Spin density wave dislocation in chromium probed by coherent X-ray diffraction

We report on the study of a magnetic dislocation in pure chromium. Coherent X-ray diffraction profiles obtained on the incommensurate Spin Density Wave (SDW) reflection are consistent with the presence of a dislocation of the magnetic order, embedded at a few micrometers from the surface of the sample. Beyond the specific case of magnetic dislocations in chromium, this work may open up a new method for the study of magnetic defects embedded in the bulk.     

Crystal optics as guard apertures for coherent x-ray diffraction imaging

A crucial issue in coherent x-ray diffraction imaging experiments is how to increase the signal-to-noise ratio when measuring relatively weak diffraction intensities from a nonperiodic object. A novel crystal guard aperture is described that makes use of a pair of multiple-bounce crystal optics to eliminate unwanted parasitic scattering background. This background is often produced by upstream optical elements such as a coherent-beam defining aperture. Recent experimental observation and theoretical analysis confirm the effectiveness of the crystal guard aperture method with coherence-preserved wave propagation through the crystal guard aperture and dramatically reduced scattering background in coherent x-ray diffraction images.     

Evaluation of threading dislocation density of strained Ge epitaxial layer by high resolution x-ray diffraction

The analysis of threading dislocation density(TDD)in Ge-on-Si layer is critical for developing lasers,light emitting diodes(LEDs),photodetectors(PDs),modulators,waveguides,metal oxide semiconductor field effect transistors(MOS-FETs),and also the integration of Si-based monolithic photonics.The TDD of Ge epitaxial layer is analyzed by etching or transmission electron microscope(TEM).However,high-resolution x-ray diffraction(HR-XRD)rocking curve provides an optional method to analyze the TDD in Ge layer.The theory model of TDD measurement from rocking curves was first used in zinc-blende semiconductors.In this paper,this method is extended to the case of strained Ge-on-Si layers.The HR-XRD 2θ/ωscan is measured and Ge(004)single crystal rocking curve is utilized to calculate the TDD in strained Ge epitaxial layer.The rocking curve full width at half maximum(FWHM)broadening by incident beam divergence of the instrument,crystal size,and curvature of the crystal specimen is subtracted.The TDDs of samples A and B are calculated to be 1.41×10~8cm~(-2)and 6.47×10~8cm~(-2),respectively.In addition,we believe the TDDs calculated by this method to be the averaged dislocation density in the Ge epitaxial layer.     

Pinning and depinning process of an incommensurate CDW as revealed by coherent X-ray diffraction

Experiments on the coherent X-ray diffraction, and their modeling, have been performed on the Charge Density Wave (CDW) system NbSe₃. The 2k_F$2k_F$ satellite reflection associated with the CDW has been measured with respect to external dc currents. Below the threshold current, reflection displays several fringes in the transverse direction which disappear when the threshold current is exceeded. In the sliding state, the transverse satellite profile has a form of two nonsymmetric peaks, one of them being centered at the same position as below the threshold and another one being shifted. The shift of the longitudinal peak position below the threshold current and the nonsymmetric peak in the transverse direction above the threshold one is interpreted as the influence of strong linear defect like a crystal step present on the sample surface, combined with induced arrays of dislocations. Coherent X-rays provide a new access to processes in a CDW driven by an external force in a random pinning potential.     

Iron contamination in ion implanted silicon,as revealed by x-ray and electron diffraction

A surface contamination effect was detected by double-crystal x-ray diffraction analysis of silicon wafers implanted with silicon ions at different doses and energies after annealing at 700 °C.The hypothesis of recoiled oxygen from the native oxide, as the impurity responsible for surface strain, was excluded by x-ray characterization of a series of samples implanted through thermally grown silicon oxides. The surface positions of the strain, resulting from x-ray analysis after 700 °C annealing and the analysis of the electron diffraction patterns, taken on particles originated from precipitation of the impurity by 1000 °C heating, allowed to conclude that the contamination phenomenon is due to iron atoms coming from the ion implanter.     

Single-pulse coherent diffraction imaging using soft x-ray laser

We report a coherent diffraction imaging (CDI) using a single 8 ps soft x-ray laser pulse at a wavelength of 13.9 nm. The soft x-ray pulse was generated by a laboratory-scale intense pumping laser providing coherent x-ray pulses up to the level of 10(11) photons/pulse. A spatial resolution below 194 nm was achieved with a single pulse, and it was shown that a resolution below 55 nm is feasible with improved detector capability. The single-pulse CDI might provide a way to investigate dynamics of nanoscale molecules or particles.     

