Yimo Han

Magnetoelectric multiferroics promise low-power switching, non-volatile memory and signal transduction, yet practical deployment requires ultrathin films in which polar and magnetic order often degrade. Uncompensated bound charge drives depolarization in ferroelectrics, whereas off-stoichiometry, reduced anisotropy and charge transfer can produce magnetic dead layers. Here we realize a multiferroic phase of BiFeO₃ with no dead layer, revealing signatures of emergent altermagnetism in the ultrathin limit. First-principles calculations, symmetry analysis, atomic-resolution imaging and magnetic imaging show that short-circuit electrostatic boundary conditions and epitaxial strain drive a continuous, thickness-induced phase transition accompanied by multiferroic topological textures. At four unit cells, these boundary conditions stabilize a collinear monoclinic phase with d-wave altermagnetic order, supported by magnetic circular dichroism and spin-resolved photoemission measurements. These results establish a pathway to stabilize unconventional multiferroicity at device-relevant thicknesses, reframing scaling limits for oxide electronics.

Phase transitions in halide perovskites critically influence their optoelectronic performance and stability, yet the nanoscale pathways by which structural transformations proceed remain elusive because of their soft ionic lattice and electron-beam sensitivity. Here, we combine in situ heating with four-dimensional scanning transmission electron microscopy (4D-STEM) to investigate the phase-transition dynamics in single CsPbBrxI3–x nanowires, from the non-perovskite to the perovskite phase, with nanometer spatial and millisecond temporal resolution. We directly visualize and track the real-time propagation of the perovskite phase front along a nanowire, while mapping local crystallographic order and diffuse scattering during the transition. The emergence of amorphous diffraction rings indicates a breakdown of long-range crystalline order, and with molecular dynamics simulations, we can conclude that local atomic coordination is preserved. More broadly, this work establishes a general framework for resolving phase transition pathways in beam-sensitive materials beyond halide perovskites, with implications for stabilizing functional phases for future applications.

Ferroelectric materials exhibit a spontaneous electric polarization that can be reversed by an electric field¹, a property central to non-volatile memories^2,3, sensors⁴ and actuators^5,6. In most conventional ferroelectrics, the polarization originates from a softening of a polar mode⁷. The amplitude of the mode along a given direction couples with the applied field along the same direction⁸. Ferroelectrics with a predominant in-plane polarization are harder to use in standard device geometries and therefore the field has mostly focused on out-of-plane ferroelectrics. Developing approaches that enable manipulation of the in-plane polarization component with an out-of-plane field would therefore provide new opportunities for device design and functionality. Here we have discovered that a trilinear coupling between the in-plane and out-of-plane polarization, mediated by means of the octahedral tilts and rotations in the layered ferroelectric Bi₄Ti₃O₁₂, naturally fulfils this challenge and enables perpendicular switching of the polarization state. In its bulk monoclinic phase, this material hosts a large in-plane polarization (about 50 μC cm^-2) driven by a proper ferroelectric instability, together with smaller out-of-plane polarization (about 5 μC cm^-2) of improper origin, induced by oxygen octahedral distortions. We demonstrate that, in c-axis-oriented epitaxial films, the in-plane polarization switches deterministically under an out-of-plane electric field. This cross-coupling between orthogonal polarization components provides a route to transverse manipulation of ferroic order parameters and establishes layered ferroelectrics as a platform for capacitive computing concepts.

Liquid crystals (LCs) provide a canonical framework to understand intermediate phases between liquids and crystals, such as nematic and smectic order, and the rich phenomenology that emerges when such orders are subjected to boundaries, confinement, and elastic frustration. Here, we reveal a hidden mesoscale liquid-crystal-like organization of spin cycloids in a room-temperature magnetoelectric antiferromagnet BiFeO3. It is observed that ferroelectric domain walls act as anchoring surfaces, locally selecting cycloid propagation vectors analogous to surface induced alignment in LCs. Interestingly, confinement imposed by multiple domain walls stabilizes coherent smectic-like cycloidal order in between domain walls through “order by confinement”, while geometrical frustration at domain boundaries induces non-local Helfrich-Hurault–type elastic instabilities in cycloid smectics. Building on these insights, we show that surface-energy engineering dramatically enhances the long range orientational coherence of spin cycloids, enabling robust non-local magnon transport in the ultra-thin limit. These findings establish self-organization as a guiding principle for engineering novel magnetic textures and emergent spin functionalities, opening new opportunities for spintronic and magnonic technologies.

