Magnetic hysteresis of the magnetoresistance and the critical current density in polycrystalline YBa2Cu3O7−δ–Ag superconductors

A systematic study of the magnetic hysteresis in transport properties of polycrystalline YBa₂Cu₃O_7−δ–Ag compounds has been made based on two kinds of measurements at 77 K and under applied magnetic fields up to 30 mT: critical current density J_c(B_a) and magnetoresistance R(B_a). The R(B_a) curves show a minimum in their decreasing branch occurring at B=B_min which was found to be both the excitation current I_ex and the maximum applied magnetic field B_am dependent. In addition, for a certain value of B_am>5 mT, we have observed that B_min increases with increasing I_ex and reaches a saturation value. The J_c(B_a) curves show a maximum in decreasing applied magnetic fields occurring at B=B_max. We have also found that B_max increases with increasing B_am and reaches a saturation value. The minimum in the R(B_a) and the maximum in J_c(B_a) curves were found to be related to the trapped flux within the grains. All the experimental results are discussed within the context of the flux dynamics and transport mechanisms in these high-T_c materials.     

煤系针状焦原料在成焦过程中的XRD结构分析

低QI(原生喹啉不溶物)含量的软沥青（SCTP）是制备煤系针状焦的优选原料,研究其在成焦过程中的结构变化有助于高品质针状焦的研制。基于样品的X射线衍射（XRD）数据,利用Smarsly团队开发的CarbX软件对其全谱拟合,定量出SCTP在不同炭化温度（400,500,600,800,1 000,1 200和1 400 ℃）下的微晶结构参数,进而在纳米尺度下研究SCTP的热致结构变化情况。结果表明,随炭化温度升高,微晶堆垛的石墨烯层大小L_a从初始沥青的10.3 Å逐步增大到1 400 ℃的47.9 Å,但在500 ℃前L_a增加缓慢,只有当温度超过800 ℃后,L_a才显著增大,这表明需要800 ℃以上的高温才能使交联石墨烯层内的原子重组,进而导致微晶长大。然而,石墨烯碳网的C—C键长l_cc受温度的影响很小,在1.41~1.42 Å范围内变化。由于SCTP在液相炭化成半焦过程中存在中间相转化,导致微晶堆垛高度L_c在500 ℃前逐步增大,在500 ℃时达到最大（L_c=31.1 Å）,随后由于半焦进一步热解缩聚,使L_c逐步减小,在1 000 ℃时达到最低点（L_c=15.4 Å）,超过1 000 ℃后又开始增大。与L_c的变化趋势相同,堆垛的石墨烯层数N从原始沥青的2.66层增加到500 ℃的约9.05层,随后减小到1 000 ℃的4.55层,超过1 000 ℃后又开始增大。由于500 ℃前样品仍处于沥青态,所以此阶段微晶的石墨烯层间距a₃都较大,约为3.50 Å。当在500 ℃变为半焦后,a₃迅速减小至3.44 Å。随后温度升高,a₃在1 000 ℃达到最小（a₃=3.39 Å）,1 000 ℃后又开始增大,这表明焦炭经历了收缩再膨胀过程。通过CarbX软件拟合样品的XRD数据,除了可得到样品炭微晶的主要尺寸（L_a,L_c,N,a₃）信息外,还可获得这些参数的分散性（k_a,k_c,σ₃,ε₃）以及堆垛的取向性（q）、均匀性（η）和无序碳含量（c_un）等信息,有利于深入了解样品的微观结构,有助于优质针状焦的生产。     

Thermodynamics of vortices in layered superconductors and the dependence of Tc on anisotropy

Data on multilayered superconductors such as YBa₂Cu₃O₇PrBa₂Cu₃O₇ show a strong dependence of T_c on anisotropy. In particular, the ratio T_cτ, where τ is the effective XY coupling constant, is found to vary by much more than the theoretical XY limit of 2.4. Layered superconductors allow for an additional anisotropy due to the core energy E_c of vortices perpendicular to the layers. It is shown that if E_c is large, such that the anisotropy is larger than exp(-E_cT_c), T_c is near a fluxon transition which is described by fluctuations of flux loops parallel to the layers. In the latter case T_cτ can vary by more than 2.4, accounting for the data.     

