New high-pressure Sr-Ca-Cu-O compounds and their superconducting properties

It is shown for (Sr, Ca)_1–y CuO_2.00+z (nominal composition) superconductors fabricated under a high pressure of 5 GPa and at 930°C that, as z increases, the Meissner fraction increases from a few percent to 20% and then starts to decrease. In the X-ray powder diffraction pattern for the sample with the maximum Meissner fraction, it is obvious that the infinite-layer structure has been decomposed and a new structure becomes the major phase. It appears that the new structure is the 0223 structure which consists of rock-salt and the infinite-layer type blocks. This material is likely to be a new superconducting cuprate.     

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.     

Thermoelectric Power Studies on YBa2?xCaxCu3O7?δ System

The temperature dependence of the thermoelectric power of calcium substituted YBa_2–x Ca _x Cu₃O_7– pellets with 0x1.5 is presented between 60 K and 300 K. A metal–insulator transition was reported earlier by us in this system and was attributed to the ionic size-dependent localization effect. While the sign of thermoelectric power of all the calcium substituted samples was found to be positive, its magnitude increases significantly with calcium content in YBCO. The normal state thermoelectric power data of substituted YBa_2–x Ca _x Cu₃O_7– (0x1.5) are discussed in light of a two-band model originally proposed by Gottwick et al. for heavy fermion systems and later modified by Forro et al.     

Heat Capacity Measurement of Oxygen-Dissolved V–Ti Alloys1

The heat capacities of (V_1–y Ti _y ) O _x alloys (x=0.13 to 0.18, y=0.06 to 0.23) were measured in the temperature range from 380 to 900K with a differential scanning calorimeter. A -type peak corresponding to the order–disorder phase transition was observed in the heat capacity curves of each (V_1–y Ti _y ) O _x alloy (x0.18, y0.09). The baseline of the measured heat capacity was estimated with the harmonic term and the anharmonic term of the lattice vibration. The entropy changes at the phase transition were determined by subtracting the baseline mentioned above from the measured heat capacities. The experimental entropy changes were in good agreement with those calculated theoretically, based on the arrangement of oxygen and the limited coordination number of oxygen around vanadium. There were no peaks in the heat capacity curves for the samples with compositions y0.16. Introduction of titanium, which has a larger ionic radius than that of vanadium, may disturb the ordering of oxygen around the metal.     

Heat Inputs to Sub-mK Temperature Cryostats and Experiments from γ-Radiation and Cosmic Ray Muons

We measured the spectrum of energies deposited by -radiation, emanating from radioactive materials in the laboratory that houses our mK cryostat, and by cosmic ray muons. This allows us to quantify the heat input that adversely affects the lowest temperature accessible in sub-mK experiments. We use our nuclear stage, stage plate and experimental cell as a prototype model system, and calculate the power deposited due to low energy (below 2.65 MeV) background radiation quanta (~20 pW). This is significantly less than the power (~120 pW) deposited in the nuclear and experimental stages by muons. Installation of a 5 cm thick lead wall around the cryostat reduced the energy due to the flux of quanta by a factor of ~10 to ~2 pW, and the number of quanta by a factor of ~20. The lower energy, soft cosmic ray component was also affected by introducing the same thickness of lead, reducing the overall count of cosmic ray derived particles by ~15% and the heat leak to ~100 pW.     

Thermoelectric power measurements of YBa2Cu3O7?δ up to 950°C and their application to test the band structure near EF

Four-probe resistivity () and thermoelectric power (TEP) measurements were carried out on samples of YBa₂Cu₃O_7– up to 950°C, in air and in flowing oxygen at 1 bar. Below 700 K the TEP is small and increases rapidly above it, reaching, at 1200 K, +140V/K in air and +120V/K in oxygen. At the changeover temperature (700 K) the slope of log vs.T changes abruptly. These results are interpreted in terms of a model of transport of carriers in a narrow band, which is full for = 1 and half-filled for = 0. Possible origins for such a narrow band are discussed in detail.     

Novel methodology to investigate superconducting dielectric resonators

An analytical method is presented for investigating the resonant behavior of a c-axis oriented YBa₂Cu₂O_7–(0) thin film on a resonator composed of LaAlO₃ (001). The concept of the negative dielectric medium for a superconductor is introduced within the framework of the two-fluid model, which permits us to treat a superconductor as any other penetrable materials so that only its electromagnetics are concerned. A conformai transformation is further suggested to map the original open boundary-value problem to a closed one. This not only makes the original problem readily solvable by using the variational technique, but is also a powerful tool for analyzing some kinds of problems such as the propagation characteristics of the superconducting microstripe and coplanar waveguide structures.     

