BaO Layers Research Articles

Explanation of high-temperature superconductivity without cuprate planes

Abstract The superconducting hole condensate resides in the SrO planes, or in the BaO planes, or in the interstitial regions of high-temperature perovskite superconductors, depending upon the crystal structure. Supporting evidence includes the following: firstly, PrBa2Cu3O7 superconducts in its BaO layers, and not in its cuprate planes; secondly, the similarity of the layer charges for YBa2Cu3Ox and PrBa2Cu3Ox indicates that both materials superconduct in their BaO layers; thirdly, Gd2-zCezSr2Cu2TiO10, Pr2-zCezSr2Cu2NbO10 and Eu2-zCezSr2Cu2TiO10 all superconduct in their SrO layers, and not in their cuprate planes; and fourthly, the superconducting hole condensate for Cu-doped Sr2YRuO6 (a material with no cuprate planes to better than 1%) must reside in its SrO layers.

Low-temperature scanning tunneling microscopy of YBa 2Cu 3O 7− δ

Low-temperature scanning tunneling microscopy studies have been performed on YBa 2Cu 3O 7− δ single crystals. We present two types of atomic images corresponding to BaO- and CuO-chain layers, which vary with applied bias voltage and temperature. At low bias voltage, the electronic modulation images of the CuO-chain layer tend to be emphasized. Warming the sample up to about 45 K alters the atomic images of the BaO layers but does not alter those of the CuO-chain layers. These results suggest that the electronic modulation images are closely related to the electronic states near the Fermi level and are attributed to the charge density wave.

Redox state analysis for understanding the high- Tc superconductivity in highly unstable Ba 2Ca 2Cu 3O 8− δ phase with Tc≈124 K and its derivative phase with Tc≈78 K

By means of wet-chemical analysis the redox state was established for a highly unstable Ba 2Ca 2Cu 3O 8− δ superconductive phase ( T c≈124 K) obtained by high-pressure synthesis under oxidizing conditions and also for its derivative phase with T c≈78 K formed immediately when exposing the as-synthesized sample to humid air or H 2O/N 2 gas. The former phase is of the 0 (Ba)2 (Ca)23 structure having a relatively thin blocking block consisting of only two BaO layers in between two superconductive CuO 2–Ca–CuO 2–Ca–CuO 2 layer blocks. On the other hand, the latter phase possesses a structure of H- m (Ba)2 (Ca)23 ( m ≈ 5 ) with an expanded blocking block due to accommodation of water in the structure. Wet-chemical analysis clearly manifested much higher oxidation state for the 0 (Ba)2 (Ca)23 phase than for its water-containing derivative. This suggests that the driving force for the phase transformation is the strong tendency of the 0 (Ba)2 (Ca)23 phase to be reduced. The possible origin of the unusually high overall oxidation state of the 0 (Ba)2 (Ca)23 phase is discussed on the basis of peroxide-type mixed-valent oxygen in its (BaO) 2 blocking block.

Maximum-entropy muon-spin-rotation study of Pr xY 1− xBa 2Cu 3O 7− δ

Muon-spin-rotation (μSR) data of Pr x Y 1− x Ba 2Cu 3O 7− δ (Pr x Y 1− x BCO) recorded at room temperature have been analyzed by a maximum-entropy (ME) technique. For different Pr doping, the obtained magnetic field distributions show distinct differences. The field distributions have been fitted using two Lorentzians, affirming trends seen for the ( x⩽0.5) superconductors. These trends are consistent with increasing Pr presence in BaO layers. Further, we are studying the vortex field distributions in (Pr x Y 1− x )BCO by means of ME-μSR.

HRTEM and EELS Study for Superconducting and Non-Superconducting (Cu,Cr)-1212 Cuprates

The crystal structures of high temperature superconducting cuprates consist of alternative stacking of charge-reservoir (CR) block and conduction block. in case of YBa2Cu3Oy, (YBCO), CR block contains one-dimensional Cu(I)-O chain and BaO layers, and conduction block contains two-dimensional Cu(II)O2 planes and Y layers. YSr2Cu3Oy (YSCO), where Ba-sites of the YBCO are replaced by Sr completely can be obtained only by high-pressure synthesis. However, by replacing Cu(I) partly by Cr, YSr2Cu3-xCrxOy can be obtained even under ambientpressure conditions. The compound YSr2Cu2.8Cr0.2Oy (x=0.2) is non-superconductive, but, superconductivity is induced by partial replacement of Sr-sites with Ba; the compounds with YSr2-xBaxCu2.8Cr0.2Oy (x≤0.5) is non-superconductive, but the compound with x≥1.0 is superconductive. in order to obtain the structural information about the transition to superconducting phases, the effect of the Ba substitution is investigated by transmission electron microscopy (TEM).In the present study, the compounds with YSr2-xBaxCu2.8Cr0.2Oy (x=0-1.5) are examined by high resolution TEM (HRTEM) and electron energy-loss spectroscopy (EELS) to elucidate structural and electrical modification induced by Ba-substitution.

