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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. ‘

A cocktail of experiments and theory provides a way of revealing complex patterns in magnetic materials, including some textures that can identify the systems as members of an elusive class known as axion insulators. Symmetries reveal an unpinned broken magnetic helix.

Antiferromagnetism has a vanishing total magnetization and thus is extremely challenging to manipulate. Now, circularly polarized light is shown to efficiently detect, induce and switch a unique class of antiferromagnets.

Weakly coupled ferroelectric/dielectric superlattice thin-film heterostructures exhibit complex nanoscale polarization configurations that arise from a balance of competing electrostatic, elastic, and domain-wall contributions to the free energy. A key feature of these configurations is that the polarization can locally have a significant component that is along the thin-film surface normal direction with an overall configuration maintaining zero net in-plane polarization.PbTiO3/SrTiO3thin-film superlattice heterostructures on a conductingSrRuO3bottom electrode onSrTiO3have a room-temperature stripe nanodomain pattern with a nanometer-scale lateral period. Ultrafast time-resolved x-ray free electron laser diffraction and scattering experiments reveal that above-bandgap optical pulses induce propagating acoustic pulses and a perturbation of the domain diffuse scattering intensity arising from the nanoscale stripe domain configuration. With 400-nm optical excitation, two separate acoustic pulses are observed: a high-amplitude pulse resulting from strong optical absorption in the bottom electrode and a weaker pulse arising from the depolarization-field-screening effect due to absorption directly within the superlattice. The picosecond scale variation of the nanodomain diffuse scattering intensity is consistent with a larger polarization change than would be expected due to the polarization-tetragonality coupling of uniformly polarized ferroelectrics. The polarization change is consistent, instead, with polarization rotation facilitated by the reorientation of the in-plane component of the polarization at the domain boundaries of the striped polarization structure. The complex steady-state configuration within these ferroelectric heterostructures leads to ultrafast polarization rotation phenomena that have previously been available only through the selection of bulk crystal composition.

Broken symmetries and electronic topology are well manifested together in the second-order nonlinear optical responses from topologically non-trivial materials. Although second-order nonlinear optical effects from the electric dipole contribution have been extensively explored in polar Weyl semimetals with broken spatial-inversion symmetry, they are rarely studied in centrosymmetric magnetic Weyl semimetals with broken time-reversal symmetry due to the complete suppression of the electric dipole contribution. Here we report the experimental demonstration of optical second-harmonic generation (SHG) in a magnetic Weyl semimetal Co 3 Sn 2 S 2 from the electric quadrupole contribution. By tracking the temperature dependence of the rotational anisotropy of SHG, we capture two magnetic phase transitions, with both SHG intensity increasing and its rotational anisotropy pattern rotating at T C,1  = 175 K and T C,2  = 120 K subsequently. The fitted critical exponents for the SHG intensity and rotational anisotropy orientation near T C,1 and T C,2 suggest that the magnetic phase at T C,1 is a three-dimensional Ising-type out-of-plane ferromagnetism, whereas the other at T C,2 is a three-dimensional XY-type all-in–all-out in-plane antiferromagnetism. Our results show the success of the detection and exploration of electric quadrupole SHG in a centrosymmetric magnetic Weyl semimetal and hence open the pathway towards future investigations into its association with band topology. Optical second-harmonic waves are generated from the electric quadrupole contribution in a centrosymmetric magnetic Weyl semimetal Co 3 Sn 2 S 2 . Two magnetic orders and phase transitions are explored in temperature-dependent rotational anisotropy measurements by second-harmonic generation.

Ultrafast first-order phase transitions exhibit distinct transition pathways and dynamical properties that are not accessible during quasi-equilibrium transitions. Phenomena arising at the ultrafast timescale are important for understanding the transition mechanisms and in applications using the fast switching of electronic properties or magnetism. These transitions are accompanied by nanoscale structural dynamics that have been challenging to explore by optical or electronic transport probes. Here, X-ray nanodiffraction imaging shows that the nanoscale structural dynamics arising in ultrafast phase transitions differ dramatically from the transitions under slowly varying parameters. The solid-solid phase transitions in a FeRh thin film involve concurrent structural and magnetic changes and can be sensitively probed by monitoring their diffraction signatures following femtosecond optical excitation. Time-dependent nanodiffraction maps with 100-ps temporal and 25-nm spatial resolutions reveal that the preexisting nanoscale variation in phase composition results in spatially inhomogeneous changes of phase fraction after ultrafast optical excitation. The spatial inhomogeneity leads to nanoscale temperature variations and subsequent in-plane heat transport, which are responsible for spatially distinct relaxation pathways on nanometer length scales. The spatial gradients of the phase composition and elastic strain increase upon excitation rather than exhibiting the decrease previously reported in quasi-equilibrium transformations. Long-range elastic interactions thus do not play significant roles in the ultrafast phase transition. These microscopic insights into first-order phase transitions provide routes to manipulate nanoscopic phases in functional materials on ultrafast time scales by engineering initial nanoscale phase distributions.

