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We employ the high-speed synchrotron hard X-ray imaging and diffraction techniques to monitor the laser powder bed fusion (LPBF) process of Ti-6Al-4V in situ and in real time. We demonstrate that many scientifically and technologically significant phenomena in LPBF, including melt pool dynamics, powder ejection, rapid solidification, and phase transformation, can be probed with unprecedented spatial and temporal resolutions. In particular, the keyhole pore formation is experimentally revealed with high spatial and temporal resolutions. The solidification rate is quantitatively measured, and the slowly decrease in solidification rate during the relatively steady state could be a manifestation of the recalescence phenomenon. The high-speed diffraction enables a reasonable estimation of the cooling rate and phase transformation rate, and the diffusionless transformation from β to α ’ phase is evident. The data present here will facilitate the understanding of dynamics and kinetics in metal LPBF process, and the experiment platform established will undoubtedly become a new paradigm for future research and development of metal additive manufacturing.

When electrons in a solid are excited by light, they can alter the free energy landscape and access phases of matter that are out of reach in thermal equilibrium. This accessibility becomes important in the presence of phase competition, when one state of matter is preferred over another by only a small energy scale that, in principle, is surmountable by the excitation. Here, we study a layered compound, LaTe 3 , where a small lattice anisotropy in the a – c plane results in a unidirectional charge density wave (CDW) along the c axis 1 , 2 . Using ultrafast electron diffraction, we find that, after photoexcitation, the CDW along the c axis is weakened and a different competing CDW along the a axis subsequently emerges. The timescales characterizing the relaxation of this new CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. The new density wave represents a transient non-equilibrium phase of matter with no equilibrium counterpart, and this study thus provides a framework for discovering similar states of matter that are ‘trapped’ under equilibrium conditions. Short pulses of light shift the balance between two competing charge density wave phases, allowing the weaker one to manifest transiently while suppressing the stronger one. This shows that competing phases can be tuned in a non-equilibrium setting.

Next-generation terahertz (THz) sources demand lightweight, low-cost, defect-tolerant, and robust components with synergistic, tunable capabilities. However, a paucity of materials systems simultaneously possessing these desirable attributes and functionalities has made device realization difficult. Here we report the observation of asymmetric spintronic-THz radiation in Two-Dimensional Hybrid Metal Halides (2D-HMH) interfaced with a ferromagnetic metal, produced by ultrafast spin current under femtosecond laser excitation. The generated THz radiation exhibits an asymmetric intensity toward forward and backward emission direction whose directionality can be mutually controlled by the direction of applied magnetic field and linear polarization of the laser pulse. Our work demonstrates the capability for the coherent control of THz emission from 2D-HMHs, enabling their promising applications on the ultrafast timescale as solution-processed material candidates for future THz emitters. Terahertz radiation has wide array of potential uses, however, finding robust and tunable sources of terahertz radiation has been challenging. Here, Cong et al demonstrate a room temperature terahertz source composed of a two-dimensional hybrid metal halide and ferromagnetic heterostructure.

The collective dynamics of topological structures 1 – 6 are of interest from both fundamental and applied perspectives. For example, studies of dynamical properties of magnetic vortices and skyrmions 3 , 4 have not only deepened our understanding of many-body physics but also offered potential applications in data processing and storage 7 . Topological structures constructed from electrical polarization, rather than electron spin, have recently been realized in ferroelectric superlattices 5 , 6 , and these are promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics underlying the functionality of such complex extended nanostructures. Here, using terahertz-field excitation and femtosecond X-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders-of-magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices 3 . A previously unseen tunable mode, hereafter referred to as a vortexon, emerges in the form of transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond timescales. Its frequency is considerably reduced (softened) at a critical strain, indicating a condensation (freezing) of structural dynamics. We use first-principles-based atomistic calculations and phase-field modelling to reveal the microscopic atomic arrangements and corroborate the frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density. A dynamical study shows that vortices of electrical polarization have higher frequencies and smaller size than their magnetic counterparts, properties that are promising for electric-field-driven data processing.