Influence of DC external magnetic field on AC transport current loss of HTS tape

We measured the AC transport current loss of Bi 2223 multifilament Ag-sheathed tape under DC external magnetic field of 0–0.2 T. There were discrepancies between the measured data and Norris' formula for elliptical model in the range of low value of I_p/I_c (I_p and I_c are peak of the AC transport current and critical current of the tape respectively), while without DC background field, the loss of the tape was close to Norris' formula. Theoretically speaking, even with the DC background field and decreased critical current the AC transport current loss of the tape follows Norris' formula which is derived from the Bean model. When DC background field is applied to the HTS tape, n value of the power law E–J characteristics decreases together with the decrease of J_c. Dependence of the AC transport current loss on the n value was analyzed by numerical calculation. The results show that the loss depends on the n value and that decrease of the n value is one of the causes of the discrepancies between the measured data and Norris' formula.     

Temperature dependence of the critical current in YBa2Cu3O7−δ and Bi2Sr2Ca1Cu2O8 Josephson junctions

We have studied the stationary Josephson effect on YBa₂Cu₃O_7−δ (T_c=90 K) and Bi₂Sr₂Ca₁Cu₂ O₈ (T_c=80 K and 87 K for two samples of different origin) ceramic based junctions. The temperature dependence of the critical current near T_c has been found as I_c≈(T_c-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi₂Sr₂Ca₁Cu₂O₈, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/k_BT_c3.5.     

Mean power transmission for one-dimensional random wave propagation using a cumulant expansion

In this paper the problem of the mean power transmission for one-dimensional wave propagation in a random medium is studied. We use a cumulant technique valid for small k₀L_c where measures the size of the fluctuations, L_c is the correlation length of the random wave number, and k₀ is the undisturbed wave number. We obtain an integral expression for the mean transmitted power. It shows exponential decay for large width, and linear decay for small width. The relevant scale to measure the width of the slab is ²k²₀∫^∞₀C(x)cos(2k₀x)dx where C(x) is the autocorrelation of the random wave number.     

Pinning model derived from resistive measurements on melt processed Bi2Sr2CaCu2O8

On melt processed samples of the 86 K superconductor Bi₂Sr₂CaCu₂O₈ we have performed resistive measurements in the low field limit B0.13 T and 40 KT77 K. The voltage drop is found to rise exponentially with current E ∝exp j/j₀, which is interpreted in terms of thermal activation of pinned flux lines. An activation energy U₀(T)90 meV is derived from the transition width j₀(T) and is related to a plausible core pinning interaction of flux lines with normal conducting precipitates. This reproduces the measured j_c(B, T) values in the whole regime investigated. We conclude that pinning centers must have a minimum size in order to control flux creep. Finally we demonstrate that conventional summation of the single site pinning forces cannot account for the observed macroscopic depinning current density.     

The angular intensity correlation functions C(1) and C(10) for the scattering of light from randomly rough dielectric and metal surfaces

We study the statistical properties of the scattering matrix S(q|k) for the problem of the scattering of light of frequency ω from a randomly rough one-dimensional surface, defined by the equation x₃=ζ(x₁), where the surface profile function ζ(x₁) constitutes a zero-mean, stationary, Gaussian random process. This is done by studying the effects of S(q|k) on the angular intensity correlation function C(q,k|q',k')=〈I(q|k)I(q'|k')〉-〈I(q|k)〉〈I(q'|k')〉, where the intensity I(q|k) is defined in terms of S(q|k) by I(q|k)=L^-1₁(ω/c)|S(q|k)|², with L₁ the length of the x₁ axis covered by the random surface. We focus our attention on the C⁽¹⁾ and C⁽¹⁰⁾ correlation functions, which are the contributions to C(q,k|q',k') proportional to δ(q-k-q'+k') and δ(q-k+q'-k'), respectively. The existence of both of these correlation functions is consistent with the amplitude of the scattered field obeying complex Gaussian statistics in the limit of a long surface and in the presence of weak surface roughness. We show that the deviation of the statistics of the scattering matrix from complex circular Gaussian statistics and the C⁽¹⁰⁾ correlation function are determined by exactly the same statistical moment of S(q|k). As the random surface becomes rougher, the amplitude of the scattered field no longer obeys complex Gaussian statistics but obeys complex circular Gaussian statistics instead. In this case the C⁽¹⁰⁾ correlation function should therefore vanish. This result is confirmed by numerical simulation calculations.     

