Chemical Control of Underdoped and Overdoped States in Y(Ba2 ? ySry)Cu3O6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba_2–y Sr _y )Cu₃O_{6 +} (0.9) compounds gives rise to high hole concentrations within both the CuO₂ planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.     

Magnetic and transport properties of PrBa2Cu4O8-δ prepared at 1 atm oxygen pressure

Measurements of physical properties were performed on the PrBa₂Cu₄O_8- sample prepared by the sol-gel method and sintered in I atm oxygen pressure. Instead of exhibiting superconductivity as for R Y or Gd in the RBa₂Cu₄O_8- compound. This compound is unstable, insulating but metallic-like at low temperatures. No superconductivity was observed down to 2 K. Since PrBa₂Cu₄O_8- can be re-written as Cu₂Ba₂PrCu₂O_8- which is a compound of two CuO₂-layers and two CuO-chains, the possibility of anomalous Pr antiferromagnetic ordering around 18 K observed from preliminary magnetic data and the effect of oxygen stoichiotmetry parameter on the metal-insulator transition are explored.     

Phonon anomalies in La2CuO4 + δ

The phonon dispersion of oxygen doped La₂CuO_{4 +} has been investigated by inelastic neutron scattering on a large single crystal with 0.02 (T_c = 31 K). Most of the phonons were found to be very similar to those of La_1.9Sr_0.1CuO₄. However, in measurements of a phonon branch which exhibits a pronounced anomaly in (La, Sr)₂CuO₄ a two-peak structure was observed: one series of peaks corresponds to the phonon frequencies of undoped La₂CuO₄, the other one to that of La_2–xSr_xCuO₄ with x 0.1. This finding is presumably associated with a phase separation into domains with low and high , respectively.     

Phase Relations in the Bi–(Pb)–Sr–Ca–Cu–Sc–O System

The phase relations in the Bi–(Pb)–Sr–Ca–Cu–Sc–O system were studied near Bi₂Sr₂CaCu₂O_{8 +} (Bi-2212) and (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +} (Bi-2223) between 850 and 930°C. The introduction of Sc led to the formation of a new compound Sr₂ScBiO₆, which coexisted with Bi-2212 and Bi-2223. Using crystallization from a peritectic melt at different cooling rates, we obtained Bi-2212 matrix composites containing finely dispersed Sr_1.9Ca_0.1ScBiO₆inclusions, with T _cattaining 89 K. The T _cof the Bi-2223–Sr_1.9Ca_0.1ScBiO₆superconducting ceramic prepared by solid-state sintering of a Bi–(Pb)–Sr–Ca–Cu–Sc–O precursor was 108.5 K.     

Metal-Insulator Transition and Cu K-Edge XANES Studies for Pr1.85Ce0.15CuO4+δ High-Tc Superconductors

Metal-insulator transition near oxygen content parameter –0.018 was observed for electron-doped Pr_1.85Ce_0.15CuO₄₊ (–0.0030.03) cuprates. Cu K-edge X-ray absorption near-edge structure (XANES) studies with nearly identical threshold edge energy E₀ of 8980.8 eV indicate a Cu formal valence smaller than 2 for all samples, which is consistent with the estimated Cu valence of 1.84 for 20.5 K superconductor Pr_1.85Ce_0.15CuO_3.997 and 1.91 for Pr_1.85Ce_0.15CuO_4.03 insulator. The XANES spectrum reflects the Cu 3d ⁿ character where low energy peak A ₁ reflects the 3d ¹⁰ configuration of Cu(I) oxidation state and A ₂ peak reflects the 3d¹⁰ ( for a oxygen ligand hole) configuration for Cu(II) oxidation state. The variation of energy separation E(A ₂–A ₁) is consistent with the observed metal-insulator transition, increases sharply from 2.42 eV for Pr_1.85Ce_0.15CuO_4.018 insulator to 2.74 eV for 15 K underdoped superconductor Pr_1.85Ce_0.15CuO_4.015.     

Substitution Effect of Sn for Cu in the Bi2Sr2CaCu2?xSnxO8+δ System

The Bi₂Sr₂CaCu₂O₈₊ system samples doped with Sn on Cu sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction and photoemission experiments have been performed. Suppression of superconductivity in the Sn-doped Bi₂Sr₂CaCu₂O₈₊ system has been observed. X-ray photoemission measurements show that Sn ion enters the lattice. Our results reveal that Sn ions are mostly in bivalent states.     

c-axis Superfluid Response and Quasiparticle Conductivity in Bi2Sr2CaCu2O8+δ and Bi2Sr2CuO6+δ

Josephson plasma resonance for underdoped Bi ₂ Sr ₂ CaCu ₂ O ₈₊ and Bi ₂ Sr ₂ CuO ₆₊ have been measured by sweeping the microwave, frequency continuously. The resonance enables us to determine the superfluid density and quasiparticle conductivity in the c-axis accurately. We show that the. superfluid response and the low energy excitations out of the condensate in the c-axis of these materials are very different from those in the ordinary Josephson multilayer tunnel junctions. The Josephson coupling energy in single layer Bi ₂ Sr ₂ CuO ₆₊ is more than 5000 times smaller than is predicted in interlayer tunneling model.     

