Disentangling charge and structural contributions during coherent atomic motions studied by ultrafast resonant x-ray diffraction

通过全X射线偏振分析解开多极共振

Disentangling multipole resonances through a full x-ray polarization analysis

完整的偏振分析应用于共振X射线散射在铬K-边缘在K2 CrO 4表明，入射线偏振的X射线可以被转换成圆偏振的X射线衍射在铬前边缘（E = 5994 eV）。这种现象背后的物理机制是纯偶极（E1-E1）和纯四极（E2-E2）跃迁之间的微妙干涉效应，导致相应散射振幅之间的相移。可以利用该效应来解开在常规能量扫描中表现为单个峰值的两个靠近的共振，以这种方式允许挑出并识别所涉及的不同多极序参量。

Probing photo-induced rearrangements in the NdNiO$_{3}$ magnetic spiral with polarization-sensitive ultrafast resonant soft x-ray scattering

We use resonant soft X-ray diffraction to track the photo-induced dynamics of the antiferromagnetic structure in a NdNiO$_{3}$ thin film. Femtosecond laser pulses with a photon energy of 0.61 eV, resonant with electron transfer between long-bond and short-bond nickel sites, are used to excite the material and drive an ultrafast insulator-metal transition. Polarization sensitive soft X-ray diffraction, resonant to the nickel L$_{3}$-edge, then probes the evolution of the underlying magnetic spiral as a function of time delay with 80 picosecond time resolution. By modelling the azimuthal depende…

Exchange-striction driven ultrafast nonthermal lattice dynamics in NiO

We use femtosecond electron diffraction to study ultrafast lattice dynamics in the highly correlated antiferromagnetic (AF) semiconductor NiO. Using the scattering vector (Q) dependence of Bragg diffraction, we introduce a Q-resolved effective lattice temperature, and identify a nonthermal lattice state with preferential displacement of O compared to Ni ions, which occurs within ~0.3 ps and persists for 25 ps. We associate this with transient changes to the AF exchange striction-induced lattice distortion, supported by the observation of a transient Q-asymmetry of Friedel pairs. Our observatio…

Ultrafast Electron Diffraction: Visualizing Dynamic States of Matter

Since the discovery of electron-wave duality, electron scattering instrumentation has developed into a powerful array of techniques for revealing the atomic structure of matter. Beyond detecting local lattice variations in equilibrium structures, recent research efforts have been directed towards the long sought-after dream of visualizing the dynamic evolution of matter in real-time. The atomic behavior at ultrafast timescales carries critical information on phase transition and chemical reaction dynamics, the coupling of electronic and nuclear degrees of freedom in materials and molecules, th…

多铁性BiFeO $_3$中局域激发载流子产生超快非均匀应变

Localized excited charge carriers generate ultrafast inhomogeneous strain in the multiferroic BiFeO$_3$

我们应用飞秒时间分辨率的超快X射线衍射来监测多铁性BiFeO $_3$薄膜在带隙以上光激发后的晶格动力学。观察到的晶格应变的声速有限的演变表示准瞬时光致应力的纳秒时间尺度上衰减。这种应力表现出不均匀的空间分布证明了布拉格峰的加宽。这些新的数据需要在BiFeO $_3$的光生应力的现有模型的实质性修改：相关的激发载流子必须保持本地化的数据是一致的。

层状锰氧化物中金属相的超快结构和电子动力学

Ultrafast structural and electronic dynamics of the metallic phase in a layered manganite

锰氧化物中不同状态之间的转变可以由各种外部刺激驱动。用光控制这些转变为研究它们进化的微观路径提供了可能性。利用飞秒透射电子显微术研究了双层锰氧化物对超快光激发的响应。我们发现，光致温度跳跃发射的压力波，挑起相干振荡的晶格参数，通过超快电子衍射检测。它们对电子结构的影响通过超快电子能量损失谱（EELS）进行监测，揭示了不同轨道响应特定结构扭曲的动力学。

Disentangling amplitude and phase dynamics of a charge density wave in a photo-induced phase transition

Upon excitation with an intense ultrafast laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways dissimilar from those in thermal equilibrium. Determining the mechanism underlying these photo-induced phase transitions (PIPTs) has been a long-standing issue in the study of condensed matter systems. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) material, LaTe$_3$. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a quick …

Probing the pathway of an ultrafast structural phase transition to illuminate the transition mechanism in Cu2S

Disentangling the primary order parameter from secondary order parameters in phase transitions is critical to the interpretation of the transition mechanisms in strongly correlated systems and quantum materials. Here we present a study of structural phase transition pathways in superionic Cu2S nanocrystals that exhibit intriguing properties. Utilizing ultrafast electron diffraction techniques sensitive in both momentum-space and the time-domain, we distinguish the dynamics of crystal symmetry breaking and lattice expansion in this system. We are able to follow the transient states along the tr…

Disentangling Electronic, Lattice and Spin Dynamics in the Chiral Helimagnet Cr1/3NbS2

We investigate the static and ultrafast magneto-optical response of the hexagonal chiral helimagnet $Cr_{1/3}NbS_{2}$ above and below the helimagnetic ordering temperature. The presence of a magnetic easy plane contained within the crystallographic ab-plane is confirmed, while degenerate optical pump-probe experiments reveal significant differences in the dynamic between the parent, $NbS_{2}$, and Cr-intercalated compounds. Time resolved magneto-optical Kerr effect measurements show a two-step demagnetization process, where an initial, sub-ps relaxation and subsequent buildup ($τ> 50$ ps) in t…