Evidence for Dielectric (e–h) and Superconducting (e–e) Pairing in Cuprates

It is demonstrated that anomalous behavior of cuprates can be described in a natural way by the model with partial dielectrization of conduction electron energy spectra (EES). In such a model, near the Fermi surface (FS), there are formed the fours of charged particles (FCP) rather than independent electron–hole (e–h)- and electron–electron (e–e)-pairs. Since the bonding energies are the same for Cooper (e–e)- and dielectric (e–h)-pairs, then the temperature of SC transition T _c due to the Bose-condensation of FCP can be much more than the temperature of Bose-condensation of Cooper pairs only. The dielectric (e–h)-transition is connected with structural phase transformation in the cuprate system, formation of stripe structure, etc. The model successfully describes the presence of maximum at T _c (x)-curve, T-dependence of pseudogap (PG) and SC-gap, effect of nonmagnetic impurities to T _c , STM and ARPES spectra and other properties of cuprates.     

The Superconductivity State Induced by Spin-Wave Exchange

We used the flow equation method to obtain from the electron–magnon Hamiltonian an effective electron–electron Hamiltonian with renormalized single-particle energy. In the Cooper channel the electron–electron interaction was found to be attractive. The energy scale of the superconducting critical temperature is determined by the spin splitting, which gives rise to the realistic energy scale.     

Theory of superconductivity. 3. 2D conduction bands for high Tc. Bose-Einstein condensation transition of the third order

A general theory of superconductivity is developed, starting with a BCS Hamiltonian in which the interaction strengths (V ₁₁,V ₂₂,V ₁₂) among and between electron (1) and hole (2) Cooper pairs are differentiated, and identifying electrons (holes) with positive (negative) masses as those Bloch electrons moving on the empty (filled) side of the Fermi surface. The supercondensate is shown to be composed of equal numbers of electron and hole ground (zero-momentum) Cooper pairs with charges ±2e and different masses. This picture of a neutral supercondensate naturally explains the London rigidity and the meta-stability of the supercurrent ring. It is proposed that for a compound conductor the supercondensate is formed between electron and hole Fermi energy sheets with the aid of optical phonons having momenta greater than the minimum distance (momentum) between the two sheets. The proposed model can account for the relatively short coherence lengths observed for the compound superconductors including intermetallic compound, organic, and cuprous superconductors. In particular, the model can explain why these compounds are type II superconductors in contrast with type I elemental superconductors whose condensate is mediated by acoustic phonons. A cuprous superconductor has 2D conduction bands due to its layered perovskite lattice structure. Excited (nonzero momentum) Cooper pairs (bound by the exchange of optical phonons) aboveT _c are shown to move like free bosons with the energy-momentum relation=1/2v_Fq. They undergo a Bose-Einstein condensation atT _c = 0.977v _F k _b ^–1 n ^1/2, wheren is the number density of the Cooper pairs. The relatively high value ofT _c (100 K) arises from the fact that the densityn is high:n ^1/2–1 10⁷ cm^–1. The phase transition is of the third order, and the heat capacity has a reversed lambda ()-like peak atT _c .     

Environmental contamination of amorphous Bi-Sr-Ca-Cu-O fibers

A study of the environmental contamination of amorphous YBa₂Cu₃O₇- fibers has been undertaken in order to determine the best method to handle these materials during the fabrication of superconducting wire. The fibers often need to be handled in organic solvents as part of the cleaning and manipulating process. In organics that are free of water, the fibers retain their mechanical strength with little carbon contamination or other ill effects. Water, however, causes premature crystallization and destroys the mechanical strength of the fibers.     

On the Bose-Einstein condensation of nonzero-momentum cooper pairs. A second-order phase transition

Based on the BCS Hamiltonian, the normal-to-super phase transition is investigated, approaching the critical temperatureT _c from the high-temperature side. Nonzero-momentum Cooper pairs, that is, pairs of electrons (holes) with antiparallel spins and nearly opposite momenta aboveT _c in the bulk limit, are shown to move like independent bosons with the energy vs. momentump relation=1/2v_F , where _F represents the Fermi velocity (1/2m* _F ² _FFermi energy). The system of free Cooper pairs undergoes a phase transition of the second order with the critical temperatureT _c given byk _B T _c=1/2(²³ _F ³ n/1.20257)^1/3 wheren is the number density of Cooper pairs. The ratio of the jump of the heat capacity, C, to the maximum heat capacity,C _s, is a universal constant: C/C _s=0.60874; this number is close to the universal constant 0.588 obtained by the finite-temperature BCS theory. The physical significance of these results is discussed, referring to the well-known BCS theory, which treats the many-Cooper-pair ground state exactly and the thermodynamic state belowT _c approximately. An explanation is proposed on the question why sodium should remain normal down to 0 K, based on the band structures with the hypothesis that the supercondensate composed of zero-momentum electron and hole Cooper pairs is electrically neutral.     

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.     

Anisotropic uniaxial pressure effects in YBa2Cu3O7?δ

We present measurements of the uniaxial pressure dependence ofT _c of untwinned YBa₂Cu₃O_7– crystals with various oxygen stoichiometries. For all samples investigated,T _c decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La_2–x Sr _x CuO₄ and YBa₂Cu₄O₈ systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.     

