E.L. Albuquerque

Localization and scaling properties of magnetostatic modes in quasiperiodic magnetic superlattices

The localization and scaling behaviour of quasiperiodic structures are studied for a geometry where the magnetization is perpendicular to the interfaces of the superlattices. Numerical results for the bulk and surface spin waves in the magnetostatic regime are presented for the Fibonacci, Thue-Morse and period-doubling sequences. The results are obtained for both ferromagnetic and antiferromagnetic ordering by using the transfer-matrix method. Interesting features of the localized modes are shown for Fe, EuS and MnF2 .

Structural and optoelectronic properties, and infrared spectrum of cubic BaSnO3 from first principles calculations

The electronic band structure, density of states, dielectric function, optical absorption, and infrared spectrum of cubic BaSnO3 were simulated using density functional theory, within both the local density and generalized gradient approximations, LDA and GGA, respectively. Dielectric optical permittivities and polarizabilities at ω=0 and ω=∞ were also estimated. Indirect band gaps E(R→Γ) of 1.01 eV (LDA) and 0.74 eV (GGA) were found, which are smaller than the experimental one (≈3.1 eV). A comparison of the calculated cubic BaSnO3 band gap with those of others stannates ASnO3 (A = Ca, Sr, Cd) already published highlights their dependence on each crystal profile. The cubic BaSnO3 effective masses of electrons and holes were computed by parabolic fittings along different directions at the conduction band minimum and valence band maximum, being anisotropic for both electrons and holes. The experimental band gap and calculated effective masses confirm the semiconductor character of cubic BaSnO3. Finally, the...

Electronic and optical properties of CaCO3 calcite, and excitons in Si@CaCO3 and CaCO3@SiO2 core-shell quantum dots

Density functional theory calculations of the electronic and optical properties of the CaCO3 calcite polymorph were performed within both the local density (LDA) and generalized gradient (GGA) approximations, respectively. The carriers effective masses are estimated, and the energy gap is shown to be indirect, with and (for comparison, the experimental value is 6.0 ? 0.35?eV). Two optical absorption regimes are predicted, and the dielectric function does not change considerably with the light polarization. The confinement features of excitons in Si@CaCO3 and CaCO3@SiO2 spherical core-shell quantum dots are also presented.

Defects in graphene-based twisted nanoribbons: structural, electronic, and optical properties.

We present some computational simulations of graphene-based nanoribbons with a number of half-twists varying from 0 to 4 and two types of defects obtained by removing a single carbon atom from two different sites. Optimized geometries are found by using a mix of classical quantum semiempirical computations. According with the simulations results, the local curvature of the nanoribbons increases at the defect sites, especially for a higher number of half-twists. The HOMO-LUMO energy gap of the nanostructures has significant variation when the number of half-twists increases for the defective nanoribbons. At the quantum semiempirical level, the first optically active transitions and oscillator strengths are calculated using the full configuration interaction (CI) framework, and the optical absorption in the UV/vis range (electronic transitions) and in the infrared (vibrational transitions) are achieved. Distinct nanoribbons show unique spectral signatures in the UV/vis range, with the first absorption peaks in wavelengths ranging from the orange to the violet. Strong absorption is observed in the ultraviolet region, although differences in their infrared spectra are hardly discernible.

Fractal spectrum of charge carriers in quasiperiodic graphene structures

In this work we investigate the interaction of charge carriers in graphene with a series of p-n-p junctions arranged according to a deterministic quasiperiodic substitutional Fibonacci sequence. The junctions create a potential landscape with quantum wells and barriers of different widths, allowing the existence of quasi-confined states. Spectra of quasi-confined states are calculated for several generations of the Fibonacci sequence as a function of the wavevector component parallel to the barrier interfaces. The results show that, as the Fibonacci generation is increased, the dispersion branches form energy bands distributed as a Cantor-like set. Besides, for a quasiperiodic set of potential barriers, we obtain the electronic tunneling probability as a function of energy, which shows a striking self-similar behavior for different generation numbers.

Nonreciprocity in the Goos-Hänchen shift on oblique incidence reflection off antiferromagnets

We investigate a lateral shift of the reflected beam on reflection of far-infrared radiation, at oblique incidence, off an antiferromagnet in an external magnetic field, at frequencies close to the magnon resonances. This shift is nonreciprocal and depends on the direction of the applied field. It occurs both at bulk and reststrahlen frequencies, with or without damping. We illustrate this effect using simulations of reflection off MnF2 at low temperature.

Specific heat properties of electrons in generalized Fibonacci quasicrystals

The purpose of this paper is to investigate the specific heat properties of electrons in one-dimensional quasiperiodic potentials, arranged in accordance with the generalized Fibonacci sequence. The electronic energy spectra are calculated using the one-dimensional Schrodinger equation in a tight-binding approximation. Both analytical and numerical results on the temperature dependence of the electrons specific heat associated with their multiscale fractal energy spectra are presented. We compare our numerical results with those found for the ordinary Fibonacci structure. A rich and varied behavior is found for the specific heat oscillations when T→0, with interesting physical consequences.

Quantum molecular modelling of ibuprofen bound to human serum albumin

The binding of the nonsteroidal anti-inflammatory drug ibuprofen (IBU) to human serum albumin (HSA) is investigated using density functional theory (DFT) calculations within a fragmentation strategy. Crystallographic data for the IBU–HSA supramolecular complex shows that the ligand is confined to a large cavity at the subdomain IIIA and at the interface between the subdomains IIA and IIB, whose binding sites are FA3/FA4 and FA6, respectively. The interaction energy between the IBU molecule and each amino acid residue of these HSA binding pockets was calculated using the Molecular Fractionation with Conjugate Caps (MFCC) approach employing a dispersion corrected exchange–correlation functional. Our investigation shows that the total interaction energy of IBU bound to HSA at binding sites of the fatty acids FA3/FA4 (FA6) converges only for a pocket radius of at least 8.5 A, mainly due to the action of residues Arg410, Lys414 and Ser489 (Lys351, Ser480 and Leu481) and residues in non-hydrophobic domains, namely Ile388, Phe395, Phe403, Leu407, Leu430, Val433, and Leu453 (Phe206, Ala210, Ala213, and Leu327), which is unusual. Our simulations are valuable for a better understanding of the binding mechanism of IBU to albumin and can lead to the rational design and the development of novel IBU-derived drugs with improved potency.

Lateral shift of far infrared radiation on normal incidence reflection off an antiferromagnet

A lateral shift, similar to a Goos-Hanchen shift, of a normally incident electromagnetic beam reflected off an antiferromagnet in the presence of an external magnetic field is predicted. This shift is interpreted in terms of nonreciprocity of the reflected phase, and is confirmed using numerical simulation. There is also a lateral displacement of the field within the antiferromagnet, but not of the beam transmitted through an antiferromagnetic slab.

Magnetostatic modes in quasiperiodic Fibonacci magnetic superlattices

The magnetostatic modes are studied in multilayer structures that exhibit deterministic disorders. Some models that have attracted particular attention are the quasiperiodic magnetic multilayers that obey a substitutional sequence of the Fibonacci type. The spin wave spectra are evaluated in the geometry where the magnetization is perpendicular to the surfaces of the layers of the superlattice by using a transfer-matrix approach. Numerical results are presented for the ferromagnets EuS and Fe and for the antiferromagnet MnF2.