arXiv subjects

Andrea Marini

Publications and source records attributed to Andrea Marini.

At least 109 records · Page 6Linked to original sources

arXiv2012

Thermo-modulational interband susceptibility and ultrafast temporal dynamics in nonlinear gold-based plasmonic devices

Starting from first principles, we theoretically model the nonlinear temporal dynamics of gold-based plasmonic devices resulting from the heating of their metallic components. At optical frequencies, the gold susceptibility is determined by the interband transitions around the X,L points in the first Brillouin zone and thermo-modulational effects ensue from Fermi smearing of the electronic energy distribution in the conduction band. As a consequence of light-induced heating of the conduction electrons, the optical susceptibility becomes nonlinear. In this paper we describe, for the first time to our knowledge, the effects of the thermo-modulational nonlinearity of gold on the propagation of surface plasmon polaritons guided on gold nanowires. We introduce a novel nonlinear Schroedinger-like equation to describe pulse propagation in such nanowires, and we predict the appearance an intense spectral red-shift caused by the delayed thermal response.

physics.optics↗

arXiv2012

Thermal string probes in AdS and finite temperature Wilson loops

We apply a new description of thermal fundamental string probes to the study of finite temperature Wilson loops in the context of the AdS/CFT correspondence. Previously this problem has been considered using extremal probes even though the background is at finite temperature. The new description of fundamental string probes is based on the blackfold approach. As a result of our analysis we find a new term in the potential between static quarks in the symmetric representation which for sufficiently small temperatures is the leading correction to the Coulomb force potential. We also find an order 1/N correction to the onset of the Debye screening of the quarks. These effects arise from including the thermal excitations of the string probe and demanding the probe to be in thermodynamic equilibrium with the Anti-de Sitter black hole background.

hep-th↗

arXiv2011

Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes

The gap oscillations caused by a magnetic flux penetrating a carbon nanotube represent one of the most spectacular observation of the Aharonov-Bohm effect at the nano--scale. Our understanding of this effect is, however, based on the assumption that the electrons are strictly confined on the tube surface, on trajectories that are not modified by curvature effects. Using an ab-initio approach based on Density Functional Theory we show that this assumption fails at the nano-scale inducing important corrections to the physics of the Aharonov-Bohm effect. Curvature effects and electronic density spilled out of the nanotube surface are shown to break the periodicity of the gap oscillations. We predict the key phenomenological features of this anomalous Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of a large metallic phase in the low flux regime of Multi-walled nanotubes, also suggesting possible experiments to validate our results.

cond-mat.mes-hall↗

arXiv2011

Effect of the quantistic zero-point atomic motion on the opto-electronic properties of diamond and trans-polyacetylene

The quantistic zero-point motion of the carbon atoms is shown to induce strong effects on the opto-electronic properties of diamond and trans-polyacetylene, a conjugated polymer. By using an ab initio approach, we interpret the sub-gap states experimentally observed in diamond in terms of entangled electron-phonon states. These states also appear in trans-polyacetylene causing the formation of strong structures in the band-structure that even call into question the accuracy of the band theory. This imposes a critical revision of the results obtained for carbon-based nano-structures by assuming the atoms frozen in their equilibrium positions.

cond-mat.mes-hall↗

arXiv2011

Implementation and testing of Lanczos-based algorithms for Random-Phase Approximation eigenproblems

The treatment of the Random-Phase Approximation Hamiltonians, encountered in different frameworks, like Time-Dependent Density Functional Theory or Bethe-Salpeter equation, is complicated by their non-Hermicity. Compared to their Hermitian Hamiltonian counterparts, computational methods for the treatment of non-Hermitian Hamiltonians are often less efficient and less stable, sometimes leading to the breakdown of the method. Recently [Grüning et al. Nano Lett. {\bf 8}, 2820 (2009)], we have identified that such Hamiltonians are usually pseudo-Hermitian. Exploiting this property, we have implemented an algorithm of the Lanczos type for random-Phase Approximation Hamiltonians that benefits from the same stability and computational load as its Hermitian counterpart, and applied it to the study of the optical response of carbon nanotubes. We present here the related theoretical grounds and technical details, and study the performance of the algorithm for the calculation of the optical absorption of a molecule within the Bethe-Salpeter equation framework.

