arXiv subjects

N. Hua

Publications and source records attributed to N. Hua.

4 recordsLinked to original sources

arXiv2025

Memory in strain-tuned insulator-metal-insulator sequence of transitions after photoexcitation in the Mott material V2O3

Memory effects during metal-insulator transitions in quantum materials reveal complex physics and potential for novel electronics mimicking biological neural systems. Nonetheless, understanding of memory and nonlinearity in sequential non-equilibrium transitions remains elusive as the full chain of transitions can involve features lasting anywhere from femtoseconds to microseconds. Here, we extend time-resolved x-ray Bragg diffraction to the dynamic range of timescales spanning 9 orders of magnitude to fully trace the pathways of photoexcited insulator-metal transition and the following relaxation through non-equilibrium metal-insulator transitions in epitaxial films of V2O3, a promising Mott material. We find 5 orders of magnitude variation in metal-insulator transition time, from nanoseconds to hundreds of microseconds, depending on pre-excitation phase state. We provide a theoretical explanation and simulations based on strain feedback to domain nucleation. The lingering transition is stretched in time by memory of spatial and energy heterogeneity and, contrary to known memory effects in vanadium oxides commonly described by power laws, follows an extremely (factor below 0.2) stretched exponential. The induced dramatic slowdown in the light-driven highly correlated system signifies unusually high heterogeneity of transition barriers similar to biological systems, and demonstrates importance of non-local correlations of structure in evolution of transitional phases in quantum materials.

cond-mat.mtrl-sci↗

arXiv2024

Imaging ultrafast electronic domain fluctuations with X-ray speckle visibility

Speckle patterns manifesting from the interaction of coherent X-rays with matter offer a glimpse into the dynamics of nanoscale domains that underpin many emergent phenomena in quantum materials. While the dynamics of the average structure can be followed with time-resolved X-ray diffraction, the ultrafast evolution of local structures in nonequilibrium conditions have thus far eluded detection due to experimental limitations, such as insufficient X-ray coherent flux. Here we demonstrate a nonequilibrium speckle visibility experiment using a split-and-delay setup at an X-ray free-electron laser. Photoinduced electronic domain fluctuations of the magnetic model material Fe$_{3}$O$_{4}$ reveal changes of the trimeron network configuration due to charge dynamics that exhibit liquid-like fluctuations, analogous to a supercooled liquid phase. This suggests that ultrafast dynamics of electronic heterogeneities under optical stimuli are fundamentally different from thermally-driven ones.

cond-mat.str-el↗

arXiv2024

Extending the Takagi-Taupin equations for x-ray nanobeam Bragg coherent diffraction

We present a new approach for simulating x-ray nanobeam Bragg coherent diffraction patterns based on the Takagi-Taupin equations. Compared to conventional methods, the current approach can be universally applied to any weakly strained system including semi-infinite crystals that diffract dynamically. It addresses issues such as the curved wavefront and re-divergence of the focused incident beam. We show excellent agreement against experimental data on a strained La0.7Sr0.3MnO3 thin film on SrTiO3 substrate, and a path to extracting physical information using automatic differentiation.

cond-mat.mes-hall↗

arXiv2017

Nucleation of dislocations and their dynamics in layered oxides cathode materials during battery charging

Defects and their interactions in crystalline solids often underpin material properties and functionality as they are decisive for stability, result in enhanced diffusion, and act as a reservoir of vacancies. Recently, lithium-rich layered oxides have emerged among the leading candidates for the next-generation energy storage cathode material, delivering 50 % excess capacity over commercially used compounds. Oxygen-redox reactions are believed to be responsible for the excess capacity, however, voltage fading has prevented commercialization of these new materials. Despite extensive research the understanding of the mechanisms underpinning oxygen-redox reactions and voltage fade remain incomplete. Here, using operando three-dimensional Bragg coherent diffractive imaging, we directly observe nucleation of a mobile dislocation network in nanoparticles of lithium-rich layered oxide material. Surprisingly, we find that dislocations form more readily in the lithium-rich layered oxide material as compared with a conventional layered oxide material, suggesting a link between the defects and the anomalously high capacity in lithium-rich layered oxides. The formation of a network of partial dislocations dramatically alters the local lithium environment and contributes to the voltage fade. Based on our findings we design and demonstrate a method to recover the original high voltage functionality. Our findings reveal that the voltage fade in lithium-rich layered oxides is reversible and call for new paradigms for improved design of oxygen-redox active materials.

cond-mat.mtrl-sci↗