Temperature and doping dependent flat-band superconductivity on the Lieb-lattice

We consider the superconducting properties of Lieb lattice, which produces a flat-band energy spectrum in the normal state under the strong electron–electron correlation. Firstly, we show the hole-doping dependent superconducting order amplitude with various electron–electron interaction strengths in the zero-temperature limit. Secondly, we obtain the superfluid weight and Berezinskii–Kosterlitz–Thouless(BKT) transition temperature with a lightly doping level. The large ratio between the gap-opening temperature and BKT transition temperature shows similar behavior to the pseudogap state in high-T_c superconductors. The BKT transition temperature versus doping level exhibits a dome-like shape in resemblance to the superconducting dome observed in the high-T_c superconductors. However, unlike the exponential dependence of T_c on the electron–electron interaction strength in the conventional high-T_c superconductors, the BKT transition temperature for a flat band system depends linearly on the electron–electron interaction strength. We also show the doping-dependent superconductivity on a lattice with the staggered hoping parameter in the end. Our predictions are amenable to verification in the ultracold atoms experiment and promote the understanding of the anomalous behavior of the superfluid weight in the high-T_c superconductors.     

Realization and properties of ramp-type YBa2Cu3O7−δ/Au/Nb junctions

All-thin-film ramp type Josephson junctions between YBa₂Cu₃O_7−δ and Nb have been fabricated. This procedure allows connections between high-T_c and low-T_c superconductors at different crystal sides of the high-T_c superconductor on one chip, which is of great interest for novel phase devices. A thin Au layer is incorporated as a chemical barrier to avoid oxygen transfer from the YBa₂Cu₃O_7−δ to the Nb. Critical current densities up to 600 A/cm² are obtained at T=4.2 K, with typical R_nA values of 0.8 μΩ cm². The variation of the magnetic field dependence of the critical current with the angle between the junction barrier and the YBa₂Cu₃O_7−δ crystal axes is explained by considering a predominant d_x²−y² order parameter symmetry of the YBa₂Cu₃O_7−δ. The successful fabrication of these junctions allows the implementation of novel superconducting electronics, such as complementary Josephson circuitry or proposed qubit concepts, using the unconventional order parameter symmetry of the high-T_c superconductor.     

Dressed Josephson junction network models and low-field magnetic response of high-Tc granular superconductors

A bare three-dimensional model, in which grains are reduced to points, cannot fully account for the magnetic properties of granular superconductors. A dressed version of these network models is proposed to discuss the quantitative link between the low-field magnetic response of high-T_c superconducting granular samples and the characteristic properties of Josephson junction network models. By means of dressed models, the temperature dependence of the d.c. field-cooled susceptibility of a simple three-dimensional granular system, consisting of eight grains in a cubic arrangement, is studied.     

Statistical modeling of inductances in high-Tc superconductor electronic structures

In the design of electronic applications, e.g. rapid single flux quantum (RSFQ) circuits, with high-T_c superconductors there is an obvious necessity of having precise knowledge of loop inductances. In general, these inductances have to be computed numerically due to the small sizes of the structures and the large contribution of the kinetic inductance. The present work is based on the analysis of a large number of inductances which have been calculated numerically for several high-T_c superconducting loop configurations. By means of statistical modeling, analytical formulae, which are customized for the considered type of structure, are derived. They can efficiently be used to simplify and accelerate the process of layout design in high-T_c RSFQ electronics.     

Superconductivity, Ca content and oxygen deficiency of Ga(Sr,Ca)2(Yb,Ca)Cu2O7

Superconducting transition temperature (T_c), Ca content and oxygen deficiency are studied on GaSr_1.8Ca_0.2Yb_1−xCa_xCu₂O₇ (x≤0.35). Superconducting samples with T_c=52 K are prepared after the annealing at 20 MPa of oxygen. The T_c is reduced through a slight oxygen loss accompanied by annealing in air above 650°C. The oxygen loss suggests the presence of short Cu–O chains in the GaO₄ slab. The formal valence of planar Cu required for the appearance of superconductivity depends on oxygen and Ca contents. The critical formal Cu valences are 2.105 and 2.125 for the samples annealed in air at 600°C and at 835°C, respectively. The values are higher than those of usual high-T_c superconductors. This can be explained by a high concentration of localized holes in the CuO₅ slab.     