Effective Łojasiewicz inequalities in semialgebraic geometry

The main result of this paper can be stated as follows: letV ⁿ be a compact semialgebraic set given by a boolean combination of inequalities involving only polynomials whose number and degrees are bounded by someD > 1. LetF, G[X₁,, X_n] be polynomials with degF, degG D inducing onV continuous semialgebraic functionsf, g:VR. Assume that the zeros off are contained in the zeros ofg. Then the following effective ojasiewicz inequality is true: there exists an universal constantc ₁ and a positive constantc ₂ (depending onV, f,g) such that for allxV. This result is generalized to arbitrary given compact semialgebraic setsV and arbitrary continuous functionsf,g:V . An effective global ojasiewicz inequality on the minimal distance of solutions of polynomial inequalities systems and an effective Finiteness Theorem (with admissible complexity bounds) for open and closed semialgebraic sets are derived.     

Isochoric p-ρ-T Measurements on 1,1-Difluoroethane (R152a) from 158 to 400 K and 1,1,1-Trifluoroethane (R143a) from 166 to 400 K at Pressures to 35 MPa

The p--T relationships have been measured for 1,1-difluoroethane (R152a) and 1,1,1-trifluoroethane (R143a) by an isochoric method with gravimetric determinations of the amount of substance. Temperatures ranged from 158 to 400 K for R152a and from 166 to 400 K for R143a, while pressures were up to 35 MPa. Measurements were conducted on compressed liquid samples. Determinations of saturated liquid densities were made by extrapolating each isochore to the vapor pressure, and determining the temperature and density at the intersection. Published p--T data are in good agreement with this study. For the p--T apparatus, the uncertainty of the temperature is ±0.03 K, and for pressure it is ±0.01% at p>3 MPa and ±0.05% at p<3 MPa. The principal source of uncertainty is the cell volume (28.5 cm³), which has a standard uncertainty of ±0.003 cm³. When all components of experimental uncertainty are considered, the expanded relative uncertainty (with a coverage factor k=2 and thus a two-standard deviation estimate) of the density measurements is estimated to be ±0.05%.     

Scientific aspects of deformability of aluminum alloys during extrusion

The value of the limiting flow rate (V _max) indicated by formation of tearing on the surface of extruded products has been selected as an indicator of deformability during extrusion of aluminum alloys. Formation of tearing is due to embrittlement of grain boundaries occurring when the alloy temperature rises and reaches subsolidus temperatures. The temperature rise to subsolidus temperatures occurs because of deformation heat (adiabatic heating) which depends on the deformation resistance of the alloy for the given temperature/strain rate conditions present during the extrusion. The deformation resistance is a fundamental feature of the alloy determining theV _max. The amount of alloying elements in solid solution have a major influence on the limiting flow rate. Second phase particles and grain and dendrite structures are not as important as the amount of alloying elements in solid solution. Scientific principles for the optimum alloying and heat-treatment of aluminum alloys intended for extrusion are presented. The second part of the article discusses the potential for using a heterogenization anneal for aluminum alloy ingots before extrusion, allowing significant increases in theV _max.     

Study on the Physical and Electrical Properties of Bi2 ? δZnδSr2Ca2Cu3O10 + y Glass-Ceramic Superconductors

We have fabricated a series of glass-ceramic (Bi_{2 –} Zn) Sr₂Ca₂Cu₃O_{10 + y} , where = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, and investigated the effect of Zn ions on the glass formation, crystallization, thermal, electrical, and on the magnetic properties of the BSCCO-2223 superconductor system. The structural symmetry was found to be tetragonal in all the substitution levels. The best electrical performance was obtained from the = 0 sample, the T _c and T _zero was obtained at 110 K and 107 K, respectively. The J _c values of the samples were determined using the magnetization hysteresis and Bean's model. The crystallization kinetics were investigated using nonisothermal models of Augis–Bennett. The calculated activation energy, E _a, of the system was found to be in the range of 258–336 kJ/mol.     

The origin of melt-texture crystal growth process in a simple tube furnace

The melt-texture growth (MTG) method has been shown to be effective in fabricating high-T _csuperconducting bulk samples with highly oriented layers of single crystals. The critical current densityJ _cof the typical samples are enhanced significantly. Furthermore, making use of the nonzero spatial temperature gradient at certain locations of a common tube furnace, we fabricated Y-123 bulk samples with various dopants allowingJ _cup to 3.0×10⁴ A cm^–2 in our recent investigations. We followed up our project by analyzing the relations among the crucial parameters involved in the modified version of the MTG process. These parameters include the temperature gradient, the time rate of change of temperature, the growth front of the crystal, etc. Our study indicates that some of the empty space regions that appear between layers are only shallow gap lines. True layers of single crystals do exist, and we present an explanation for the growth of crystal layers along special directions as observed.     

