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Using epitaxy and the misfit strain imposed by an underlying substrate, it is possible to elastically strain oxide thin films to percent levels—far beyond where they would crack in bulk. Under such strains, the properties of oxides can be dramatically altered. In this article, we review the use of elastic strain to enhance ferroics, materials containing domains that can be moved through the application of an electric field (ferroelectric), a magnetic field (ferromagnetic), or stress (ferroelastic). We describe examples of transmuting oxides that are neither ferroelectric nor ferromagnetic in their unstrained state into ferroelectrics, ferromagnets, or materials that are both at the same time (multiferroics). Elastic strain can also be used to enhance the properties of known ferroic oxides or to create new tunable microwave dielectrics with performance that rivals that of existing materials. Results show that for thin films of ferroic oxides, elastic strain is a viable alternative to the traditional method of chemical substitution to lower the energy of a desired ground state relative to that of competing ground states to create materials with superior properties.

Nonlinear Hall effect (NLHE) is a new type of Hall effect with wide application prospects. Practical device applications require strong NLHE at room temperature (RT). However, previously reported NLHEs are all low-temperature phenomena except for the surface NLHE of TaIrTe 4 . Bulk RT NLHE is highly desired due to its ability to generate large photocurrent. Here, we show the spin-valley locked Dirac state in BaMnSb 2 can generate a strong bulk NLHE at RT. In the microscale devices, we observe the typical signature of an intrinsic NLHE, i.e. the transverse Hall voltage quadratically scales with the longitudinal current as the current is applied to the Berry curvature dipole direction. Furthermore, we also demonstrate our nonlinear Hall device’s functionality in wireless microwave detection and frequency doubling. These findings broaden the coupled spin and valley physics from 2D systems into a 3D system and lay a foundation for exploring bulk NLHE’s applications. The nonlinear Hall effect (NLHE) results in a second-harmonic transverse voltage in response to alternating longitudinal current in zero magnetic field and has so far only been observed at low temperatures in bulk materials. Here, the authors observe bulk NLHE at room temperature in the Dirac material BaMnSb 2 , which will provide a large photocurrent for applications in THz detection.

The emergence of magnetism in quantum materials creates a platform to realize spin-based applications in spintronics, magnetic memory, and quantum information science. A key to unlocking new functionalities in these materials is the discovery of tunable coupling between spins and other microscopic degrees of freedom. We present evidence for interlayer magnetophononic coupling in the layered magnetic topological insulator MnBi 2 Te 4 . Employing magneto-Raman spectroscopy, we observe anomalies in phonon scattering intensities across magnetic field-driven phase transitions, despite the absence of discernible static structural changes. This behavior is a consequence of a magnetophononic wave-mixing process that allows for the excitation of zone-boundary phonons that are otherwise ‘forbidden’ by momentum conservation. Our microscopic model based on density functional theory calculations reveals that this phenomenon can be attributed to phonons modulating the interlayer exchange coupling. Moreover, signatures of magnetophononic coupling are also observed in the time domain through the ultrafast excitation and detection of coherent phonons across magnetic transitions. In light of the intimate connection between magnetism and topology in MnBi 2 Te 4 , the magnetophononic coupling represents an important step towards coherent on-demand manipulation of magnetic topological phases. Tunable coupling between magnetism and the lattice is important for on-demand manipulation of magnetic phases. Here, the authors demonstrate that lattice vibrations can coherently modulate the interlayer magnetic exchange coupling in the magnetic topological insulator MnBi 2 Te 4 .

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.

