameters a =5.451 (2)A, b =5.432 (2)A and c =15.826 (7) A and a sharp superconducting transition (magnetically determined) at ∼112 K as-synthesized and as high as ∼135 K after oxygenation. The resistivity vanishes below 134 K, the highest reported for any superconductor to date. No superstructures of the types previously reported in the > 130 K samples were detected in this study. A linear T -dependent resistivity which is interpolated to zero at 0 K was also observed above ∼240 K. Below ∼240 K, deviation from such a linearity appears, which may be attributable to the fluctuation or spin-scattering effect, assuming that the behavior is intrinsic to HgBa 2 Ca 2 Cu 3 O 8+δ .

We have measured the micro-Raman scattering on the mercury-based cuprate superconductors HgBa 2 CuO 4+δ (Hg-1201), HgBa 2 CaCu 2 O 6+δ (Hg-1212) and HgBa 2 Ca 2 Cu 3 O 8+δ (Hg-1223) in ceramic forms. The observed Raman spectra showed similar features for all the samples. A strong peak at ∼585 cm -1 was tentatively attributed to the apical oxygen vibrating along the c -direction. A shoulder peak at ∼570 cm -1 was speculated to be due to the vibration of the interstitial oxygen. Weak features around 180 cm -1 were observed for all the samples. The Raman spectrum from CaHgO 2 was also reported. The power dependence of the Raman peaks was studied on Hg-1212. Raman scattering on a Hg-1212 thin film was also measured.

Several samples of PrBa 2 Cu 3 O 0.67 , Pr 1+ x Ba 2− x Cu 3 O 7−δ and La 1+ x Ba 2− x Cu 3 O 7−δ have been synthesized and examined by X-ray diffraction and polarized micro-Raman scattering. The X-ray diffraction refinement results on these samples show a decrease of the Cu(2)-O(4)-Cu(1) bond length due to partial Ba-site substitution by large radius RE-ions. The Raman scattering shows an increase of the apical O(4) stretching frequency in agreement with the structure data. The increasing of the O(4) vibration frequency with the rare-earth induced bond-length changes can account for previous seemingly contradictory Raman results on PrBa 2 Cu 3 O 7 . An empirical relation was found between the effective force constant k for O(4) and the Cu(1)-Cu(2) spacing d : k ∝ d -6 . The oxygen disorder induced peak at ∼ 600 cm -1 is also discussed.

We report the induction of superconductivity in Pb 2 Sr 2 (Ln 2− y Ce y )Cu 3 O 10+ d (Pb-2223) with T c ~ 30 K and a Meissner effect indicating a superconducting volume fraction ~ 15%. A combination of cation substitution and fine adjustment of oxygen content is necessary to induce superconductivity in this system. The superconducting compounds are prepared by a two-step solid state reaction and subsequent annealing at temperatures in the range 650–700°C and in oxygen partial pressures P (O 2 ) in the range 1–8 Torr, with y =0.67, where Ln=Sm, Eu, and Gd. Using identical preparation and annealing procedures, samples with Ln=Pr, Nd, Tb, Dy, Ho, and Er can be prepared phase pure, but they are not superconducting down to liquid helium temperature.

The magnetic and multiferroic phase diagram of ${\text{Mn}}_{0.85}{\text{Co}}_{0.15}{\text{WO}}_{4}$ single crystals is investigated by means of magnetic, heat-capacity, dielectric, polarization, and neutron-scattering experiments. Three magnetic phase transitions are detected through distinct anomalies in all physical quantities. The ferroelectric polarization is observed only along the $b$ axis below 10 K but not along the $a$ axis as recently suggested. The magnetic phases studied by neutron scattering are very complex. Up to four different magnetic structures, partially coexisting at certain temperature ranges, have been identified. Upon decreasing temperature two commensurate phases (AF4, AF1) are followed by an incommensurate phase (AF5) and a second incommensurate phase (AF2) is detected as a minor phase. The ferroelectric polarization is possibly associated with both (AF2 and AF5) phases.

The thermodynamic, magnetic, dielectric, and magnetoelectric properties of ${\text{HoFe}}_{3}{({\text{BO}}_{3})}_{4}$ and ${\text{Ho}}_{0.5}{\text{Nd}}_{0.5}{\text{Fe}}_{3}{({\text{BO}}_{3})}_{4}$ are investigated. Both compounds show a second order Ne\'el transition above 30 K and a first-order spin reorientation transition below 10 K. ${\text{HoFe}}_{3}{({\text{BO}}_{3})}_{4}$ develops a spontaneous electrical polarization below the Ne\'el temperature $({T}_{N})$ which is diminished in external magnetic fields. No magnetic-field induced increase of the polarization could be observed in ${\text{HoFe}}_{3}{({\text{BO}}_{3})}_{4}$. In contrast, the solid solution ${\text{Ho}}_{0.5}{\text{Nd}}_{0.5}{\text{Fe}}_{3}{({\text{BO}}_{3})}_{4}$ exhibits both, a spontaneous polarization below ${T}_{N}$ and a positive magnetoelectric effect at higher fields that extends to high temperatures. The superposition of spontaneous polarization, induced by the internal magnetic field in the ordered state, and the magnetoelectric polarizations due to the external field results in a complex behavior of the total polarization measured as a function of temperature and field.