Aluminum. II. Derivation of C v0from C p and comparison to C v0 calculated from anharmonic models

The specific heat at constant pressure, C _p, of aluminum measured by Ditmars, Plint, and Shukla has been reduced to the volume V ₀ appropriate for 0 K employing the Murnaghan equation. The C _v0 thus obtained is compared with the theoretical C _v0 calculated in the harmonic and the lowest-order anharmonic approximation from three different pseudopotentials (Harrison, Ashcroft, and Dagens-Rasolt-Taylor) as well as a phenomenological Morse potential. The higher-order ( ⁴) anharmonic contributions are calculated from the same nearest-neighbor Morse potential as in the lowest-order anharmonic theory. The role of the vacancy and the higher-order anharmonic contributions to C _v0 has been examined and we conclude that the ⁴ contributions to C _v0 are much smaller than the vacancy contribution. After removal of the vacancy contribution, the reduced C _v0 is found to be in excellent agreement with the Ashcroft and Harrison pseudopotentials as well as the Morse potential including the ² and ⁴ contributions to C _v0.     

Extraordinary behaviour of the Y1?xPrxBa2Cu3O7?δ system

Investigations of Y_1–x M _x Ba₂Cu₃O_7– (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofT _c withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals reveal aT ² dependence of the cotangent of the Hall angle in the normal state and a negative Hall anomaly belowT _c in the superconducting state, in agreement with recent reports. Our research shows that the depth, , of the negative Hall signal scales withT/T _c and that the maximum value of decreases linearly withx and vanishes atx0.24. Magnetoresistance measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals indicate that the irreversibility lineH(T ^*) obeys a universal scaling relation characterized by anm=3/2 power law nearT _c, with a crossover to a more rapid temperature dependence of belowT/T _c 0.6, similar to that observed for polycrystalline specimens.     

Role of electron-phonon interaction in lattice dynamics and superconductivity of oxide spinel LiTi2O4

The lattice dynamics of an oxide spinel LiTi₂O₄ is studied by taking account of the electron-phonon (EP) interaction derived on the basis of the realistic tight-binding bands fitted to the first-principles bands. Due to the characteristic dependences of the EP interaction on wavevectors and vibrational modes, a remarkable frequency renormalization of the O vibrational modes, which hybridize with the Ti vibrational modes, is obtained over a wide region of the Brillouin zone. The overall features of the calculated phonon density of states are in agreement with those observed by the inelastic neutron scattering measurements. By using the EP interaction and the renormalized phonon frequencies we have calculated the EP spectral function ² F(). The superconducting transition temperature, gap function, and tunneling spectra are calculated by solving the Éliashberg equation. The results agree well with the observations.     

Magnetic torque of κ-(BEDT-TTF)2Cu(NCS)2 single crystals

The magnetic torque of-(BEDT-TFF)₂Cu(NCS)₂ was measured as a function of field direction with respect to thea ^*-axis under constant magnetic fields,H, up to 8 kOe in the temperature range from 1.3 to 8 K. A sharp cusp, C₁, in the irreversible region was found at _c1 near theHbc-plane between 1.3 and 7 K. In addition, extra cusps, C₂ and C₃, were observed at _c2 and _c3, respectively, between 2.5 and 6 K. At each temperature, the perpendicular component ofH giving each cusp is kept constant as i.e., cusps C₁, C₂, and C₃ are ruled by the characteristic field perpendicular to thebc-planeH _cp1,H _cp2, andH _cp3, respectively. These behaviors are almost the same as those we found in the oxide superconductor Bi₂Sr₂CaCu₂O₈. These results suggest that the cusps are intrinsic for irreversible vortex states of these layered superconductors.     

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.     

Phonon Raman scattering of R2CuO4 (R = Pr, Nd, Sm, and Gd)

For Gd₂CuO₄, a new symmetry-forbidden phonon for the tetragonal T structure is observed. This implies that oxygen in the CuO₂ plane is locally distorted along the CuO₂ plane. Such distortion has never been observed for other T-type superconductors, which show superconductivity by Ce doping. The disappearance of superconductivity for Gd can be correlated with the appearance of the distortion due to oxygen in the CuO₂ plane.