Orthorhombic R MnO 3 ( R  = rare-earth cation) compounds are type-II multiferroics induced by inversion-symmetry-breaking of spin order. They hold promise for magneto-electric devices. However, no spontaneous room-temperature ferroic property has been observed to date in orthorhombic R MnO 3 . Here, using 3D straining in nanocomposite films of (SmMnO 3 ) 0.5 ((Bi,Sm) 2 O 3 ) 0.5 , we demonstrate room temperature ferroelectricity and ferromagnetism with T C,FM  ~ 90 K, matching exactly with theoretical predictions for the induced strain levels. Large in-plane compressive and out-of-plane tensile strains (−3.6% and +4.9%, respectively) were induced by the stiff (Bi,Sm) 2 O 3 nanopillars embedded. The room temperature electric polarization is comparable to other spin-driven ferroelectric R MnO 3 films. Also, while bulk SmMnO 3 is antiferromagnetic, ferromagnetism was induced in the composite films. The Mn-O bond angles and lengths determined from density functional theory explain the origin of the ferroelectricity, i.e. modification of the exchange coupling. Our structural tuning method gives a route to designing multiferroics. Multiferroic materials exhibiting ferromagnetism (FM) and ferroeletricity (FE) at room temperature (RT) are promising for applications. Here, the authors demonstrate by inducing strain in SmMnO3 via introducing vertically aligned nanocomposites that exchange coupling can be modified tuning the system from anti-FM to FM and simultaneously inducing FE at RT.

Few materials have been identified as high-performance transparent conductors in the visible regime (400–700 nm). Even fewer conductors are known to be transparent in ultraviolet (UV) spectrum, especially at wavelengths below 320 nm. Doped wide-bandgap semiconductors employed currently as UV transparent conductors have insufficient electrical conductivities, posing a significant challenge for achieving low resistance electrodes. Here, we propose SrNbO 3 as an alternative transparent conductor material with excellent performance not only in the visible, but also in the UV spectrum. The high transparency to UV light originates from energetic isolation of the conduction band, which shifts the absorption edge into the UV regime. The standard figure of merit measured for SrNbO 3 in the UV spectral range of 260–320 nm is on par with indium tin oxide in the visible, making SrNbO 3 an ideal electrode material in high-performance UV light emitting diodes relevant in sanitation application, food packaging, UV photochemotherapy, and biomolecule sensing. Optically transparent electrodes with high electrical conductance are essential for the implementation of optoelectronics, but current technology performs poorly in the ultraviolet regime. Here, SrNbO 3 is proposed as an alternative material due to its high figure of merit in the ultraviolet range.

Incipient ferroelectricity bridges traditional dielectrics and true ferroelectrics, enabling advanced electronic and memory devices. Firstly, we report incipient ferroelectricity in freestanding SrTiO 3 nanomembranes integrated with monolayer MoS 2 to create multifunctional devices, demonstrating stable ferroelectric order at low temperatures for cryogenic memory devices. Our observation includes ultra-fast polarization switching (~10 ns), low switching voltage (<6 V), over 10 years of nonvolatile retention, 100,000 endurance cycles, and 32 conductance states (5-bit memory) in SrTiO 3 -gated MoS 2 transistors at 15 K and up to 100 K. Additionally, we exploit room-temperature weak polarization switching, a feature of incipient ferroelectricity, to construct a physical reservoir for pattern recognition. Our results showcase the potential of utilizing perovskite material properties enabled by advancements in freestanding film growth and heterogeneous integration, for diverse functional applications. Notably, the low 180 °C thermal budget for fabricating the 3D-SrTiO 3 /2D-MoS 2 device stack enables the integration of diverse materials into silicon complementary metal-oxide-semiconductor technology, addressing challenges in compute-in-memory and neuromorphic applications. Sen et al. report the stacking of a perovskite incipient ferroelectric nanomembrane with atomically thin 2D material for a back-end-of-line compatible ferroelectric-like field effect transistors, functioning as a cryogenic memory at 15 K and as an inference engine at room temperature.

Polar metals, commonly defined by the coexistence of polar crystal structure and metallicity, are thought to be scarce because the long-range electrostatic fields favoring the polar structure are expected to be fully screened by the conduction electrons of a metal. Moreover, reducing from three to two dimensions, it remains an open question whether a polar metal can exist. Here we report on the realization of a room temperature two-dimensional polar metal of the B-site type in tri-color (tri-layer) superlattices BaTiO 3 /SrTiO 3 /LaTiO 3 . A combination of atomic resolution scanning transmission electron microscopy with electron energy-loss spectroscopy, optical second harmonic generation, electrical transport, and first-principles calculations have revealed the microscopic mechanisms of periodic electric polarization, charge distribution, and orbital symmetry. Our results provide a route to creating all-oxide artificial non-centrosymmetric quasi-two-dimensional metals with exotic quantum states including coexisting ferroelectric, ferromagnetic, and superconducting phases. Materials that combine metallic behaviour with stable electric polarization are scarce despite being proposed in the 1960s. Here the authors engineer a perovskite heterostructure where 2D polar metallic behavior coexists with built-in electric polarization from the displacement of B-site titanium cations.

