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J. Ortega

Publications and source records attributed to J. Ortega.

16 recordsLinked to original sources
arXiv2025

High second harmonic generation in ferroelectric nematic liquid crystals by doping with optimally oriented chromophores

We report on the second harmonic generation performance of a ferroelectric nematic fluid consisting of the prototype ferroelectric nematic liquid crystal, RM734 mixed with the classical chromophore Dispersed-Orange 3 at 5 %wt. The mixture exhibits an interesting mesomorphic behavior with a wide temperature range of the ferroelectric mesophase, high nematic order parameter and ease of alignment in cells. In the study, two fundamental wavelengths (1064 nm and 1574 nm) have been used in order to account for the second harmonic generation performance in transparent and absorbing regimens. The results have been very outstanding and are among the highest for liquid crystals so far. Specifically, second order susceptibility tensor components of up to 25 pmV-1 and 8.5 pmV-1 have been obtained in the absorbing and transparent regimens, respectively. The mixing of chromophores with ferroelectric nematic liquid crystals is then stablished as a promising strategy in the search of high-performance nonlinear-optical mesogens.

cond-mat.soft↗
arXiv2023

Second harmonic generation in anisotropic stratified media: A generalization of the Berreman method and its application to photonic materials

We have developed a numerical method for calculating the second harmonic generation (SHG) generated by an anisotropic material whose optical properties present an arbitrary modulation in one dimension. The method is based on the Berreman 4x4 matrix formalism, which is generalized to include nonlinear optical phenomena. It can be used under oblique incidences of the input beam, and is valid even when the SHG frequency is close to photonic bands, where the usual slowly-varying-amplitude approximation breaks down. As an example of application, we have studied the SHG performance of ferroelectric and helielectric nematic liquid crystals. The latter present a helicoidal structure that can be distorted under electric field. In the different tests of the method we have analyzed the conditions for the most efficient SHG, and compared with previous results in the case there were any. The obtained results indicate that the present procedure may contribute to improve the structural design and enlarge the variety of nonlinear optical materials for their application in optical devices.

physics.optics↗
arXiv2022

Evidence for chiral superconductivity on a silicon surface

Sn adatoms on a Si(111) substrate with 1/3 monolayer coverage form a two-dimensional triangular adatom lattice with one unpaired electron per site and an antiferromagnetic Mott insulating state. The Sn layers can be modulation hole-doped and metallized using heavily-doped $p$-type Si(111) substrates, and become superconducting at low temperatures. While the pairing symmetry of the superconducting state is currently unknown, the combination of repulsive interactions and frustration inherent to the triangular adatom lattice opens up the possibility for a chiral order parameter. Here, we study the superconducting state of Sn/Si(111) using scanning tunneling microscopy/spectroscopy and quasi-particle interference imaging. We find evidence for a doping-dependent $T_c$ with a fully gapped order parameter, the presence of time-reversal symmetry breaking, and a strong enhancement of the zero-bias conductance near the edges of the superconducting domains. While each individual piece of evidence could have a more mundane interpretation, our combined results suggest the tantalizing possibility that Sn/Si(111) is an unconventional chiral d-wave superconductor.

cond-mat.supr-con↗
arXiv2022

Ferroelectric chiral nematic liquid crystals: New photonic materials with multiple bandgaps controllable by low electric fields

We have carried out a spectroscopic study on a prototype ferroelectric nematic material (RM7343) doped with a non-polar chiral compound. The mixture presents two chiral nematic phases, one conventional (N*) and the other polar (NF*), whose behaviours under electric fields E are totally different. On the one hand, the N* phase shows a single reflection band if E is not very high, while fields perpendicular to the helix of a few V/mm are already sufficient to produce multiple bands in the NF* phase. On the other hand, the pitch of the NF* phase grows notably on increasing fields, however remaining practically constant in the N* phase. Both effects have been explained in terms of the different type of interaction with E in each phase (ferroelectric in NF* and dielectric in N*). We argue that the NF* phase behaves as a photonic material with multiple gaps tunable by small fields, which presents important potentialities for applications. A study of the bandgaps has been carried out through the analysis of the dispersion relation of the optical eigenmodes in the NF* phase under field. Finally, an example of the potentialities of the NF* phase within the field of nonlinear optics is briefly presented.

