The control of electrons through patterning of superstructures

随着人们对环境、能源发电和气候变化的担忧与日俱增，对性能更好的新材料的需求将越来越大，以使技术应用更加清洁和高效。优化材料性能的一种方法是从简单的母体化合物开始，以连续和系统的方式改变其化学成分，例如，用另一种化学成分取代它的一种化学成分。这种被称为兴奋剂的策略非常成功。例如，1988年，J.G.Bednorz和K.A.Müler用Ba取代了绝缘陶瓷La2CuO4中约15%的La离子，发现该产品在前所未有的35K温度下成为超导体(即失去所有电阻)，显著高于此前已知的最高超导温度。这一发现是开发高温氧化铜超导体的起点，现在这种超导体的工作温度高达135K，并越来越多地被用于需要强磁场或电流的应用中。尽管掺杂的后果可能是壮观的(比如铜氧化物超导体)，但它们也可能是复杂的，而且人们并不总是很好地理解掺杂与物理性质变化的联系。一种可能发挥重要作用的效应是超结构的形成，在超结构中，掺杂原子或它们转移到宿主的电荷将自己组织成延伸到很长距离的图案，并在纳米尺度上进行周期性调制。超结构改变了主体材料中的静电势，这反过来可以强烈地影响材料的物理性质。这就提出了一个有趣的可能性：如果一个人能够控制超结构的形成，那么就应该有可能调整一种材料的性能，从而提高它的性能。这个项目的目的有两个，第一，了解某些材料中为什么会出现超结构，第二，研究超结构形成对这些材料的物理性质的影响。为了为这些想法提供一个试验场，我们已经确定了几种不同的材料，在这些材料中，超结构似乎发挥了突出的作用。其中包括钴酸钠(一种非常有前途的p型热电材料)、钴酸锂(用于手机和笔记本电脑的充电电池的主要成分)，以及最近发现的两种铁基高温超导体。这些系统不仅是进行实验的良好模型，还因为它们具有良好的前景，可以通过提高能量收集和存储设备的效率来支持环境和社会问题的技术解决方案。我们将制备单晶样品，其组成可以通过不同的掺杂策略来改变。X射线和中子散射将被用来探测晶体内部的深处，以揭示超结构的存在，并改进相关的结构和电子图案，我们将把结果与材料的电、热和磁性质的整体测量相关联。在理论建模的帮助下，我们的计划将使我们更清楚地了解上部结构可以在多大程度上控制物理行为，并将有助于为实际应用开发性能更好的材料。

As concern grows over the environment, energy generation and climate change, there will be an increasing demand for new materials with improved performance to make technological applications cleaner and more efficient. One way to go about optimizing a material's performance is to start with a simple 'parent' compound and vary its chemical composition in a continuous and systematic way, for example, by substituting one of its chemical constituents by another. This strategy, known as doping, has been extremely successful. For example, in 1988, J.G. Bednorz and K.A. Müller replaced about 15% of the La ions in the insulating ceramic La2CuO4 with Ba and found that the product became a superconductor (i.e. lost all its electrical resistance) at an unprecedented temperature of 35 K, significantly higher than the previous highest known superconducting temperature. This discovery was the starting point for the development of the high temperature copper oxide superconductors, which now have operating temperatures as high as 135 K and which are increasingly being used in applications where high magnetic fields or electric currents are required. Although the consequences of doping can be spectacular (witness the copper oxide superconductors) they can also be complex, and the link to changes in physical properties is not always well understood. One effect that can play an important role is the formation of superstructures, in which either the dopant atoms or the charges they transfer to the host organise themselves into patterns which extend over long distances and are periodically modulated on a nanometre scale. Superstructures modify the electrostatic potential in the host material, which can in turn strongly influence the physical properties of the material. This raises an interesting possibility: If one can control the formation of superstructures then it should be possible to tune the properties of a material and thereby enhance its performance.The aims of this project are twofold, first, to understand why superstructures occur in certain materials and, secondly, to study the consequences of superstructure formation for the physical properties of those materials. To provide a testing ground for these ideas we have identified several different materials in which superstructures appear to play a prominent role. These include sodium cobaltate (a very promising p-type thermoelectric material), lithium cobaltate (the main component of the type of rechargeable batteries used in mobile phones and laptops), and two recently-discovered iron-based high temperature superconductors. As well as being good models on which to conduct experiments, these systems are chosen because they offer good prospects to underpin technological solutions for environmental and societal issues through their potential to improve the efficiency of energy harvesting and storage devices.We will prepare single crystal samples whose composition can be varied via different doping strategies. X-ray and neutron scattering will be employed to probe deep inside the crystals to reveal the presence of superstructures and to refine the associated structural and electronic patterns, and we will correlate the results with bulk measurements of the electrical, thermal and magnetic properties of the materials. With the help of theoretical modelling, our programme will lead to a clearer understanding of the degree to which superstructures can be used control physical behaviour, and will contribute towards the development of materials with improved performance for practical applications.