In situ observation of crystallization from a superconductor melt of composition Bi1.9Sb0.1Sr2Ca2Cu3δ

The surface appearance of a pellet of composition Bi_1.9Sb_0.1Sr₂Ca₂Cu₃O, and crystallization from a melt of this composition, were examined by thermo-microscopy and thermal analysis. The Bi_1.9Sb_0.1Sr₂Ca₂Cu₃O mixture melted incongruently at 865 to 906°C. As the temperature dropped below 835°C, the melt solidified into plate-like crystals. Analytical results on a videotape indicated that the growth rate of these plate-like single crystals was 6.6×10^–3 mms^–1. Surface changes caused by heating, and the formation process of the plate-like single crystals with cooling, were clarified by the present investigation.     

Superconductivity in the tetragonal phase of La1.9Bi0.1CuO4+δ annealed under high oxygen pressure

We have investigated the relation between the crystal structure and superconductivity in La_1.9Bi_0.1CuO₄₊ , in which the phase separation observed in La₂CuO₄₊ is suppressed. A phase diagram in theT– plane is given for La_1.9Bi_0.1CuO₄₊ with excess oxygen. For very small values, the crystal structure is orthorhombic, and an orthorhombic-tetragonal phase transition occurs markedly at 0.03 in the measured temperature range between 13 and 293 K. Superconductivity is observed in the range of 0.04<<0.11. This is clear evidence thathigh-T _c superconductivity also appears in the tetragonal phase.     

Low-temperature synthesis of binary cuprates from hydroxide precursor

Binary cuprates of Sr₂CuO₃₊ and SrCuO₂ have been synthesized at low temperatures below 500 °C under various values of partial oxygen-pressure by thermal decomposition of hydroxide precursors, Sr₂Cu(OH)₆ and SrCu(OH)₄, respectively. The tetragonal Sr₂CuO₃₊ ( 0.3) is obtained by the heat-treatment at 400 ° C and 0.2 atm. The value of appears to be independent of and unchangeable. On the other hand, the orthorhombic Sr₂CuO₃₊ ( 0) is obtained by the heat-treatment at 400 °C in flowing gas of N₂. For 0 < < 0.2 atm, a mixture of the tetragonal and orthorhombic phases is obtained. The orthorhombic SrCuO₂ is obtained by the heat-treatment at 500 ° C in flowing gas of N₂, although the tetragonal SrCuO₂, namely, the so-called infinite-layer compound is not obtained.     

Stable and Metastable Phase Equilibria in the Bi2O3–BiPO4 System

The stable and metastable phase equilibria in the Bi₂O₃–P₂O₅ system were studied in the range 0–50 mol % P₂O₅ by differential thermal analysis and x-ray diffraction. The results were used to construct the corresponding phase diagrams. In the equilibrium state, the system contains one sillenite phase with Bi₂O₃ : P₂O₅ = 12 : 1 and four other bismuth phosphates (2 : 1, 3 : 1, 5 : 1, and 1 : 1). In a metastable state, resulting from solidification of metastable melts, there exist * solid solutions (0–25 mol % P₂O₅) based on the high-temperature form of Bi₂O₃. At lower temperatures, the * phase transforms eutectoidally into the metastable phase, which has the same structural basis as the high-temperature solid solutions. At room temperature, the phase exists in a narrow composition range around 15 mol % P₂O₅. At lower P₂O₅ contents, the * phase decomposes at 868 K by a monotectoid reaction to form a mixture of the metastable and phases. The phases 3Bi₂O₃ · P₂O₅ () and 2Bi₂O₃ · P₂O₅ (), melting incongruently at 1193 and 1223 K, respectively, appear in both the equilibrium and metastable phase diagrams.     

On the nano-size intergrowth of Sr4Fe6O13±δ and (Sr1?xLax)FeO3?δ phases in the mixed conductor Sr3.6La0.4Fe6Oz

The microstructures of mixed-conducting Sr₄Fe₆O_13±, and Sr_3.6La_0.4Fe₆O _z (nominal composition) have been studied by electron microscopy. The lanthanum free material shows a microstructure containing mainly Sr₄Fe₆O_13±, with some 5 vol% Sr_1–y La _y Fe₁₂O₁₉embedded as micron-size inclusions. The lanthanum-containing material revealed a significantly different microstructure consisting of 20 vol% micron-size Sr_1–y La _y Fe₁₂O₁₉embedded in 40 vol% (Sr_0.85La_0.15)FeO_3–tetragonal perovskite, and 40 vol% of plate-shaped nano-scale intergrowths between (Sr_1–x La _x )FeO_3–and Sr₄Fe₆O_13±phases. Domains with dimensions of 20–50 nm are observed in the tetragonal perovskite when viewed along its fourfold axis. From compositional analysis it is concluded that there is little or no solubility of La in the Sr₄Fe₆O_13±phase. The observed microstructure is important input in explaining the significant effect of La addition on the transport properties of Sr₄Fe₆O_13±materials.     

