Spin-phonon coupling across a magnetic quantum critical point in Mn1-xFexSi

这一建议解决了Mn1-xFexSi中自旋-声子耦合的存在及其意义，其中通过增加Fe浓度到x=0.17处的量子临界点来抑制日磁有序。X=0时的母体化合物MnSi是一种开创性的化合物，用于竞争磁相互作用，从而产生具有新晶序的复杂相图。在x=1的另一端，FeSi在低温下是一种窄间隙的非磁性绝缘体，但在高温下表现出温度激活的顺磁性，这让科学界困惑了近50年。整个掺杂系列的非中心对称晶体结构导致了有限的自旋-轨道相互作用，提供了磁矩和晶格之间的自然耦合。最近对晶格动力学性质的研究-包括我们自己的工作-揭示了FeSi中磁自由度和晶格自由度之间的密切但意想不到的联系：(1)介导传统电声子耦合的电子态只有在存在强烈的磁涨落时才被激活。(2)Fe-Fe距离变化很大的声子通过与温度感生磁矩的动态耦合被抑制，突出了FeSi是一种具有直接自旋-声子耦合和多个相互作用路径的材料。我们提出了一个工作方案，通过声子谱研究了Mn1-xFexSi中自旋-声子耦合的能量、动量和成分依赖关系。基于FeSi的结果和MNSI的初步数据，我们将重点研究R点的纵向声子的演化，即动量空间中沿[111]方向的区域边界，它表现出强烈的变化的Mn/Fe-Mn/Fe距离。从头算晶格动力学计算预测了R点上声子重整化的壮观场面，它随Fe浓度的增加而增加，但与磁有序基态密切相关。在x=0.17时，磁序的抑制可能会对这一行为产生关键影响，并可能导致对所谓的磁量子临界点的强烈晶格动力学响应。在实验上，我们已经有10个表征良好的样品，掺杂水平为0.03≤x≤0.32。我们将利用拉曼散射获得区域中心光学声子的全x依赖关系，然后通过动量和能量分辨的高分辨率非弹性x射线散射和非弹性中子光谱来研究最有趣的样品，以测量上述讨论的声子在R点处的演化。我们的研究将仔细审查迄今为止被忽视的自旋和晶格自由度之间的相互作用，在自旋电子器件中具有潜在的应用前景。在这里，了解这类材料对掺杂等外部参数的响应方式是开发新材料和使其功能化的关键挑战。

This proposal addresses the presence and implications of spin-phonon coupling in Mn1-xFexSi where helimagnetic order is suppressed by increasing Fe concentration to a quantum critical point at x = 0.17. The parent compound at x = 0, MnSi, is a seminal compound for competing magnetic interactions resulting in a complex phase diagram with novel crystalline orders. On the other end at x = 1, FeSi is a narrow-gap and non-magnetic insulator at low temperatures but exhibits temperature-activated paramagnetism at elevated temperatures puzzling the scientific community for nearly 50 years. The noncentrosymmetric crystal structure present for the whole doping series results in a finite spin-orbit interaction providing a natural coupling between magnetic moments and the crystal lattice. Recent investigations of lattice dynamical properties – including our own work – revealed a close but unexpected link between magnetic and lattice degrees of freedom in FeSi: (1) Electronic states mediating conventional electron-phonon coupling are only activated in the presence of strong magnetic fluctuations. (2) Furthermore, phonons entailing strongly varying Fe-Fe distances are damped via dynamic coupling to the temperature-induced magnetic moments, highlighting FeSi as a material with direct spin-phonon coupling and multiple interaction paths. We propose a work program to investigate the energy, momentum and compositional dependence of spin-phonon coupling in Mn1-xFexSi via phonon spectroscopy. Based on the results for FeSi and preliminary data on MnSi, we will focus on the evolution of longitudinal phonons at the R point, i.e., the zone boundary along the [111] direction in momentum space, which exhibit strongly varying Mn/Fe – Mn/Fe distances. Ab-initio lattice-dynamical calculations predict spectacular phonon renormalization at the R point with increasing Fe concentration but closely linked to a magnetically ordered ground state. The suppression of magnetic order at x = 0.17 may have a critical impact on this behavior and could result in a strong lattice dynamical response to the alleged magnetic quantum critical point. Experimentally, we already have 10 well-characterized samples with doping levels 0.03 ≤ x ≤ 0.32. We will employ Raman scattering to obtain a full x dependence of zone center optical phonons and then investigate the most interesting samples via momentum and energy resolved high resolution inelastic x-ray scattering and inelastic neutron spectroscopy in order to measure the evolution of the above discussed phonons at the R point.Our investigation will scrutinize the so far overlooked interaction between spin and lattice degrees of freedom in a seminal material family with potential applications in spintronic devices. Here, understanding the ways in which such materials respond to extrinsic parameters such as doping is a key challenge for developing and functionalizing new materials.