The origin of multiple superconducting gaps in MgB2

Magnesium diboride, MgB2, has the highest transition temperature (T(c) = 39 K) of the known metallic superconductors. Whether the anomalously high T(c) can be described within the conventional BCS (Bardeen-Cooper-Schrieffer) framework has been debated. The key to understanding superconductivity lies with the 'superconducting energy gap' associated with the formation of the superconducting pairs. Recently, the existence of two kinds of superconducting gaps in MgB2 has been suggested by several experiments; this is in contrast to both conventional and high-T(c) superconductors. A clear demonstration of two gaps has not yet been made because the previous experiments lacked the ability to resolve the momentum of the superconducting electrons. Here we report direct experimental evidence for the two-band superconductivity in MgB2, by separately observing the superconducting gaps of the sigma and pi bands (as well as a surface band). The gaps have distinctly different sizes, which unambiguously establishes MgB2 as a two-gap superconductor.     

MgB2制备过程中退火效应和热稳定性的实验研究

报道了关于 MgB₂ 超导体制备过程中的退火效应和热稳定性的实验研究。把硼片在不同的温度 Mg 气氛中退火不同时间得到 MgB₂,制备样品的测量结果显示制备 MgB₂ 的合适温度范围是 700～1000℃,并且较高的制备温度下只需要相对短的退火时间内就能得到较高转变温度的样品。热稳定性实验的结果显示在没有 Mg 的气氛中 MgB₂ 在 700℃ 下是稳定的,从 800℃ 开始分解,直到完全失去超导电性。实验还观测到利用 MgB₂ 混合物薄膜前驱代替硼片制备 MgB₂ 时,在 600℃ 退火时样品就显示超导电性。     

Superconductivity in MgB2

In January of 2001 the superconductivity of the compound MgB2 with a critical temperature Tc of up to 39 K was discovered. This Tc is the highest in all intermetallic compound and alloy superconductors. MgB2 has a simple structure and its manufacturing capital cost is lower, therefore it could become a practical superconductor in the future. The recent progress is reviewed here which covers the progress in electronic structure, high Tc mechanism, superconducting parameters (Debye temperature, specific heat coefficient of electron, critical fields, coherent length, penetration depth, energy gap, critical current and relaxation rate of flux). Moreover the issue on power transmission is discussed.     

Superconducting Properties and Microstructure in MgB2 Bulks, Wires and Tapes

We prepared a series of MgB2 bulk samples under different temperatures, holding time and increasing rates in temperature by the solid state reaction. The thermodynamic behavior and phase formation in the Mg-B system were studied by using DTA, XRD and SEM. The results indicate that the formation of the MgB2 phase is very fast and the high increasing rate in temperature is necessary to obtain high quality MgB2. In addition, the effects of the Zr-doping in Mg1-xZrxB2 bulk samples fabricated by the solid state reaction at ambient pressure on phase compositions, microstructure and flux pinning behavior were investigated by using XRD, SQUID magnetometer, SEM and TEM. Critical current density Jc can be significantly enhanced by the Zr-doping and the best data are achieved in Mg0.9Zr0.1B2. For this sample, Jc values are remarkably improved to 1. 83 × 106 A/cm2 in self-field and 5. 51 × 105 A/cm2in 1T at 20K. Also, high quality MgB2/Ta/Cu wires and tapes with and without Ti-doping, MgB2/Fe wires and 18 filamen     

多晶MgB2相形成中纳米尺寸硼粉的作用

原料硼粉在MgB2相形成中的作用已经利用原位高温电阻率(HT-ρT)的测量技术进行了研究.MgB2相形成中的起始转变温度Tonset和相转变完成温度TPF在完成了的热处理过程中已直接确定了.用纳米硼粉和镁粉制备MgB2样品(简称为NanoB-MgB2)时的Tonset和TPF这2个温度值分别是440和 490 ℃,类似于用微米硼粉和纳米镁粉制备MgB2样品(简称为MicroB-MgB2)的情况.这表明MgB2相形成的温度并不特别依赖于硼粉的尺寸.另外,烧结温度的上极限TN是750 ℃,在此温度下NanoB-MgB2样品的超导电性将丧失.此温度远低于用微米硼粉和毫米尺寸的镁粉制备的MgB2样品(简称为DM-MgB2)的TN= 980 ℃.把直接在650 ℃＜T N,烧结的样品与先在750 ℃下烧结,之后又再次重磨和处理后,并于富镁的环境中在650 ℃下再烧结处理的样品相比较,观察到了MgB2的不可逆的晶体结构转变.     