Multiferroics made easier Ferroelectric ferromagnets, or multiferroics, are of significant technological interest because they combine the low power and high speed of field-effect electronics with the permanence and routability of voltage-controlled ferromagnetism. Unfortunately, they are rare, and those that do exist have ferroelectric and ferromagnetic properties that are typically weak compared with conventional useful ferroelectrics and ferromagnets. A new route to fabricating multiferroics was recently predicted: in theory, magnetically ordered insulators that are neither ferroelectric nor ferromagnetic — of which there are many — can be turned into ferroelectric multiferroics by strain from the underlying substrate. June Hyuk Lee et al . now realize this route experimentally for EuTiO3. Their demonstration that a single experimental parameter, strain, can simultaneously control multiple order parameters opens up exciting possibilities for creating useful multiferroic materials. Ferroelectric ferromagnets — materials that are both ferroelectric and ferromagnetic — are of significant technological interest. But they are rare, and those that do exist have weak ferroelectric and ferromagnetic properties. Recently a new way of fabricating such materials was proposed, involving strain from the underlying substrate. This route has now been realized experimentally for EuTiO 3 . The work shows that a single experimental parameter, strain, can simultaneously control multiple order parameters. Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials that could give rise to new technologies in which the low power and high speed of field-effect electronics are combined with the permanence and routability of voltage-controlled ferromagnetism 1 , 2 . Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena 1 , 2 , 3 , 4 , 5 are insignificant in comparison with those of useful ferroelectrics or ferromagnets: their spontaneous polarizations or magnetizations are smaller by a factor of 1,000 or more. The same holds for magnetic- or electric-field-induced multiferroics 6 , 7 , 8 . Owing to the weak properties of single-phase multiferroics, composite and multilayer approaches involving strain-coupled piezoelectric and magnetostrictive components are the closest to application today 1 , 2 . Recently, however, a new route to ferroelectric ferromagnets was proposed 9 by which magnetically ordered insulators that are neither ferroelectric nor ferromagnetic are transformed into ferroelectric ferromagnets using a single control parameter, strain. The system targeted, EuTiO 3 , was predicted to exhibit strong ferromagnetism (spontaneous magnetization, ∼7 Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, ∼10 µC cm −2 ) simultaneously under large biaxial compressive strain 9 . These values are orders of magnitude higher than those of any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression we turned to tensile strain. Here we show both experimentally and theoretically the emergence of a multiferroic state under biaxial tension with the unexpected benefit that even lower strains are required, thereby allowing thicker high-quality crystalline films. This realization of a strong ferromagnetic ferroelectric points the way to high-temperature manifestations of this spin–lattice coupling mechanism 10 . Our work demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition 11 for creating multiferroics.

Oxynitrides have been explored extensively in the past decade because of their interesting properties, such as visible-light absorption, photocatalytic activity and high dielectric permittivity. Their synthesis typically requires high-temperature NH 3 treatment (800–1,300 °C) of precursors, such as oxides, but the highly reducing conditions and the low mobility of N 3− species in the lattice place significant constraints on the composition and structure—and hence the properties—of the resulting oxynitrides. Here we show a topochemical route that enables the preparation of an oxynitride at low temperatures (<500 °C), using a perovskite oxyhydride as a host. The lability of H − in BaTiO 3− x H x ( x ≤ 0.6) allows H − /N 3− exchange to occur, and yields a room-temperature ferroelectric BaTiO 3− x N 2 x /3 . This anion exchange is accompanied by a metal-to-insulator crossover via mixed O–H–N intermediates. These findings suggest that this ‘labile hydride’ strategy can be used to explore various oxynitrides, and perhaps other mixed anionic compounds. Oxynitrides are garnering interest because of their variety of novel properties, but their synthesis has typically involved highly reducing conditions that put significant constraints on their composition, structure and properties. Now, the lability of H − in perovskite oxyhydride BaTiO 3− x H x has enabled H – /N 3– exchange at a lower temperature, yielding a ferroelectric oxynitride BaTiO 3− x N 2 x /3 .