cond-mat.soft↗
arXiv2022

Generalization of the Maier-Saupe theory to the ferroelectric nematic phase

We present a generalization of the Maier-Saupe (MS) theory that incorporates the description of the ferroelectric nematic phase (NF) within its scope of application. The extension of the theory is carried out in a natural way, by adding to the nematic potential of MS a term proportional to cosθ and to the polar order parameter , where θ is the angle between the long molecular axis and the spontaneous polarization. The NF phase can be reached by cooling from the isotropic phase with or without the formation of an intermediate normal nematic phase, depending on the relative intensity of the polar and nonpolar terms of the nematic potential. For both phase sequences all the transitions are first order. The temperature dependence of the polar and nonpolar order parameters is calculated, and some results derived from the theory are compared with the experimental observations. It is argued that the theory is a valuable first approximation to describe some of the properties of ferroelectric nematics.

cond-mat.soft↗
arXiv2021

The ferroelectric nematic phase: An optimum liquid crystal candidate for nonlinear optics

Materials that exhibit high nonlinear optical (NLO) susceptibilities are considered as promising candidates for a wide range of photonic and electronic applications. Here we argue that the ferroelectric nematic (NF) materials have sufficient potentialities to become materials for the next-generation of NLO devices. We have carried out a study of the efficiency of optical second-harmonic generation in a prototype NF material, finding a nonlinear susceptibility of 5.6 pmV-1 in the transparent regime, one of the highest ever reported in ferroelectric liquid crystals. Given the fact that the studied molecule was not specifically designed for NLO applications we conclude there is still margin to obtain NF materials with enhanced properties that should allow their practical use.

cond-mat.soft↗
arXiv2017

The additional-mode garden of RR Lyrae stars

Space-based photometric missions revealed a surprising abundance of millimagnitude-level additional modes in RR Lyrae stars. The modes that appear in the modulated fundamental-mode (RRab) stars can be ordered into four major categories. Here we present the distribution of these groups in the Petersen diagram, and discuss their characteristics and connections to additional modes observed in other RR Lyrae stars.

astro-ph.SR↗
arXiv2015

Time Series Clustering using the Total Variation Distance with Applications in Oceanography

An algorithm for determining stationary periods for time series of random sea waves is proposed in this work. This is a problem in which changes between stationary sea states are usually slow and segmentation procedures based on change-point detection frequently give poor results. The method is based on a new procedure for time series clustering, built on the use of the total variation distance between normalized spectra as a measure of dissimilarity. The oscillatory behavior of the series is thus considered the central characteristic for classification purposes. The proposed algorithm is compared to several other methods which are also based on features extracted from the original series and the results show that its performance is comparable to the best methods available and in some tests it performs better. This clustering method may be of independent interest.

stat.ME↗
arXiv2010

Profile Likelihood Intervals for Quantiles in Extreme Value Distributions

Profile likelihood intervals of large quantiles in Extreme Value distributions provide a good way to estimate these parameters of interest since they take into account the asymmetry of the likelihood surface in the case of small and moderate sample sizes; however they are seldom used in practice. In contrast, maximum likelihood asymptotic (mla) intervals are commonly used without respect to sample size. It is shown here that profile likelihood intervals actually are a good alternative for the estimation of quantiles for sample sizes $25 \leq n\leq 100$ of block maxima, since they presented adequate coverage frequencies in contrast to the poor coverage frequencies of mla intervals for these sample sizes, which also tended to underestimate the quantile and therefore might be a dangerous statistical practice. In addition, maximum likelihood estimation can present problems when Weibull models are considered for moderate or small sample sizes due to singularities of the corresponding density function when the shape parameter is smaller than one. These estimation problems can be traced to the commonly used continuous approximation to the likelihood function and could be avoided by using the exact or correct likelihood function, at least for the settings considered here. A rainfall data example is presented to exemplify the suggested inferential procedure based on the analyses of profile likelihoods.

stat.AP↗
arXiv2005

Ga-induced atom wire formation and passivation of stepped Si(112)