N=2 6-dimensional supersymmetric E6 breaking

We study the N=2 supersymmetric E₆ models on the 6-dimensional space–time where the supersymmetry and gauge symmetry can be broken by the discrete symmetry. On the space–time M⁴×S¹/(Z₂×Z₂′)×S¹/(Z₂×Z₂′), for the zero modes, we obtain the 4-dimensional N=1 supersymmetric models with gauge groups SU(3)×SU(2)×SU(2)×U(1)², SU(4)×SU(2)×SU(2)×U(1), and SU(3)×SU(2)×U(1)³ with one extra pair of Higgs doublets from the vector multiplet. In addition, considering that the extra space manifold is the annulus A² and disc D², we list all the constraints on constructing the 4-dimensional N=1 supersymmetric SU(3)×SU(2)×U(1)³ models for the zero modes, and give the simplest model with Z₉ symmetry. We also comment on the extra gauge symmetry breaking and its generalization.     

Intergranular properties of YBCO and BSCCO ceramic superconductors at low fields

Isothermal low-field AC susceptibility measurements have been used to analyze the intergranular critical current density J_c(T) on sintered, non-oriented YBa₂Cu₃O_7−δ and Bi₂Sr₂CaCu₂O_8+δ ceramic samples at zero field. Below the critical temperature, potential variations, J_c(T) ≈ (1−t_j)^m with t_j = T/T_j, have been found, T_j being the onset of grain's coherence, but with different exponents, supporting that different mechanisms limit the intergranular J_c values. Moreover, the effect of texture has been also considered on Bi₂Sr₂CaCu₂O_8+δ ceramics grown by the laser floating zone method, which have stronger intergranular junctions. Their high-temperature behaviour is limited by intrinsic effects, while at low temperatures the quality of the junctions is the limiting factor. The temperature dependence of the χ′(h₀) extrapolation at zero filed has also been correlated with the evolution of the intergranular penetration depth, λ_J(T).     

Upper bound to the charge-transfer energy on a 2D Wigner lattice in high-Tc cuprates

Assuming that the energy of a 2D Wigner lattice, which has been frequently observed on the CuO₂ plane in layered high-T_c cuprates, is composed of Coulomb energy, kinetic energy, and charge-transfer energy which is necessary to transfer electrons or holes onto the CuO₂ plane (the magnitude of the energy is expected to be of the order of the work function of the compound constituting the plane from which the electrons are transferred, if the CuO₂ plane easily accepts the electrons or holes, as if the CuO₂ plane were crystal surface), we find in the mean-field approximation, that the total energy for a unit lattice, E₀, can be given by a function of the charge-transfer energy E_φ as E₀= (E_φ−E_φ⁰+O((E_φ−E_φ⁰)₂)(>0), where the value of E_φ⁰(1.5 in the ground state, for example) is independent of the lattice configuration. This relation implies that the upper bound to the charge-transfer energy is given by E_φ⁰. The smallness of E_φ⁰, compared with the work function of ordinary metals and alloys (with the work function around 4 eV), is necessary for the realization of the 2D Wigner lattice in many layered high-T_c cuprates.     

