arXiv subjects

Cecilia Noguez

Publications and source records attributed to Cecilia Noguez.

15 recordsLinked to original sources

arXiv2023

Excitons in twisted AA' hexagonal boron nitride bilayers

The twisted hexagonal boron nitride (hBN) bilayer has demonstrated exceptional properties, particularly the existence of electronic flat bands without needing a magic angle, suggesting strong excitonic effects. Therefore, a systematic approach is presented to study the excitonic properties of twisted AA' hBN using the Bethe-Salpeter equation based on single-particle tight-binding wave functions. These are provided by a one-particle Hamiltonian that is parameterized to describe the main features of {\it ab initio} calculations. The Bethe-Salpeter equation is then solved in the so-called excitonic transition representation, which significantly reduces the problem dimensionality by exploiting the system's symmetries. Consequently, the excitonic energies and the excitonic wave functions are obtained from the direct diagonalization of the effective two-particle Hamiltonian of the Bethe-Salpeter equation. We have studied rotation angles as low as $7.34^{\circ}$. The model allows the study of commensurate and incommensurate moiré patterns at much lower computational cost than the {\it ab initio} version of the Bethe-Salpeter equation. Here, using the model and effective screening of the Keldysh type, we could obtain the absorption spectra and characterize the excitonic properties of twisted hBN bilayers for different rotation angles, demonstrating how this property affects the excitonic energies and localizations of their wavefunctions.

cond-mat.str-el↗

arXiv2021

Optical band engineering via vertical stacking of honeycomb plasmonic lattices

Inspired by recent advances in atomic homo and heterostructures, we consider the vertical stacking of plasmonic lattices as a new degree of freedom to create a coupled system showing a modified optical response concerning the monolayer. The precise design of the stacking and the geometrical parameters of two honeycomb plasmonic lattices tailors the interaction among their metallic nanoparticles. Based on the similarity of the lattice symmetry, analogies can be drawn with stacked atomic crystals, such as graphene. We use the multipolar spectral representation to study the plasmonic vertical stack's optical response in the near-field regime, emphasizing symmetry properties. The strong coupling of certain optical bands shows up as anticrossings in the dispersion diagram, resulting in the polarization exchange of the interacting bands. By leveraging these effects, we engineer the near-field intensity distribution. Additionally, lifting band degeneracy at specific points of the Brillouin zone is obtained with the consequent opening of minigaps. These effects are understood by quantifying the multipolar coupling among nanospheres belonging to the same and different sublattices, as well as the interlayer and intralayer nanoparticle interactions. Differences with the atomic case are also analyzed and explained in terms of the stack's interaction matrix. Finally, we predict the absorption spectrum projected on the two orthogonal linear polarizations.

physics.optics↗

arXiv2021

Angle-dependent electron confinement in graphene moiré superlattices

In graphene moiré superlattices, electronic interactions between layers are mostly hidden as band structures get crowded because of folding, making their interpretation cumbersome. Here, the evolution of the electronic band structure as a function of the interlayer rotation angle is studied using Density Functional Theory followed by unfolding bands and then comparing them to their corresponding individual components. We observe interactions at regions not theoretically elucidated so far, where for small interlayer angles, gaps turn into discrete-like states that are evenly spaced in energy. We find that $V_{ppσ}$ attractive interactions between out-of-plane orbitals from different layers are responsible for the discretization. Furthermore, when the interlayer angle becomes small, these discrete evenly-spaced states have energy differences comparable to graphene phonons. Thus, they might be relevant to explain electron-phonon-assisted effects, which have been experimentally observed in graphene moiré superlattices.

cond-mat.mes-hall↗

arXiv2006

Controlled Anisotropic Deformation of Ag Nanoparticles by Si Ion Irradiation

The shape and alignment of silver nanoparticles embedded in a glass matrix is controlled using silicon ion irradiation. Symmetric silver nanoparticles are transformed into anisotropic particles whose larger axis is along the ion beam. Upon irradiation, the surface plasmon resonance of symmetric particles splits into two resonances whose separation depends on the fluence of the ion irradiation. Simulations of the optical absorbance show that the anisotropy is caused by the deformation and alignment of the nanoparticles, and that both properties are controlled with the irradiation fluence.

cond-mat.mtrl-sci↗

arXiv2004

Optical Properties of Isolated and Supported Metal Nanoparticles

A review of the main phenomena related with the linear optical properties of isolated and supported metal nanoparticles is presented. The extinction, absorption and scattering efficiencies are calculated using the Mie theory and the Discrete Dipole Approximation. The origin of the optical spectra is discussed in terms of the size, shape and environment for each nanoparticle. The main optical features of each nanoparticle are identified, showing the tremendous potentiality of optical spectroscopy as a tool of characterization.

cond-mat.mtrl-sci↗

arXiv2004

Dispersive force between dissimilar materials: geometrical effects

We calculate the Casimir force or dispersive van der Waals force between a spherical nanoparticle and a planar substrate, both with arbitrary dielectric properties. We show that the force between a sphere and a plane can be calculated through the interacting surface plasmons of the bodies. Using a Spectral Representation formalism, we show that the force of a sphere made of a material A and a plane made of a material B, differ from the case when the sphere is made of B, and the plane is made of A. We found that the difference depends on the plasma frequency of the materials, the geometry, and the distance of separation between sphere and plane. The differences show the importance of the geometry, and make evident the necessity of realistic descriptions of the sphere-plane system beyond the Derjaguin Approximation or Proximity Theorem Approximation.

