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Researchers from UCLA have used ultrafast electron diffraction to reveal a hidden electronic phase that cannot be reached by ordinary heating. The study focuses on 1T-TaS₂, a layered material in which electrons naturally organize into a periodic charge pattern known as a charge density wave. Theory has long suggested that such ordered electronic patterns could “melt” into a liquid-like state, but in this material a competing structural transformation occurs near 600 K and normally prevents that regime from being observed. By using femtosecond laser pulses, the experiment bypassed this limitation: the material was driven into a transient, superheated state while compressed electron pulses captured how the electronic order evolved in real time.

The diffraction measurements show that, after laser excitation, the charge order first disappears and then partially reforms through a sequence of intermediate states controlled by defects. At lower starting temperature, the diffraction pattern indicates a partially melted state: the regular spacing of the charge pattern is lost, but a preferred angular alignment remains. At higher temperature, the diffraction peaks spread into a nearly uniform ring, showing that both positional and angular order have disappeared. This is the experimental signature of a liquid charge density wave. More broadly, the work demonstrates how ultrafast electron diffraction can uncover short-lived phases of matter that are hidden from conventional equilibrium experiments.

Dichroic X-ray tomography resolves a sample’s 3D crystal orientation (linear dichroism) or magnetization (circular dichroism) from projections taken at varying beam directions and polarizations. Like most tomographic methods, its reconstruction assumes that absorption along a ray is a simple line integral of a local, voxel-independent absorption coefficient. This work showed that for linear dichroism that assumption can fail: as the beam crosses an anisotropic material whose optic axis is misaligned with the polarization, the polarization itself rotates dynamically. Using a finite-element Maxwell solver, full-field tomography of a 2000 Å aragonite Voronoi polycrystal was simulated at the O K-edge. Reconstructions indicated that polarization rotation through propagation (PRP) introduces real artifacts, including spurious grain boundaries, false orientation gradients, and the apparent splitting of single grains. The authors found that the reconstruction error increased by three orders of magnitude as the dichroism was tuned from weak (0.1) to full strength (1.0). Qualitatively, the observed reconstruction artifacts mimic genuine microstructure, demonstrating a potential for false conclusions to be drawn about a material’s orientational statistics or grain-boundary structure.

These results identify a previously underappreciated, physics-based limitation of linear dichroic orientation tomography, a technique of growing importance for imaging anisotropic functional materials. This work also supplies practical diagnostics, including degraded sinogram fit residuals and a rotationally invariant biaxiality measure that flags voxels where the uniaxial reconstruction is likely to break down. The authors chart concrete mitigation paths: acquiring redundant data across additional tilt and polarization conditions so the reconstruction can down-weight the worst-affected angles, operating at energies where dichroism is weak, or folding a beam-propagation model directly into the reconstruction. Additionally, they outline a fixed-point iterative scheme and a more robust model-based iterative reconstruction (MBIR) framework for performing propagation-aware reconstructions that keep dichroic tomography quantitatively reliable.

Colloidal semiconductor nanocrystals (NCs) offer remarkable tunability of optoelectronic properties through synthetic control of their size, shape, and composition. An emerging strategy is the synthesis of III-V nanocrystals in molten salts, where high temperatures and unique redox chemistry enable the production of highly crystalline binary, ternary, and quaternary semiconductor NCs. A notable example of the compositional complexity that can be achieved by these methods is the cation exchange of InP_1-yAs_y in a gallium-containing molten salt to yield In_1-xGa_xP_1-yAs_y NCs. However, the process by which Ga atoms migrate through the NC lattice, and the resulting elemental distributions, was not fully understood.

STROBE scientists, together with collaborators at the University of Chicago, employed advanced scanning transmission electron microscopy (STEM) methods to reveal the atomic structure and elemental distribution of NCs taken throughout the cation exchange process. High angle annular dark field (HAADF-STEM) imaging revealed only subtle changes in NC morphology, with the tetrahedral shape and zinc blende crystal structure largely maintained throughout the exchange. Energy dispersive x-ray spectroscopy (STEM-EDS) was used to map the elemental distribution of the NCs. To overcome the beam sensitivity and low STEM-EDS signal resulting from the NCs, extensive experimental optimization and novel image processing methods were applied to produce elemental maps representative of individual In_1-xGa_xP_1-yAs_y NCs. The elemental maps revealed that Ga first exchanges as the surface of NCs, before diffusing inward. The In/Ga distribution becomes more, but never completely homogeneous, resulting in a compositional gradient that persists at long annealing times. Modeling in COMSOL suggests that the rate of diffusion becomes progressively slower as the cation exchange progresses.

These findings demonstrate that cation exchange occurs through a diffusion-limited process, resulting in a graded elemental distribution and incomplete exchange even at long annealing times. The intentional use of graded elemental distributions in II-VI NCs for superior optoelectronic performance suggests that diffusion-limited exchange may be desirable for producing high-performance III-V NCs. Furthermore, the image processing methods developed in this work may be generally useful for the analysis of NCs by methods like EDS or electron energy loss spectroscopy.