Moiré superlattices formed at the interfaces of mismatched lattices have attracted significant interest over the past decade due to their large tunability of band parameters and interactions among electrons, spins, and lattices. Superlattices made from twisted perovskite oxides may have strong structure and potential modulation, but evidence of such modulation over macroscopic areas, particularly at large twisting angles, has not been clearly demonstrated so far. Here, millimeter‐scale twisted oxide bilayers are fabricated at angle, close to the simple coincidence site lattice condition , from freestanding SrTiO ₃ membranes. We observe new low‐frequency vibrational modes whose Raman activity, according to molecular dynamics simulations, is greatly enhanced by an asymmetric, twisted interface between the SrO and TiO ₂ layers. Such an interface is energetically favorable from first‐principles calculations and is corroborated by the observation of strong second‐harmonic generation from the interface, whose amplitude is comparable to that from the bare SrTiO ₃ surface throughout the bilayer region. The results are consistent with interlayer coupling, which is enhanced by high‐temperature annealing and confirmed by cross‐sectional scanning transmission electron microscopy. Our work sheds light on the structural behavior of twisted oxides and provides directions for tuning their phononic and nonlinear optical properties in future studies.

Van der Waals (vdW) epitaxy can integrate lattice-mismatched crystals with atomically sharp, pristine interfaces. However, translating this concept to atomic layer deposition (ALD), a standard for low-temperature, layer-by-layer growth, remains challenging because precursors adsorb transiently on chemically inert vdW basal planes, leading to physisorption-limited nucleation and poor crystalline ordering. Here, we introduce diffusion-steered epitaxial ALD (Epi-ALD), redefining vdW surfaces as programmable kinetic-thermodynamic landscapes and demonstrate highly crystalline tellurium at 150°C. Epi-ALD couples surface-potential-encoded physisorption with long-range diffusion to promote ordered nucleation and epitaxial alignment. This “soft” pathway enables strain-free tellurium epitaxy with a pristine vdW gap (∼1.4 angstrom) despite lattice mismatch (>3.6%) and generalizes across multiple vdW templates. Retaining hallmark advantages of ALD including scalability and uniformity, we further program in-plane orientation through vdW symmetry engineering to achieve quasi-single-crystalline tellurium films with pronounced anisotropy and a chiral anomaly signature. Our work establishes a universal, low-thermal-budget approach for integrating vdW materials and expands the scope of epitaxy within the ALD paradigm.

BiFeO ₃ is a model multiferroic in which the ferroelectric polarization is coupled to ferroelastic lattice distortions, yet deterministic control of its domain structure remains limited by high switching fields and competing polarization variants. Here, we identify a mechanically assisted polarization switching pathway in epitaxial BiFeO ₃ thin films that fundamentally alters the switching energetics. Using just out-of-plane electric fields, polarization reversal requires voltages of approximately 4 volts and stabilizes coexisting polarization states. In contrast, when mechanical pressure is applied concurrently, the positive coercive voltage can be substantially reduced, even to 0 volts, resulting in spontaneous switching. Piezoresponse force microscopy measurements reveal that applied mechanical pressure suppresses ferroelastic domain competition, indicating a decrease in the required electrical energy barrier associated with polarization rotation and domain wall motion. This frames the strain field from the AFM tip directly as an effective voltage that assists in ferroelectric switching. By directly coupling lattice distortions to polarization reversal, mechanically assisted switching provides a general framework for controlling coupled order parameters in multiferroic oxides, which can be directly applied in the device-level architecture, where a small mechanical pressure can help in achieving a lower switching energy of ferroelectric polarization. This work advances the fundamental understanding of electromechanical coupling in complex ferroics and establishes mechanical energy as a powerful tool for probing and manipulating ferroelastic-ferroelectric interactions.

Weyl semimetals, characterized by topologically nontrivial band structures, exhibit unconventional transport properties and hold great promise for next-generation electronic devices. When reduced to the nanoscale, the contribution from the topological surface states becomes markedly enhanced, leading to unusual resistivity scaling behaviors. However, the synthesis of nanosized single-crystalline Weyl semimetals remains limited, hindering in-depth investigations of size-dependent transport phenomena. Here, we introduce a novel vapor-liquid-liquid-solid (VLLS) synthesis method for the growth of high-quality single-crystalline NbP nanowires with small diameters and controllable orientations. The VLLS process enables the stable and precise crystallization of NbP guided by Au seeds, favoring growth along the a axis, which has intrinsically lower resistivity. Room-temperature electrical measurements reveal a pronounced decrease in resistivity with decreasing diameter, indicating surface-dominant transport. We achieve a high breakdown current density of 116 MA cm ^–2 , comparable to that of copper nanowires, and a low resistivity of 21 µΩ cm at ~ 30 nm equivalent diameter, surpassing bulk NbP and many other semimetals. The dominance of topological surface states in charge transport is strongly supported by scanning microwave impedance microscopy measurements and simulations. The VLLS synthesis of NbP nanowires thus not only offers a pathway for precise structural control of Weyl semimetals but also opens new opportunities to tune their transport properties and enable advanced technological applications.