The reference chiral helimagnet MnSi is the first system where skyrmion lattice correlations have been reported. At zero magnetic field the transition at $T_C$ to the helimagnetic state is of first order. Above $T_C$, in a region dominated by precursor phenomena, neutron scattering shows the build up of strong chiral fluctuating correlations over the surface of a sphere with radius $2π/\ell$, where $\ell$ is the pitch of the helix. It has been suggested that these fluctuating correlations drive the helical transition to first order following a scenario proposed by Brazovskii for liquid crystals. We present a comprehensive neutron scattering study under magnetic fields, which provides evidence that this is not the case. The sharp first order transition persists for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and start to concentrate along the field direction already above 0.2 T. Our results thus disconnect the first order nature of the transition from the precursor fluctuating correlations. They also show no indication for a tricritical point, where the first order transition crosses over to second order with increasing magnetic field. In this light, the nature of the first order helical transition and the precursor phenomena above $T_C$, both of general relevance to chiral magnetism, remain an open question.
Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite
跨越相变的对称性破坏通常会导致选择规则的变化和光学模式的出现，这些光学模式可以通过光谱技术检测到或在泵浦-探测实验中相干产生。在二阶或弱一阶跃迁中，序参量的波动出现在有序温度以上，从而产生有趣的前体现象，如临界乳光。在这里，我们表明，在磁铁矿（Fe $_3$ O $_4$）光激发耦合到电荷顺序的临界波动和相干产生的有序相的结构模式高于Verwey过渡的临界温度。我们的研究结果是通过超快宽带光谱检测光学常数的相干振荡，并分析它们对温度的依赖性。为了揭示结构模式和电子激发之间的耦合，在Verwey跃迁的起源，我们结合我们的结果从泵浦-探测实验与自发拉曼散射数据和声子色散曲线和光学常数的理论计算。我们的方法是一个有效的工具来研究跨相变的临界波动的实时动态。
Quantum incommensurate Skyrmion crystals and Commensurate to In-commensurate transitions in cold atoms and materials with spin orbit couplings in a Zeeman field
本文研究了二维各向异性Rashba型自旋轨道耦合（SOC）和塞曼场作用下晶格中强相互作用的旋量原子。我们发现塞曼场和SOC之间的相互作用提供了一个新的平台，主机丰富和新颖类的量子公度和非公度相位，激发和相变。这些公度相分别包括低塞曼场和高塞曼场下的两个共线态和镜反射SOC参数下的两个共面倾斜态。最重要的是，存在由4个公度相包围的非共面的非公度Skyrmion（IC-SkX）晶体相。计算了所有5个相位以上的新激发谱，特别是IC-SKX相位。三个不同类别的量子相称的不相称的过渡从IC-SKX到其4个相邻的相称的阶段被确定。有限的温度行为和转变进行了讨论。所有相的临界温度可以通过简单地调整每个位点的原子数$ N $而提高到高于当前冷原子冷却技术所能达到的温度。鉴于最近在光晶格中产生冷原子的2d SOC方面取得的令人印象深刻的实验进展，这些新的多体现象可以在当前和不久的将来的冷原子实验中探索。给出了在MnSi、Fe $_{0.5}$ Co $_{0.5}$ Si等材料中的应用，特别是Li $_2$ IrO $_3$中的复杂无公度磁序.
First-order phase transition driven by competing charge-order fluctuations in 1T'-TaTe$_{2}$
UGC-DAE Consortium for Scientific Research（UGC-DAE 科学研究联合体）；Deutsches Elektronen-Synchrotron DESY（德国电子同步加速器研究所）；Donostia International Physics Center (DIPC)（多诺斯蒂亚国际物理中心）；University of Trento（特伦托大学）；University of the Basque Country (UPV/EHU)（巴斯克地区大学）；Diamond Light Source Ltd（钻石光源有限公司）；RIKEN SPring-8 Center（理化学研究所SPring-8中心）；Aalto University（阿尔托大学）
