Upper Critical Field Hc2 on c Axis of Ag/Bi-2223 Polycrystal Tape

The upper critical field H _c2, based on Ginzburg–Landau theory at medial magnetic fields, can be calculated from the relationship of the magnetization intensity M(H) versus magnetic field H if M is linear with ln₀ H. For Ag/Bi-2223 tapes, the measured M(H) was found to be linear with ln₀ H. In this case, the values of H _c2(T) may in principle be determined. To do this, we will meet another problem in that the obliquity of crystal planes is not equal for different grains in tapes, and values of grain-oblique angle will appear in the calculated H _c2. Obviously, for Ag/Bi-2223 tapes is an uncertain parameter and hard to determine. To avoid the effects of , we only study the H _c2 in the c axis direction and the projections of H onto the c axis is taken as the actual applied fields. Thus, the effects of grain-oblique angle may be counteracted when measuring critical current density J, if the external magnetic fields are applied to the tape along both the parallel to and perpendicular to the c axes directions (on the narrow surfaces of the tape). On medial fields (H = 0 – 3 T), the upper critical fields H _c2(T) on c axis for the Ag/Bi-2223 tape are obtained and fitted as ₀ H _c2(T) = 830 e ^–0.07T . The average slope d[₀ H _c2(T)]/dT – 8T/K is found to be as large as that of Bi-2212. On extrapolating the relation to T = 0 K, the value of ₀ H _c2(0) 800T. The coherence length _ab is determined from H _c2(c), and _ab (0) = 0.63 nm at T = 0 K.     

Simultaneous ac Susceptibility and Specific Heat Measurements of YBCO Ceramics

Simultaneous ac magnetic susceptibility and specific heat studies of YBCO ceramics permitted the correct correlation of the corresponding critical temperatures. The midpoint of the real component () and the maximum of the imaginary component () of ac susceptibility curves confirmed the T _c = 92 K critical temperature value determined by dc resistance measurements. The specific heat maximum's temperature (T _cc 93.6 K)—-taken as the calorimetric critical one—fell between T _c and T* (the pseudogap onset temperature). We suggest that T _cc signals the appearance of the first localized coherent pairs, while the temperature T _c marks their percolation. The existence of two different critical temperature values confirms the coexistence of partial coherent and coherent electron pairing phenomena in the region below T _cc of the superconducting phase diagram.     

Upper critical field of aD-wave superconductor stabilized by antiferromagnetic fluctuations

We have derived microscopic equations for the upper critical magnetic field of aD-wave superconductor stabilized by antiferromagnetic spin fluctuations. We present numerical results for the reduced fieldh _c2(t) as a function of reduced temperaturet when the magnetic field is along thez axis and also zero-order results when it is along thex ory axis. Two differentD-wave models are considered. The angular dependence ofH _c2 forT nearT _c is given as a function of polar angleϑ in thez−x orz−y plane.     

Superconductivities of Quasi-One-Dimensional and Quasi-Two-Dimensional Tight-Binding Electrons in Magnetic Field

The superconductivity of the tight binding electrons in a magnetic field is studied. We can treat both cases of quasi-one-dimension and quasi-two-dimension in the same manner. We study a various kind of anisotropic superconductivity with line nodes of the energy gap by taking attractive interaction between electrons in nearest sites along each axis. The magnetic field dependence of the transition temperature is calculated for a various pairing symmetry of superconductivity. When a magnetic field is applied in the conducting plane, the transition temperature is shown to increase as the magnetic field increases. In the strong magnetic field the eigenstates approach to those in the absence of hopping between planes, resulting in no orbital frustration.     

Convergence of Theory and Experiments on Electron-Lattice Interactions in Cuprates: An Overview of SILS

A brief overview of some of main highlights of the Symposium on Localised and Itinerant States (SILS) is presented.     

机械合金化合成高临界场Sm0.85Nd0.15FeAsO0.85F0.15超导体（英文）

通过采用机械合金化方法制备的高活性的粉体,可以高度可重复性地制备高质量的铁基超导材料Sm0.85Nd0.15FeAsO0.85F0.15.样品具有高临界温度Tc(约51 K)和高临界场Hc2(达到377 T).由WHH公式确定的Hc2显著高于常规固相方法制备样品的典型值(<200 T).高的临界磁场Hc2与样品微结构有很大关系.机械合金化处理的原始粉体包含大量的晶格畸变缺陷,在快速升温和低温退火制备的小晶粒陶瓷样品中这些缺陷会部分残留,因此形成有效的磁通钉扎,从而提高样品的临界场.     