Operation of high-Tc SFQ devices at near liquid nitrogen temperature

As the operating temperature of the SFQ logic circuits gets higher by using high-T_c superconductors, the effect of noise on switching a Josephson junction to the voltage state becomes more important. In this paper, we report our work on high-T_c SFQ RS flip-flop which was made with YBCO thin film deposited on a SrTiO₃ bi-crystal. The circuit operated correctly at 71 K over the 200 computer-generated clock cycles without making errors, where a reset or a set operation was made over one clock cycle. Good agreement between the measured data and the calculation based on the thermal activation theory was obtained. The effective noise temperature used to fit the data was much higher than the physical temperature. This could be due to the instrument noise. Improvement in the measurement set-up might reduce the effective noise temperature. Also our measurement results indicate that the elevation of the operating temperature near the liquid nitrogen temperature may not affect the margin of the circuit.     

Multi-stable static states of Bi-based superconducting composites and current instabilities at various operating temperatures

Static thermal and electric states before current instability in the current-carrying conductor like Bi-based superconducting composite are theoretically studied under the assumption that the critical current density of a superconductor and matrix resistivity are the non-linear functions of the temperature at fixed applied magnetic field. The simulation was made for the conduction-cooling conditions at different operating temperatures. The proposed analysis was based on the investigation of the non-isothermal voltage–current characteristics of composite superconductors. It is shown that they may have many-valued stable and unstable branches appearing in accordance with the non-trivial variation of the differential resistivity with increasing temperature. These states, first of all, are due to the temperature change of ∂J_c/∂T and are accompanied with the jump-like current-sharing mechanism. The parameters of the current instability onset as a function of operating temperature are numerically derived accounting for the additional stable branches of the voltage–current characteristics. In particular, it is revealed the existence of the static states when Bi-based composite superconductors may have a stable current distribution in the temperature range increasing up to the critical temperature of a superconductor without redistribution of all transport current into the matrix. The peculiarities of these phenomena are discussed.     

The effect of nonmagnetic doping on high-Tc superconductivity

In this paper, we introduce an S-N (superconducting-normal) model with d wave Cooper pairing in the S layer and indicate that the interlayer hopping between the S and N layers has pair-weakening effect on the high-T_c superconductivity. Also, we respectively study the influence of the nonmagnetic impurities sitting in the superconducting or nonsuperconducting layers on the high-T_c superconductivity based on the S-N model. The obtained results show the differences in the suppression effects on the transition temperature depending on where the impurities are located. The experimental observations are qualitatively consistent with our theoretical results.     

Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing

Highly dense sintered YBa₂Cu₄O₈ has been produced by hot isostatic pressing (HIP). The electrical resistivity of this material has been measured as a function of temperature T and pressure in the range 40–650 K and 0–0.7 GPa. Both the temperature dependence and the pressure dependence of are found to be well described by a model based on the standard Bloch-Grüneisen theory. It is pointed out that is liner in T only under isobaric conditions, while is strongly nonlinear in all high-T_c superconductors under isochoric (constant volume) conditions. The critical current density of the material is 900 A/cm² at 4 K, while the resistivity is 630 μΩ cm at 294 K.     