The interplay between a multitude of electronic, spin, and lattice degrees of freedom underlies the complex phase diagrams of quantum materials. Layer stacking in van der Waals (vdW) heterostructures is responsible for exotic electronic and magnetic properties, which inspires stacking control of two-dimensional magnetism. Beyond the interplay between stacking order and interlayer magnetism, we discover a spin-shear coupling mechanism in which a subtle shear of the atomic layers can have a profound effect on the intralayer magnetic order in a family of vdW antiferromagnets. Using time-resolved X-ray diffraction and optical linear dichroism measurements, interlayer shear is identified as the primary structural degree of freedom that couples with magnetic order. The recovery times of both shear and magnetic order upon optical excitation diverge at the magnetic ordering temperature with the same critical exponent. The time-dependent Ginzburg-Landau theory shows that this concurrent critical slowing down arises from a linear coupling of the interlayer shear to the magnetic order, which is dictated by the broken mirror symmetry intrinsic to the monoclinic stacking. Our results highlight the importance of interlayer shear in ultrafast control of magnetic order via spin-mechanical coupling. Van der Waals materials are characterized by two dimensional layers weakly held together by interlayer van der Waals forces. Here, the authors study how shear motions between these layers influence the magnetic properties of the van der Waals antiferromagnets FePS3, MnPS3, and NiPS3. ‘

Flatbands, characterized by their dispersionless energy levels in electronic, magnetic, and phononic systems, hold substantial potential for advancements in electronics and quantum information processing. Most flatbands exist in thermal equilibrium and cannot be easily created or annihilated externally, limiting their flexibility as switchable knobs for use in microelectronics and quantum applications. In our work, we demonstrate the generation of a coherent phonon flatband in a GaAs/AlAs superlattice using 800 nm femtosecond laser pulses. This coherent phonon flatband does not correspond to a phonon eigenmode at equilibrium and exhibits strong coupling with two branches of coherently excited longitudinal phonon modes. With molecular dynamics simulations, we show more generally that the coherent phonon flatband can be induced by coherently and spatially modulated optical excitations of superlattice structures. Our results highlight a pathway for coherent phonon flatband creation in the time domain that can be generalized to various superlattice systems, potentially inspiring the realization of coherent flatband generation of other quasiparticles. Above-bandgap femtosecond laser pulses generate a coherent phonon flatband in a GaAs/AlAs superlattice captured using ultrafast X-ray diffraction, providing a way to control atomic vibrations for future electronic and quantum technologies.

Ultrafast X-ray scattering is one of the primary tools to track intrinsic crystallographic evolution with atomic accuracy in real time. However, its application to study nonequilibrium structural properties at the two-dimensional limit remains a long-standing challenge due to a significant reduction of diffraction volume and complexity of data analysis. Here, we report femtosecond surface X-ray diffraction in combination with crystallographic model-refinement calculations to quantify the ultrafast structural dynamics of monolayer WSe2 crystals supported on a substrate. We found the absorbed optical photon energy is preferably coupled to the in-plane lattice vibrations within one picosecond whereas the out-of-plane lattice vibration amplitude remains unchanged during the first ten picoseconds. The model-assisted fitting suggests an asymmetric intralayer spacing change upon excitation. The observed nonequilibrium anisotropic structural dynamics agrees with first-principles modelling in both real and momentum space, marking the distinct structural dynamics of monolayer crystals from their bulk counterparts.Electron–phonon coupling in a monolayer WSe2 on a substrate is investigated by femtosecond surface X-ray scattering. Counterintuitively, the absorbed optical photon energy is dominantly coupled to the in-plane lattice vibrations within 1 ps.

