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

Electronic and Magnetic Changes in a Finite-Sized Single-Walled Zigzag Carbon Nanotube Embedded in Water

In vacuum an open-ended finite-sized zigzag and hydrogen atom terminated carbon nanotube (FS-CNT) has a ground state with antiferromagnetic configuration, and the α and β gaps are degenerated with a magnitude inversely proportional to the nanotube length. However, when a FS-CNT is embedded in a box of water molecules, a single-file hydrogen bonded chain of water molecules (confined water inside) flows through it from one side to the other, while a spatially varying density profile occurs for the bulk water molecules (unconfined water outside). As a consequence, we have observed for an embedded FS-CNT­(11,0,<i>L</i>) with <i>L</i> < 2.0 nm important changes in its electronic and magnetic properties. The electronic gap degeneracy is broken, and the gap value for each spin state fluctuates around a mean value which depends on the CNT length. We rationalized these changes by decomposing the fluctuating electric field produced by the water molecules as due to molecules of unconfined water outside and confined water inside the FS-CNT. The confined water inside produces an electric field nearly constant in magnitude and pointing almost along the axial axis of the tube, equivalent to an external uniform electric field with a mean value of 0.56 ± 0.05 V/nm. Meanwhile, the unconfined water outside produces an electric field that fluctuates randomly in direction and magnitude, and it is equivalent to an external uniform electric field with a mean value of 0.7 ± 0.4 V/nm. The maximum electric field observed was 1.7 ± 0.2 V/nm which occurs when both confined water inside and unconfined water outside the electric fields have the same direction. The maximum electric field is three times smaller than the one necessary to change the CNT from semiconductor to half-metallic. The findings are important in devices where solvent molecules change the electronic properties of the CNT

Population biology of Aegla platensis (Decapoda: Anomura: Aeglidae) in a tributary of the Uruguay River, state of Rio Grande do Sul, Brazil

Aeglids are freshwater anomurans that are endemic from southern South America. While their population biology at the species-level is relatively well understood, intraspecific variation within populations has been poorly investigated. Our goal was to investigate the population biology of Aegla platensis Schmitt, 1942 from the Uruguay River Basin, and compare our data with data from other populations. We estimated biometric data, sex ratio, population density and size-class frequencies, and frequencies of ovigerous females and juveniles, from the austral spring of 2007 until autumn 2008. Sexual dimorphism was present in adults, with males being larger than females. Furthermore, males and females were significantly larger than previously recorded for the species. The overall sex ratio was 1.33:1 (male:female), and population density ranged from 1.8 (spring) to 3.83 ind.m-² (winter). Data from this population differ from published information about A. platensis in almost all parameters quantified except for the reproductive period, which happens in the coldest months, and a population structure with two distinct cohorts. Difference among studies, however, may be in part due to methodological differences and should be further investigated in order to determine their cause. In addition to different methodologies, they may result from ecological plasticity or from the fact that the different populations actually correspond to more than one species