Temperature pulse induced thermal instability in Y1Ba2Cu3O7−δ superconductors

As a common effect of a large and variable temperature gradient and a temperature pulse, a thermal instability of a new type has been observed in high-T_c superconductors. The primary effect is the temporal change of local energy density, that causes a dynamic state of simultaneously coexisting normal and superconducting domains, establishing a spatial and temporal variation of superconducting and normal states. The fluctuation of the ratio of the superconducting and normal volume fractions can be detected by the oscillation of DC and AC voltages measured on the samples. Supposing that every jump of the AC voltage represents the transformation of a single coherent domain, it leads to the average value of the linear dimension of domains as 2.1 mm, characteristic time of the transformation as 7.5 s and velocity of the propagation of this effect as 0.3 mm/s in the case of Y₁Ba₂Cu₃O_7−δ specimens. The macroscopic consequence of the thermal disturbance introduced into the sample is the appearance of a giant flux creep. Its existence is confirmed by comparison of our measurements to others, by analytical and numerical calculations of the forces acting on the vortices and by evaluation of the effect of the huge fluctuation created in the samples by our experiments.     

MgB2超导膜的厚度与其Jc(5K,0T)的关系

通过混合物理化学气相沉积法 (hybrid physical-chemical vapor deposition, HPCVD), 在(000l) SiC 衬底上制得一系列从10 nm到8 μm的MgB₂超导膜样品, 并对它们的形貌、超导转变温度T_c 和临界电流密度J_c与膜厚度的关系进行了研究. 观察到T_c随膜厚度增加上升到最大值后, 尽管膜继续增厚, 但T_c值保持近乎平稳, 而J_c则先随膜厚度增加上升到最高值后, 继而则随膜的厚度的增加而下降. MgB₂膜的T_c(0)和T_c(onset)值与膜厚的关系基本一致, T_c(0)在膜厚为230 nm处达到最大值T_c(0)=41.4 K, 而J_c(5K,0T)在膜厚为100 nm时达到最大值, J_c (5 K, 0 T)=2.3×10⁸A·cm^-2, 这也说明了我们能用HPCVD方法制备出高质量干净MgB₂超导膜. 本文研究的超导膜厚度变化跨度非常大, 从10 nm级的超薄膜到100 nm级的薄膜, 再到几微米的厚膜, 如此T_c和J_c对膜厚度变化的依赖就有了较完整、成体系的研究. 并且本文的工作对MgB₂超导薄膜制备的厚度选取具有实际应用意义. 关键词： 2超导膜')" href="#">MgB₂超导膜 混合物理化学气相沉积法 厚度 临界电流密度     

Influence of the Tl- and Hg-content on magnetic and transport properties of the Pb, Sr-doped Tl-1223 and Hg-1223 superconductors

The effects of the thallium and mercury content x on the as-sintered and post annealed samples of M_xPb_0.4Sr_1.6Ba_0.4Ca₂Cu₃O_8+δ {M: Tl (0.32≤x≤0.74) or Hg (0.18≤x≤0.68)} have been studied by magnetization and transport measurements. For Tl-1223 we have found the optimum Tl doping level to be x=0.53 regarding the grain properties, the content of superconducting phase, the first penetration field H_pl^wl, the transport (J_c^tr), magnetic intergrain (J_c^M) and intragrain (J_c^g) critical current densities. The critical temperature T_c of the as-sintered Tl-1223 sample decreased with increasing Tl content. Post-annealing in oxygen improved the T_c for Tl contents of x≥0.53 and had generally positive effects on the critical current densities. The intergrain properties of the Hg-1223 samples were much worse than those of the Tl-based superconductors.     

New gap equation for strongly correlated metals

Assuming a phenomenological self-energy ImΣ(ω)|ω|^β, (β=1), which becomes gapped below T_c, we derived a new gap equation. The new gap equation contains the effect of the kinetic energy gain upon developing a superconducting order parameter. However, this new kinetic energy gain mechanism works only for a repulsive pairing potential leading to a s-wave state. In this case, compared to the usual potential energy gain in the superconducting state as in the BCS gap equation, the kinetic energy gain is more effective to easily achieve a high critical temperature T_c, since it is naturally Fermi energy scale. In view of the experimental evidences of a d-wave pairing state in the hole-doped copper-oxide high-T_c superconductors, we discuss the implications of our results.     

