Field Induced Quantum Ordering

对不同材料低温性质的传统理论描述始于对量子基态的识别和对其激发的考虑。然而，当材料被调谐到量子临界点(QCP)时，两个基态之间发生连续变化，需要一种新的方法来适当地解释这两个相互竞争的宏观构型之间的局域量子涨落。例如，当通过施加压力将连续相变到基态的临界温度(例如反铁磁性)驱动到零开尔文时，就会出现这样的点。当临界温度被抑制到零时，临界涨落变成量子涨落而不是热涨落，从而引起比热和其他性质的异常温度依赖关系。最终，当清洁材料中的温度降低时，要么发生两个竞争基态的宏观量子纠缠，要么出现一个全新的基态。可以说，后一种过程是许多最近观察到的状态形成的基础，包括高温超导和在几个重费米子反铁磁体中看到的不寻常的超导形式。这一建议涉及铁磁性金属中的QCP，如URhGe和UGe2，其中传导电子与磁性密切相关。它建立在我们最近发现的既是铁磁又是超导的新基态的基础上。显示铁磁QCP的材料特别值得研究，因为它们可以通过施加磁场连续地调谐到交叉QCP。值得注意的是，在URhGe中，超导电性在很大的磁场范围内(从8特斯拉到28特斯拉以上)在接近QCP的低温下被诱导。在传统的三维超导体中，超导转变温度(0.4K)是完全未知的。通过对不同铁磁材料(包括URhGe和UGe2)的磁跃迁和量子临界行为的实验研究，我们的目标是展示这种不寻常的超导形式是如何产生的，并寻找与未来应用相关的新行为。要做到这一点，我们必须首先了解产生QCP的竞争磁基态的性质，然后是电子成对束缚以提供超导电性的态，最后这些成对态是如何由竞争磁态之间的涨落引起的。我们还计划研究铁磁超导体的一些潜在的不寻常性质。例如，为了确定超导电性是否在磁区壁上被抑制，以形成被称为约瑟夫森结的器件，这种器件可能被用来构建量子计算机。如果这种结确实形成在磁畴壁上，就有可能像在磁盘上写入和擦除数据一样容易地创建和修改它们的位置和互连。新的超导状态只出现在几乎完美的晶体中，因此必须做出重大努力来生长高质量的晶体。对于我们想要研究的许多材料，必须施加很大的压力(高达大气压的10万倍)来驱动它们，使它们达到实验室磁场可以达到的QCPs的程度。为了做到这一点，我们将开发一种仪器，使用专门生长的设计师钻石来排列和挤压晶体，以进行研究。

The conventional theoretical description of the low temperature properties of different materials begins with identifying the quantum ground state and considering excitations from it. However, when a material is tuned to a Quantum Critical Point (QCP), where a continuous change between two ground states occurs, a new approach is required to properly account for local quantum fluctuations between these two competing macroscopic configurations. Such a point occurs, for example, when the critical temperature for a continuous phase transition to a primary state, such as anti-ferromagnetism, is driven to zero Kelvin by applying pressure. When the critical temperature is suppressed to zero, the critical fluctuations become quantum instead of thermal giving rise to unusual temperature dependences of the specific heat and other properties. Ultimately as the temperature is reduced in a clean material either a macroscopic quantum entanglement of the two competing ground states will occur or a completely new ground state will emerge. The latter process arguably underlies the formation of many recently observed states including high temperature superconductivity and unusual forms of superconductivity seen in several 'heavy fermion' antiferromagnets. This proposal concerns QCPs in ferromagnetic metals, such as URhGe and UGe2, where the conduction electrons are intimately involved in the magnetism. It builds on our recent discovery of new ground states that are both ferromagnetic and superconducting. Materials displaying ferromagnetic QCPs are particularly interesting to study because they can be continuously tuned to cross QCPs by applying magnetic field. Remarkably, in URhGe superconductivity is induced close to a QCP over a very large range of fields (from 8 to above 28 Tesla) at low temperature. Superconductivity over such a large field range with such a low superconducting transition temperature (0.4 K) is completely unknown in conventional 3-dimensional superconductors.Through experimental investigation of the magnetic transitions and quantum critical behaviour of different ferromagnetic materials, including URhGe and UGe2, we aim to show how such unusual forms of superconductivity can be brought about, and to look for novel behaviour relevant for future applications. To do this we have to understand, firstly, the natures of the competing magnetic ground states that give rise to the QCPs, then the states in which electrons are bound in pairs to give superconductivity, and finally how these paired states are brought about by fluctuations between the competing magnetic states. We also plan to investigate some potentially unusual properties of ferromagnetic superconductors. For example, to establish whether superconductivity is suppressed at magnetic domain walls to form devices known as Josephson junctions, which could potentially be used to construct a quantum computer. If such junctions do form at magnetic domain walls, it might be possible to create and modify their positions and interconnections as easily as writing and erasing data on a magnetic disc.The new superconducting states occur only in almost perfect crystals, so a significant effort has to be made to grow high quality crystals. For many of the materials we wish to investigate, large pressures (up to 100,000 times atmospheric pressure) must be applied to drive them to the point where QCPs can be reached with laboratory magnetic fields. To do this we will develop apparatus, using specially grown designer-diamonds, to align and squash the crystals to carry out the studies.