cond-mat.other↗

arXiv2003

Casimir force between a sphere and a plane: spectral representation formalism

We develop a spectral representation formalism to calculate the Casimir force in the non-retarded limit, between a spherical particle and a substrate, both with arbitrary local dielectric properties. This spectral formalism allows one to do a systematic study of the force as a function of the geometrical variables separately from the dielectric properties. We found that the force does not follow a simple power-law as a function of the separation between the sphere and substrate. As a consequence, the non-retarded Casimir force is enhanced by several orders of magnitude as the sphere approaches the substrate, while at large separations the dipolar term dominates the force.

quant-ph↗

arXiv2003

Electronic and optical properties of InAs(110)

The electronic and optical properties of the cleavage InAs(110) surface are studied using a semi-empirical tight-binding method which employs an extended atomic-like basis set. We describe and discuss the electronic character of the surface electronic states and we compare with other theoretical approaches, and with experimental observations. We calculate the surface electronic band structure and the Reflectance Anisotropy Spectrum, which are described and discussed in terms of the surface electronic states and the atomic structure.

cond-mat.mtrl-sci↗

arXiv2003

Circular dichroism simulated spectra of chiral gold nanoclusters: A dipole approximation

Circular dichroism (CD) spectra of chiral bare and thiol-passivated gold nanoclusters have been calculated within the dipole approximation. The calculated CD spectra show features that allow us to distinguish between clusters with different indexes of chirality. The main factor responsible of the differences in the CD lineshapes is the distribution of interatomic distances that characterize the chiral cluster geometry. These results provide theoretical support for the quantification of chirality and its measurement, using the CD lineshapes of chiral metal nanoclusters.

cond-mat.mtrl-sci↗

arXiv2003

High-Multipolar Effects on Dispersive Forces

We show that the dispersive force between a spherical nanoparticle (with a radius $\le$ 100 nm) and a substrate is enhanced by several orders of magnitude when the sphere is near to the substrate. We calculate exactly the dispersive force in the non-retarded limit by incorporating the contributions to the interaction from of all the multipolar electromagnetic modes. We show that as the sphere approaches the substrate, the fluctuations of the electromagnetic field, induced by the vacuum and the presence of the substrate, the dispersive force is enhanced by orders of magnitude. We discuss this effect as a function of the size of the sphere.

quant-ph↗

arXiv2003

Atomic structure and electronic properties of the cleavage InAs(110) surface

The electronic properties of the cleavage InAs(110) surface are studied using a semi-empirical tight-binding method which employs an extended atomic-like basis set. We calculate the surface electronic band structure which is studied as a function of the structural parameters of the surface. We describe and discuss the electronic character of the surface electronic states and we compare with other theoretical approaches, and with experimental observations. Finally, we discuss which atomic model better resembles the available experimental data.

cond-mat.mtrl-sci↗

arXiv2003

Optical properties of metal nanoparticles with arbitrary shapes

We have studied the optical properties of metallic nanoparticles with arbitrary shape. We performed theoretical calculations of the absorption, extinction and scattering efficiencies, which can be directly compared with experiments, using the Discrete Dipole Approximation (DDA). In this work, the main features in the optical spectra have been investigated depending of the geometry and size of the nanoparticles. The origin of the optical spectra are discussed in terms of the size, shape and material properties of each nanoparticle, showing that a nanoparticle can be distinguish by its optical signature.

cond-mat.mtrl-sci↗

arXiv2003

Spectral representation of the Casimir Force Between a Sphere and a Substrate

We calculate the Casimir force in the non-retarded limit between a spherical nanoparticle and a substrate, and we found that high-multipolar contributions are very important when the sphere is very close to the substrate. We show that the highly inhomegenous electromagnetic field induced by the presence of the substrate, can enhance the Casimir force by orders of magnitude, compared with the classical dipolar approximation.

quant-ph↗

arXiv2002

Casimir Forces between Nanoparticles and Substrates

We study the Casimir force between a nanoparticle and a substrate. We consider the interaction of metal nanoparticles with different substrates within the dipolar approximation. We study the force as a function of the distance for gold and potassium spheres, which are over a substrate of titanium dioxide, sapphire and a perfect conductor. We show that Casimir force is important in systems at the nanometer scale. We study the force as a function of the material properties, radii of the spheres, and the distance between the sphere and the substrate.

quant-ph↗

arXiv1995

Anisotropic optical response of the diamond (111)-2x1 surface

The optical properties of the 2$\times$1 reconstruction of the diamond (111) surface are investigated. The electronic structure and optical properties of the surface are studied using a microscopic tight-binding approach. We calculate the dielectric response describing the surface region and investigate the origin of the electronic transitions involving surface and bulk states. A large anisotropy in the surface dielectric response appears as a consequence of the asymmetric reconstruction on the surface plane, which gives rise to the zigzag Pandey chains. The results are presented in terms of the reflectance anisotropy and electron energy loss spectra. While our results are in good agreement with available experimental data, additional experiments are proposed in order to unambiguously determine the surface electronic structure of this interesting surface.

cond-mat↗