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

该提案解决了Mn 1-xFexSi中自旋-声子耦合的存在和影响，其中螺旋磁序通过增加Fe浓度到x = 0.17的量子临界点来抑制。母体化合物在x = 0，MnSi，是一种开创性的化合物竞争的磁相互作用，导致复杂的相图与新的结晶顺序。在x = 1的另一端，FeSi在低温下是一种窄隙和非磁性绝缘体，但在高温下表现出温度激活的顺磁性，困扰了科学界近50年。非中心对称的晶体结构存在于整个掺杂系列的结果在一个有限的自旋-轨道相互作用提供了一个自然的磁矩和晶格之间的耦合。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）介导常规电子-声子耦合的电子态仅在存在强磁波动的情况下被激活。(2)此外，声子引起强烈变化的Fe-Fe距离通过动态耦合到温度诱导的磁矩被阻尼，突出FeSi作为具有直接自旋-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计划，通过声子谱研究Mn 1-xFexSi中自旋-声子耦合的能量，动量和成分依赖性。基于FeSi的结果和MnSi的初步数据，我们将重点关注R点处的纵向声子的演化，即，沿[111]方向沿着动量空间的带边界，表现出强烈变化的Mn/Fe-Mn/Fe距离。从头算晶格动力学计算预测壮观的声子重整化在R点与铁浓度的增加，但密切相关的磁有序的基态。在x = 0.17时磁序的抑制可能对这种行为产生关键影响，并可能导致对所谓的磁量子临界点的强晶格动力学响应。 在实验上，我们已经有10个掺杂水平为0.03 ≤ x ≤ 0.32的良好表征的样品。我们将采用拉曼散射获得完整的x依赖区中心光学声子，然后研究最有趣的样品通过动量和能量分辨高分辨率非弹性x-射线散射和非弹性中子能谱，以测量上述讨论的声子在R点的演化。我们的研究将仔细研究迄今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应用的材料族。在这里，了解这些材料对掺杂等外部参数的响应方式是开发和功能化新材料的关键挑战。

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.