We present an in-depth analysis of the atomic and electronic structure of the quasi one-dimensional (1D) surface reconstruction of Ga on Si(112) based on Scanning Tunneling Microscopy and Spectroscopy (STM and STS), Rutherford Backscattering Spectrometry (RBS) and Density Functional Theory (DFT) calculations. A new structural model of the Si(112)6 x 1-Ga surface is inferred. It consists of Ga zig-zag chains that are intersected by quasi-periodic vacancy lines or misfit dislocations. The experimentally observed meandering of the vacancy lines is caused by the co-existence of competing 6 x 1 and 5 x 1 unit cells and by the orientational disorder of symmetry breaking Si-Ga dimers inside the vacancy lines. The Ga atoms are fully coordinated, and the surface is chemically passivated. STS data reveal a semiconducting surface and show excellent agreement with calculated Local Density of States (LDOS) and STS curves. The energy gain obtained by fully passivating the surface calls the idea of step-edge decoration as a viable growth method toward 1D metallic structures into question.

cond-mat.other↗
arXiv2004

A new multi-center approach to the exchange-correlation interactions in ab initio tight-binding methods

A new approximate method to calculate exchange-correlation contributions in the framework of first-principles tight-binding molecular dynamics methods has been developed. In the proposed scheme on-site (off-site) exchange-correlation matrix elements are expressed as a one-center (two-center) term plus a {\it correction} due to the rest of the atoms. The one-center (two-center) term is evaluated directly, while the {\it correction} is calculated using a variation of the Sankey-Niklewski \cite{Sankey89} approach generalized for arbitrary atomic-like basis sets. The proposed scheme for exchange-correlation part permits the accurate and computationally efficient calculation of corresponding tight-binding matrices and atomic forces for complex systems. We calculate bulk properties of selected transition (W,Pd), noble (Au) or simple (Al) metals, a semiconductor (Si) and the transition metal oxide Ti$O_2$ with the new method to demonstrate its flexibility and good accuracy.

cond-mat.mtrl-sci↗
arXiv2004

Barrier formation at metal/organic interfaces: dipole formation and the Charge Neutrality Level

The barrier formation for metal/organic semiconductor interfaces is analyzed within the Induced Density of Interface States (IDIS) model. Using weak chemisorption theory, we calculate the induced density of states in the organic energy gap and show that it is high enough to control the barrier formation. We calculate the Charge Neutrality Levels of several organic molecules (PTCDA, PTCBI and CBP) and the interface Fermi level for their contact with a Au(111) surface. We find an excellent agreement with the experimental evidence and conclude that the barrier formation is due to the charge transfer between the metal and the states induced in the organic energy gap.

cond-mat.mtrl-sci↗
arXiv2004

Dipole formation at metal/PTCDA interfaces: Role of the Charge Neutrality Level

The formation of a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic dianhydride) interface barrier is analyzed using weak-chemisorption theory. The electronic structure of the uncoupled PTCDA molecule and of the metal surface is calculated. Then, the induced density of interface states is obtained as a function of these two electronic structures and the interaction between both systems. This induced density of states is found to be large enough (even if the metal/PTCDA interaction is weak) for the definition of a Charge Neutrality Level for PTCDA, located 2.45 eV above the highest occupied molecular orbital. We conclude that the metal/PTCDA interface molecular level alignment is due to the electrostatic dipole created by the charge transfer between the two solids.

cond-mat.mtrl-sci↗
arXiv2004

Formation of atom wires on vicinal silicon

The formation of atomic wires via pseudomorphic step-edge decoration on vicinal silicon surfaces has been analyzed for Ga on the Si(112) surface using Scanning Tunneling Microscopy and Density Functional Theory calculations. Based on a chemical potential analysis involving more than thirty candidate structures and considering various fabrication procedures, it is concluded that pseudomorphic growth on stepped Si(112), both under equilibrium and non-equilibrium conditions, must favor formation of Ga zig-zag chains rather than linear atom chains. The surface is non-metallic and presents quasi-one dimensional character in the lowest conduction band.

cond-mat.mtrl-sci↗
arXiv1995

On the Informativeness of the DNA Promoter Sequences Domain Theory

The DNA promoter sequences domain theory and database have become popular for testing systems that integrate empirical and analytical learning. This note reports a simple change and reinterpretation of the domain theory in terms of M-of-N concepts, involving no learning, that results in an accuracy of 93.4% on the 106 items of the database. Moreover, an exhaustive search of the space of M-of-N domain theory interpretations indicates that the expected accuracy of a randomly chosen interpretation is 76.5%, and that a maximum accuracy of 97.2% is achieved in 12 cases. This demonstrates the informativeness of the domain theory, without the complications of understanding the interactions between various learning algorithms and the theory. In addition, our results help characterize the difficulty of learning using the DNA promoters theory.

cs.AI↗