My thesis research investigates photoinduced phenomena in ferroelectric electronic oxide materials, with a focus on changes that occur on sub-nanosecond timescales and length scales ranging from single-unit cells to the mesoscopic ferroelectric polarization pattern. The experiments employ time-resolved synchrotron X-ray diffraction and microscopy. A series of X-ray diffraction experiments and theoretical calculations were employed in order to understand photoinduced phenomena from a structural perspective. The dynamics of the photoinduced phenomena were analyzed to obtain insight into the mechanisms of light-induced structural effects in ferroelectric materials. Taken together, the results show that bound charges due to the polarization discontinuity at domain boundaries or interfaces with non-polar materials have an important role in photoinduced structural phenomena in ferroelectrics with nanoscale domain patterns. The photoexcitation phenomena reported in this thesis are excited by ultrashort optical pulse. Optical absorption is followed by rapid deformation of the equilibrium structure via a series of pathways that are reported in detail.This thesis reports studies of photoinduced phenomena in three ferroelectric material systems. The first results involve the dynamics of ferroelectric nanodomain patterns within PbTiO3/SrTiO3 superlattices. Previous studies have found that the steady-state domain pattern can be modified by changing mechanical and electrostatic boundary conditions. In a series of in-situ X-ray diffraction experiments, we have discovered a photoinduced transformation to uniform polarization state. Thermodynamic calculations reveal that the uniform polarization state is energetically stabilized by the screening of bound charges. An analysis of the relaxation dynamics indicates that trapped charge carriers have an important role in setting the concentration of mobile charge carriers. The results are reported in Ahn et al., Phys. Rev. Lett. 119, 057601 (2017).A second study involves low-strain BaTiO3 thin films. Photoexcitation leads to a reorientation of the domain walls in this system on a sub-nanosecond timescale. The reorientation is observed only in the room-temperature regime in which two phases of the domain pattern coexist. Electrostatic calculations show that the domain wall reorientation results from the screening of the bound charges at domain walls. An alternative model based on an elastic response to optically induced expansion is not consistent with the experimental results. The bound charges can arise due to roughness and disorder of the domain walls and from weak in-plane polarization components, both of which are reduced in the high-temperature single-domain phase.Finally, we report an optically induced transformation between structural phases of a compressively strained BiFeO3 film. These structural phases have distinct electronic and magnetic properties coupled to the crystal structures and thus the system has a potential to enable properties to be manipulated significantly via this phase transformation. Time-resolved synchrotron X-ray diffraction microscopy showed the photoinduced phase transformation on nanosecond timescale. Thermodynamic free energy calculations provide insight into the phase transformation [Ahn et al., Phys. Rev. Lett. 123, 045703 (2019)].

Terahertz resonances embedded in crystalline heterostructures could close a spectral gap between conventional electronics and photonics while opening new windows on phenomena in non‐equilibrium lattice dynamics. We show that femtosecond optical screening of the depolarization field in epitaxial PbTiO 3 /SrTiO 3 superlattices launches a collective polar mode that oscillates near 1 THz and coherently spans the entire mini‐Brillouin zone. Wave‐vector‐resolved pump–probe X‐ray diffraction resolves a nearly dispersion‐less oscillation at 0.87 and 0.94 THz at the zone boundary and zone center, respectively, persisting for ∼2.5 ps, corresponding to a weakly damped resonance. Dynamical phase‐field simulations reveal the origin of the mode to mesoscopic rotation of closure‐domain textures during the photoexcited transition from an unscreened to a screened electrostatic state. Varying the PbTiO 3 and SrTiO 3 ratio tunes the mode frequency continuously from 0.9 to 1.4 THz, providing a quantitative design rule for frequency‐selectable THz oscillators in ferroelectric heterostructures. By coupling nanoscale polarization reconfiguration to long‐wavelength coherent dynamics, this work establishes depolarization‐field engineering to topology‐driven THz functionality and expanding the landscape of collective lattice dynamics.