Estimation of Jc(B) dependence from self-field alternating current (AC) losses measured on Bi-2223/Ag tapes

Transport AC losses measured in self-field conditions on multifilamentary Bi-2223 tapes are often found to be lower than those calculated within the framework of the critical state model for a bulk wire with elliptical cross section, though generally higher than predicted for a strip. This effect is sometimes ascribed to the non-ideal geometry of the tapes, which does not exactly reproduce either shape. Here we propose an alternative explanation assuming that the critical current density of superconducting material depends on magnetic field. In practice, we analyzed the AC loss curve and deduced different I_c values for the individual data points, using the standard Norris equation for elliptical conductor. This gives the relation between ‘calculated' I_c and the self-field associated to AC transport current, which can be regarded as an alternative way to qualify the dependence of J_c on magnetic field. Important is that this procedure covers the range of fields below the self-field at I_c where the measurement in background DC field can not be used to determine J_c(B).     

Electrical properties of pure and Li2O-doped CuO/Fe2O3 system precalcined at different temperatures

The electrical properties of pure and Li₂O-doped CuO/Fe₂O₃ solids were investigated. Pure and variously doped solids were subjected to thermal treatment at 1073–1273 K and the amount of dopant was varied between 0.84 to 3.36 mol%. The effect of precalcination temperature and amount of Li₂O added on the electrical conductivity σ, activation energy E_a and dielectric constant * were studied. The variation of ′ and ″ as a function of frequency for pure and variously doped solids precalcined at different temperatures was also investigated. The results obtained were discussed.     

The thermodynamics of purely elastic scattering theories and conformal perturbation theory

We discuss the thermodynamic Bethe ansatz, and explain how it allows one to reduce the infinite-volume thermodynamics of a (1 + 1)-dimensional purely elastic scattering theory to the solution of a set of integral equations for the one-particle excitation energies. The free energy at zero chemical potential(s) and temperature T is related to the ground state energy E₀(R) of the theory on a cylinder of circumference R = 1/T. E₀(R) determines properties of the CFT describing the UV limit of the given massive theory. These include the central charge (which we investigated in earlier work), the scaling dimension d of the conformal field whose perturbation leads to the massive theory, the coefficients in the conformal perturbation theory (CPT) expansion of E₀(R) in powers of R^2−d, and the bulk term in the CPT calculation of the ground-state energy. We determine the bulk term analytically, and obtain numerically the first six coefficients in the expansion of E₀(R) for many purely elastic scattering theories, including the scaling limit of the T = T_c Ising model in a magnetic field. The perfect agreement with (more limited) direct CPT results provides further strong support for the identification of these theories as specific perturbed CFTs. We suggest that the singularities of E₀(R), the first of which is responsible for the finite radius of convergence of CPT, are square-root branch points and related to the zeros of the partition function of the corresponding lattice model.     

Anisotropic dielectric properties of PbYb1/2Ta1/2O3 single crystals

PbYb_1/2Ta_1/2O₃ single crystals were obtained for the first time. They were grown by the flux method. The PbOPbF₂B₂O₃ system was used as a solvent. Dielectric investigations were carried out in 1 0 0_c, 1 1 0_c and 1 1 1_c pseudocubic directions. These studies pointed to anisotropy of dielectric properties. Frequency-independent ε′(T) and ε″(T) maxima related to the antiferroelectric–paraelectric (AFE—PE) phase transition are observed for all directions at 562 K. The frequency-dependent ε′(T) and ε″(T) maxima near 400 K related to the ferroelectric (FE)–AFE phase transition are observed only in 1 1 1_c direction. The hysteresis loops were observed in this direction only. These results point that ferroelectric relaxor properties appear only in 1 1 1_c direction. We propose to consider the ferroelectric phase as ferrielectric one.     

Asymmetric double-barrier S–I1–N–I2–S Josephson heterojunctions: experiment and theory

Double-barrier highly asymmetric Nb–Al oxide–Al–Nb oxide–Nb structures with reproducible characteristics were fabricated. The heterocontacts with the middle Al layer thickness ranging from 4 to 6 nm exhibited a well-defined d.c. Josephson supercurrent I_c at 4.2 K and characteristic voltages V_c=I_cR_N (R_N is the normal resistance, V_c defines the response time of the junction) from 0.3 to 0.4 mV. Two prominent features in the quasiparticle current–voltage curves have been observed: a so-called ‘knee' in the energy-gap region and an additional (to the linear voltage dependence) current at higher biases. They are discussed within a simple Landauer–Büttiker scattering approach to the phase-coherent quasiparticle transport in a quasiballistic S–I₁–N–I₂–S heterostructure with an extremely great difference between the barrier transparencies.     

