Direct measurement of the AC loss of an ITER relevant coil

In the ENEA Frascati Laboratory, an ITER relevant coil is being tested in pulsed regime. One item of the testing program is the search for possible ramp rate limitations. The range explored (up to 3.2 T/s) covers amply the ITER Central Solenoid operating regime (0.5–1 T/s). An interesting outcome of these tests is that repeated run sets, performed in similar conditions, show an increasing value of the ramp rate that produces a quench. This behaviour is attributed to a continuous decrease of the value of the AC loss constant nτ, due to the increase of the cable transversal resistivity. An explanation on the basis of a ‘classical' training was also considered, but for a number of reasons reported in the paper, it was discarded. Direct measurements of the AC loss on the whole coil confirm that the original value of nτ (140 ms), as measured on a ‘virgin' piece of conductor, has gone below 40 ms. The measurements are made difficult by the presence of a heavy mechanical structure and of a background coil. The techniques applied and the results are reported in detail.     

Segregation of copper in the ITER cable and ramp rate limitation

The quench of a superconducting cable containing a defect and subjected to a current increase is studied. The cable is represented by a model which consists of a strand with a local decrease in the critical current I_c and the rest of the cable lumped together. It is assumed that the quench takes place when the electric field in the defect reaches the instability level. An important feature of the process is the noticeable portion of current expelled from the defect to the rest of the cable prior to the quench. It is shown that given the strong inequality in the strand currents during the ramp, the influence of the defect on the quench current is proportional to I_c⁴. This criterion is applied to the problem of copper separation in the triplets of the ITER cables.     

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.     

Normal state magnetoresistance in stable magnetic fields up to 20 T in single crystalline La1.76Sr0.24CuO4

The in-plane normal state resistance R_n(T, H) in an overdoped La_2−xSr_xCuO₄ crystal (x = 0.24) has been measured in magnetic fields up to 20 T parallel to the c-axis. The R_n(T) curves in constant fields show a quadratic behavior in a wide range of temperature above T_c(H). Some characteristic features in R_n(H) are observed. In the low-field region R_n(H) increases with increasing H, reaches a maximum and then decreases with further increasing H. Possible origins for the observed unusual R_n(T, H) behavior are discussed.     

Spectator fermion binding of bosons

We consider a two-component mixture of charged fermions on neutralizing background with all sign combinations and arbitrarily small mass ratios. In the two-impurity limit for the heavier component we show that the pair forms a bound state for all charge combinations. In the lowest order approximation we derive a closed-form expression for V_eff(r), the binding potential which has short-range repulsion followed by attraction. In the classical limit, when the mass of embedded particles is large, m₂ å m, we can calculate from V_eff(r) also the cohesive energy E and the bond length R of a metallic crystal such as lithium. For lithium metal the lowest-order result is R = 3.1 Å, E = −1.8 eV, while the corresponding experimental values are 3.4 Å and −1.6 eV. The same interaction for two holes on a parabolic band with m₂ > m gives the quantum mechanical bound state which one may interpret as a boson or local pair in the case of high-T_c and heavy-fermion superconductors.     

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.     

Scale-dependent analysis of magnetic flux condensation in QCD

The structure of the vacuum is studied in the Yang-Mills theory as a function of the scale length R. As a candidate of the vacuum we analyze explicitly the Savvidy state. We obtain the following results: (i) there is no gluon condensation for R<R′_c (perturbative phase); (ii) there is condensation of a uniform magnetic flux for R′_c<R<R′_c (Savvidy phase); (iii) the Savvidy state becomes unstable for R>R_c (Copenhagen phase). Here, R_c and R′_c are calculable in terms of magnetic flux condensation H².     

外力驱动作用下高分子链在表面吸附性质的计算机模拟

采用退火法模拟研究受外力F驱动的高分子链在吸引表面的吸附特性.通过高分子链的平均表面接触数〈M〉与温度T之间的关系计算临界吸附温度T_c,并发现T_c随着F的增加而减小;进而通过高分子链的均方回转半径分析外力驱动作用对高分子链构象的影响,并从回转半径极小值或者垂直外力方向的y和z分量的变化交叉校验临界吸附点T_c.模拟计算了处于吸附状态的高分子链随着外力F的增加是否会发生吸附状态到脱附状态的相变以及发生相变所需施加的外力是否由温度所决定.模拟结果表明:两种不同温度下高分子链的吸附性质和构象性质受外力驱动作用而产生不同现象,在温度区间T*_cTT_c时会发生脱附现象,而在TT*_c时不会发生脱附现象.     

