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Recently a metallic state was discovered at the interface between insulating oxides, most notably LaAlO 3 and SrTiO 3 . Properties of this two-dimensional electron gas (2DEG) have attracted significant interest due to its potential applications in nanoelectronics. Control over this carrier density and mobility of the 2DEG is essential for applications of these unique systems, and may be achieved by epitaxial strain. However, despite the rich nature of strain effects on oxide materials properties, such as ferroelectricity, magnetism, and superconductivity, the relationship between the strain and electrical properties of the 2DEG at the LaAlO 3 ∕SrTiO 3 heterointerface remains largely unexplored. Here, we use different lattice constant single-crystal substrates to produce LaAlO 3 ∕SrTiO 3 interfaces with controlled levels of biaxial epitaxial strain. We have found that tensile-strained SrTiO 3 destroys the conducting 2DEG, while compressively strained SrTiO 3 retains the 2DEG, but with a carrier concentration reduced in comparison to the unstrained LaAlO 3 ∕SrTiO 3 interface. We have also found that the critical LaAlO 3 overlayer thickness for 2DEG formation increases with SrTiO 3 compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO 3 layer is responsible for this behavior. The polarization is directed away from the interface and hence creates a negative polarization charge opposing that of the polar LaAlO 3 layer. This behavior both increases the critical thickness of the LaAlO 3 layer, and reduces carrier concentration above the critical thickness, in agreement with our experimental results. Our findings suggest that epitaxial strain can be used to tailor 2DEGs properties of the LaAlO 3 ∕SrTiO 3 heterointerface.oxide interface | electronic transport | polar discontinuity S train has been used to engineer and enhance numerous properties of materials. For example, mobility in semiconductors (1,2), and transition temperatures in ferroelectric materials (3-6), and superconductors (7) have been controlled by strain. A recently discovered two-dimensional electron gas (2DEG) at the LaAlO 3 ∕SrTiO 3 interface (8,9) has attracted great interest due to its unique application to nanoscale oxide devices (10). So far, most studies of 2DEGs at oxide interfaces were performed using TiO 2 -terminated SrTiO 3 bulk single-crystal substrates. Despite the rich nature of strain effects on oxide materials properties, the relationship between the strain and electrical properties of the 2DEG at the LaAlO 3 ∕SrTiO 3 heterointerface remains largely unexplored.One important effect of strain arises from the constraint that integrating 2DEGs to other functional devices or substrates always involves strain. Understanding the effect of strain on a 2DEG at the LaAlO 3 ∕SrTiO 3 interface is essential for these considerations. In addition, incorporation of strain might lead to unique functional properties. For example, strain can induce an electric polarization in otherwise nonpol...

Heterostructured material systems devoid of ferroic components are presumed not to display ordering associated with ferroelectricity. In heterostructures composed of transition metal oxides, however, the disruption introduced by an interface can affect the balance of the competing interactions among electronic spins, charges and orbitals. This has led to the emergence of properties absent in the original building blocks of a heterostructure, including metallicity, magnetism and superconductivity. Here we report the discovery of ferroelectricity in artificial tri-layer superlattices consisting solely of non-ferroelectric ndmno 3 /srmno 3 / Lamno 3 layers. Ferroelectricity was observed below 40 K exhibiting strong tunability by superlattice periodicity. Furthermore, magnetoelectric coupling resulted in 150% magnetic modulation of the polarization. Density functional calculations indicate that broken space inversion symmetry and mixed valency, because of cationic asymmetry and interfacial polar discontinuity, respectively, give rise to the observed behaviour. our results demonstrate the engineering of asymmetric layered structures with emergent ferroelectric and magnetic field tunable functions distinct from that of normal devices, for which the components are typically ferroelectrics.

Based on first-principles and model calculations we investigate the effect of polar interfaces on the ferroelectric stability of thin-film ferroelectrics. As a representative model, we consider a TiO 2 -terminated BaTiO 3 film with LaO monolayers at the two interfaces that serve as doping layers. We find that the polar interfaces create an intrinsic electric field that is screened by the electron charge leaking into the BaTiO 3 layer. The amount of the leaking charge is controlled by the boundary conditions which are different for three heterostructures considered, namely, vacuum/LaO/ BaTiO 3 / LaO, LaO/ BaTiO 3 , and SrRuO 3 / LaO/ BaTiO 3 / LaO. The intrinsic electric field forces ionic displacements in BaTiO 3 to produce the electric polarization directed into the interior of the BaTiO 3 layer. This creates a ferroelectric dead layer near the interfaces that is nonswitchable and thus detrimental to ferroelectricity. Our first-principles and model calculations demonstrate that the effect is stronger for a larger effective ionic charge at the interface and longer screening length due to a stronger intrinsic electric field that penetrates deeper into the ferroelectric. The predicted mechanism for a ferroelectric dead layer at the interface controls the critical thickness for ferroelectricity in systems with polar interfaces.

Topological edge states have recently garnered a lot of attention across various fields of physics. The topological edge soliton is a hybrid edge state that is both topologically protected and immune to defects or disorders, and a localized bound state that is diffraction-free, owing to the self-balance of diffraction by nonlinearity. Topological edge solitons hold great potential for on-chip optical functional device fabrication. In this report, we present the discovery of vector valley Hall edge (VHE) solitons in type-II Dirac photonic lattices, formed by breaking lattice inversion symmetry with distortion operations. The distorted lattice features a two-layer domain wall that supports both in-phase and out-of-phase VHE states, appearing in two different band gaps. Superposing soliton envelopes onto VHE states generates bright-bright and bright-dipole vector VHE solitons. The propagation dynamics of such vector solitons reveal a periodic change in their profiles, accompanied by the energy periodically transferring between the layers of the domain wall. The reported vector VHE solitons are found to be metastable.