Fragile-to-strong transition and polyamorphism in the energy landscape of liquid silica

Liquid silica is the archetypal glass former, and compounds based on silica are ubiquitous as natural and man-made amorphous materials. Liquid silica is also the extreme case of a 'strong' liquid, in that the variation of viscosity with temperature closely follows the Arrhenius law as the liquid is cooled toward its glass transition temperature. In contrast, most liquids are to some degree 'fragile', showing significantly faster increases in their viscosity as the glass transition temperature is approached. Recent studies have demonstrated the controlling influence of the potential energy hypersurface (or 'energy landscape') of the liquid on the transport properties near the glass transition. But the origin of strong liquid behaviour in terms of the energy landscape has not yet been resolved. Here we study the static and dynamic properties of liquid silica over a wide range of temperature and density using computer simulations. The results reveal a change in the energy landscape with decreasing temperature, which underlies a transition from a fragile liquid at high temperature to a strong liquid at low temperature. We also show that a specific heat anomaly is associated with this fragile-to-strong transition, and suggest that this anomaly is related to the polyamorphic behaviour of amorphous solid silica.     

Enhancement of the high-magnetic-field critical current density of superconducting MgB2 by proton irradiation

Magnesium diboride, MgB2, has a relatively high superconducting transition temperature, placing it between the families of low- and high-temperature (copper oxide based) superconductors. Supercurrent flow in MgB2 is unhindered by grain boundaries, making it potentially attractive for technological applications in the temperature range 20-30 K. But in the bulk material, the critical current density (Jc) drops rapidly with increasing magnetic field strength. The magnitude and field dependence of the critical current are related to the presence of structural defects that can 'pin' the quantized magnetic vortices that permeate the material, and a lack of natural defects in MgB2 may be responsible for the rapid decline of Jc with increasing field strength. Here we show that modest levels of atomic disorder induced by proton irradiation enhance the pinning of vortices, thereby significantly increasing Jc at high field strengths. We anticipate that either chemical doping or mechanical processing should generate similar levels of disorder, and so achieve performance that is technologically attractive in an economically viable way.     

Structural transformation in supercooled water controls the crystallization rate of ice

One of water's unsolved puzzles is the question of what determines the lowest temperature to which it can be cooled before freezing to ice. The supercooled liquid has been probed experimentally to near the homogeneous nucleation temperature, T(H) ≈ 232 K, yet the mechanism of ice crystallization-including the size and structure of critical nuclei-has not yet been resolved. The heat capacity and compressibility of liquid water anomalously increase on moving into the supercooled region, according to power laws that would diverge (that is, approach infinity) at ~225 K (refs 1, 2), so there may be a link between water's thermodynamic anomalies and the crystallization rate of ice. But probing this link is challenging because fast crystallization prevents experimental studies of the liquid below T(H). And although atomistic studies have captured water crystallization, high computational costs have so far prevented an assessment of the rates and mechanism involved. Here we report coarse-grained molecular simulations with the mW water model in the supercooled regime around T(H) which reveal that a sharp increase in the fraction of four-coordinated molecules in supercooled liquid water explains its anomalous thermodynamics and also controls the rate and mechanisms of ice formation. The results of the simulations and classical nucleation theory using experimental data suggest that the crystallization rate of water reaches a maximum around 225 K, below which ice nuclei form faster than liquid water can equilibrate. This implies a lower limit of metastability of liquid water just below T(H) and well above its glass transition temperature, 136 K. By establishing a relationship between the structural transformation in liquid water and its anomalous thermodynamics and crystallization rate, our findings also provide mechanistic insight into the observed dependence of homogeneous ice nucleation rates on the thermodynamics of water.     