Preparation and characterization of Y1−xPrxBa2Cu3O7−δ single crystals

We present a reliable method for growing single crystals of Y_1−xPr_xBa₂Cu₃O_7−δ high-T_c superconductors in ZrO₂ crucibles. This method results in crystals with greatly improved superconducting properties compared to crystals grown with the previously reported methods which use Al₂O₃ crucibles. We describe techniques for crystal growth in both Al₂O₃ and ZrO₂ crucibles using an excess of BaCo₃ and CuO as the flux. The crystals were characterized by means of DC magnetic-susceptibility measurements, electrical-resistivity measurements, and electron microprobe analysis. The effects of Al contamination on the conditions for crystal growth and on the superconducting properties of the crystals are found to be quite significant.     

Electronic behavior of spatially distributed junction small inductance dc π-SQUIDs

As investigated theoretically [Annalen der Physik (Leipzig) 8 (1999) 511; Phys. B, in press] spatially distributed junction small inductance dc π-SQUIDs exhibit unusual electric transport properties in both zero-voltage and voltage states. The purpose of this paper is to summarize the peculiarities of this electronic behavior in order to (a) reveal some of the advantages the device has over other known Josephson junctions based configurations when used as a tool to investigate the order parameter symmetry in high-T_c superconductors, and (b) to emphasize its potential for applications in superconducting electronics.     

Thermodynamic stability of cuprate perovskite superconductors

The oxygen-partial-pressure versus temperature phase behaviour of a variety of cuprate perovskites, including high-T_c superconductors has been studied. For the n=1, 2, 3 and the ∞ Bi-compounds the temperature above which the perovskite structure becomes unstable lies on the same boundary of thermodynamic stability. This boundary coincides with that separating cupric and cuprous oxide implying that stability of the compound is determined by the stability of the copper oxide (CuO₂) planes. The variety of other cuprate perovskites which remain stable to well beyond this boundary appear to be those in which the CuO₂ planes are subject to substantial internal strain whether compressive or tensile. For these, stability is probably determined by the non-copper-oxide substructures.     

Quantum phase fluctuations responsible for pseudogap

The effect of ordering field phase fluctuations on the normal and superconducting properties of a simple 2D model with a local four-fermion attraction is studied. Neglecting the coupling between the spin and charge degrees of freedom an analytical expression has been obtained for the fermion spectral function as a single integral over a simple function. From this we show that, as the temperature increases through the 2D critical temperature and a nontrivial damping for a phase correlator develops, quantum fluctuations fill the gap in the quasiparticle spectrum. Simultaneously the quasiparticle peaks broaden significantly above the critical temperature, resembling the observed pseudogap behavior in high-T_c superconductors.     

Angle-dependent broadening and anomalies of the resistivity transition in YBa2Cu3O7 single crystal

In a further study of the field-broadening of the resistive transition in high-T_c superconductors, resistivity measurements have been performed as a function of the direction of the applied field with respect to the crystallographic axes on a YBa₂Cu₃O₇ single-crystalline sample for field and current directions within the ab-plane over the full range of field orientations. The angular modulation in resistivity is compared wit predictions of flux-creep and flux-flow theories. Magneto-resistivity measurements have been performed at specific angles of the applied field with respect to the crystallographic directions. Saturation of the Lorentz-force effect for perpendicular field and current directions is found for fields larger than a critical value that is strongly temperature dependent. A complex resistivity anomaly is observed for fields parallel to the twin boundary.     

Influence of fluctuations on thermal conductivity of high-Tc YBa2Cu3O7−δ superconductor

To study a behavior of the thermal conductivity near T_c specific heat and thermal diffusivity of the YBa₂Cu₃O_7−δ high-T_c ceramics were simultaneously measured. Close to T_c = 92.30 K the thermal diffusivity and the thermal conductivity discovered minima and the specific heat – maximum. Quantitative analysis of the influence of thermodynamical fluctuations showed the same power laws with Gaussian exponent equal to 0.5 and existing of crossover from the 3D Gaussian to 3D XY critical behavior in the specific heat and thermal conductivity at the approach to T_c. To explain the minimum in thermal conductivity at T_c we propose a mechanism of scattering of phonons on the superconducting fluctuations.