Field Induced Quantum Ordering

对不同材料低温特性的传统理论描述始于确定量子基态并考虑其激发。然而，当一种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适当地解释这两个相互竞争的宏观配置之间的局部量子波动。例如，当连续相变到初级态（如反铁磁性）的临界温度通过施加压力被驱动到零开尔文时，就会出现这样一个点。当临界温度被抑制到零时，临界波动变成量子波动而不是热波动，从而引起比热和其他性质对温度的不寻常依赖。最终，当清洁材料的温度降低时，要么会出现两个相互竞争的基态的宏观量子纠缠，要么会出现一个全新的基态。后一个过程可以说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量子cps，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铁磁性和超导的新基态之上。显示铁磁性量子cps的材料是特别有趣的研究，因为它们可以通过施加磁场连续调谐到交叉量子cps。值得注意的是，在低温下，在非常大的场范围内（从8到28特斯拉以上），超导性被诱导出接近QCP。在如此大的场范围内，在如此低的超导转变温度（0.4 K）下，超导性在传统的三维超导体中是完全未知的。通过对不同铁磁材料（包括uhge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量子粒子粒子的相互竞争的磁性基态的性质，然后了解电子成对结合以产生超导性的状态，最后了解这些成对状态是如何通过相互竞争的磁性状态之间的波动而产生的。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被抑制以形成称为约瑟夫森结的器件，这可能被用于构建量子计算机。如果这样的连接确实在磁畴壁上形成，那么就有可能创建和修改它们的位置和相互连接，就像在磁盘上写入和擦除数据一样容易。新的超导状态只出现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（高达大气压力的100,000倍）才能将它们驱动到可以通过实验室磁场达到量子cps的位置。为了做到这一点，我们将开发一种仪器，使用特殊生长的设计师钻石，来排列和压碎晶体，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.