We examine the low-energy excitations in the vicinity of the quantum critical point in LiHoF$_4$, a physical realization of the Transverse Field Ising Model, focusing on the long-range fluctuations which soften to zero energy at the ferromagnetic quantum phase transition. Microwave spectroscopy in tunable loop-gap resonator structures identifies and characterizes the soft mode and higher-energy electronuclear states. We study these modes as a function of frequency and magnetic fields applied transverse and parallel to the Ising axis. These are understood in the context of a theoretical model of a soft electronuclear mode that interacts with soft photons as well as soft phonons. We identify competing infrared divergences at the quantum critical point, coming from the photons and the electronuclear soft mode. It is an incomplete cancellation of these divergences that leads to the muted but distinct signatures observed in the experiments. The application of a longitudinal magnetic field gaps the soft mode. Measurements well away from the quantum critical point reveal a set of ``Walker'' modes associated with ferromagnetic domain dynamics.
Electronic Structure of the Ferromagnetic Semiconductor Fe-doped Ge Revealed by Soft X-ray Angle-Resolved Photoemission Spectroscopy
Ge $_{1-x}$ Fe $_{x}$（Ge：Fe）在高达210 K的相对高温下显示出铁磁行为，因此是与Si技术兼容的自旋电子学应用的有前景的材料。我们用软X射线角分辨光电子能谱（SX-ARPES）研究了它的电子结构，以阐明其铁磁性的机制。我们在动量空间的宽范围内观察到费米能级态（$E_{F}$）的有限Fe 3 $d$分量，其中$E_{F}$位于价带顶（VBM）之上.第一性原理超原胞计算还表明，$E_{F}$位于VBM之上，在窄的自旋向下的$d$（$e$）带内，并且在源自强$p$（$t_{2}$）杂化的深受体能级起源的自旋向上的杂质带内。结果表明，Ge：Fe的$d$（$e$）窄能带是Fe原子间铁磁耦合的主要原因，而受主能级起源的能带是Ge：Fe的输运性质的主要原因.
Comment on "Instability of the ferromagnetic quantum critical point and symmetry of the ferromagnetic ground state in two-dimensional and three-dimensional electron gases with arbitrary spin-orbit splitting"
University of Oregon（俄勒冈大学）；University of Maryland（马里兰大学）
Magneto-optical investigation of the field-induced spin-glass insulator to ferromagnetic metallic transition of the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$
为了研究磁场驱动自旋玻璃绝缘体到铁磁金属的转变，我们测量了（$_{0.4}$ Pr $_{0.6}$）$_{1.2}$ Sr $_{1.8}$ Mn $_2$ O $_7$的磁光响应.磁场的应用恢复铁磁状态的电子结构的整体红移，增长的束缚载流子本地化与铁磁域，发展的赝能隙，软化的Mn-O的伸缩和弯曲模式，表明结构的变化。我们讨论了领域和温度引起的趋势的框架内的富冈Tokura全球电子相图图片，并建议控制无序附近的相边界可以用来调整磁介电响应。自旋玻璃绝缘体到铁磁金属转变的恢复也可以驱动（La $_{0.4}$ Pr $_{0.6}$）$_{1.2}$ Sr $_{1.8}$ Mn $_2$ O $_7$中的300 K颜色变化。
Quantum critical scaling in the disordered itinerant ferromagnet UCo$_{1-x}$Fe$_x$Ge
Belitz-Kirkpatrick-Vojta（BKV）理论与实验非常吻合地表明，清洁金属中的铁磁量子相变（QPT）通常是一阶的，这是由于磁化强度与电子软模的耦合，与典型的二阶相变形成对比。对于无序金属，BKV理论预测，QPT的二阶性质恢复，因为电子软模式改变其性质从弹道扩散。我们的低温磁化研究确定了无序金属UCo $_{1-x}$ Fe $_x$ Ge中的铁磁QPT作为第一个明确的例子，表现出相关的临界指数预测的BKV理论。
