Prediction of thickness limits of ideal polar ultrathin films

Competition between electronic and atomic reconstruction is a constantly recurring theme in transition-metal oxides. We use density functional theory calculations to study this competition for a model system consisting of a thin film of the polar, infinite-layer structure ACuO2 (A=Ca, Sr, Ba) grown on a nonpolar, perovskite SrTiO3 substrate. A transition from the bulk planar structure to a chain-type thin film accompanied by substantial changes to the electronic structure is predicted for a SrCuO2 film fewer than five unit cells thick. An analytical model explains why atomic reconstruction becomes more favorable than electronic reconstruction as the film becomes thinner, and suggests that similar considerations should be valid for other polar films

Predicted band structures of III-V semiconductors in wurtzite phase

While non-nitride III-V semiconductors typically have a zincblende structure, they may also form wurtzite crystals under pressure or when grown as nanowhiskers. This makes electronic structure calculation difficult since the band structures of wurtzite III-V semiconductors are poorly characterized. We have calculated the electronic band structure for nine III-V semiconductors in the wurtzite phase using transferable empirical pseudopotentials including spin-orbit coupling. We find that all the materials have direct gaps. Our results differ significantly from earlier {\it ab initio} calculations, and where experimental results are available (InP, InAs and GaAs) our calculated band gaps are in good agreement. We tabulate energies, effective masses, and linear and cubic Dresselhaus zero-field spin-splitting coefficients for the zone-center states. The large zero-field spin-splitting coefficients we find may lead to new functionalities for designing devices that manipulate spin degrees of freedom

Imaging Pulsed Laser Deposition oxide growth by in-situ Atomic Force Microscopy

To visualize the topography of thin oxide films during growth, thereby enabling to study its growth behavior quasi real-time, we have designed and integrated an atomic force microscope (AFM) in a pulsed laser deposition (PLD) vacuum setup. The AFM scanner and PLD target are integrated in a single support frame, combined with a fast sample transfer method, such that in-situ microscopy can be utilized after subsequent deposition pulses. The in-situ microscope can be operated from room temperature (RT) up to 700$^\circ$C and at (process) pressures ranging from the vacuum base pressure of 10$^{-6}$ mbar up to 1 mbar, typical PLD conditions for the growth of oxide films. The performance of this instrument is demonstrated by resolving unit cell height surface steps and surface topography under typical oxide PLD growth conditions.Comment: 8 pages, 8 figure

Active multilayer mirrors for reflectance tuning at extreme ultraviolet (EUV) wavelengths

We propose an active multilayer mirror structure for EUV wavelengths which can be adjusted to compensate for reflectance changes. The multilayer structure tunes the reflectance via an integrated piezoelectric layer that can change its dimension due to an externally applied voltage. Here, we present design and optimization of the mirror structure for maximum reflectance tuning. In addition, we present preliminary results showing that the deposition of piezoelectric thin films with the requisite layer smoothness and crystal structure are possible. Finally, polarization switching of the smoothest piezoelectric film is presented

Dehydration of solute-lipid systems: Hydration forces analysis

Sorption isotherms were obtained for a range of lipid/sugar/water mixtures. These were analysed using a simple hydration forces formalism. The results demonstrate that this simple analysis can be used to estimate dehydration parameters for these relatively complex systems. This in turn provides some insight into the location and role of sugars in the hydration behaviour of lipid systems. The relevance of these results to the phase behaviour of lipid/sugar mixtures during dehydration are discussed

Theory of spin-orbit coupling in bilayer graphene

Theory of spin-orbit coupling in bilayer graphene is presented. The electronic band structure of the AB bilayer in the presence of spin-orbit coupling and a transverse electric field is calculated from first-principles using the linearized augmented plane wave method implemented in the WIEN2k code. The first-principles results around the K points are fitted to a tight-binding model. The main conclusion is that the spin-orbit effects in bilayer graphene derive essentially from the single-layer spin-orbit coupling which comes almost solely from the d orbitals. The intrinsic spin-orbit splitting (anticrossing) around the K points is about 24\mu eV for the low-energy valence and conduction bands, which are closest to the Fermi level, similarly as in the single layer graphene. An applied transverse electric field breaks space inversion symmetry and leads to an extrinsic (also called Bychkov-Rashba) spin-orbit splitting. This splitting is usually linearly proportional to the electric field. The peculiarity of graphene bilayer is that the low-energy bands remain split by 24\mu eV independently of the applied external field. The electric field, instead, opens a semiconducting band gap separating these low-energy bands. The remaining two high-energy bands are spin-split in proportion to the electric field; the proportionality coefficient is given by the second intrinsic spin-orbit coupling, whose value is 20\mu eV. All the band-structure effects and their spin splittings can be explained by our tight-binding model, in which the spin-orbit Hamiltonian is derived from symmetry considerations. The magnitudes of intra- and interlayer couplings---their values are similar to the single-layer graphene ones---are determined by fitting to first-principles results.Comment: 16 pages, 13 figures, 5 tables, typos corrected, published versio

A Backtracking-Based Algorithm for Computing Hypertree-Decompositions

Hypertree decompositions of hypergraphs are a generalization of tree decompositions of graphs. The corresponding hypertree-width is a measure for the cyclicity and therefore tractability of the encoded computation problem. Many NP-hard decision and computation problems are known to be tractable on instances whose structure corresponds to hypergraphs of bounded hypertree-width. Intuitively, the smaller the hypertree-width, the faster the computation problem can be solved. In this paper, we present the new backtracking-based algorithm det-k-decomp for computing hypertree decompositions of small width. Our benchmark evaluations have shown that det-k-decomp significantly outperforms opt-k-decomp, the only exact hypertree decomposition algorithm so far. Even compared to the best heuristic algorithm, we obtained competitive results as long as the hypergraphs are not too large.Comment: 19 pages, 6 figures, 3 table

Exclusion of maltodextrins from phosphatidylcholine multilayers during dehydration: effects on membrane phase behaviour

The effect of increasing solute size on phosphatidylcholine phase behaviour at a range of hydrations was investigated using differential scanning calorimetry. Dehydration of phospholipid membranes gives rise to a compressive stress within the bilayers that promotes fluid-to-gel phase transitions. According to the Hydration Forces Explanation, sugars in the intermembrane space minimize the compressive stress and limit increases in the fluid-gel transition temperature, Tm, by acting as osmotic and volumetric spacers that hinder the close approach of membranes. However, the sugars must remain between the bilayers in order to limit the rise in Tm. Large polymers are excluded from the interlamellar space during dehydration and do not limit the dehydration-induced rise in Tm. In this study, we used maltodextrins with a range of molecular weights to investigate the size-exclusion limit for polymers between phosphatidylcholine bilayers. Solutes with sizes ranging from glucose to dextran 1000 limited the rise in lipid Tm during dehydration, suggesting that they remain between dehydrated bilayers. At the lowest hydrations the solutions vitrified, and Tm was further depressed to about 20 °C below the transition temperature for the lipid in excess water, To. The depression of Tm below To occurs when the interlamellar solution vitrifies between fluid phase bilayers. The larger maltodextrins, dextran 5000 and 12,000, had little effect on the Tm of the PCs at any hydration, nor did vitrification of these larger polymers affect the lipid phase behaviour. This suggests that the larger maltodextrins are excluded from the interlamellar region during dehydration