Quantum fluctuations and criticality in model magnets

相变是自然界中普遍存在的现象。事实上，人们的日常生活模式与一种特别重要的物质的相变密切相关，尽管他们很少再考虑这一问题。无论是等着茶壶烧开来喝早茶，还是等着一天结束后喝完水里的冰融化，人们都取决于水在冰、水或蒸汽中的不同阶段的性质。这些相的稳定性一方面来自H2O分子之间的引力，另一方面是随着温度的升高而变得更加强烈的热涨落的无序效应之间的平衡。当引力从热波动中胜出时，H2O分子的气体首先凝结成水，然后冻结成冰。这是否意味着所有材料在冷却到较低温度时都将是无菌的，或者相应地，相变应该只在温度较高时才能预期？也许令人惊讶的是，对于某些类型的材料，这个问题的答案是否定的，事实上，即使在绝对零度，T=0开尔文(-273.15摄氏度)，所有热运动停止的点上，这种物质也会融化。在T=0K可以熔化的材料是那些经历强烈量子涨落的材料。这就是由于适用于原子尺度上的物质的量子规则而产生的波动。粗略地说，量子涨落是海森伯格提出的著名的不确定性原理中的不确定性。尽管量子涨落最初可能被认为会破坏系统的稳定，就像热涨落破坏经典(热)相变中的秩序一样，但过去十年左右的实验表明，在所谓的量子临界点附近，可能会出现全新的物质状态。描述热相变的理论的发展产生了20世纪科学的顶峰之一--重整化群理论，肯尼思·威尔逊因此而被授予诺贝尔奖。相比之下，量子相变的研究可以说还处于起步阶段。我们的工作集中在通过在一些最简单的系统上进行实验来对快速发展的量子相变领域做出重大贡献：相互作用的原子磁铁(自旋)阵列。化学的独创性使不同结构和维度的磁性阵列，如量子自旋链、梯子、牌匾、平面等，可以在三维晶体内生长。然后，通过发射中子束穿过阵列，并监测当系统通过量子相变(例如，通过施加磁场、压力等)驱动系统时，中子如何偏转，可以精确地监测自旋配置的变化。以这种方式重建自旋系统的全部空间和时间相关性，从而可以回答一些问题，例如自旋是结晶形成有序阵列，还是保持无序形成量子自旋液体(如果是的话，是哪种类型)。这正是大量研究量子相变的理论物理学家需要的信息类型，以测试和发展他们的理论。因此，我们对模型磁铁中的量子涨落和临界性的中子散射实验不仅将有助于回答物质如何在绝对零度下熔化这一重要的基本问题，而且还将为揭示发生在量子临界点附近的物质的迷人状态的本质提供新的见解。

Phase transitions are ubiquitous in nature. Indeed, the daily pattern of peoples lives is intimately concerned with the phase transitions of one particularly important substance even though they rarely give it a second thought. Whether it is waiting for the kettle to boil for the morning cup of tea, or for the ice to melt in ones drink at the end of a long day, people depend on the properties of H2O in its different phases of ice, water or steam. The stability of these phases arises from a balance between, on the one hand, attractive forces between the H2O molecules, and, on the other hand, the disordering effects of thermal fluctuations which become more intense with increasing temperature. When attractive forces win out of thermal fluctuations a gas of H2O molecules first condenses to form water before freezing into ice.Does this mean that all materials will be icely sterile when cooled down to low temperatures, or correspondingly that phase transitions should only be expected at elevated temperatures? Surprisingly, perhaps, the answer to this question is no for certain classes of materials, and that matter can in fact melt even at the absolute zero of temperature, T= 0 Kelvin (-273.15 degrees centigrade), the point at which all thermal motion ceases. Materials that can melt at T= 0 K are those that experience strong quantum fluctuations. That is fluctuations that arise because of the quantum rules that apply to matter at the atomic scale. Roughly speaking, quantum fluctuations are the uncertainty in the famous Uncertainty Principle proposed by Heisenberg.Although quantum fluctuations might at first be thought to destabilise a system as much the same way as thermal fluctuations destabilise order at a classical (thermal) phase transition, experiments over the last decade or so have revealed in fact that entirely new states of matter can arise in the vicinity of so-called quantum critical points. The development of the theory describing thermal phase transitions produced one of the pinnacles of 20th century science, the Renormalisation Group Theory, for which Kenneth Wilson was awarded the Noble prize. In comparison, the study of quantum phase transitions can be said to be in its infancy. Our work is focussed on making significant contributions to the rapidly developing field of quantum phase transitions by performing experiments on some of the simplest systems that display such phenomena: arrays of interacting atomic magnets ( spins ). Chemical ingenuity allows magnetic arrays of different architecture and dimensionality, such as quantum spin chains, ladders, plaquettes, planes, etc., to be grown within three dimensional crystals. Changes to the spin configuration can then be monitored in exquisite detail by firing beams of neutrons through the arrays and monitoring how the neutrons are deflected as the system is driven through a quantum phase transition (e.g. by applying a magnetic field, pressure, etc.). The full spatial and temporal correlations of the spin system are reconstructed in this way, allowing questions to be answered such as whether the spins crystallise to form an ordered array, or remain disordered to form a quantum spin liquid (and if so what type). This is exactly the type of information required by the large community of theoretical physicists who study quantum phase transitions to test and develop their theories.Our neutron scattering experiments on quantum fluctuations and criticality in model magnets will thus not only help to answer the important fundamental question of how matter can melt at the absolute zero of temperature, but will also provide new insights into the nature of the fascinating states of matter that occur in the vicinity of quantum critical points.