Quantum fluctuations and criticality in model magnets

模型磁体中的量子涨落和临界性

基本信息

批准号：
EP/F032293/1
负责人：
Desmond McMorrow
金额：
$ 59.7万
依托单位：
University College London
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2008
资助国家：
英国
起止时间：
2008 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FF032293%2F1
关键词：
Quantum fluctuations criticality model magnets

项目摘要

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.

相变在自然界中普遍存在。事实上，人们的日常生活模式与一种特别重要物质的相变密切相关，尽管他们很少认真考虑这一点。无论是等待水壶烧开来喝早茶，还是在漫长的一天结束后等待饮料中的冰融化，人们都依赖于 H2O 在冰、水或蒸汽不同状态下的特性。这些相的稳定性一方面来自于水分子之间的吸引力，另一方面来自于热波动的无序效应之间的平衡，热波动的无序效应随着温度的升高而变得更加强烈。当吸引力战胜热波动时，H2O分子气体在冻结成冰之前首先凝结成水。这是否意味着所有材料在冷却到低温时都将是冰冷无菌的，或者相应地，相变应该只发生在温度升高？令人惊讶的是，对于某些类别的材料来说，这个问题的答案也许是否定的，而且事实上，即使在绝对零温度下，T= 0 开尔文（-273.15 摄氏度），即所有热运动发生的点，物质也可以熔化。停止。在 T= 0 K 时可以熔化的材料是那些经历强烈量子涨落的材料。这是由于适用于原子尺度物质的量子规则而产生的波动。粗略地说，量子涨落是海森堡提出的著名的不确定性原理中的不确定性。虽然量子涨落一开始可能被认为会破坏系统的稳定性，就像热涨落破坏经典（热）相变秩序的稳定一样，但实验超过事实上，过去十年左右的时间表明，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。我们的工作重点是通过在一些显示此类现象的最简单系统上进行实验：相互作用的原子磁体（自旋）阵列，为快速发展的量子相变领域做出重大贡献。化学独创性允许不同结构和维度的磁性阵列，例如量子自旋链、梯子、小片、平面等，在三维晶体内生长。然后，可以通过通过阵列发射中子束并监测中子在系统驱动通过量子相变时如何偏转（例如通过施加磁场、压力等）来详细监测自旋构型的变化。。自旋系统的完整空间和时间相关性以这种方式重建，从而可以回答诸如自旋是否结晶形成有序阵列，或保持无序形成量子自旋液体（如果是的话是什么类型）等问题。这正是研究量子相变以测试和发展其理论的广大理论物理学家所需的信息类型。因此，我们关于模型磁体中的量子涨落和临界性的中子散射实验不仅有助于回答重要的基本问题研究物质如何在绝对零温度下熔化，而且还将为量子临界点附近发生的令人着迷的物质状态的本质提供新的见解。

项目成果

期刊论文数量（10）

专著数量（0）

科研奖励数量（0）

会议论文数量（0）

专利数量（0）

Quantum magnets under pressure: controlling elementary excitations in TlCuCl3.

DOI：
10.1103/physrevlett.100.205701
发表时间：
2008-03-26
期刊：
Physical review letters
影响因子：
8.6
作者：
C. Rüegg;B. Norm;M. Matsumoto;A. Furrer;D. McMorrow;K. Krämer;H. Güdel;S. Gvasaliya;H. Mutka
通讯作者：
H. Mutka

Pressure dependence of phonon modes across the tetragonal to collapsed-tetragonal phase transition in CaFe 2 As 2

CaFe 2 As 2 中四方相到塌陷四方相变声子模式的压力依赖性

DOI：
10.1103/physrevb.81.144502
发表时间：
2009-11-09
期刊：
Physical Review B
影响因子：
3.7
作者：
R. Mittal;R. Heid;A. Bosak;T. Forrest;S. Chaplot;D. Lamago;D. Reznik;K. Bohnen;Y. Su;N. Kumar;S. Dhar;A. Thamizhavel;Christian Ruegg;M. Krisch;D. McMorrow;T. Brueckel;L. Pintschovius
通讯作者：
L. Pintschovius

Ubiquity of amplitude-modulated magnetic ordering in the H - T phase diagram of the frustrated non-Fermi-liquid YbAgGe

受抑非费米液体 YbAgGe 的 H-T 相图中普遍存在调幅磁序