Surface diffusion of dysprosium on the W(1 1 1) facet

Diffusion of dysprosium on the (1 1 1) facet of a tungsten micromonocrystal was investigated by means of spectral analysis of field emission current fluctuations. The experimental spectral density functions of the current fluctuations were analysed by using Gesley and Swanson’s theoretical spectral density function, which enables to determine the surface diffusion coefficient D for dysprosium. Derived from the temperature dependence of D, the diffusion activation energy E is presented for some Dy coverages θ_(1 1 1). In the temperature range 400–600 K, the E first drops from 1.25 eV per atom at θ₍₁₁₁₎≈0.25 ML to 0.48 eV per atom at θ₍₁₁₁₎≈1 ML (corresponding to the minimum of the work function of the system), then increases to 1.03 eV per atom at θ₍₁₁₁₎≈1.3 ML. The results are discussed from the aspects of the substrate structure and interaction in the adsorbed layer.     

The structure of the surface compound CrN formed by cosegregation on a Fe–15%Cr–N(100) single crystal surface

Cosegregation is known to cause the formation of two-dimensional chemical compounds (surface compounds) which can be epitaxed to substrate surfaces of a suitable structure. In the present work the cosegregation-induced formation of the CrN surface compound on nitrided Fe–15%Cr–N(100) single crystal surfaces was studied by means of Auger electron spectroscopy and low-energy electron diffraction. Intensity versus energy spectra (I(E)) were measured and analysed fully dynamically to investigate the structural details of the CrN surface compound. It is found that nitrogen is segregated to the surface forming the sample's top layer and substantial amounts of chromium are cosegregated with nitrogen. Nitrogen atoms reside in four-fold symmetric hollow sites about 0.1 Å above the metallic substrate. There is a huge relative expansion of the distance between the first and second metal atom layers (Δd₁₂/d₀≈26%), while the distances between deeper layers are almost bulk-like. The small distance between the nitrogen and the top metal layer as well as the huge layer expansion Δd₁₂/d₀ are in agreement with results found for N/Cr(100).     

Optical absorption, disorder and the Anderson transition

The optical gap, E_g, of amorphous indium-oxide films is measured as a function of static disorder near the metal-insulator transition. On the insulating side of the transition the optical gap obeys a scaling relation, ΔE_g = -E_*Δg where E_* is of the order of the Fermi energy of the given sample and g≡K_Fl. These results are ascribed to the continuous shift of the mobility-edge in the conduction band with disorder.     

A new 9 K superconducting organic salt composed of the bis (ethylenedithio) tetrathiafulvalene (ET) electron-donor molecule and the tetrakis (trifluoromethyl) cuprate (III) anion, [Cu(CF3)4]

We report the discovery of a second, higher-T_c superconducting organic charge-transfer salt derived from the electron-donor molecule BEDT-TTF (or ET), the novel organometallic anion [Cu(CF₃)₄]⁻, and the neutral solvent molecule 1, 1, 2-trichloroethane (TCE). We have very recently reported that this charge-transfer system yields a new superconducting phase salt, _L- (ET)₂Cu(CF₃)₄·TCE, with inductive onset T_crmc=4.0 K at ambient pressure. This phase salt ( denotes a particular packing arrangement of the ET organic donor molecules) is electrocrystallized in the habit of hexagonal plates. Crystals possessing a needle-like habit electrocrystallize simultaneously with these plates, and we find these needles to be a distinctly different superconducting phase with diamagnetic onset T_c=9.2±0.1 K at ambient pressure. On the basis of our experiments, we denote this new superconducting phase as _H−(ET)₂Cu(CF₃)₄·(TCE)_x,X<1.