High critical current density in powder-in-tube processed MgB2/Ta/Cu wire

The magnetization of dense MgB2/Ta/Cu wires prepared by the powder-in-tube method is measured by a SQUID magnetometer. The results indicate that the critical temperature of MgB2/Ta/Cu is around 38.4 K with a sharp transition width of 0.6 K. The MgB2/Ta/Cu wire shows a strong flux pinning and the critical current density is higher than 105 A/cm2 (5 K, self-field) and 104 A/cm2 (20 K, 1 T). Also, the irreversibility field of the sample reaches 6.6 T at 5 K.     

Mg1-xNaxB2化合物的结构和超导特性（英文）

报道了Mg1-xNaxB2化合物的结构和超导特性.块状样品的X射线衍射结果表明样品具有六边形的晶态结构.由于镁粉的化学活性,X射线衍射检测到了少量的MgO杂相.磁测量结果表明非化学计量比的MgB2并没有影响到超导的转变温度及转变温度的宽度.同时,钠的引入几乎没有影响到超导体转变温度.     

MgB2超导体的相图及其对薄膜制备的指导作用

MgB2适合于制备约瑟夫森结,在超导电子学领域有很好的应用前景。制备高质量的MgB2薄膜至关重要,应用Mg-B/Mg-B-O体系的相图指导MgB2薄膜生长意义重大。总结MgB2相体系及相关系,详细对比分析Mg-B/Mg-B-O体系的热力学相图,总结分析富氧区杂项MgO的生成机理及其对MgB2薄膜质量和性能的影响,研究分析有氧体系下Mg-B/Mg-B-O热力学相图对MgB2薄膜材料制备生长的指导意义,探讨HPCVD环境下采用原位生长技术制备MgB2超导薄膜时热力学相图的指导作用及相关制备工艺。     

Strongly linked current flow in polycrystalline forms of the superconductor MgB2

The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field-temperature (H-T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.     

Vortex dynamics in superconducting MgB2 and prospects for applications

The recently discovered superconductor magnesium diboride, MgB2, has a transition temperature, Tc, approaching 40 K, placing it intermediate between the families of low- and high-temperature superconductors. In practical applications, superconductors are permeated by quantized vortices of magnetic flux. When a supercurrent flows, there is dissipation of energy unless these vortices are 'pinned' in some way, and so inhibited from moving under the influence of the Lorentz force. Such vortex motion ultimately determines the critical current density, Jc, which the superconductor can support. Vortex behaviour has proved to be more complicated in high-temperature superconductors than in low-temperature superconductors and, although this has stimulated extensive theoretical and experimental research, it has also impeded applications. Here we describe the vortex behaviour in MgB2, as reflected in Jc and in the vortex creep rate, S, the latter being a measure of how fast the 'persistent' supercurrents decay. Our results show that naturally occurring grain boundaries are highly transparent to supercurrents, a desirable property which contrasts with the behaviour of the high-temperature superconductors. On the other hand, we observe a steep, practically deleterious decline in Jc with increasing magnetic field, which is likely to reflect the high degree of crystalline perfection in our samples, and hence a low vortex pinning energy.     

新型超导体材料MgB_2的电子结构随压强的变化关系

根据密度泛函理论运用全势能线性muffin tin轨道 (FP LMTO :full potentiallinear muffin tin orbital)方法计算了不同压强下MgB2 晶格常数发生变化时的电子结构的变化情况 ,得出态密度的倒数与临界温度的自然对数成线性关系 ,符合BCS理论 ,从而进一步证实了MgB2 的超导机制为电—声子耦合BCS机制     

Interfacial behavior of Fe76Si9B10P5/Zn0.5Ni0.5Fe2O4 amorphous soft magnetic composite during spark plasma sintering process

