Gauss's law dictates that the net electric field inside a conductor in electrostatic equilibrium is zero by effective charge screening; free carriers within a metal eliminate internal dipoles that may arise owing to asymmetric charge distributions. Quantum physics supports this view, demonstrating that delocalized electrons make a static macroscopic polarization, an ill-defined quantity in metals--it is exceedingly unusual to find a polar metal that exhibits long-range ordered dipoles owing to cooperative atomic displacements aligned from dipolar interactions as in insulating phases. Here we describe the quantum mechanical design and experimental realization of room-temperature polar metals in thin-film ANiO3 perovskite nickelates using a strategy based on atomic-scale control of inversion-preserving (centric) displacements. We predict with ab initio calculations that cooperative polar A cation displacements are geometrically stabilized with a non-equilibrium amplitude and tilt pattern of the corner-connected NiO6 octahedral--the structural signatures of perovskites--owing to geometric constraints imposed by the underlying substrate. Heteroepitaxial thin-films grown on LaAlO3 (111) substrates fulfil the design principles. We achieve both a conducting polar monoclinic oxide that is inaccessible in compositionally identical films grown on (001) substrates, and observe a hidden, previously unreported, non-equilibrium structure in thin-film geometries. We expect that the geometric stabilization approach will provide novel avenues for realizing new multifunctional materials with unusual coexisting properties.

The metal-insulator transition in correlated materials is usually coupled to a symmetrylowering structural phase transition. This coupling not only complicates the understanding of the basic mechanism of this phenomenon but also limits the speed and endurance of prospective electronic devices. We demonstrate an isostructural, purely electronically driven metal-insulator transition in epitaxial heterostructures of an archetypal correlated material, vanadium dioxide. A combination of thin-film synthesis, structural and electrical characterizations, and theoretical modeling reveals that an interface interaction suppresses the electronic correlations without changing the crystal structure in this otherwise correlated insulator. This interaction stabilizes a nonequilibrium metallic phase and leads to an isostructural metal-insulator transition. This discovery will provide insights into phase transitions of correlated materials and may aid the design of device functionalities.

The design of efficient thermoelectric (TE) devices for energy harvesting and advanced cooling applications is one of the current challenges in materials science. [1] So far, the most common materials used in commercial TE devices are rock-salt IV-VI (PbTe, PbSe) and distorted rock-salt V2-VI3 (Bi2Te3, Bi2Se3) semiconductors. [2] One of the key factors behind the high TE performance of these materials is their abnormally low lattice thermal conductivity (κl), 2 which is one of the fundamental parameters that define the dimensionless TE figure of merit zT = S 2 σT/(κl+κe), in which S is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κe the electronic thermal conductivity.In a recent paper, Lee et al. [3] suggested that the main reason for the low lattice thermal conductivity in rock-salt IV-VI compounds is the resonant bonding (RB) effect: the p-orbitals with 3 electrons per atom cannot form the six saturated bonds of the rock-salt lattice, and therefore an RB structure is established. [ 4 ] Using first-principles calculations, they demonstrated that the large electronic polarizability of the resonant bonds introduces long-range interactions and a softening of the transverse optical phonon mode. This ultimately causes acoustic phonon scattering and is responsible for the low lattice thermal conductivity in IV-VI and V2-VI3 compounds. An interesting question is whether or not the concept of RB can be extrapolated to transition-metal (TM) compounds with a rock-salt structure. The versatility of the oxidation states and ionic sizes shown by TM ions would offer enormous possibilities for tuning their TE properties, which would allow for new approaches regarding the design of TE materials with improved capabilities.In this communication we demonstrate that rock-salt CrN shows intrinsic lattice instabilities that suppress its thermal conductivity. Using ab-initio calculations, we determined that the origin of these instabilities is similar to that observed in IV-VI compounds with RB states. [3,5] Through the fabrication of high quality epitaxial (001) CrN thin films we report a 250% increase in the zT at room temperature compared to bulk CrN. [6] These results along with its high thermal stability, resistance to corrosion, and exceptional mechanical properties, make CrN a promising n-type material for high-temperature TE applications.The presence of extrinsic factors, such as N-vacancies or epitaxial constrains, are likely behind the large variety of structural and transport properties previously reported for CrN films. [7] In the case of polycrystalline bulk CrN, the intergrain contribution to the electrical and 3 thermal conductivities can be significant enough to mask its intrinsic transport properties and, ultimately, its thermoelectric performance. Therefore, in order to access the intrinsic thermoelectric properties of CrN, it is necessary to develop the fabrication of epitaxial, stoichiometric, and fully relaxed CrN films. The results discussed in this pap...

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), 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 largely determines the MIT dynamics critical to the device performance. We develop an approach to control the MIT dynamics in epitaxial VO 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 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.

Exploiting multiferroic BiFeO3 thin films in spintronic devices requires deterministic and robust control of both internal magnetoelectric coupling in BiFeO3, as well as exchange coupling of its antiferromagnetic order to a ferromagnetic overlayer. Previous reports utilized approaches based on multi-step ferroelectric switching with multiple ferroelectric domains. Because domain walls can be responsible for fatigue, contain localized charges intrinsically or via defects, and present problems for device reproducibility and scaling, an alternative approach using a monodomain magnetoelectric state with single-step switching is desirable. Here we demonstrate room temperature, deterministic and robust, exchange coupling between monodomain BiFeO3 films and Co overlayer that is intrinsic (i.e., not dependent on domain walls). Direct coupling between BiFeO3 antiferromagnetic order and Co magnetization is observed, with ~ 90° in-plane Co moment rotation upon single-step switching that is reproducible for hundreds of cycles. This has important consequences for practical, low power non-volatile magnetoelectric devices utilizing BiFeO3.