He-3 Cryostat for Muon Spin Rotation/Relaxation Studies of Quantum Materials

传统的有序和无序状态之间的相变是由热涨落驱动的。在以竞争相为特征的强关联电子材料中，可能会发生一种截然不同的转变。当其中一个相变温度被非热控制参数(外加压力、外加磁场或电子密度的化学操纵)驱动到绝对零度时，可能会出现分隔物质两个不同量子相的量子临界点(QCP)。在QCP附近，可能会出现异常现象(例如高温超导、非费米液体行为、磁性边缘的偏序)，量子涨落可以影响材料的性质，使其达到非常高的温度。 理解量子临界性构成了现代凝聚态物理学的一个重大挑战。在许多情况下，QCP的发生可以通过在临界区产生相分离的无序(外在或内在驱动)来绕过。需要局部实验探头来识别和表征相分离。Muon自旋旋转/弛豫(SR)技术就是这样一个探测器。这种方法是基于在材料中注入自旋极化的正µ子(+)，并随后探测衰变正电子，以揭示局部磁场对µ子自旋运动的影响。作为一种纯磁探头，SR方法对各种磁场敏感，对空间偏析相敏感。 位于TRIUMF的分子和材料科学中心(CMMS)拥有一台独特的SR光谱仪，名为“NuTime”，它提供了一种探测受强磁场影响的材料的方法。目前，它在量子材料研究中的适用性受到温度的限制。加拿大和国际研究人员建议，通过为NuTime配备He-3低温恒温器，将材料冷却到亚开尔文温度，就可以将SR应用于量子临界性和通过外加磁场可调的新态物质的研究。拟议中的设备将在西半球建立量子材料研究的独特能力，这是加拿大公认的战略优先研究领域。

Conventional phase transitions between an ordered and disordered state are driven by thermal fluctuations. A profoundly different kind of transition can occur in strongly-correlated electronic materials characterized by competing phases. When one of the phase transition temperatures is driven to absolute zero by a non-thermal control parameter (applied pressure, applied magnetic field or chemical manipulation of electron density), a quantum critical point (QCP) separating two distinct quantum phases of matter may occur. Near a QCP unusual phenomena can emerge (e.g. high-temperature superconductivity, non-Fermi liquid behaviour, partial order at the verge of magnetism) and quantum fluctuations can affect the properties of a material to remarkably high temperatures. Understanding quantum criticality constitutes a grand challenge of modern condensed matter physics. In many cases the occurrence of a QCP may be circumvented by disorder (extrinsically or intrinsically driven) that produces phase separation in the critical region. Local experimental probes are required to recognize and characterize phase separation. The muon spin rotation/relaxation (SR) technique is such a probe. This method is based on the implantation of spin-polarized positive muons (+) in a material and the subsequent detection of the decay positrons to reveal the influence of the local magnetic fields on the motion of the muon spin. As a pure magnetic probe, the SR method is sensitive to various kinds of magnetism and is sensitive to spatially segregated phases. The Centre for Molecular and Materials Science (CMMS) at TRIUMF is home to a unique SR spectrometer, called “NuTime”, which provides a means of probing materials subject to a high magnetic field. Its applicability to the study of quantum materials is currently limited by temperature. It is proposed, that by outfitting NuTime with a He-3 cryostat to cool materials to sub-Kelvin temperatures, Canadian and International researchers can apply SR to the study of quantum criticality and novel states of matter tunable by applying magnetic field. The proposed equipment will establish a unique capability in the Western Hemisphere for quantum materials’ research, a recognized strategic priority research area in Canada.