The enhancement of the functional properties of materials at reduced dimensions is crucial for continuous advancements in nanoelectronic applications. Here, we report that the scale reduction leads ...to the emergence of an important functional property, ferroelectricity, challenging the long-standing notion that ferroelectricity is inevitably suppressed at the scale of a few nanometers. A combination of theoretical calculations, electrical measurements, and structural analyses provides evidence of room-temperature ferroelectricity in strain-free epitaxial nanometer-thick films of otherwise nonferroelectric strontium titanate (SrTiO3). We show that electrically induced alignment of naturally existing polar nanoregions is responsible for the appearance of a stable net ferroelectric polarization in these films. This finding can be useful for the development of low-dimensional material systems with enhanced functional properties relevant to emerging nanoelectronic devices.
Ferromagnetism is usually considered to be incompatible with conventional superconductivity, as it destroys the singlet correlations responsible for the pairing interaction. Superconductivity and ...ferromagnetism are known to coexist in only a few bulk rare-earth materials. Here we report evidence for their coexistence in a two-dimensional system: the interface between two bulk insulators, LaAlO(3) (LAO) and SrTiO(3) (STO), a system that has been studied intensively recently. Magnetoresistance, Hall, and electric-field dependence measurements suggest that there are two distinct bands of charge carriers that contribute to the interface conductivity. The sensitivity of properties of the interface to an electric field makes this a fascinating system for the study of the interplay between superconductivity and magnetism.
In recent years, complex-oxide heterostructures and their interfaces have become the focus of significant research activity, primarily driven by the discovery of emerging states and functionalities ...that open up opportunities for the development of new oxide-based nanoelectronic devices. The highly conductive state at the interface between insulators LaAlO3 and SrTiO3 is a prime example of such emergent functionality, with potential application in high electron density transistors. In this report, we demonstrate a new paradigm for voltage-free tuning of LaAlO3/SrTiO3 (LAO/STO) interface conductivity, which involves the mechanical gating of interface conductance through stress exerted by the tip of a scanning probe microscope. The mechanical control of channel conductivity and the long retention time of the induced resistance states enable transistor functionality with zero gate voltage.
Light-induced supercurrents chart a path forward for the electromagnetic design of emergent materials phases and collective modes for quantum engineering applications. However, controlled ...spatial–temporal modulation of the complex order parameter characterizing such non-equilibrium macroscopic quantum states remains elusive. Such ultrafast phase-amplitude modulation can manifest via high harmonic modes beyond those allowed by equilibrium symmetries. Here, we drive moving condensate states via subcycle dynamical symmetry breaking achieved with nonlinear oscillating terahertz photocurrents. These non-equilibrium macroscopic quantum states with broken inversion symmetry are controlled via Cooper pair acceleration by asymmetric and multi-cycle terahertz photoexcitations. The observed supercurrent-carrying states evolve during a lightwave cycle and exhibit three distinguishing features: Anderson pseudo-spin precessions forbidden by equilibrium symmetry, strong high harmonic coherent oscillations assisted by pairing and long-lived gapless superfluidity with minimal condensate quench. Lightwave tuning of persistent photocurrents can be extended for quantum control of unconventional superconductors and topological matter, with implications on quantum gate and sensing functionalities.
The Fe-based superconductors (FBSs) are an important new class of superconducting materials. As with any new superconductor with a high transition temperature and upper critical field, there is a ...need to establish what their applications potential might be. Applications require high critical current densities, so the usefulness of any new superconductor is determined both by the capability to develop strong vortex pinning and by the absence or ability to overcome any strong current-limiting mechanisms of which grain boundaries (GBs) in the cuprates are a cautionary example. In this review we first consider the positive role that GB properties play in the metallic, low-temperature superconductors and then review the theoretical background and current experimental data relating to the properties of GBs in FBS polycrystals, bicrystal thin films and wires. Based on this evidence, we conclude that GBs in FBS are weak linked in a qualitatively similar way to GBs in the cuprate superconductors, but also that the effects are a little less marked. Initial experiments with the textured substrates used for cuprate coated conductors show similar benefit for the critical current density of FBS thin films too. We also note that the particular richness of the pairing symmetry and the multiband parent state in FBS may provide opportunities for GB modification as a better understanding of their pairing state and GB properties are developed.
Recent advances in atomic-precision processing of oxide ferroelectricsmaterials with a stable polarization that can be switched by an external electric fieldhave generated considerable interest due ...to rich physics associated with their fundamental properties and high potential for application in devices with enhanced functionality. One of the particularly promising phenomena is the tunneling electroresistance (TER) effectpolarization-dependent bistable resistance behavior of ferroelectric tunnel junctions (FTJ). Conventionally, the application of an electric field above the coercive field of the ferroelectric barrier is required to observe this phenomenon. Here, we report a mechanically induced TER effect in ultrathin ferroelectric films of BaTiO3 facilitated by a large strain gradient induced by a tip of a scanning probe microscope (SPM). The obtained results represent a new paradigm for voltage-free control of electronic properties of nanoscale ferroelectrics and, more generally, complex oxide materials.
Polar metals by geometric design Kim, T H; Puggioni, D; Yuan, Y ...
Nature (London),
2016-May-05, 2016-05-05, 20160505, Letnik:
533, Številka:
7601
Journal Article
Recenzirano
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.
Multiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. In this work, we demonstrate the first ...observation of electrical control of antiferromagnetic domain structure in a single-phase multiferroic material at room temperature. High-resolution images of both antiferromagnetic and ferroelectric domain structures of (001)-oriented multiferroic BiFeO3 films revealed a clear domain correlation, indicating a strong coupling between the two types of order. The ferroelectric structure was measured using piezo force microscopy, whereas X-ray photoemission electron microscopy as well as its temperature dependence was used to detect the antiferromagnetic configuration. Antiferromagnetic domain switching induced by ferroelectric polarization switching was observed, in agreement with theoretical predictions.
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