Ni2O3掺杂对新固相源顶部籽晶熔渗生长法制备单畴GdBCO超导块材超导性能的影响

本文通过在新固相源中添加Ni₂O₃的方法, 采用顶部籽晶熔渗生长工艺(TSIG)制备出组分为(1-x) (Gd₂O₃+1.2BaCuO₂)+x Ni₂O₃、直径为20 mm的单畴GdBCO 超导块材(其中x = 0, 0.02, 0.06, 0.10, 0.14, 0.18, 0.30, 0.50 wt%), 并研究了Ni₂O₃的掺杂量x对样品的表面生长形貌、微观结构、临界温度T_c、磁悬浮力以及俘获磁通密度的影响. 研究结果表明, 当Ni₂O₃的掺杂量x在0–0.50 wt%的范围内时, 均可制备出单畴性良好的样品, 且Ni₂O₃的掺杂对样品中Gd211粒子的分布和粒径没有明显的影响. 在Ni₂O₃的掺杂量x从0增加到0.50 wt%的过程中, 样品的临界温度T_c呈现下降的趋势, 从x=0时的92.5 K下降到x=0.50 wt%时的86.5 K, 这是由于Ni^{3 +}替代GdBCO晶体中Cu^{2 +}所致; 样品磁悬浮力和俘获磁通密度均呈现先增大后减小的变化规律, x=0.14 wt%时, 磁悬浮力达到最大值34.2 N, x=0.10 wt%时, 俘获磁通密度达到最大值0.354 T. 样品磁悬浮力和俘获磁通密度的变化规律与Ni₂O₃的掺杂量x有密切关系, 只有当掺杂量x合适时, Ni³⁺对Cu^{2 +}的替代既不会造成T_c的明显下降, 但又能产生适量的Ni^{3 +}/Cu²⁺ 晶格畸变, 从而达到提高样品磁通钉扎能力和超导性能的效果.     

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.     

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.     

Proper In deposition amount for on-demand epitaxy of InAs/GaAs single quantum dots

The test-QD in-situ annealing method could surmount the critical nucleation condition of InAs/GaAs single quantum dots(SQDs) to raise the growth repeatability.Here,through many growth tests on rotating substrates,we develop a proper In deposition amount(θ) for SQD growth,according to the measured critical θ for test QD nucleation(θ_c).The proper ratio θ/θ_c,with a large tolerance of the variation of the real substrate temperature(T_(sub)),is 0.964-0.971 at the edge and 0.989 but 0.996 in the center of a 1/4-piece semi-insulating wafer,and around 0.9709 but 0.9714 in the center of a 1/4-piece N~+ wafer as shown in the evolution of QD size and density as θ/θ_c varies.Bright SQDs with spectral lines at 905 nm-935 nm nucleate at the edge and correlate with individual 7 nm-8 nm-height QDs in atomic force microscopy,among dense 1 nm-5 nm-height small QDs with a strong spectral profile around 860 nm-880 nm.The higher T_(sub) in the center forms diluter,taller and uniform QDs,and very dilute SQDs for a proper θ/θ_c:only one 7-nm-height SQD in25 μm~2.On a 2-inch(1 inch = 2.54 cm) semi-insulating wafer,by using θ/θ_c = 0.961,SQDs nucleate in a circle in 22%of the whole area.More SQDs will form in the broad high-T_(sub) region in the center by using a proper θ/θ_c.     

Giant pressure effect in oxygen deficient YBa2Cu3Ox

The pressure effect on T_c of polycrystalline and single crystalline YBa₂Cu₃O_x investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x> 6.93 the single crystals show a negative dT_c/d p, as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find dT_c/d P = 0.4 K/GPa which points to pressure effects on T_c aside from charge transfer. In the underdoped region x < 6.93 the dT_c/d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, dT_c/d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the dT_c/d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa₂Cu₃O_x structure and can be explained by considering the anisotropic structure of YBa₂Cu₃O_x. The decrease of the c-axis lattice parameter results in a charge transfer to the CuO₂-planes mainly [1], whereas the compression of the a- and b-axis lattice parameter is known to produce different pressure effects which are responsible for the peak in dT_c/d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of T_c to the equilibrium value T_c(p) at this pressure with a time constant depending on the oxygen content x. A decrease x results in a peak effect in dT_c/d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, dT_c/d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x < 6.42. These giant pressure effects at low oxygen contents are mainly caused by a reversible T_c increase (dT_c/d p)_O due to pressure induced oxygen ordering via oxygen motion between unit cells.     

Fabrication and characteristics of all YBCO sandwich-type junctions using highly crystalline DC sputtered superconducting a-axis oriented Y1Ba2Cu3O7-x films

We report on the fabrication and characteristics of sandwich-type tunnel junctions with highly crystalline sputtered a-axis oriented thin film of Y₁Ba₂Cu₃O_7-x (YBC) as the base and the counter electrode. The junctions have been fabricated on SrTiO₃ (100) and MgO (100) substrates. A non-superconducting phase of YBC corresponding to a lattice constant of 4.08 Å is used as the barrier layer making this an all YBC sandwich junction. For all temperatures below T_ce (R=0) of the device, a zero voltage current was observed. The critical current density (J_c) of the device was found to be dependent on the thickness of the barrier layer and the crystallinity of the a-axis oriented YBC electrodes. At 40 K, such a junction fabricated on a SrTiO₃ (100) substrate was found to have a J_c of 1.8 X 10⁴ A/cm² and an I_cR_n product of 0.2 mV.     