Bose condensation of the localized pairs in two-band superconductors

Superconductor with two singularities inherent in high-T_c materials, low carrier density and overlapping of the energy bands on the Fermi surface, is considered. Provided T=0, the order parameter ‡_n and chemical potential Μ are determined in the mean-field approximation for the state with Bose condensation of the localized pairs (Μ < 0). The equation for the energy of the bound state ε_b is also obtained and the relationship ε_b = 2Μ is established. The method of functional integration concerning the two-band model is developed. On the basis of this method, the crossover between the Fermi picture and the Bose one of the elementary excitations is demonstrated for the system in the presence of the two-particle bound state. The expression for the temperature of Bose condensationT _k is obtained and the contribution of the residual interaction between bosons for the systems with arbitrary dimension is determined.     

Complementary Analysis of the Field Perturbation Theory of Superconductivity

We reexamine the Nambu–Gorkov perturbation theory of superconductivity. We suggest that any field perturbation theory of superconductivity should be based on the Bogoliubov–Valatin (BV) quasi-particles. We show that two such different fields (and two additional analogous fields) may be constructed on the basis of this suggestion. The Nambu field is only one of them. For the field that is different than Nambu’s, the coherence field, the interaction is given by means of two interaction vertices that are based on the Pauli matrices τ₁ and τ₃. Consequently, the Hartree integral for the off-diagonal pairing self-energy may be finite, and in some cases large. We interpret the results in terms of conventional superconductivity and also discuss briefly the possible implications to HTSC.     

Electronic and Phonon Contributions to the Pairing in the Cuprates

Thermal Difference Reflectance (TDR) Spectroscopy has been used to determinethe superconducting gap parameter for several of the superconducting cupratesover a wide range of energies, extending from the infrared (0.3 eV)to the ultraviolet (5.3 eV). A contribution to the pairing is found in eachcase from the phonons, and from an electronic excitation with energy thatranges from 1.6 eV and 2.3 eV for the different compounds attributed to thed ⁹–d ¹⁰ L charge-transfer excitation between Cu and O. In every case thereflectance ratio between the superconducting and normal state, Rs/Rnplotted as a function of photon energy can be well described using theEliashberg theory. The theory also predicted a characteristic shape for thelow energy part of such spectra due to the phonons. We report theobservation of this feature in measurements on films of Tl₂Ba₂CaCu₂O₈. Wediscuss the significance of the success of the Eliashberg theory inexplaining these results and successfully predicting new effects in thelight of the correlations that had been thought to invalidate such atheoretical approach.     

Origin of Superconductivity and Construction of Doping Curves for Cuprates on an Elasticity-Exchange Competition Model

Superconducting T _c is modeled over the doping range for the three prototype cuprate families (with respect to apical coordination). The model is based on a competition of elastic and exchange energies (ExEl model). Data for these energies are taken from direct observation such as T _c and the pseudo-gap T _p. An additional penalty for charge concentration in pairs is introduced for the low pair concentration region. These energies are internally calibrated. The rather different observed curve shapes and quantitative features in the doping curves of different materials can be related to the relative magnitude of T _c and T _p. Both linear and parabolic curve shapes near the T _c optimum can be explained on the ExEl model. The basis of the relevant phenomenology is indicated in bond ordering effects, for which new concepts, e.g. for resonating pair kernels, are developed. Accordingly, stripes of single holes are dimerized into pair strands through super-exchange. Conclusions are drawn about the limits of phenomenology for cuprates and other high T _c materials.     

Lattice and Polarizability Effects on Singlet and Triplet States in an Effective Hamiltonian for High-Tc Cuprates

From various theoretical approaches it seems to be rather well established that the Zhang–Rice (ZR) singlet state should play an important role in doped high-temperature superconducting (HTSC) oxides. Yet the conclusion that a one-band model is capable of describing many of the unusual physics observed in HTSC compounds has been questioned because the p _z states of the oxygen atoms in the planes as well as those of the apical oxygen atoms hybridize with the Cu d_3z2-_r2 orbitals and induce a mixing of the in-plane states with the out-of-plane bands. It is shown that electron–lattice coupling and polarizability effects modify the electronic structure of the effective Hamiltonian considered here.     

Evidence for Polaron Formation in High-Temperature Superconducting Cuprates: Experiment and Theory

We provide compelling support for the key role played by polaron formation to the physics of cuprate superconductors, which is evidenced above the pseudogap temperature T ^*, is the origin of the pseudogap phase itself and persists in the superconducting phase. Experimental and theoretical results are compared and show convincing agreement with each other.     