Framework for three-dimensional coherent diffraction imaging by focused beam x-ray Bragg ptychography

We present the framework for convergent beam Bragg ptychography, and, using simulations, we demonstrate that nanocrystals can be ptychographically reconstructed from highly convergent x-ray Bragg diffraction. The ptychographic iterative engine is extended to three dimensions and shown to successfully reconstruct a simulated nanocrystal using overlapping raster scans with a defocused curved beam, the diameter of which matches the crystal size. This object reconstruction strategy can serve as the basis for coherent diffraction imaging experiments at coherent scanning nanoprobe x-ray sources.     

Three-dimensional GaN-Ga2O3 core shell structure revealed by x-ray diffraction microscopy

In combination of direct phase retrieval of coherent x-ray diffraction patterns with a novel tomographic reconstruction algorithm, we, for the first time, carried out quantitative 3D imaging of a heat-treated GaN particle with each voxel corresponding to 17 x 17 x 17 nm3. We observed the platelet structure of GaN and the formation of small islands on the surface of the platelets, and successfully captured the internal GaN-Ga2O3 core shell structure in three dimensions. This work opens the door for nondestructive and quantitative imaging of 3D morphology and 3D internal structure of a wide range of materials at the nanometer scale resolution that are amorphous or possess only short-range atomic organization.     

Order in amorphous polystyrenes as revealed by electron diffraction and diffraction microscopy

Ordered regions ranging in size between 15 and 45 Å have been established in several atactic polystyrenes of various molecular weights (4800; 51,000; and 1,800,000) as well as in an amorphous isotactic polystyrene by means of electron diffraction and high-resolution diffraction microscopy. The order is due to long-chain polymer molecules which tend to align more-or-less parallel to one another with a constant average spacing. The orderly chain packing can be disturbed upon cross-linking by electron irradiation, the disturbance being manifested in the irreversible changes in the d spacing and broadening of one of the diffuse rings which is inter-molecular in origin. From the results obtained, the high-resolution diffraction microscopy technique appears to have the potential for resolving the question regarding the presence of order in unoriented amorphous polymers. The technique can also differentiate between a broadened diffuse ring due to small crystallites and another due to paracrystals containing numerous defects.     

基于三轴X射线衍射方法的n-GaN位错密度的测试条件分析

三轴X射线衍射技术广泛应用于半导体材料参数的精确测试,然而应用于纤锌矿n-GaN位错密度的测试却可能隐藏极大的误差.本文采用三轴X射线衍射技术测试了两个氢化物气相外延方法生长的n-GaN样品,发现两样品对应衍射面的半高全宽都基本一致,按照镶嵌结构模型,采用Srikant方法或Williamson-Hall方法,两样品的位错密度也应基本一致.但van der Pauw变温霍尔效应测试表明,其中的非故意掺杂样品是莫特相变材料,而掺Si样品则是非莫特相变材料,位错密度有数量级的差别.实验表明,位错沿晶界生长导致的晶粒尺寸效应,表现为三轴X射线衍射技术检测不到晶界晶格畸变区域的位错,给测试带来极大误差,这对正确使用Srikant方法和Williamson-Hall方法提出了测试要求.分析表明,当扭转角与倾转角之比β_(twist)/β_(tiit)≥2.0时,Srikant方法是准确的,否则需进一步由Williamson-Hall方法确定晶粒大小(面内共格长度L//),当L//≥1.5μm时,Srikant方法是准确的.     

Charge density wave fluctuations in one dimensional metals

The magnetic susceptibility χ(T) for several quasi one-dimensional (1-D) inorganic metals was recently found to agree quantitatively with that predicted in 1973 by Lee, Rice and Anderson for 1-D chains exhibiting charge density wave amplitude fluctuations. We derive here a physical interpretation of their predicted pseudogap in the electronic density-of-states (from which they derived χ(T)) and examine applications of the theory to the electronic heat capacity.     

Local plasticity of Al thin films as revealed by x-ray microdiffraction

Grain-to-grain interactions dominate the plasticity of Al thin films and establish effective length scales smaller than the grain size. We have measured large strain distributions and their changes under plastic strain in 1.5-microm-thick Al 0.5% Cu films using a 0.8-microm-diameter white x-ray probe at the Advanced Light Source. Strain distributions arise not only from the distribution of grain sizes and orientation, but also from the differences in grain shape and from stress environment. Multiple active glide plane domains have been found within single grains. Large grains behave like multiple smaller grains even before a dislocation substructure can evolve.     