Identification of Atomic Layers of YBa2Cu3O7-δby Scanning Tunneling Microscopy and Spectroscopy

Low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) measurements have been performed on YBa 2 Cu 3 O 7-δ single crystals cleaved below 20 K in order to obtain clean surfaces. The observed STM images are classified into two types of atomic images corresponding to BaO and CuO-chain layers, and the superconducting STS spectra are observed only on the latter layer. In the BaO layer, barium atomic images are clearly observed for the first time in addition to oxygen atomic images. In the CuO-chain layer, on the other hand, we observe the oxygen atomic images, in which a number of oxygen vacancies form clusters along the b -axis. Moreover, the CuO-chain layer shows elliptically modulated electronic images which are attributed to the charge density wave in the CuO chains.

Effect of oxygen on the chemical reactions and electron work function in Ba-Si and BaO-Si structures

Effects of the annealing in oxygen on the chemical composition and electron work function of Ba-Si and BaO-Si structures were studied. Thin Ba or BaO layers deposited onto silicon stimulate oxidation processes on the semiconductor surface. Treatment of the Ba-Si and BaO-Si samples at temperatures Ts≥500°C leads to the formation of a barium orthosilicate (Ba2SiO4) layer. A decrease in the electron work function observed for the silicon surface coated with both barium oxide and barium silicate layers provides for a more than tenfold increase in the electron emission current.

Onset of dielectric modes at 110 K and 60 K due to local lattice distortions in nonsuperconductingYBa2Cu3O6.0crystals

We report the observation of two dielectric transitions at 110K and 60K in the microwave response of non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals. The transitions are characterized by a change in polarizability and presence of loss peaks, associated with overdamped dielectric modes. An explanation is presented in terms of changes in polarizability of the apical O atoms in the Ba-O layer, affected by lattice softening at 110K, due to change in buckling of the Cu-O layer. The onset of another mode at 60K strongly suggests an additional local lattice change at this temperature. Thus microwave dielectric measurements are sensitive indicators of lattice softening which may be relevant to superconductivity.

Nanoparticles of Barium Hexaferrite by Gel to Crystallite Conversion and their Magnetic Properties

A novel method for the preparation of nanoparticles of barium hexaferrite is realized by the gel-to-crystallite (G-C) conversion method. Here, gels of Fe(OH)(3). xH(2)O, 70 < x < 110, were reacted with Ba(OH)(2). 8H(2)O in ethanol/water medium at 80-95 degreesC yielding the precursor, barium iron (III) oxy hydroxide hydrate which is x-ray amorphous but crystalline by electron diffraction (ED). Thermal analyses showed dehydroxylation of the precursor around 600 degreesC to barium hexaferrite which exhibits ED with spotty ring patterns. Samples heat-treated at 650 degreesC are X-ray crystalline with average particle size of 17 nm, which on recrystallization at 780-930 degreesC gives monocrystallites with spot patterns by ED. By varying the wet chemical conditions, precursors of variable Fe2O3/BaO ratios could be prepared which on heat-treatment yield monophasic hexaferrite of Fe2O3/BaO ratio ranging from 4.51 to 6. In hyperbarium compositions, annealing at 1350 degreesC leads to ordering of excess barium in anti-BR sites within Ba-O layers of beta -alumina type unit cells. Nanoparticles of barium hexaferrite with superparamagnetic as revealed by Mossbauer spectra, the temperature vs. magnetization plots and the absence of hysteresis in B-H curves. With increasing temperature of heat treatment, the area under the B-H loop increases continuously, with the magnetization increasing from 2 to 52 emu/g. The conversion from superparamagnetic to ferrimagnetic state is continuous because of the out-diffusion of cation vacancies, created to charge compensate hydroxyl ions, which, in turn, affects Fe3+-O2--Fe3+ superexchange interactions, with the addition of surface and size factors.