Measurement of Thermal Conductivity of TiO2 Thin Films Using 3ω Method

TiO₂ film has been used in many industrial components such as laser filters, protection mirrors, chemical sensors, and optical catalysts. Therefore, the thermal properties of TiO₂ thin films are important in, e.g., reducing the thermal conductivity of ceramic coatings in gas turbines and increasing the laser damage threshold of antireflection coatings. The thermal conductivity of four kinds of TiO₂ thin films, prepared by dc magnetron sputtering, was measured using the 3 method in the temperature range from 80 K to room temperature. The results showed that the thermal conductivity of TiO₂ thin films strongly depends on the thickness and the microstructure of the films. The films with smaller grain size and thinner thickness have smaller thermal conductivities.     

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.     

On structure and stability of (Hg, Mo)-1212 superconductors based on SAED and EDS analysis

A superconductor with nominal composition Hg_0.8Mo_0.2Sr₂Y_0.5Ca_0.5Cu₆₊ withT _c up to 96 K has been isolated and carefully examined by electron diffraction and X-ray energy dispersive spectrum analyses. Its structure can be attributed to space groupP4/mmm and lattice constantsa=3.82 å andc=11.91 å. Inhomogeneous distribution of the cations was observed, which is considered to be the key to the broadening ofT _c in theR-T curve. The effect of humid atmosphere on the stability of the sample has also been investigated.     

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.     

SHS/PHIP制备TiC-Al2O3/Fe复合材料的性能

对SHS/PHIP技术制备出的TiC-Al₂O₃/Fe复合材料的性能进行了测试和分析。结果表明,TiC-Al₂O₃/Fe复合材料具有良好的综合力学性能。材料具有很高的比刚度。金属Fe相的加入,较大地提高了材料的抗弯强度和断裂韧性。TiC-Al₂O₃复相陶瓷为典型的脆性断裂;随着Fe含量的增加,材料具有明显韧性断裂的特征。      

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.     

Physical Vacuum in Superconductors

Although experiments carried out by Jain et al. showed that the Cooper pairs obey the strong equivalence principle, The measurement of the Cooper pairs inertial mass by Tate et al. revealed an anomalous excess of mass. In the present paper, we interpret these experimental results in the framework of an electromagnetic model of dark energy for the superconductors’ vacuum. We argue that this physical vacuum is associated with a preferred frame. Ultimately from the conservation of energy for Cooper pairs, we derive a model for a variable vacuum speed of light in the superconductors physical vacuum in relation with a possible breaking of the weak equivalence principle for Cooper pairs.     

Structural properties and phase diagram of La2MO4+δ (M=Ni, Cu)

The phase diagram of La₂MO₄₊ (M=Ni, Cu) and the temperature dependence of orthorhombicity for several orthorhombic phases were investigated byin situ X-ray powder diffraction. A successive phase transition, tetragonalI4/mmm-[second-order] orthorhombicBmab-[ first-order] orthorhombicPccn, was observed in La₂NiO_4.00. The orthorhombicity of thePccn phase decreases on cooling in contrast to that of theBmab phase. The orthorhombic La₂NiO_4.15 (Fmmm) shows a weak temperature dependence of orthorhombicity and exhibits a first-order phase transition to the tetragonalI4/mmm phase on heating. As judged from the temperature dependence of orthorhombicity, superconducting La₂CuO_4.07, which results from the phase separation in the high-pressure oxygenated sample, isBmab, while La₂CuO_4.07 prepared by electrochemical oxidation at room temperature isFmmm.     

Fabrication of bulk superconductors YBa2Cu3?xTexO7?δ with highly oriented layer structure using the melt-texture growth process

We report on the successful fabrication of bulk samples satisfying the nominal composition YBa₂Cu_3–x Te _x O_7– and containing highly single-crystal-like texture. While this type of crystal growing process is not new, our process requires only the existence of spatial temperature gradient in some parts of the common tube furnace. The samples with tellurium were found to be much more stable and better superconductors than the Y-123 specimen. Large layer textures with cross-section 50×500m² were observed to grow inside the usual bulk sample. The critical currents were measured by the d.c. magnetization method and found to be easily greater than 6000 A cm^–2 for the samples with Te.     

SrCuO2 and related high-pressure phases

Treatment at high pressures and temperatures of 6 GPa and 1120 K, typically, have proved to be useful to find new layered complex cupric oxides. Among these, SrCuO₂ crystallizing in the infinite-layer structure may be considered to be the key material not only because of its simple composition and structure but also because of the fact that interesting homologous series of high-pressure phases like Sr _n+1Cu _n O_2n+1+ and Sr _n–1Cu _n+1O _2n converge to SrCuO₂ asn increases.     

X-Ray Diffraction Studies on LaBa2Cu3Ox Cuprates with Iron Substitution

LaBa₂Cu_3–y Fe _y O _x ceramic samples with y = 0.00–1.50 are synthesized by the solid-state reaction technique. Rietveld analysis for X-ray diffraction is performed on these iron-doped samples. A BaCuO₂ impurity phase and a ceramic cuprate phase coexist in each sample. An orthorhombic-to-tetragonal (OT) phase transition occurs in the doping range of 0.03y0.06, and a tetragonal-to-orthorhombic (TO) one occurs in the doping level of 0.10y0.25. There is a jump in the structural parameters due to the iron doping. The occupancy of oxygen at the O(4) site, which is in the La plane at z = 1/2, increases with increase in iron content. These results may relate to the iron preferential occupancy for the Cu(1) site at the lower doping level, and for Cu(2) sites at the higher doping level.