Fe_(76)Si_9B_(10)P_5/Zn_(0.5)Ni_(0.5)Fe_2O_4 amorphous composite with micro-cellular structure and high electrical resistivity was prepared by spark plasma sintering(SPS) at 487 °C. XRD and SEM results showed that the Fe_(76)Si_9B_(10)P_5 alloy powders remained the amorphous state and the composite was dense. A fusion zone at interface of Fe_(76)Si_9B_(10)P_5 cell body and Zn_(0.5)Ni_(0.5)Fe_2O_4 cell wall was observed by TEM, which also indicates the formation of local high temperature. The interface bonding based on the formation of local high temperature in SPS process was observed. It is believed that the tip effect of Zn_(0.5)Ni_(0.5)Fe_2O_4 nanoparticles promotes the local discharging and plasmas creation in the gaps, and the discharging energy forms an instantaneous local high temperature to complete the local sintering and the densification of Zn_(0.5)Ni_(0.5)Fe_2O_4 particles at a low nominal sinter temperature. Simultaneously, the local high temperature stimulates the adjacent gaps discharging, thus facilitate the continuous formation of new discharging path. Finally, sintering and densification of the amorphous composite is complete.     

Thermal physical properties and key influence factors of phase change emulsion

Latent functionally thermal fluids (LFTF) are a novel kind of heat storage and heat transfer fluids that include phase change microcapsule slurry and phase change emulsion (PCE). They have much greater apparent specific heats and higher heat transfer abilities in the phase change temperature range than conventional single-phase heat transfer fluids such as water. Thus they are advantageous in the field of the convective heat transfer enhancement and energy transport. In this paper, some thermal physical properties such as viscosity, fusion heat and apparent specific heat (Cp) are measured, and the influences of some factors (such as selection of surfactants, preparation method, temperature, mixing ratio of surfactants and mass concentration of phase change material) on them are discussed. The study shows that: 1) the viscosity of the PCE prepared in the present work is lower than that reported in the literature; 2) its apparent specific heat value for the phase change temperatu reregion is high and proportionally increases with the concentration of phase change material.     

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.     

Superconductivity at 39 K in magnesium diboride

In the light of the tremendous progress that has been made in raising the transition temperature of the copper oxide superconductors (for a review, see ref. 1), it is natural to wonder how high the transition temperature, Tc, can be pushed in other classes of materials. At present, the highest reported values of Tc for non-copper-oxide bulk superconductivity are 33 K in electron-doped Cs(x)Rb(y)C60 (ref. 2), and 30 K in Ba(1-x)K(x)BiO3 (ref. 3). (Hole-doped C60 was recently found to be superconducting with a Tc as high as 52 K, although the nature of the experiment meant that the supercurrents were confined to the surface of the C60 crystal, rather than probing the bulk.) Here we report the discovery of bulk superconductivity in magnesium diboride, MgB2. Magnetization and resistivity measurements establish a transition temperature of 39 K, which we believe to be the highest yet determined for a non-copper-oxide bulk superconductor.     

Doping effects of ZrC and ZrB2 in the powder-in-tube processed MgB2 tapes

We have investigated the effects of ZrC and ZrB2 doping on the superconducting properties of the powder-in-tube processed MgB2/Fe tapes. Sam- ples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM/EDX), transport and magnetic measurements. We confirmed the fol- lowing quite different roles of ZrC and ZrB2 in MgB2. ZrC doping was found to decrease the transport critical current density (Jc) at 4.2 K, while the critical temperature (Tc) kept constant. In contrast, the Jc values in magnetic fields were enhanced greatly by the ZrB2 addition, which resulted in a decrease in Tc by only 0.5 K. The reason for different effects of two dopants is also discussed.     

超导的纯电子机制

从电子晶格的相互作用，受晶格屏蔽的电子间的相互作用，以及电子自旋的磁力作用等纯电子机制阐明正常超导和高温超导共同的机理，说明MgB2的超导和陶瓷性超导的机理相同，都是由于自由电子少的原因，以此对BCS公式作以修正，并预言Li2O的临界温度高于MgB2。