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Register a new userPolar morphologies from first principles: PbTiO$_3$ films on SrTiO$_3$ substrates and the $p(2 times Lambda)$ surface reconstruction
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Low dimensional structures comprised of ferroelectric (FE) PbTiO$_3$ (PTO) and quantum paraelectric SrTiO$_3$ (STO) are hosts to complex polarization textures such as polar waves, flux-closure domains and polar skyrmion phases. Density functional theory (DFT) simulations can provide insight into this order, but, are limited by the computational effort needed to simulate the thousands of required atoms. To relieve this issue, we use the novel multi-site support function (MSSF) method within DFT to reduce the solution time for the electronic groundstate whilst preserving high accuracy. Using MSSFs, we simulate thin PTO films on STO substrates with system sizes $>2000$ atoms. In the ultrathin limit, the polar wave texture with cylindrical chiral bubbles emerges as an intermediate phase between full flux closure domains and in-plane polarization. This is driven by an internal bias field born of the compositionally broken inversion symmetry in the [001] direction. Since the exact nature of this bias field depends sensitively on the film boundary conditions, this informs a new principle of design for manipulating chiral order on the nanoscale through the careful choice of substrate, surface termination or use of overlayers. Antiferrodistortive (AFD) order locally interacts with these polar textures giving rise to strong FE/AFD coupling at the PbO terminated surface driving a $p(2 times Lambda)$ surface reconstruction. This offers another pathway for the local control of ferroelectricity.
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We demonstrate how the quantum paraelectric ground state of SrTiO$_3$ can be accessed via a microscopic $ab~initio$ approach based on density functional theory. At low temperature the quantum fluctuations are strong enough to stabilize the paraelectric phase even though a classical description would predict a ferroelectric phase. We find that accounting for quantum fluctuations of the lattice and for the strong coupling between the ferroelectric soft mode and lattice elongation is necessary to achieve quantitative agreement with experimental frequency of the ferroelectric soft mode. The temperature dependent properties in SrTiO$_3$ are also well captured by the present microscopic framework.
The intrinsic magnetic state (ferromagnetic or antiferromagnetic) of ultra-thin LaMnO$_3$ films on the mostly used SrTiO$_3$ substrate is a long-existing question under debate. Either strain effect or non-stoichiometry was argued to be responsible for the experimental ferromagnetism. In a recent experiment [Science textbf{349}, 716 (2015)], one more mechanism, namely the self-doping due to polar discontinuity, was argued to be the driving force of ferromagnetism beyond the critical thickness. Here systematic first-principles calculations have been performed to check these mechanisms in ultra-thin LaMnO$_3$ films as well as superlattices. Starting from the very precise descriptions of both LaMnO$_3$ and SrTiO$_3$, it is found that the compressive strain is the dominant force for the appearance of ferromagnetism, while the open surface with oxygen vacancies leads to the suppression of ferromagnetism. Within LaMnO$_3$ layers, the charge reconstructions involve many competitive factors and certainly go beyond the intuitive polar catastrophe model established for LaAlO$_3$/SrTiO$_3$ heterostructures. Our study not only explains the long-term puzzle regarding the magnetism of ultra-thin LaMnO$_3$ films, but also shed light on how to overcome the notorious magnetic dead layer in ultra-thin manganites.
We report first-principles density-functional pseudopotential calculations on the atomic structures, electronic properties, and band offsets of BaO/BaTiO$_3$ and SrO/SrTiO$_3$ nanosized heterojunctions grown on top of a silicon substrate. The density of states at the junction does not reveal any electronic induced interface states. A dominant perovskite character is found at the interface layer. The tunability of the band offset with the strain conditions imposed by the substrate is studied. Using previously reported theoretical data available for Si/SrO, Si/BaO and BaTiO$_{3}$/SrRuO$_{3}$ interfaces we extrapolate a value for the band alignments along the whole gate stacks of technological interest: Si/SrO/SrTiO$_3$ and Si/BaO/BaTiO$_3$/SrRuO$_3$ heterostructures.
PbTiO$_3$ is a simple but very important ferroelectric oxide that has been extensively studied and widely used in various technological applications. However, most previous studies and applications were based on the bulk material or the conventional [$001$]-orientated films. There are few studies on PbTiO$_3$ films grown along other crystalline axes. In this study, a first-principles calculation was performed to compute the polarization of PbTiO$_3$ films strained by SrTiO$_3$ and LaAlO$_3$ substrates. Our results show that the polarization of PbTiO$_3$ films strongly depends on the growth orientation as well as the monoclinic angles. Further, it is suggested that the ferroelectricity of PbTiO$_3$ mainly depends on the tetragonality of the lattice, instead of the simple strain.
We studied surface and electronic structures of barium stannate (BaSnO$_3$) thin-film by low energy electron diffraction (LEED), and angle-resolved photoemission spectroscopy (ARPES) techniques. BaSnO$_3$/Ba$_{0.96}$La$_{0.04}$SnO$_3$/SrTiO$_3$ (10 nm/100 nm/0.5 mm) samples were grown using pulsed-laser deposition (PLD) method and were emph{ex-situ} transferred from PLD chamber to ultra-high vacuum (UHV) chambers for annealing, LEED and ARPES studies. UHV annealing starting from 300$^{circ}$C up to 550$^{circ}$C, followed by LEED and ARPES measurements show 1$times$1 surfaces with non-dispersive energy-momentum bands. The 1$times$1 surface reconstructs into a $sqrt{2}$$times$$sqrt{2}R45^circ$ one at the annealing temperature of 700$^{circ}$C where the ARPES data shows clear dispersive bands with valence band maximum located around 3.3 eV below Fermi level. While the $sqrt{2}$$times$$sqrt{2}R45^circ$ surface reconstruction is stable under further UHV annealing, it is reversed to 1$times$1 surface by annealing the sample in 400 mTorr oxygen at 600$^{circ}$C. Another UHV annealing at 600$^{circ}$C followed by LEED and ARPES measurements, suggests that LEED $sqrt{2}$$times$$sqrt{2}R45^circ$ surface reconstruction and ARPES dispersive bands are reproduced. Our results provide a better picture of electronic structure of BaSnO$_3$ surface and are suggestive of role of oxygen vacancies in the reversible $sqrt{2}$$times$$sqrt{2}R45^circ$ surface reconstruction.
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