Field Induced Quantum Ordering

传统的理论描述不同材料的低温特性是从确定量子基态和考虑激发开始的。然而，当材料被调谐到量子临界点（QCP）时，在两个基态之间发生连续变化，需要一种新的方法来正确地解释这两个竞争宏观配置之间的局部量子涨落。例如，当通过施加压力将连续相变到初级状态（例如反铁磁性）的临界温度驱动到零开尔文时，会出现这样的点。当临界温度被抑制到零时，临界波动变成量子而不是热的，从而引起比热和其他性质的不寻常的温度依赖性。最终，当温度在干净的材料中降低时，两个竞争基态的宏观量子纠缠将发生，或者一个全新的基态将出现。后一个过程可以说是许多最近观察到的状态，包括高温超导性和不寻常的超导形式在几个'重费米子'反铁磁体的形成的基础。该提案涉及铁磁金属（例如URhGe和UGe 2）中的QCP，其中导电电子与磁性密切相关。它建立在我们最近发现的新的基态，即铁磁和超导。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调谐以穿过QCP。值得注意的是，在URhGe超导诱导接近QCP在一个非常大的范围内的领域（从8到28特斯拉以上）在低温下。在如此低的超导转变温度下在如此大的场范围内的超导性（0.4 K）是传统三维超导体中完全未知的。通过对不同铁磁材料（包括URhGe和UGe 2）的磁跃迁和量子临界行为的实验研究，我们的目标是展示如何实现这种不寻常的超导形式，并寻找与未来应用相关的新行为。要做到这一点，我们必须首先了解引起QCP的竞争磁性基态的性质，然后是电子成对结合以产生超导性的状态，最后是这些成对状态是如何由竞争磁性状态之间的波动引起的。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，为了确定超导性是否在磁畴壁处被抑制，以形成被称为约瑟夫森结的设备，该设备可能用于构建量子计算机。如果这样的结确实在磁畴壁上形成，那么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。新的超导状态只发生在几乎完美的晶体中，因此必须付出巨大的努力来生长高质量的晶体。对于我们希望研究的许多材料，必须施加大压力（高达100，000倍大气压），以将它们驱动到实验室磁场可以达到QCP的程度。为了做到这一点，我们将开发设备，使用特别生长的设计师钻石，对齐和挤压晶体进行研究。

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.