cond-mat.mtrl-sci↗

arXiv2011

Heating up the BIon

We propose a new method to consider D-brane probes in thermal backgrounds. The method builds on the recently developed blackfold approach to higher-dimensional black holes. While D-brane probes in zero-temperature backgrounds are well-described by the Dirac-Born-Infeld (DBI) action, this method addresses how to probe thermal backgrounds. A particularly important feature is that the probe is in thermal equilibrium with the background. We apply our new method to study the thermal generalization of the BIon solution of the DBI action. The BIon solution is a configuration in flat space of a D-brane and a parallel anti-D-brane connected by a wormhole with F-string charge. In our thermal generalization, we put this configuration in hot flat space. We find that the finite temperature system behaves qualitatively different than its zero-temperature counterpart. In particular, for a given separation between the D-brane and anti-D-brane, while at zero temperature there are two phases, at finite temperature there are either one or three phases available.

hep-th↗

arXiv2011

Thermodynamics of the hot BIon

We investigate the thermodynamics of the recently obtained finite temperature BIon solution of arXiv:1012.1494, focusing on two aspects. The first concerns comparison of the free energy of the three available phases for the finite temperature brane-antibrane wormhole configuration. Based on this we propose a heuristic picture for the dynamics of the phases that involves a critical temperature below which a stable phase exists. This stable phase is the finite temperature analogue of the thin throat branch of the extremal brane anti-brane wormhole configuration. The second aspect that we consider is the possibility of constructing a finite temperature generalization of the infinite spike configuration of the extremal BIon. To this end we identify a correspondence point at the end of the throat where the thermodynamics of the D3-F1 blackfold configuration can be matched to that of k non-extremal black fundamental strings.

hep-th↗

arXiv2010

Many body effects in the excitation spectrum of a defect in SiC

We show that electron correlations control the photophysics of defects in SiC through both renormalization of the quasiparticle bandstructure and exciton effects. We consider the carbon vacancy, which is a well-identified defect with two possible excitation channels that involve conduction and valence band states. Corrections to the Kohn-Sham ionization levels are found to strongly depend on the occupation of the defect state. Excitonic effects introduce a red shift of 0.23 eV. The analysis unambigiously re-assigns excitation mechanism at the thresholds in photo-induced paramagnetic resonance measurements [J. Dashdorj \emph{et al.}, J. Appl. Phys. \textbf{104}, 113707 (2008)].

cond-mat.mtrl-sci↗

arXiv2009

New Penrose Limits and AdS/CFT

We find a new Penrose limit of AdS_5 x S^5 giving the maximally supersymmetric pp-wave background with two explicit space-like isometries. This is an important missing piece in studying the AdS/CFT correspondence in certain subsectors. In particular whereas the Penrose limit giving one space-like isometry is useful for the SU(2) sector of N=4 SYM, this new Penrose limit is instead useful for studying the SU(2|3) and SU(1,2|3) sectors. In addition to the new Penrose limit of AdS_5 x S^5 we also find a new Penrose limit of AdS_4 x CP^3.

hep-th↗

arXiv2009

Yambo: an \textit{ab initio} tool for excited state calculations

{\tt yambo} is an {\it ab initio} code for calculating quasiparticle energies and optical properties of electronic systems within the framework of many-body perturbation theory and time-dependent density functional theory. Quasiparticle energies are calculated within the $GW$ approximation for the self-energy. Optical properties are evaluated either by solving the Bethe--Salpeter equation or by using the adiabatic local density approximation. {\tt yambo} is a plane-wave code that, although particularly suited for calculations of periodic bulk systems, has been applied to a large variety of physical systems. {\tt yambo} relies on efficient numerical techniques devised to treat systems with reduced dimensionality, or with a large number of degrees of freedom. The code has a user-friendly command-line based interface, flexible I/O procedures and is interfaced to several publicly available density functional ground-state codes.

cond-mat.mtrl-sci↗

arXiv2008

Exciton-plasmon states in nanoscale materials: breakdown of the Tamm-Dancoff approximation

Within the Tamm-Dancoff approximation ab initio approaches describe excitons as packets of electron-hole pairs propagating only forward in time. However, we show that in nanoscale materials excitons and plasmons hybridize, creating exciton--plasmon states where the electron-hole pairs oscillate back and forth in time. Then, as exemplified by the trans-azobenzene molecule and carbon nanotubes, the Tamm-Dancoff approximation yields errors as large as the accuracy claimed in ab initio calculations. Instead, we propose a general and efficient approach that avoids the Tamm--Dancoff approximation, and correctly describes excitons, plasmons and exciton-plasmon states.