稳态热度场对转子系统临界转速的影响

汽轮机转子系统在频繁启停时，将产生较大的热应力和热变形，使转子系统的振动特性发生变化。采用有限单元法，利用热-结构-动力学耦合理论，对稳态温度场对某汽轮机转子系统的临界转速的影响进行了探讨。结果表明，随着转子系统轴向温差和径向温差的增大，热变形程度逐渐增大，系统振动的临界转速逐渐降低。稳态温度场对低阶临界转速的影响较大，而对于高阶临界转速的影响较小，特别对第一阶临界转速具有较大的影响，但对各阶临界转速的振型影响很小。在对转子系统的振动研究中，不应当忽略温度的影响。     

Superconductivity from Repulsion: Ginzburg–Landau Phenomenology of Cuprates

We develop Ginzburg–Landau approach to the problem of superconducting pairing with large momentum under screened Coulomb repulsion (η_K-pairing). Two-component order parameter arising in this scheme can be associated with charge and orbital current degrees of freedom of the relative motion of η_K-pair corresponding to superconducting and orbital antiferromagnetic ordered states, respectively. All basic features of the phase diagram of cuprate superconductors result directly from the η_K-pairing concept. PACS numbers: 74.20.-z, 74.20.De, 74.72.-h.     

Upper Critical Magnetic Field for a Modified Maltifiamental Nb3A1Wire

We measured the upper critical field Μ₀H_c2 for a modified Nb_3A.l sample above 10 K and evaluated the values using a different criterion. The values from theI ^c measurement, converting from the ac susceptibility and direct judging from the transition point on the magnetization curve, were close and larger than that extrapolated from the magnetization basedon the theory of Kramer and the critical state model.     

Ginzburg-Landau Analysis of the Critical Temperature and the Upper Critical Field for Three-Band Superconductors

Using the microscopic formalism of Eilenberger equations, a three-band Ginzburg-Landau theory for the intraband dirty limit and clean interband scattering case is derived. Within the framework of this three-band Ginzburg-Landau theory, expressions for the critical temperature T _c and the temperature dependence of the upper critical field H _c2 are obtained. Based on some special cases of the matrix of interaction constants, we demonstrate the influence of the sign of the interband interaction on the critical temperature and the upper critical field as compared with a two-band superconductor where it plays no role. We study also analytically and numerically the effect of its magnitude.     

Scaling of Hc2⊥(T) in Nb/CuMn Multilayers

Measurements of the perpendicular upper critical magnetic field H _c2(T) are reported for several Nb/CuMn multilayers. It is found that, despite the magnetic nature of the samples, the data for samples with low Mn percentage in the CuMn layers are simply described by the Werthamer–Helfand–Honenberg theory for conventional type-II superconductors, neglecting both Pauli spin paramagnetism and spin orbit impurity scattering. For high Mn concentration a different theoretical approach is needed.     

Charge Complementarity and Interlayer Coupling Between Charge Ordered Layers of Cuprates

The interlayer Coulomb coupling between charge ordered superlattices with charge antisymmetry has been studied. The 2D pair-condensate can be characterized by a charge ordered state with a checkerboard like pattern seen recently by scanning tunneling microscopy. Interlayer charge complementarity is essential for getting interlayer energy gain which is found to be proportional to the condensation energy. The correlation between the condensation energy and the critical temperature is studied within our charge ordered superlattice bilayer model in which pairing is supported by interlayer Coulomb energy gain (potential energy-driven superconductivity). The static c-axis dielectric constant _c is calculated for various cuprates and compared with the available experimental data.     

二氧化钒薄膜材料相变临界场强调控方法研究

采用无机溶胶-凝胶法并结合真空退火工艺在Al_2O_3陶瓷基片上制备了二氧化钒及其他价态钒氧化物共存的薄膜材料。研究了退火时间对VO_2、V_2O_5、V_6O_(13)、V_6O_(11)等价态成分和含量的影响以及对薄膜的相变临界温度和相变临界电场强度的影响。实验采用的退火时间分别为10h、8h及6h,得到的薄膜的相变临界电场强度分别为1.8 MV/m、0.8 MV/m及0.4 MV/m,相变场强降低75%以上,且随着电场强度相变点的降低,薄膜材料相变点前后电阻变化倍数也降低,但相变临界温度没有明显变化。研究结果表明:通过控制真空退火时间能够实现对电场强度相变点的有效调控,利用该方法可以研制不同相变临界场强的薄膜材料,以适应不同电磁环境的防护应用要求。