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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 Co3Sn2S2from 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 TC,1= 175 K and TC,2= 120 K subsequently. The fitted critical exponents for the SHG intensity and rotational anisotropy orientation near TC,1and TC,2suggest that the magnetic phase at TC,1is a three-dimensional Ising-type out-of-plane ferromagnetism, whereas the other at TC,2is 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.

Dynamical control of thermal transport at the nanoscale provides a time-domain strategy for optimizing thermal management in nanoelectronics, magnetic devices, and thermoelectric devices. However, the rate of change available for thermal switches and regulators is limited to millisecond time scales, calling for a faster modulation speed. Here, time-resolved X-ray diffraction measurements and thermal transport modeling reveal an ultrafast modulation of the interfacial thermal conductance of an FeRh/MgO heterostructure as a result of a structural phase transition driven by optical excitation. Within 90 ps after optical excitation, the interfacial thermal conductance is reduced by a factor of 5 and lasts for a few nanoseconds, in comparison to the value at the equilibrium FeRh/MgO interface. The experimental results combined with thermal transport calculations suggest that the reduced interfacial thermal conductance results from enhanced phonon scattering at the interface where the lattice experiences transient in-plane biaxial stress due to the structural phase transition of FeRh. Our results suggest that optically driven phase transitions can be utilized for ultrafast nanoscale thermal switches for device application.

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.

Optical excitation leads to ultrafast stress generation in the prototypical multiferroic BiFeO3. The time scales of stress generation are set by the dynamics of the population of excited electronic states and the coupling of the electronic configuration to the structure. X-ray free-electron laser diffraction reveals high-wavevector subpicosecond-time scale stress generation following ultraviolet excitation of a BiFeO3thin film. Stress generation includes a fast component with a 1/e rise time with an upper limit of 300 fs and longer-rise time components extending to 1.5 ps. The contributions of the fast and delayed components vary as a function of optical fluence, with a reduced a fast-component contribution at high fluence. The results provide insight into stress-generation mechanisms linked to the population of excited electrons and point to new directions in the application of nanoscale multiferroics and related ferroic complex oxides. The fast component of the stress indicates that structural parameters and properties of ferroelectric thin film materials can be optically modulated with 3 dB bandwidths of at least 0.5 THz.

The collective dynamics of topological structures1–6are of interest from both fundamental and applied perspectives. For example, studies of dynamical properties of magnetic vortices and skyrmions3,4have not only deepened our understanding of many-body physics but also offered potential applications in data processing and storage7. Topological structures constructed from electrical polarization, rather than electron spin, have recently been realized in ferroelectric superlattices5,6, 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 vortices3. 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.

The functional properties of complex oxides, including magnetism and ferroelectricity, are closely linked to subtle structural distortions. Ultrafast optical excitations provide the means to manipulate structural features and ultimately to affect the functional properties of complex oxides with picosecond-scale precision. We report that the lattice expansion of multiferroic BiFeO3following above-bandgap optical excitation leads to distortion of the oxygen octahedral rotation (OOR) pattern. The continuous coupling between OOR and strain was probed using time-resolved X-ray free-electron laser diffraction with femtosecond time resolution. Density functional theory calculations predict a relationship between the OOR and the elastic strain consistent with the experiment, demonstrating a route to employing this approach in a wider range of systems. Ultrafast control of the functional properties of BiFeO3thin films is enabled by this approach because the OOR phenomena are related to ferroelectricity, and via the Fe–O–Fe bond angles, the superexchange interaction between Fe atoms.

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.

Crystallization from an amorphous precursor presents a new route to control the properties of complex oxides by selecting their nanoscale morphology. A key challenge in crystal growth from the amorphous form is to select the locations of nucleation and the crystallographic orientation of the resulting crystals. Nucleation sites for crystallization of the prototypical perovskite complex oxide SrTiO3(STO) from its amorphous form can be reproducibly introduced using nanoscale seed crystals. The results of two seeding strategies are reported here: (i) SrRuO3(SRO) (001)-oriented nanomembranes and (ii) STO nanocrystalline seeds. Amorphous STO crystallizes laterally over distances of several microns from the seeds before encountering separately nucleated crystals. The lateral crystallization rates for both types of seeding methods are close to the values measured in solid-phase epitaxy (SPE) of STO on single-crystal substrates. The lateral crystallization distances along different crystallographic orientations are equal, indicating that the lateral crystallization rate is isotropic. The isotropic crystallization suggests that the rate-limiting steps for crystallization occur within the amorphous STO away from the amorphous/crystalline interface. In addition to the lateral crystallization, STO crystallizes on top of the planar SRO nanomembrane seeds via SPE, forming partially relaxed [001]-oriented heteroepitaxial STO layers. The in-plane orientation of laterally crystallized STO near the SRO nanomembrane seeds exhibits overall polycrystallinity with regions in which micron-scale grains of laterally crystallized STO share the same in-plane orientation as the SRO nanomembrane seeds. Crystallization from seed crystals provides opportunities to create a wide range of other perovskite oxides in nanoscale geometries.

Molecular monolayers that can be reconfigured through the use of external stimuli promise to enable the creation of interfaces with precisely selected dynamically adjustable physical and electronic properties with potential impact ranging from electronics to energy storage. Azobenzene-containing molecular monolayers have multiple stable molecular conformations but face a challenging nanoscale problem associated with understanding the basic mechanisms of reconfiguration. Time-resolved X-ray reflectivity studies show that the reconfiguration of a densely packed rhenium–azobenzene monolayer occurs in a period of many seconds. The degree of reconfiguration from trans to cis forms depends on the integrated UV fluence and has kinetics that are consistent with a mechanism in which the transformation occurs through the nucleation and growth of nanoscale two-dimensional regions of the cis isomer.

Li metal anodes and Ni-rich layered oxide cathodes with high reversible capacities are promising candidates for the fabrication of high energy density batteries. However, low Coulombic efficiency, safety hazards from likely vertical Li growth, and morphological instability of Ni-rich cathodes hinder the practical applications of these electrodes. Here, we report that fluorinated compounds can be employed as interface modifiers to extend the applicable voltage range of ether-based electrolytes, which have been used specifically so far for lithium metal batteries with charging cut-off voltages lower than 4 V (vs. Li/Li+). A complementary electrolyte design using both 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether and fluoroethylene carbonate in concentrated ether-based electrolytes significantly improves the capacity retention (99.1%) in a Li|LiNi0.8Co0.1Mn0.1O2full cell, with a high Coulombic efficiency of 99.98% after 100 cycles at 25 °C. Thus, the modified electrolyte system is promising for addressing the reductive and oxidative decompositions of labile ether-based electrolytes in high energy density Li metal batteries with Ni-rich cathodes.