Charge density wave states in phase-engineered monolayer VTe2

Charge density wave (CDW) strongly affects the electronic properties of two-dimensional (2D) materials and can be tuned by phase engineering. Among 2D transitional metal dichalcogenides (TMDs), VTe$_{2}$ was predicted to require small energy for its phase transition and shows unexpected CDW states in its T-phase. However, the CDW state of H-VTe$_{2}$ has been barely reported. Here, we investigate the CDW states in monolayer (ML) H-VTe$_{2}$, induced by phase-engineering from T-phase VTe$_{2}$. The phase transition between T- and H-VTe$_{2}$ is revealed with x-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) measurements. For H-VTe$_{2}$, scanning tunneling microscope (STM) and low-energy electron diffraction (LEED) results show a robust $2sqrt 3 times 2sqrt 3 $ CDW superlattice with a transition temperature above 450 K. Our findings provide a promising way for manipulating the CDWs in 2D materials and show great potential in its application of nanoelectronics.     

Charge density wave in graphene: Magnetic-field-induced Peierls instability

We suggest that a magnetic-field-induced Peierls instability accounts for the recent experiment of Zhang et al. in which unexpected quantum Hall plateaus were observed at high magnetic fields in graphene on a substrate. This Peierls instability leads to an out-of-plane lattice distortion resulting in a charge density wave (CDW) on sublattices A and B of the graphene honeycomb lattice. We also discuss alternative microscopic scenarios proposed in the literature and leading to a similar CDW ground state in graphene.     

Electron density distribution of LiMn_2O_4 cathode investigated by synchrotron powder x-ray diffraction

Electron density plays an important role in determining the properties of functional materials. Revealing the electron density distribution experimentally in real space can help to tune the properties of materials. Spinel Li Mn2 O4 is one of the most promising cathode candidates because of its high voltage, low cost, and non-toxicity, but suffers severe capacity fading during electrochemical cycling due to the Mn dissolution. Real-space measurement of electron distribution of Li Mn2 O4 experimentally can provide direct evaluation on the strength of Mn–O bond and give an explanation of the structure stability.Here, through high energy synchrotron powder x-ray diffraction(SPXRD), accurate electron density distribution in spinel Li Mn2 O4 has been investigated based on the multipole model. The electron accumulation between Mn and O atoms in deformation density map indicates the shared interaction of Mn–O bond. The quantitative topological analysis at bond critical points shows that the Mn–O bond is relatively weak covalent interaction due to the oxygen loss. These findings suggest that oxygen stoichiometry is the key factor for preventing the Mn dissolution and capacity fading.     

Classical phase space revealed by coherent light

We study the far-field characteristics of oval-resonator laser diodes made of an GaAs/Al(x)Ga(1-x)As quantum well. The resonator shapes are various oval geometries, thereby probing chaotic and mixed classical dynamics. The far-field pattern shows a pronounced fine structure that strongly depends on the cavity shape. Comparing the experimental data with ray-model simulations for a Fresnel billiard yields convincing agreement for all geometries and reveals the importance of the underlying classical phase space for the lasing characteristics.     

Enhanced x-ray phase determination by three-beam diffraction

For decades, solving the phase problem of x-ray scattering has been a goal that, in principle, could be achieved by means of n-beam diffraction (n-BD). However, the phases extracted by the actual n-BD phasing techniques are not very precise, mainly due to systematic errors that are difficult to estimate. We present an innovative theoretical approach and experimental procedure that, combined, eliminate two major sources of error. It is a high precision phasing technique that provides the triplet-phase angle with an error of about 2 degrees.     

Three-dimensional coherent x-ray diffraction imaging of a ceramic nanofoam: determination of structural deformation mechanisms

Ultralow density polymers, metals, and ceramic nanofoams are valued for their high strength-to-weight ratio, high surface area, and insulating properties ascribed to their structural geometry. We obtain the labrynthine internal structure of a tantalum oxide nanofoam by x-ray diffractive imaging. Finite-element analysis from the structure reveals mechanical properties consistent with bulk samples and with a diffusion-limited cluster aggregation model, while excess mass on the nodes discounts the dangling fragments hypothesis of percolation theory.