cond-mat.mtrl-sci↗

arXiv2008

{\it Ab--initio} finite temperature excitons

The coupling with the lattice vibrations is shown to drastically modify the state--of--the--art picture of the excitonic states based on a frozen atom approximation. The zero--point vibrations renormalize the bare energies and optical strengths. Excitons acquire a non--radiative lifetime that decreases with increasing temperature. The optical brightness and efficiency turn out to be strongly temperature dependent such as to induce bright to dark (and vice versa) transitions. The finite temperature experimental optical absorption spectra of bulk Si and hexagonal BN are successfully explained without using any external parameter.

cond-mat.mtrl-sci↗

arXiv2008

Tailoring wave nonlinearity through spatial composites

We propose and demonstrate theoretically a method to achieve and design optical nonlinear responses through a light-mediated spatial hybridization of different standard nonlinearities. The mechanism is based on the fact that optical propagation through a spatial composite of different nonlinear media is governed by an effective nonlinear response if the spatial scale of the sequence is much smaller than the light diffraction length. We apply our general approach to the significant case of centrosymmetric photorefractive crystal biased by a periodically modulated external voltage to predict strictly bending-free miniaturized soliton propagation.

physics.optics↗

arXiv2007

Optical saturation driven by exciton confinement in molecular-chains: a TDDFT study

We have identified excitonic confinement in one-dimensional molecular chains (i.e. polyacetylene and H$_2$) as the main driving force for the saturation of the chain polarizability as a function of the number of molecular units. This conclusion is based on first principles time--dependent density functional theory calculations performed with a new derived exchange--correlation kernel. The failure of simple local and semi--local functionals is shown to be related to the lack of memory effects, spatial ultranonlocality, and self--interaction corrections. These effects get smaller as the gap of the system reduces, in which case such simple approximations do perform better.

cond-mat.mtrl-sci↗

arXiv2006

First-Principle Description of Correlation Effects in Layered Materials

We present a first-principles description of anisotropic materials characterized by having both weak (dispersion-like) and strong covalent bonds, based on the Adiabatic--Connection Fluctuation--Dissipation Theorem within Density Functional Theory. For hexagonal boron nitride the in-plane and out of plane bonding as well as vibrational dynamics are well described both at equilibrium and when the layers are pulled apart. Also bonding in covalent and ionic solids is described. The formalism allows to ping-down the deficiencies of common exchange-correlation functionals and provides insight towards the inclusion of dispersion interactions into the correlation functional.

cond-mat.mtrl-sci↗

arXiv2006

An exact Coulomb cutoff technique for supercell calculations

We present a new reciprocal space analytical method to cutoff the long range interactions in supercell calculations for systems that are infinite and periodic in 1 or 2 dimensions, extending previous works for finite systems. The proposed cutoffs are functions in Fourier space, that are used as a multiplicative factor to screen the bare Coulomb interaction. The functions are analytic everywhere but in a sub-domain of the Fourier space that depends on the periodic dimensionality. We show that the divergences that lead to the non-analytical behaviour can be exactly cancelled when both the ionic and the Hartree potential are properly screened. This technique is exact, fast, and very easy to implement in already existing supercell codes. To illustrate the performance of the new scheme, we apply it to the case of the Coulomb interaction in systems with reduced periodicity (as one-dimensional chains and layers). For those test cases we address the impact of the cutoff in different relevant quantities for ground and excited state properties, namely: the convergence of the ground state properties, the static polarisability of the system, the quasiparticle corrections in the GW scheme and in the binding energy of the excitonic states in the Bethe-Salpeter equation. The results are very promising.

cond-mat.other↗

arXiv2005

Excitons in boron nitride nanotubes: dimensionality effects

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2,2) tube. The strongly localized nature of this exciton dictates the fast convergence of its binding energy with increasing tube diameter towards the sheet value. The absolute position of the first excitonic peak is almost independent of the tube radius and system dimensionality. This provides an explanation for the observed "optical gap" constancy for different tubes and bulk hBN [R. Arenal et al., to appear in Phys. Rev. Lett. (2005)].

cond-mat.mtrl-sci↗

arXiv2005

Excitonic effects in optical absorption and electron-energy loss spectra of hexagonal boron nitride

A new interpretation of the optical and energy-loss spectra of hexagonal boron nitride is provided based on first-principle calculations. We show that both spectra cannot be explained by independent-particle transitions but are strongly dominated by excitonic effects. The lowest direct and indirect gaps are much larger than previously reported. The direct gap amounts to 6.8 eV. The first absorption peak at 6.1 eV is due to an exciton with a binding energy of 0.7 eV. We show that this strongly bound Frenkel exciton is also responsible for the low frequency shoulder of the pi plasmon in the energy-loss function. Implications for nanotube studies are discussed.

cond-mat.mtrl-sci↗