Magnetic flux line structures and phase transitions in unconventional and conventional superconductors

当磁场施加到任何超导体上时，它的初始反应是排斥场：这导致超导体悬浮磁体的现象。然而，许多超导体允许更大的磁场穿透，它们作为单独的磁通线。这些磁通线的一个实际重要方面是，通过超导体的电流可能会迫使它们移动。如果发生这种情况，能量将被耗散，超导体将变成电阻性的。然而，如果磁通线被钉扎到材料中的杂质上，超导体可以携带大电流而不会耗散。我们的一些研究是针对理解钉扎，通过测量磁通线如何与钉扎中心和彼此相互作用。另一个重要的目的是利用磁通线的观测来告诉我们更多关于超导体本身的信息。一个很好的例子是每条线中的磁通量，它由普朗克常数（量子粒子的波长与其动量之间的关系）与两倍的电子电荷（这表明在超导体中，电子是成对耦合在一起的）之比给出。在所有已知的超导体中，电子都是成对的，但在不同的材料中，配对的类型可能会有很大的不同。例如，在高温超导体中，每对电子都在彼此周围循环。这将导致通量线的核心具有四叶苜蓿状结构，由组成电子对的驻波产生。当磁通线通过增加磁场而被压缩时，这些奇怪形状的芯之间的相互作用变得重要。我们使用中子衍射来观察这些通量线;如果在一个雨夜，你通过雨伞的材料观察钠路灯的光，你会发现它被衍射斑点的图案所包围。这是由于光波通过伞材料中规则间隔的纤维时发生干涉而产生的。类似地，如果你透过一个含有通量线的超导体观察一个慢中子源，比如一个研究反应堆，就会发现主中子束被衍射中子包围。中子衍射图告诉我们物质中通量线的排列。在最简单的情况下，这将是一个六边形的包装，就像一把铅笔所采用的那样。在实践中，我们发现了许多其他的排列，它们随着磁通线密度的变化而变化，并告诉我们它们之间的相互作用以及它们与超导体底层晶体结构的相互作用。我们的测量依赖于中子的两个性质：第一，它们具有类似量子波的性质;第二，它们的行为就像微观磁铁，因此它们会被磁场这样明显不存在的东西衍射。在这项研究中，我们将这些方法应用于各种超导体，并将其扩展到超低温下的高磁场的新极端。这里我们有一种新的磁通线的迹象，它的磁芯就像铅笔中的铅芯。我们还对温度升高的作用感兴趣，温度升高会使通量线从平衡位置移动，并可能形成像煮熟的意大利面条一样的排列，这肯定与电阻的出现有关。这种现象在高温超导材料中已经被清楚地观察到，但到目前为止，它还没有在传统超导体中得到明确的证明，这表明我们的理解是不完善的。总之，我们正在推进一个环环相扣的计划，这将增加我们对超导现象发生的许多方式的理解。

When a magnetic field is applied to any superconductor, its initial response is to exclude the field: this leads to the phenomenon of levitation of magnets by superconductors. However, many superconductors allow larger magnetic fields to penetrate, which they do as individual lines of magnetic flux. One of the practically important aspects of these flux lines is that currents passed through the superconductor may force them to move. If this occurs, energy will be dissipated, and the superconductor will become resistive. However, if the flux lines are pinned to impurities in the material, the superconductor may carry large currents without dissipation. Some of our research is directed towards understanding pinning, by measuring how the flux lines interact with the pinning centres and with each other. Another important aim is to use the observation of flux lines to tell us more about the superconductors themselves. A well-established example of this is the amount of magnetic flux in each line, which is given by the ratio of Planck's constant (which relates the wavelength of quantum particles to their momentum) to twice the electronic charge (which demonstrates that in superconductors, the electrons are coupled together in pairs). Electrons pair up in all known superconductors, but the type of pairing can be greatly different in different materials. For instance, in high-temperature superconductors, the electrons in each pair are circulating around each other. This should cause the cores of the flux lines to have a four-leaved clover-like structure, arising from the standing electron waves making up the pairs. When the flux lines are squeezed by increasing the magnetic field, the interaction between these odd-shaped cores becomes important. We are using neutron diffraction to observe these flux lines; if on a rainy night, you view the light from a sodium street lamp through the material of an umbrella, it is found to be surrounded by a pattern of diffraction spots,. These arise from the interference of light waves passing through the regularly-spaced fibres in the umbrella material. In a similar way, if one views a source of slow neutrons, such as a research reactor, through a superconductor containing flux lines, the main neutron beam is found to be surrounded by diffracted neutrons. The neutron diffraction pattern tells us about the arrangement of flux lines in the material. In the simplest case, this would be a hexagonal packing, like that adopted by a handful of pencils. In practice, we are finding many other arrangements, which change with the density of flux lines and tell us about their interactions with each other and with the underlying crystal structure of the superconductor. Our measurements rely on two properties of neutrons: firstly, that they have quantum-wave-like properties, and secondly, that they behave like microscopic magnets, so that they are diffracted by something as apparently insubstantial as a magnetic field. In this research, we are applying these methods to a wide variety of superconductors, and extending them to new extremes of high magnetic field at ultra-low temperatures. Here we have indications of a new kind of flux line, with a magnetic core like the lead in a pencil. We are also interested in the role of increasing temperature, which will tend to make the flux lines move from their equilibrium positions, and possibly get into an arrangement like cooked spaghetti, which is certainly associated with the appearance of electrical resistance. This phenomenon has been clearly observed at elevated temperatures in High-Tc materials, but so far it has evaded clear demonstration in conventional superconductors, which suggests that our understanding is imperfect. In summary, we are pushing forward an interlocking programme, which will increase our understanding of the many ways in which the fascinating and useful phenomenon of superconductivity can occur.