Search

We report intense, narrow line-width, surface chemisorption-activated and reversible ultraviolet (UV) photoluminescence from radiative recombination of the two-dimensional electron gas (2DEG) with photoexcited holes at LaAlO3/SrTiO3. The switchable luminescence arises from an electron transfer-driven modification of the electronic structure via H-chemisorption onto the AlO2-terminated surface of LaAlO3, at least 2 nm away from the interface. The control of the onset of emission and its intensity are functionalities that go beyond the luminescence of compound semiconductor quantum wells. Connections between reversible chemisorption, fast electron transfer, and quantum-well luminescence suggest a new model for surface chemically reconfigurable solid-state UV optoelectronics and molecular sensing.

The next technological leap forward will be enabled by new materials and inventive means of manipulating them. Among the array of candidate materials, graphene has garnered much attention; however, due to the absence of a semiconducting gap, the realization of graphene-based devices often requires complex processing and design. Spatially controlled local potentials, for example, achieved through lithographically defined split-gate configurations, present a possible route to take advantage of this exciting two-dimensional material. Here we demonstrate carrier density modulation in graphene through coupling to an adjacent ferroelectric polarization to create spatially defined potential steps at 180°-domain walls rather than fabrication of local gate electrodes. Periodic arrays of p-i junctions are demonstrated in air (gate tunable to p-n junctions) and density functional theory reveals that the origin of the potential steps is a complex interplay between polarization, chemistry, and defect structures in the graphene/ferroelectric couple.

We report a density functional theory investigation of B-site doped CaFeO3, a prototypical charge ordered perovskite. At 290 K, CaFeO3 undergoes a metal-insulator transition and a charge disproportionation reaction 2Fe4+→Fe5++Fe3+. We observe that when Zr dopants occupy a (001) layer, the band gap of the resulting solid solution increases to 0.93 eV due to a two-dimensional Jahn-Teller-type distortion, where FeO6 cages on the xy plane elongate along x and y alternatively between neighboring Fe sites. Furthermore, we show that the rock-salt ordering of the Fe5+ and Fe3+ cations can be enhanced when the B-site dopants are arranged in a (111) plane due to a collective steric effect that facilitates the size discrepancy between the Fe5+O6 and Fe3+O6 octahedra and therefore gives rise to a larger band gap. The enhanced charge disproportionation in these solid solutions is verified by rigorously calculating the oxidation states of the Fe cations with different octahedral cage sizes. We therefore predict that the corresponding transition temperature will increase due to the enhanced charge ordering and larger band gap. The compositional, structural, and electrical relationships exploited in this paper can be extended to a variety of perovskites and nonperovskite oxides, providing guidance in the structural manipulation of electrical properties of functional materials. [ABSTRACT FROM AUTHOR]

Helicoidal magnetic order breaks inversion symmetry in quadruple perovskite CaMn7O12, generating one of the largest spin-induced ferroelectric polarizations measured to date. Here, the microscopic origin of the polarization, including exchange interactions, coupling to the spin helicity, and charge density redistribution, is explored via first-principles calculations. The B-site Mn4+ (Mn3) spin adopts a noncentrosymmetric configuration, stabilized not only by spin-orbit coupling (SOC), but also by the fully anisotropic Hubbard J parameter in the absence of SOC, to break inversion symmetry and generate polarization. Berry phase computed polarization (Pelec = 2169 µ C / m²) exhibits nearly pure electronic behavior, with negligible Mn displacements ( ≈ 0.7 m Å ). Orbital-resolved density of states shows that p - d orbital mixing is microscopically driven by nonrelativistic exchange striction within the commensurate ionic structure. Persistent electronic polarization induced by helical spin order in the nearly inversion-symmetric ionic crystal lattice suggests opportunities for ultrafast magnetoelectric response. [ABSTRACT FROM AUTHOR]

We employ the fully anisotropic DFT+U+J approach with the PBEsol functional to investigate ground-state magnetic and electronic properties of bulk binary manganese oxides: MnO, Mn3O4, α-Mn2O3, and β-MnO2, in order of increasing Mn valence. The computed crystal structures, noncollinear magnetic ground states, and corresponding electronic structures are in good agreement with the experimental data and hybrid functional calculations available in the literature. We take into account the nonlinear core-valence interaction in our Mn pseudopotential designed by ourselves, as it has been proven to be important for transition-metal systems. Although the Hubbard U term is capable by itself of opening a band gap, the explicitly defined exchange parameter J plays an important role in improving the detailed electronic and noncollinear magnetic structure profiles. Appropriate band gaps are obtained with U values smaller than those used in previously reported calculations. Our results suggest that pseudopotential design together with DFT+U+J enables the acquisition of accurate properties of complex magnetic systems using a nonhybrid density functional. [ABSTRACT FROM AUTHOR]

We model short-period superlattices of WO3 and ReO3 with first-principles calculations. In fully relaxed superlattices we observe that octahedral tilts about an axis in the planes of the superlattices do not propagate from one material, despite the presence of the comer-shared oxygen atoms. However, we find that octahedral rotation is enhanced within WO3 layers in cases in which strain couples with native antiferroelectric displacements of tungsten within their octahedral cages. Resulting structures remain antiferroelectric with low net global polarization. Thermodynamic analysis reveals that superlattices with sufficiently thick ReO3 layers, the absolute number being three or more layers and the Re fraction ⩾50%, tend to be more stable than the separated material phases and also show enhanced octahedral rotations in the WO3 layers. [ABSTRACT FROM AUTHOR]