43 research outputs found
Replica Higher-Order Topology of Hofstadter Butterflies in Twisted Bilayer Graphene
The Hofstadter energy spectrum of twisted bilayer graphene is found to have
recursive higher-order topological properties. We demonstrate that higher-order
topological insulator (HOTI) phases, characterized by localized corner states,
occur as replicas of the original HOTIs to fulfill the self-similarity of the
Hofstadter spectrum. We show the existence of the exact flux translational
symmetry of twisted bilayer graphene at all commensurate angles. Based on this
result, we carefully identify that the original HOTI phase at zero flux is
re-entrant at a half-flux periodicity, where the effective twofold rotation is
preserved. In addition, numerous replicas of the original HOTIs are found for
fluxes without protecting symmetries. Similar to the original HOTIs, replica
HOTIs feature both localized corner states and edge-localized real-space
topological markers. The replica HOTIs originate from the different interaction
scales, namely, intralayer and interlayer couplings, in twisted bilayer
graphene. The topological aspect of Hofstadter butterflies revealed in our
results highlights symmetry-protected topology in quantum fractals.Comment: 6 pages, 4 figures + Supplemental Materia
Visualization of a mammalian mitochondrion by coherent x-ray diffractive imaging
We report a three dimensional (3D) quantitative visualization of a mammalian mitochondrion by coherent x-ray diffractive imaging (CXDI) using synchrotron radiation. The internal structures of a mitochondrion from a mouse embryonic fibroblast cell line (NIH3T3) were visualized by tomographic imaging at approximately 60 nm resolution without the need for sectioning or staining. The overall structure consisted of a high electron density region, composed of the outer and inner membranes and the cristae cluster, which enclosed the lower density mitochondrial matrix. The average mass density of the mitochondrion was about 1.36 g/cm3. Sectioned images of the cristae reveal that they have neither a baffle nor septa shape but were instead irregular. In addition, a high resolution, about 14 nm, 2D projection image was captured of a similar mitochondrion with the aid of strongly scattering Au reference objects. Obtaining 3D images at this improved resolution will allow CXDI to be an effective and nondestructive method for investigating the innate structure of mitochondria and other important life supporting organelles. ? 2017 The Author(s).11Ysciescopu
All-optical seeding of a light-induced phase transition with correlated disorder
Ultrafast manipulation of vibrational coherence is an emergent route to
control the structure of solids. However, this strategy can only induce
long-range correlations and cannot modify atomic structure locally, which is
required in many technologically-relevant phase transitions. Here, we
demonstrate that ultrafast lasers can generate incoherent structural
fluctuations which are more efficient for material control than coherent
vibrations, extending optical control to a wider range of materials. We observe
that local, non-equilibrium lattice distortions generated by a weak laser pulse
reduce the energy barrier to switch between insulating and metallic states in
vanadium dioxide by 6%. Seeding inhomogeneous structural-fluctuations presents
an alternative, more energy efficient, route for controlling materials that may
be applicable to all solids, including those used in data and energy storage
devices
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Single-shot 3D coherent diffractive imaging of core-shell nanoparticles with elemental specificity.
We report 3D coherent diffractive imaging (CDI) of Au/Pd core-shell nanoparticles with 6.1 nm spatial resolution with elemental specificity. We measured single-shot diffraction patterns of the nanoparticles using intense x-ray free electron laser pulses. By exploiting the curvature of the Ewald sphere and the symmetry of the nanoparticle, we reconstructed the 3D electron density of 34 core-shell structures from these diffraction patterns. To extract 3D structural information beyond the diffraction signal, we implemented a super-resolution technique by taking advantage of CDI's quantitative reconstruction capabilities. We used high-resolution model fitting to determine the Au core size and the Pd shell thickness to be 65.0 ± 1.0 nm and 4.0 ± 0.5 nm, respectively. We also identified the 3D elemental distribution inside the nanoparticles with an accuracy of 3%. To further examine the model fitting procedure, we simulated noisy diffraction patterns from a Au/Pd core-shell model and a solid Au model and confirmed the validity of the method. We anticipate this super-resolution CDI method can be generally used for quantitative 3D imaging of symmetrical nanostructures with elemental specificity
Emergence of liquid following laser melting of gold thin films
X-ray structural science is undergoing a revolution driven by the emergence of X-ray Free-electron Laser (XFEL) facilities. The structures of crystalline solids can now be studied on the picosecond time scale relevant to phonons, atomic vibrations which travel at acoustic velocities. In the work presented here, X-ray diffuse scattering is employed to characterize the time dependence of the liquid phase emerging from femtosecond laser-induced melting of polycrystalline gold thin films using an XFEL. In a previous analysis of Bragg peak profiles, we showed the supersonic disappearance of the solid phase and presented a model of pumped hot electrons carrying energy from the gold surface to scatter at internal grain boundaries. This generates melt fronts propagating relatively slowly into the crystal grains. By conversion of diffuse scattering to a partial X-ray pair distribution function, we demonstrate that it has the characteristic shape obtained by Fourier transformation of the measured F(Q). The diffuse signal fraction increases with a characteristic rise-time of 13 ps, roughly independent of the incident pump fluence and consequent final liquid fraction. This suggests the role of further melt-front nucleation processes beyond grain boundaries
An Online Dynamic Amplitude-Correcting Gradient Estimation Technique to Align X-ray Focusing Optics
High-brightness X-ray pulses, as generated at synchrotrons and X-ray free
electron lasers (XFEL), are used in a variety of scientific experiments. Many
experimental testbeds require optical equipment, e.g Compound Refractive Lenses
(CRLs), to be precisely aligned and focused. The lateral alignment of CRLs to a
beamline requires precise positioning along four axes: two translational, and
the two rotational. At a synchrotron, alignment is often accomplished manually.
However, XFEL beamlines present a beam brightness that fluctuates in time,
making manual alignment a time-consuming endeavor. Automation using classic
stochastic methods often fail, given the errant gradient estimates. We present
an online correction based on the combination of a generalized finite
difference stencil and a time-dependent sampling pattern. Error expectation is
analyzed, and efficacy is demonstrated. We provide a proof of concept by
laterally aligning optics on a simulated XFEL beamline