Fitting of transport measurements in polycrystalline La2/3Ca1/3MnO3

Using a previous qualitative explanation to describe the transport properties of polycrystalline La_2/3Ca_1/3MnO₃ thick films, we achieved a good fit of the temperature dependence of the resistance R(T). Depending on the sample, we have observed different metal–insulator (MI) transitions while the magnetic behavior is always similar. Small regions of depleted T_c adjacent to the grain boundary could have an important resistance contribution without affecting the magnetic properties in an appreciable manner. In this work, we achieve a quantitative explanation for the different transport behaviors that we have observed experimentally.     

国产MOCVD-YBCO带材高温超导线圈研制与磁场温度特性研究

超导磁体的场强与超导材料的载流能力、磁体口径和低温环境有密切关系.为了在中高温区域实现高磁场强度的超导磁体,采用国产第二代高温超导带材,成功绕制出内直径为100 mm的高温超导线圈.该超导线圈在77,65,55和46 K不同温区下进行了性能测试,其最大运行电流分别为65,147,257和338 A,对应的中心磁场强度分别为0.78,1.77,3.1和4.08 T.所研制的超导线圈的中平面上磁场基本一致.     

ITER TF导体制造研究

ITER聚变反应堆纵场磁体系统由采用CICC结构的TF导体构造。通过短距离TF导体集成试验,文章报道了电缆伸长率、铠甲冷作变形比例、穿缆拉力测量等结果,从而验证TF导体制造在中国实现的背景可能。     

基于稳态电热拉曼技术的碳纳米管纤维导热系数测量及传热研究

利用稳态电热拉曼技术测量了碳纳米管纤维对流换热环境下的导热系数. 该方法基于材料拉曼信号与温度之间的关系, 实时探测一维材料在不同电加热(内热源)下的中心点温度, 利用对流环境下的稳态导热模型推导出材料的导热系数, 实现了一维微纳材料热物性的无损化和非接触式测量. 实验发现: 碳纳米管纤维的导热系数远低于单根碳纳米管的导热系数, 但高于碳纳米管堆积床的导热系数. 这表明碳纳米管体材料的热物性主要取决于内部管束的列阵和管束间的接触热阻.     

Temperature dependence of thermal and electrical conductivity of Bi-based high-Tc (2 2 2 3) superconductor

Superconducting samples with nominal composition Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_δ were prepared by the conventional solid-state reaction technique. The samples have been characterized by X-ray diffraction, dc electrical resistivity, ac magnetic susceptibility and thermal conductivity. The X-ray diffraction studies were done at room temperature and the lattice constants of the material were determined by indexing all the peaks. All the above measurements show that, there exists two phases i.e. high-T_c (2 2 2 3) and low-T_c (2 2 1 2). The information obtained from dc electrical resistivity data agrees with ac magnetic susceptibility measurements. The onset temperature T_c (onset) and zero resistivity temperature T_c (R = 0) of the samples remains within the temperature 120 ± 1 K and 103 ± 1 K. Thermal conductivity has been measured with a transient plane source (TPS) technique in the temperature range 77–300 K. The estimation of the electrical resistivity change due to scattering by phonons and impurities has been discussed. An increase in thermal conductivity is observed above and below T_c (R = 0). The electron–phonon scattering time, phonon-limited mobility and the size of the electron–phonon constant are also calculated. Wiedemann–Franz law is applied to gain prediction about the magnitude of electronic and phonon contribution to the total thermal conductivity of the samples. It is observed that heat is mainly conducted by the phonons in this system.     

X-ray and high resolution selenium-77 solid state NMR spectroscopy as complementary probes to structural studies of organophosphorus diselenides

high resolution solid state NMR spectroscopy was employed to study structural properties of bis(diisopropoxyphosphorothioyl) diselenide 1 and bis(dineopentoxyphosphorothioyl) diselenide 2. The principal elements T_ii of effective dipolar/chemical shift tensor were calculated from spinning sideband intensities employing the WIN-MAS program. The values of anisotropy and asymmetry parameters reflect the distortion of the selenium environment. It was found that the T₃₃ component mostly contributes to changes in the isotropic chemical shifts. CP/MAS experiments were used to decide the assignment of space group by counting the number of crystallographically unique selenium centers in the unit cell. Crystals of diselenide 1 are triclinic, space group P with a=8.485(3) Å, b=8.508(1) Å, c=8.511(2) Å, =98.835(15)°, β=111.653(24)°, γ=93.524(21)°, V=559.5(3) ų, D_c=1.544(2) g/cm³ and Z=1. Refinement using 2222 reflections for 157 variables gives R=0.037. Crystals of diselenide 2 are triclinic, space group P1 with a=9.1418(8) Å, b=9.1465(8) Å, c=9.9200(9) Å, =74.751(8)°, β=74.629(7)°, γ=82.216(7)°, V=769.7(1) ų, D_c=1.365(2) g/cm³ and Z=1. Refinement using 3316 reflections for 297 variables gives R=0.0272.