SUPERCONDUCTIVITY OF PrBa2Cu3O7 AND OF Pr2-zCezSr2Cu2NbO10

The data for superconducting PrBa 2 Cu 3 O 7 and Pr 2-z Ce z Sr 2 Cu 2 NbO 10 indicate that the superconductivity of these compounds is normally unaffected by rare-earth-site Pr but is quenched by a Pr Ba or Pr Sr defect on the other side of an adjacent cuprate-plane. Hence the primary superconducting condensate must reside not in the cuprate planes, but in the charge-reservoir regions containing hypocharged oxygen, namely in the BaO layers or the CuO chain layers of PrBa 2 Cu 3 O 7, and in the SrO layers of Pr 2-z Ce z Sr 2 Cu 2 NbO 10.

The 1 : 2 : 3 phases in the Y(Ho, Gd, Nd)–Ba–Cu–O: calorimetric investigations, correlations between thermodynamic and structural properties

On the basis of dissolution enthalpies of Y 2O 3, Gd 2O 3, Nd 2O 3, Ho 2O 3, CuO, BaCO 3, YBa 2Cu 3O 6.9, HoBa 2Cu 3O 6.9, GdBa 2Cu 3O 6.9, NdBa 2Cu 3O 6.98 measured by solution calorimetry in 6 N HCl at 323.15 K the following values were determined. (1) enthalpies of formation of Re1 2 3 phases from binary oxides: Δ ox H 0( Y123, 323.15 K)=−127.5±4.6 kJ mol −1 ; Δ ox H 0( Ho123, 323.15 K)=−131.4±8.6 kJ mol −1 ; Δ ox H 0( Gd123, 323.15 K)=−180.0±7.3 kJ mol −1 ; Δ ox H 0( Nd123, 323.15 K)=−165.0±8.8 kJ mol −1 ; (2) enthalpies of formation of Re1 2 3 phases from mixtures of Re 2O 3, BaCO 3, CuO; (3)enthalpies of formation of Re1 2 3 phases from mixtures of Re 2O 3, BaCuO 2, CuO; (4) standard formation enthalpies of Re1 2 3 phases Δ f H 0( Y123, 323.15 K)=−2643.5±9.6 kJ mol −1 ; Δ f H 0( Ho123, 323.15 K)=−2635.3±12.2 kJ mol −1 ; Δ f H 0( Gd123, 323.15 K)=−2663.4±11.0 kJ mol −1 ; Δ f H 0( Y123, 323.15 K)=−2632.6±12.0 kJ mol −1. The correlations between thermodynamic data and structural parameters of Re1 2 3 phases were established. It had been noted that the stability increase was correlated with the relative difference decrease of lattice parameters of Re 2O 3 and BaO layers from 4.65% for BaO–Ho 2O 3 to 4.37% for BaO–Y 2O 3, 3.86% for BaO–Nd 2O 3, 2.42% for BaO–Gd 2O 3.

Magnetic irreversibility characteristics of highly unstable superconducting Ba 2Ca 2Cu 3O y and its derivative phase

We have characterized the magnetic irreversibility field, H irr, for an unstable phase with T c=124 K of the Ba–Ca–Cu–O system prepared by means of high-pressure synthesis and also for its derivative phase with T c=78 K obtained through exposing the as-synthesized sample to H 2O/N 2 gas. The former phase is of the 0223 structure having a relatively thin “blocking” block consisting of only two BaO layers in between two superconductive Ca 2Cu 3O 6 “infinite-layer” blocks, while the latter has been proposed to have the structure of H- m223:I or (H,C)- m223 with an expanded blocking block. In the log( H irr) vs. log(1− T/ T c) plot, the irreversibility-field lines for the two phases are found to be located at nearly the same position, comparable to the position reported previously for optimally-doped Hg-1223.

Magnetic and M-I Transitions in YBaCo2O5+x(\({\bf 0} \leQQ \mib{x} \leQQ {\bf 0.5}\))

Three compounds, orthorhombic YBaCo 2 O 5.5 , tetragonal YBaCo 2 O 5.5 and YBaCo 2 O 5.0 , have been synthesized and their structural, magnetic and electrical properties have been investigated. The structures of all three compounds have a CoO 2 –Y(O)–CoO 2 block layer sandwiched between the BaO layers. Magnetic and metal-insulator transitions have been observed in orthorhombic YBaCo 2 O 5.5 which has a one-dimensional ordering of oxygen atoms along the b -axis in the Y(O) layers. Tetragonal YBaCo 2 O 5.5 and YBaCo 2 O 5.0 , which are both semiconductive, show magnetic transitions. The physical properties of YBaCo 2 O 5+ x strongly depend on the structure and oxygen content.

