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The recent discovery of polar topological structures has opened the door for exciting physics and emergent properties. There is, however, little methodology to engineer stability and ordering in these systems, properties of interest for engineering emergent functionalities. Notably, when the surface area is extended to arbitrary thicknesses, the topological polar texture becomes unstable. Here we show that this instability of the phase is due to electrical coupling between successive layers. We demonstrate that this electrical coupling is indicative of an effective screening length in the dielectric, similar to the conductor-ferroelectric interface. Controlling the electrostatics of the superlattice interfaces, the system can be tuned between a pure topological vortex state and a mixed classical-topological phase. This coupling also enables engineering coherency among the vortices, not only tuning the bulk phase diagram but also enabling the emergence of a 3D lattice of polar textures.

Relaxor ferroelectrics have been intensely studied for decades based on their unique electromechanical responses which arise from local structural heterogeneity involving polar nanoregions or domains. Here, we report first studies of the ultrafast dynamics and reconfigurability of the polarization in freestanding films of the prototypical relaxor 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 (PMN-0.32PT) by probing its atomic-scale response via femtosecond-resolution, electron-scattering approaches. By combining these structural measurements with dynamic phase-field simulations, we show that femtosecond light pulses drive a change in both the magnitude and direction of the polarization vector within polar nanodomains on few-picosecond time scales. This study defines new opportunities for dynamic reconfigurable control of the polarization in nanoscale relaxor ferroelectrics.

Controlling the domain formation in ferroelectric materials at the nanoscale is a fertile ground to explore emergent phenomena and their technological prospects. For example, charged ferroelectric domain walls in BiFeO3 and ErMnO3 exhibit significantly enhanced conductivity which could serve as the foundation for next-generation circuits (Estévez and Laurson, Phys. Rev. B 2015, 91, 054407). Here, we describe a concept in which polar vortices perform the same role as a ferroelectric domain wall in classical domain structures with the key difference being that the polar vortices can accommodate charged (i.e., head-to-head and tail-to-tail) domains, for example, in ferroelectric PbTiO3/dielectric SrTiO3 superlattices. Such a vortex domain wall structure can be manipulated in a reversible fashion under an external applied field.

A novel mesoscale state comprising of an ordered polar vortex lattice has been demonstrated in ferroelectric superlattices of PbTiO3/SrTiO3. Here, we employ phase-field simulations, analytical theory, and experimental observations to evaluate thermodynamic conditions and geometric length scales that are critical for the formation of such exotic vortex states. We show that the stability of these vortex lattices involves an intimate competition between long-range electrostatic, long-range elastic, and short-range polarization gradient-related interactions leading to both an upper and a lower bound to the length scale at which these states can be observed. We found that the critical length is related to the intrinsic domain wall width, which could serve as a simple intuitive design rule for the discovery of novel ultrafine topological structures in ferroic systems.

Polarization switching in ferroelectric capacitors is typically realized by application of an electrical bias to the capacitor electrodes and occurs via a complex process of domain structure reorganization. As the domain evolution in real devices is governed by the distribution of the nucleation centers, obtaining a domain structure of a desired configuration by electrical pulsing is challenging, if not impossible. Recent discovery of polarization reversal via the flexoelectric effect has opened a possibility for deterministic control of polarization in ferroelectric capacitors. In this paper, we demonstrate mechanical writing of arbitrary-shaped nanoscale domains in thin-film ferroelectric capacitors with graphene electrodes facilitated by a strain gradient induced by a tip of an atomic force microscope (AFM). A phase-field modeling prediction of a strong effect of graphene thickness on the threshold load required to initiate mechanical switching has been confirmed experimentally. Deliberate voltage-free domain writing represents a viable approach for development of functional devices based on domain topology and electronic properties of the domains and domain walls.

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using "phonon currents". With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO 3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the room-temperature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10 -9 K m 2 W -1 ), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO 3 . This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO 3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics.

Atomic layer controlled growth of epitaxial thin films of unconventional superconductors opens the opportunity to discover novel high temperature superconductors. For instance, the interfacial atomic configurations may play an important role in superconducting behavior of monolayer FeSe on SrTiO 3 and other Fe-based superconducting thin films. Here, we demonstrate a selective control of the atomic configurations in Co-doped BaFe 2 As 2 epitaxial thin films and its strong influence on superconducting transition temperatures by manipulating surface termination of (001) SrTiO 3 substrates. In a combination of first-principles calculations and high-resolution scanning transmission electron microscopy imaging, we show that Co-doped BaFe 2 As 2 on TiO 2 -terminated SrTiO 3 is a tetragonal structure with an atomically sharp interface and with an initial Ba layer. In contrast, Co-doped BaFe 2 As 2 on SrO-terminated SrTiO 3 has a monoclinic distortion and a BaFeO 3- x initial layer. Furthermore, the superconducting transition temperature of Co-doped BaFe 2 As 2 ultrathin films on TiO 2 -terminated SrTiO 3 is significantly higher than that on SrO-terminated SrTiO 3 , which we attribute to shaper interfaces with no lattice distortions. This study allows the design of the interfacial atomic configurations and the effects of the interface on superconductivity in Fe-based superconductors.