Piezoelectric and mechanical properties of ceria-doped lead zirconate titanate ceramics

Phases, microstructures and properties of lead-zirconate-titanate (PZT) ceramics with the compositions Pb(Zr_0.535– Ce Ti_0.465) O₃ where =0.0, 0.001, 0.01, 0.02 and 0.05 were studied. Rhombohedral and tetragonal phases were present at =0.0. The amount of the rhombohedral phase increased with increasing , and only the tetragonal phase was present for >0.001. Thec/a ratio of the tetragonal phase also increases with increasing . Particles of CeO₂ were found to be present in compositions with >0.01, indicating that the solubility of CeO₂ is less than 1a/o on the metals basis. The piezoelectric and electromechanical constants achieved maximum values for =0.001. The hardness increased monotonically with increasing . The modulus of rupture and the fracture toughness, however, went through a minimum and both stayed lower than their values for =0.     

63Cu NMR in the normal state of HgBa2Ca2Cu3O8+δ

We present a⁶³Cu NMR study of HgBa₂Ca₂Cu₃O₈₊ underdoped single crystals with T_c 115 K. While the uniform spin susceptibility decreases below T_o 370 K, relaxation rate measurements demonstrate the opening of a spingap at Q = (, ) below T* 230 K, the highest temperature reported so far. The characteristic energy of spin fluctuations is shown to be higher than in underdoped YBa₂Cu₃O_7–, and the analysis of the quadrupole and hyperfine couplings suggests that the in-plane Cu-O hybridization is also stronger. The T-dependence of T₁ is the same in the three CuO₂ planes which seems hardly compatible with the pure interlayer spin-pairing picture.     

Heat transfer in plane film formation

A mathematical model is obtained for the process of cooling with formation of a planar film. The solution obtained is verified experimentally.Notation mean axial velocity gradient - v_x current axial velocity - v_o initial polymer velocity - v₁ sampling velocity - K draw ratio - deformation rate tensor - x, y, z spatial coordinates - X, Y dimensionless coordinates - L() differential operator - T temperature - T_o initial temperature - T_c temperature of surrounding medium - dimensionless temperature - dimensionless temperature averaged over film thickness - thermal-diffusivity coefficient - 2_o initial film thickness - thermal conductivity - heat-transfer coefficient - f(X) distance function - Bi Biot criterion, Bi_o, Biot criterion calculated for initial film thickness - Gz* modified Graetz criterion - V dimensionless velocity - ₁, ₂, ₃ heat-transfer coefficients produced by radiation, free convection, and forced convection - v_c, _c mean velocity and film half-thickness in formation zone - T₁ calculated temperature value - T₂ experimental temperature value - l formation zone length Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 5, pp. 854–858, November, 1979.     

Optical Study of the Pseudogap State in Single Layer Bi2Sr2CuO6+δ

We present infrared reflectivity measurements of the ab-plane response of Pb- and La-doped Bi ₂ Sr ₂ CuO ₆₊ single crystals. A suppression of scattering rate is observed for La-doped samples below and above T _c , suggestive of a pseudogap. Compared with double-layer Bi ₂ Sr ₂ CaCu ₂ O ₈₊ , the suppression is weak and appears at lower frequencies. This may reflect a weak pairing interaction and/or smaller gap amplitude.     

Isotone policies for the value iteration method for Markov decision processes

Summary This paper considers the value iteration process for countable state discounted Markov decision processes and shows that under certain conditions there will exist anN-isotone sequence of optimal decision rules and value functions, whereN-isotonicity of a sequence of decision rules { _n }, n {1,2,...}=N requires that, for a specified partial order overK=UK(i) (K(i) being the feasible action space fori) then _n–1(i) _n (i), n2 and alli I, with a similar definition ofN-isotonic for the value functions {v _n },n1.

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Zusammenfassung Wir betrachten die Wertiteration bei diskontierten Markovschen Entscheidungsprozessen mit abzählbarem Zustandsraum. Wir zeigen, daß unter gewissen Bedingungen eineN-isotone Folge von optimalen Entscheidungsregeln und Wertfunktionen existiert.N-isoton heißt eine Folge von Entscheidungsregeln { _n },n {1,2,...}=N, dann, wenn für eine Halbordnung überK=UK(i) gilt _n–1 (i) _n–1(i) _n (i) für allen2 undi I. (K(i is die Menge der zulässigen Aktionen im Zustandi). Eine analoge Definition derN-Isotonie gilt für die Wertfunktionen {v_n},n1.