Layer-by-layer heteroepitaxial growth process of a BaO layer on SrTiO3(001) as investigated by molecular dynamics

Layer-by-layer heteroepitaxial growth processes of a BaO layer on SrTiO3(001) were simulated in order to predict an appropriate buffer layer for a YBa2Cu3O7−x(YBCO)/SrTiO3(001) heterojunction by using our crystal growth molecular dynamics (MD) simulation code. The SrTiO3(001) terminated by a TiO2 atomic plane was employed as the substrate in the present simulations. BaO molecules were continuously deposited on the SrTiO3(001) one by one, and finally a two-dimensional and epitaxial growth of a BaO layer was observed at 700 K. Moreover, the constructed BaO layer was atomically flat and smooth without defects, retaining a NaCl-type structure and (001) oriented configuration. However, the stress of the BaO/SrTiO3(001) heterojunction gradually increased and finally reached to approximately 1.2 GPa during the epitaxial growth process. It is expected that the large stress disturbs the subsequent fabrication of the uniform YBCO/SrTiO3(001) heterojunction. We also simulated the epitaxial growth process of a BaO layer on a [single SrO layer/SrTiO3(001)]. An atomically flat and smooth BaO layer without defects was also obtained at 700 K. Surprisingly, the stress of the BaO/[single SrO layer/SrTiO3(001)] heterojunction was almost 0.0 GPa after epitaxial growth. We have already suggested that [BaO layers/single SrO layer] are suitable buffer layers for the YBCO/SrTiO3(001) heterojunction on the basis of regular MD simulations [M. Kubo et al., Phys. Rev. B 56, 13535 (1997)]. From the present crystal growth simulations, we confirmed that the above atomically uniform and smooth BaO/SrO/SrTiO3(001) can be fabricated and almost no heterointerface stress was induced after the epitaxial growth. Moreover, the effect of substrate temperature on the heteroepitaxial growth process of the BaO layer on the SrO/SrTiO3(001) was discussed.

Surface structures and dielectric response of ultrafine BaTiO3 particles

Characteristic differences are observed for the dielectric response and microstructures of BaTiO3 nanoscale fine powders prepared using sol gel (SG) and steric acid gel (SAG) methods. The former exhibit a critical size below which there is no paraelectric/ferroelectric phase transition whereas BaTiO3 prepared via the SAG route remained cubic for all conditions. Atomic resolution images of both varieties showed a high density of interesting surface steps and facets. Computer simulated images of surface structure models showed that the outer (100) surface was typically a BaO layer and that at corners and ledges the steps are typically finished with Ba+2 ions; i.e., the surfaces and steps are Ba-rich. Otherwise the surfaces were typically clean and free of amorphous layers. The relationship between the observed surfaces structures and theoretical models for size effects on the dielectric properties is discussed.

Atomic control of layer-by-layer epitaxial growth onSrTiO3(001):Molecular-dynamics simulations

Molecular-dynamics simulations were performed to clarify the structures of SrO and BaO layers on a ${\mathrm{SrTiO}}_{3}(001)$ substrate at the atomic level, and to predict an appropriate buffer layer for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}/{\mathrm{SrTiO}}_{3}$ heterojunction. The atomic structure of these layers grown on a ${\mathrm{SrTiO}}_{3}(001)$ substrate terminated on the ${\mathrm{TiO}}_{2}$ atomic plane was investigated. From the analysis of the angle distribution of Sr-O-Sr and the radial distribution between Sr and O, the first single SrO layer on the ${\mathrm{SrTiO}}_{3}(001)$ substrate was found to keep a perfect NaCl-type structure. However, the structure of the second SrO layer deviated from a NaCl-type structure. This result suggests that only a single SrO layer is able to grow epitaxially and uniformly on the ${\mathrm{SrTiO}}_{3}(001)$ substrate terminated on the ${\mathrm{TiO}}_{2}$ atomic plane. Since a BaO layer is one component of the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ layered structure, a detailed understanding of the ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}(001)$ heterojunction has been desired. Here, the stress induced by the lattice mismatch of the ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}(001)$ and ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}(001)$ heterojunction was evaluated. The ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}(001)$ gained 1.2 GPa stress, while surprisingly the ${\mathrm{B}\mathrm{a}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}(001)$ did not have any stress. Moreover, the BaO layer was found to grow epitaxially and uniformly on the ${\mathrm{S}\mathrm{r}\mathrm{O}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}(001).$ Note that ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ is expected to grow epitaxially on a BaO layer since the BaO layer is a part of the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ layered structure. Hence, we suggest that BaO/SrO is a suitable buffer layer for the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}/{\mathrm{SrTiO}}_{3}$ heterojunction.