Strontium titanate (SrTiO 3 ) is the "silicon" in the emerging field of oxide electronics. While bulk properties of this material have been studied for decades, new unexpected phenomena have recently been discovered at the nanoscale, when SrTiO 3 forms an ultrathin film or an atomically sharp interface with other materials. One of the striking discoveries is room-temperature ferroelectricity in strain-free ultrathin films of SrTiO 3 driven by the Ti Sr antisite defects, which generate a local dipole moment polarizing the surrounding nanoregion. Here, we demonstrate that these polar defects are not only responsible for ferroelectricity, but also propel the appearance of highly conductive channels, "hot spots", in the ultrathin SrTiO 3 films. Using a combination of scanning probe microscopy experimental studies and theoretical modeling, we show that the hot spots emerge due to resonant tunneling through localized electronic states created by the polar defects and that the tunneling conductance of the hot spots is controlled by ferroelectric polarization. Our finding of the polarization-controlled defect-assisted tunneling reveals a new mechanism of resistive switching in oxide heterostructures and may have technological implications for ferroelectric tunnel junctions. It is also shown that the conductivity of the hot spots can be modulated by mechanical stress, opening a possibility for development of conceptually new electronic devices with mechanically tunable resistive states.

Phase transitions in correlated materials can be manipulated at the nanoscale to yield emergent functional properties, promising new paradigms for nanoelectronics and nanophotonics. Vanadium dioxide (VO 2 ), an archetypal correlated material, exhibits a metal-insulator transition (MIT) above room temperature. At the thicknesses required for heterostructure applications, such as an optical modulator discussed here, the strain state of VO 2 largely determines the MIT dynamics critical to the device performance. We develop an approach to control the MIT dynamics in epitaxial VO 2 films by employing an intermediate template layer with large lattice mismatch to relieve the interfacial lattice constraints, contrary to conventional thin film epitaxy that favors lattice match between the substrate and the growing film. A combination of phase-field simulation, in situ real-time nanoscale imaging, and electrical measurements reveals robust undisturbed MIT dynamics even at preexisting structural domain boundaries and significantly sharpened MIT in the templated VO 2 films. Utilizing the sharp MIT, we demonstrate a fast, electrically switchable optical waveguide. This study offers unconventional design principles for heteroepitaxial correlated materials, as well as novel insight into their nanoscale phase transitions.

Phase transitions in 2D materials can lead to massive changes in electronic properties that enable novel electronic devices. Tantalum disulfide (TaS 2 ), specifically the "1T" phase (1T-TaS 2 ), exhibits a phase transition based on the formation of commensurate charge density waves (CCDW) at 180 K. In this work, we investigate the impact of substrate choice on the phase transitions in ultrathin 1T-TaS 2 . Doping and charge transfer from the substrate has little impact on CDW phase transitions. On the contrary, we demonstrated that substrate surface roughness is a primary extrinsic factor in CCDW transition temperature and hysteresis, where higher roughness leads to smaller transition hysteresis. Such roughness can be simulated via surface texturing of SiO 2 /Si substrates, which controllably and reproducibly induces periodic strain in the 1T-TaS 2 and thereby enables the potential for engineering CDW phase transitions.

Substitutional doping of transition metal dichalcogenides (TMDs) may provide routes to achieving tunable p-n junctions, bandgaps, chemical sensitivity, and magnetism in these materials. In this study, we demonstrate in situ doping of monolayer molybdenum disulfide (MoS2) with manganese (Mn) via vapor phase deposition techniques. Successful incorporation of Mn in MoS2 leads to modifications of the band structure as evidenced by photoluminescence and X-ray photoelectron spectroscopy, but this is heavily dependent on the choice of substrate. We show that inert substrates (i.e., graphene) permit the incorporation of several percent Mn in MoS2, while substrates with reactive surface terminations (i.e., SiO2 and sapphire) preclude Mn incorporation and merely lead to defective MoS2. The results presented here demonstrate that tailoring the substrate surface could be the most significant factor in substitutional doping of TMDs with non-TMD elements.

We report a discovery that self-assembled perovskite-spinel nanostructures can be controlled simply by selecting single-crystal substrates with different orientations. In a model BiFeO(3)-CoFe(2)O(4) system, a (001) substrate results in rectangular-shaped CoFe(2)O(4) nanopillars in a BiFeO(3) matrix; in contrast, a (111) substrate leads to triangular-shaped BiFeO(3) nanopillars in a CoFe(2)O(4) matrix, irrespective of the volume fraction of the two phases. This dramatic reversal is attributed to the surface energy anisotropy as an intrinsic property of a crystal.