In this paper the mechanism of phase transition and structural relaxation are elucidated by means of model experiments in the differential scanning calorimeter. Materials for this study were varied in structure and oxygen content by the introduction of Ni for Cu, through co-precipitation of oxolate gels, calcination, and sintering in ambient atmosphere. Both relaxation and phase transition are revealed by scrutinizing the heating rate dependence of heat capacity, and they seem to occur contemporaneously. Two higher order transitions are observed and are characterized by activation energies of 186 kJ/mole and 550 kJ/mole at the low and high temperatures, respectively. Perhaps the first transition is of a displacive type and occurs in anticipation of a reconstructive structural change from a fourfold chain to a sixfold sheet-like configuration. It is delineated by a change in relaxation kinetics from a localized type to a cooperative type with activation energies of 36 and 10 kJ/mole, respectively. The cooperative relaxation occurs at higher temperatures and is kinetically continuous across the reconstructive transition region. The structure characteristic exponent has a value of 0.70 for both localized and cooperative relaxation and resembles the two-dimensional relaxation often observed in depolymerized glasses. With kinetic impediment the transition region seems to approach a lambda-like behavior. In the subtransitional region, impediments in relaxation are useful in delineating oxygen stoichiometric differences between samples. The above discussion is supported by the structural and chemical characteristics that were gathered by x-ray powder diffraction, Raman spectroscopy, scanning electron microscopy and microprobe, and low temperature resistance measurements on Y123 samples containing 5 and 10 at. % Ni, respectively.

Several parameters affecting the grain growth in monolithic YBa2Cu3O7 using the melt-texturing and continuous processing technique have been identified and investigated. Steps have been found to promote grain-growth, to improve the grain-orientation, to control microcracks, to lower the processing temperature, to increase the processing rates, and to improve the overall chemical uniformity. The results will be presented and discussed.

The low-temperature ferroelectric polarization of multiferroic $YMn_2O_5$ is completely reversed at a critical pressure of 10 kbar and the phase transition from the incommensurate to the commensurate magnetic phase is induced by pressures above 14 kbar. The high-pressure data correlate with thermal expansion measurements indicating a significant lattice strain at the low-temperature transition into the incommensurate phase. The results support the exchange striction model for the ferroelectricity in multiferroic $RMn_2O_5$ compounds and they show the importance of magnetic frustration as well as the spin-lattice coupling.

We report the polarized Raman spectra of undoped $R\mathrm{FeAsO}$ ($R$=Sm, La) collected at room temperature from $ab$ surfaces of impurity-free microcrystals. The spectra exhibit sharp phonon lines on very weak electronic scattering background. The frequency and symmetry of the four Raman phonons involving out-of-plane atomic vibrations are found at $170\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ (${A}_{1g}$, Sm), $201\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ (${A}_{1g}$, As), $208\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ (${B}_{1g}$, Fe), and $345\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ (${B}_{1g}$, O) for $\mathrm{SmFeAsO}$; and $162\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ (${A}_{1g}$, La), $208\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ (${A}_{1g}$, As), $201\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ (${B}_{1g}$, Fe), and $316\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ (${B}_{1g}$, O) for $\mathrm{LaFeAsO}$.

We report the discovery of a complete suppression of ferroelectricity in $\mathrm{Mn}\mathrm{W}{\mathrm{O}}_{4}$ by 10% iron substitution and its restoration in external magnetic fields. The spontaneous polarization in ${\mathrm{Mn}}_{0.9}{\mathrm{Fe}}_{0.1}\mathrm{W}{\mathrm{O}}_{4}$ arises below $12\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in external fields above $4\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The magnetic and/or ferroelectric phase diagram is constructed from the anomalies of the dielectric constant, polarization, magnetization, and heat capacity. The observations are qualitatively described by a mean-field model with competing interactions and strong anisotropy. We propose that the magnetic field induces a noncollinear inversion symmetry breaking magnetic structure in ${\mathrm{Mn}}_{0.9}{\mathrm{Fe}}_{0.1}\mathrm{W}{\mathrm{O}}_{4}$.

The compound Bi2Sr2(GdxCa1−x)0.8Cu2O8+δ is examined by electron diffraction for several values of x. The incommensurate modulation which has been reported for x - 0 persists for all values of x with the wave vector increasing as x increases. A secondary modulation with a wave vector equal to one-half that of the primary is observed for x  0.85. This modulation manifests itself through satellite reflections that are highly streaked normal to the (001) planes and that flank certain select positions in reciprocal space. The appearance of this secondary modulation is accompanied by a change in the average crystal structure from B-centered to F-centered orthorhombic.

Several new compounds have been synthesized. By analyzing the new compounds and reviewing data from existing compounds, we conclude that the atomic radius of R in R 2 CuO 4 dictates the stability of and the type of doping permitted in the various homomorphies of the compounds. Our efforts have yet to induce a direct electron-to-hole HTS transition or to achieve 90 K-electrons HTS.