Charge transfer from Cu in YBa2Cu3O7−δ and Ba2Cu3O4Cl2 crystals determined by Auger electron spectroscopy

A formulation is proposed to estimate the charge transfer ΔqCu from Cu to some anions in metal-oxide crystals, using Auger electron spectroscopy. In the formulation, a parameter, the Auger transition probability ω, is introduced, which is defined as the ratio of Auger intensity due to a certain transition to the total Auger intensity due to other transitions when the same core electrons are primarily excited. By applying the formulation to vacuum-cleaved surfaces of YBa2Cu3O7−δ (YBCO) and Ba2Cu3O4Cl2 (BCOC) crystals, we obtained the following results: (1) YBCO cleaves between CuO2 and BaO layers, and the amount of ΔqCu is 2.3 in units of elementary electric charge. The deviation of ΔqCu from 2.0 suggests the existence of Cu3+-like atoms; (2) for BCOC, ΔqCu varies from 3.3 to 1.7 depending on the Ar ion beam sputtering time.

Heteroepitaxy of perovskite-type oxides on oxygen-annealed SrTiO3(1 0 0). Important factors for preparation of atomically flat oxide thin films

Abstract Thin film growth of perovskite-type oxides, CaTiO 3 and La 0.7 Sr 0.3 MnO 3 , on atomically smooth SrTiO 3 (1 0 0) substrate has been studied by in-situ reflection high-energy electron diffraction (RHEED) and ex-situ atomic force microscope (AFM) in order to understand the key points to keep the surface flatness. It is found out that the termination of substrate surface by AO layer of perovskite structure (ABO 3 ), i.e., CaO, SrO and BaO, is quite effective to prepare the atomically flat thin films. Among these top layers, the SrO termination gives the best result. CaTiO 3 film on SrTiO 3 (1 0 0) can keep the atomically flat surface in spite of large lattice mismatch (∼ 2.3%), while La 0.7 Sr 0.3 MnO 3 needs more strict lattice matching to keep the flat surface. Structure and orientation of SrO are analyzed by only in-situ RHEED observation as the first step of real-time growth control. Furthermore, the migration process of Sr supplied by laser ablation is studied by time-resolved measurement of RHEED intensity. The growth process of thin film by laser ablation method is discussed with this experiment.

The structures of the 126 Ba 2Ca 2Cu 3(Ca xCu y)O 8+δ the 116 K Ba 2Ca 3Cu 4(Ca xCu y)O 10+δ and their transformed products

Two new superconducting Ba Ca Cu oxides were identified as Ba 2Ca 2Cu 3(Ca xCu y)O 8+δ and Ba 2Ca 3Cu 4(Ca xCu y)O 10+δ with T c of 126 K and 116 K, respectively. The cations (Ca xCu y), 0.4≤x≤0.9and0≤v≤0.7, are inserted between two adjacent BaO layers to stabilize the structures and to adjust the Cu-valences. Both compounds show the body-centered symmetry, but with different lattice constants of ( a,c) = (3.8, 28.2) and (3.8, 34.8) Å, respectively. These compounds are unstable against the incorporation of CO 2 and H 2O in open air. The transformed products (Ca u/2Cu v/2C xH y) 2Ba 2Ca 2Cu 3O 8+δ and (Ca u/2Cu v/2C xH y) 2Ba 2Ca 3Cu 4O 10+δ have an expanded c of 33.8 and 40.2Å, and a lower T c of 90 and 114 K, respectively. The structure models for these decay products are proposed.

Fabrication of growth of textured YBCO thin film on very thin metallic substrate

A well textured YBCO thin film was grown on a 8000Åthick Pt tape with a STO buffer layer. In order to obtain the tape, a multilayer STO/Pt/STO/BaO was deposited on a YSZ(100) substrate and then the STO/Pt/STO layer was separated from the substrate by solving the BaO layer in water. A YBCO film was deposited on the STO/Pt/STO tape. It was confirmed by XRD 2θ-scan and φ-scan measurements that the single crystalline Pt layer, the single crystalline STO buffer layer, and the well textured c-axis normal YBCO film were grown. The R-T measurement showed T c was 79K.