Strain-gradient-induced switching of nanoscale domains in free-standing ultrathin films

We report first-principle atomistic simulations on the effect of local strain gradients on the nanoscale domain morphology of free-standing PbTiO$_3$ ultrathin films. First, the ferroelectric properties of free films at the atomic level are reviewed. For the explored thicknesses (10 to 23 unit cells), we find flux-closure domain structures whose morphology is thickness dependent. A critical value of 20 unit cells is observed: thinner films show structures with 90$^\circ$ domain loops, whereas thicker ones develop, in addition, 180$^\circ$ domain walls, giving rise to structures of the Landau-Lifshitz type. When a local and compressive strain gradient at the top surface is imposed, the gradient is able to switch the polarization of the downward domains, but not to the opposite ones. The evolution of the domain pattern as a function of the strain gradient strength consequently depends on the film thickness. Our simulations indicate that in thinner films, first the 90$^\circ$ domain loops migrate towards the strain-gradient region, and then the polarization in that zone is gradually switched. In thicker films, instead, the switching in the strain-gradient region is progressive, not involving domain-wall motion, which is attributed to less mobile 180$^\circ$ domain walls. The ferroelectric switching is understood based on the knowledge of the local atomic properties, and the results confirm that mechanical flexoelectricity provides a means to control the nanodomain pattern in ferroelectric systems.Comment: 9 pages, 6 figure

Effects of interladder couplings in the trellis lattice

Strongly correlated models on coupled ladders in the presence of frustration, in particular the trellis lattice, are studied by numerical techniques. For the undoped case, the possibility of incommensurate peaks in the magnetic structure factor at low temperatures is suggested. In the doped case, our main conclusion for the trellis lattice is that by increasing the interladder coupling, the balance between the magnetic energy in the ladders and the kinetic energy in the zig-zag chains is altered leading eventually to the destruction of the hole pairs initially formed and localized in the ladders.Comment: final version, to appear in Physical Review

Optical to UV spectra and birefringence of SiO2_2 and TiO2_2: First-principles calculations with excitonic effects

A first principles approach is presented for calculations of optical -- ultraviolet (UV) spectra including excitonic effects. The approach is based on Bethe-Salpeter equation calculations using the \textsc{NBSE} code combined with ground-state density-functional theory calculations from the electronic structure code \textsc{ABINIT}. Test calculations for bulk Si are presented, and the approach is illustrated with calculations of the optical spectra and birefringence of $\alpha$-phase SiO$_2$ and the rutile and anatase phases of TiO$_2$. An interpretation of the strong birefringence in TiO$_2$ is presented.Comment: 8 figure

Magnetic order in ferromagnetically coupled spin ladders

A model of coupled antiferromagnetic spin-1/2 Heisenberg ladders is studied with numerical techniques. In the case of ferromagnetic interladder coupling we find that the dynamic and static structure factor has a peak at $(\pi,\pi/2)$ where the first (second) direction is along (transversal) to the ladders. Besides, we suggest that the intensity of this peak and the spin-spin correlation at the maximum distance along the ladder direction remain finite in the bulk limit for strong enough interladder coupling. We discuss the relevance of these results for magnetic compounds containing ladders coupled in a trellis lattice and for the stripe scenario in high-T$_c$ superconducting cuprates.Comment: 6 pages, 7 psfigs- Final version to be published in Phys. Rev.

Properties of Graphene: A Theoretical Perspective

In this review, we provide an in-depth description of the physics of monolayer and bilayer graphene from a theorist's perspective. We discuss the physical properties of graphene in an external magnetic field, reflecting the chiral nature of the quasiparticles near the Dirac point with a Landau level at zero energy. We address the unique integer quantum Hall effects, the role of electron correlations, and the recent observation of the fractional quantum Hall effect in the monolayer graphene. The quantum Hall effect in bilayer graphene is fundamentally different from that of a monolayer, reflecting the unique band structure of this system. The theory of transport in the absence of an external magnetic field is discussed in detail, along with the role of disorder studied in various theoretical models. We highlight the differences and similarities between monolayer and bilayer graphene, and focus on thermodynamic properties such as the compressibility, the plasmon spectra, the weak localization correction, quantum Hall effect, and optical properties. Confinement of electrons in graphene is nontrivial due to Klein tunneling. We review various theoretical and experimental studies of quantum confined structures made from graphene. The band structure of graphene nanoribbons and the role of the sublattice symmetry, edge geometry and the size of the nanoribbon on the electronic and magnetic properties are very active areas of research, and a detailed review of these topics is presented. Also, the effects of substrate interactions, adsorbed atoms, lattice defects and doping on the band structure of finite-sized graphene systems are discussed. We also include a brief description of graphane -- gapped material obtained from graphene by attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic

Water channel of horseradish peroxidase studied by the charge-transfer absorption band of ferric heme

The heme of horseradish peroxidase is buried in the protein, but a channel from the protein surface connects the aqueous solution to the heme site. Ferric horseradish peroxidase has an absorption band at 640 nm that is attributed to a charge-transfer (CT) transition between the a(2u) HOMO of T electrons of the porphyrin ring and the d(xy)/d(yz) orbital of the ferric ion. Because the water channel extends to the Fe, it seems likely that the CT band will be sensitive to the hydration of the protein. To study this premise, the protein was incorporated into trehalose/sucrose glasses and the hydration of the sugar glasses was varied. Absorption spectra of HRP in sugar glasses and in glycerol/water were taken in the range 10-300 K. The CT absorption band shows vibronic fine structure. The peak positions are the same in hydrated sugar and glycerol/water but the peak positions change in desiccated sugar glass. The data suggest that in hydrated, but not desiccated, sugar glass, water is retained in the heme pocket. Binding of the competitive inhibitor benzohydroxamic acid to the protein increases the CT absorption and resolution. The effect of benzohydroxamic acid on the Fe as calculated using a combination of density functional theory and molecular mechanics is to stabilize the spin state 3/2 with respect to 5/2. At low temperature the widths of the lines in the CT band are narrower for the protein in glycerol/water (glass transition at similar to150 K) than in trehalose/sucrose (glass formation at 65 degreesC). This indicates that the CT band is inhomogeneously broadened and sensitive to the solvent. The spectral narrowing of the CT absorption occurs as the temperature decreases over the temperature range studied. Water, as indicated by the OH stretch, also shifts in this range. The findings are discussed in terms of how buried water and nearby charges can modulate the activity of the heme

Solvent dependent and independent motions of CO-horseradish peroxidase examined by infrared spectroscopy and molecular dynamics calculations

The role of the solvent matrix in affecting CO bound to ferrous horseradish peroxidase was examined by comparing band-widths of v(CO) for the protein in aqueous solutions and in trehalose/sucrose glasses. We have previously observed that the optical absorption band and the CO stretching mode respond to the glass transition of glycerol/water in ways that depend upon the presence of substrate (Biochemistry 40 (2001) 3483). It is now demonstrated that the CO group band-width for the protein with bound inhibitor benzhydroxamic acid is relatively insensitive to temperature or the glass transition of the solvent. In contrast, in the absence of inhibitor, the band-width varies with the temperature that the glass is formed. The results show that solvent dependent and independent motions can be distinguished, and that the presence of substrate changes the protein such that the Fe-CO site is occluded from the solvent conditions. Molecular dynamic calculations, based upon X-ray structures, showed that the presence of benzhydroxamic acid decreases the distance between His42 and Arg38 and this leads for closer distances to the O of the CO from these residues. These results are invoked to account for the observed line width changes of the CO band. (C) 2003 Elsevier Science B.V. All rights reserved