Photovoltaic conversion efficiency (PCE) of halide perovskite solar cells has risen spectacularly, yet the very crystalline structure of CH 3 NH 3 PbI 3 remains ambiguous after extensive researches, and its polar nature remains hotly debated. Here we present compelling evidences that CH 3 NH 3 PbI 3 crystals self-grown on FTO/TiO 2 substrate consist of ferroic domains with alternating polar and nonpolar orders, in contrast to previous experimental and theoretical expectations, and polar domains possess reduced photocurrent. It is found that polar and nonpolar orders of CH 3 NH 3 PbI 3 can be distinguished from their distinct lateral piezoresponse, energy dissipation, first and second harmonic electromechanical couplings, and temperature variation, even though their difference in crystalline lattice is very subtle, and they possess two-way memory effect through cubic-tetragonal phase transition. We hope these findings resolve key questions regarding polar nature of CH 3 NH 3 PbI 3 and its implication on photovoltaics, reconcile contradictory data widely reported, and point a direction toward engineering ferroic domains for enhanced PCE. Perovskite solar cells: Polar ordering The crystalline structure and polar nature of a hybrid organic−inorganic perovskite widely used in solar cells — CH 3 NH 3 PbI 3 — is still a matter of debate. Now, an international team of researchers led by Jiangyu Li and Jinjin Zhao used a combination of experimental methods, including scanning probe and transmission electron microscopy, to show that CH 3 NH 3 PbI 3 crystals grown on a FTO/TiO 2 substrate exhibit ferroic domains with alternating polar and nonpolar order. The structural differences between the polar and nonpolar phase are very subtle, and scanning probe techniques are well suited to resolve their functional responses with high resolution. The polar domains were observed to have a lower photocurrent than that expected from theoretical predictions. Building on these results, it should be possible to engineer ferroic domains to enhance the efficiency of halide perovskite solar cells.

Scattering of energetic charge carriers and their coupling to lattice vibrations (phonons) in dielectric materials and semiconductors are crucial processes that determine the functional limits of optoelectronics, photovoltaics, and photocatalysts. The strength of these energy exchanges is often described by the electron-phonon coupling coefficient, which is difficult to measure due to the microscopic time- and length-scales involved. In the present study, we propose an alternate means to quantify the coupling parameter along with thermal boundary resistance and electron conductivity by performing a high angular-resolution time-resolved X-ray diffraction measurement of propagating lattice deformation following laser excitation of a nanoscale, polycrystalline metal film on a semiconductor substrate. Our data present direct experimental evidence for identifying the ballistic and diffusive transport components occurring at the interface, where only the latter participates in thermal diffusion. This approach provides a robust measurement that can be applied to investigate microscopic energy transport in various solid-state materials.

Unraveling collective modes arising from coupled degrees of freedom is crucial for understanding complex interactions in solids and developing new functionalities. Unique collective behaviors emerge when two degrees of freedom, ordered on distinct length scales, interact. Polar skyrmions, three-dimensional electric polarization textures in ferroelectric superlattices, disrupt the lattice continuity at the nanometer scale with nontrivial topology, leading to previously unexplored collective modes. Here, using terahertz-field excitation and femtosecond x-ray diffraction, we discover subterahertz collective modes, dubbed “skyrons”, which appear as swirling patterns of atomic displacements functioning as atomic-scale gearsets. The key to activating skyrons is the use of the THz field that couples primarily to skyrmion domain walls. Momentum-resolved time-domain measurements of diffuse scattering reveal an avoided crossing in the dispersion relation of skyrons. Atomistic simulations and dynamical phase-field modeling provide microscopic insights into the three-dimensional crystallographic and polarization dynamics. The amplitude and dispersion of skyrons are demonstrated to be controlled by sample temperature and electric-field bias. The discovery of skyrons and their coupling with terahertz fields opens avenues for ultrafast control of topological polar structures. Polar skyrmions are nanoscale topological structures of electric polarizations. Their collective modes, dubbed as “skyrons”, are discovered by the terahertz-field-excitation, femtosecond x-ray diffraction measurements and advanced modeling.