arXiv subjects

A. Kara

Publications and source records attributed to A. Kara.

8 recordsLinked to original sources

arXiv2024

Euclid preparation: XLVIII. The pre-launch Science Ground Segment simulation framework

The European Space Agency's Euclid mission is one of the upcoming generation of large-scale cosmology surveys, which will map the large-scale structure in the Universe with unprecedented precision. The development and validation of the SGS pipeline requires state-of-the-art simulations with a high level of complexity and accuracy that include subtle instrumental features not accounted for previously as well as faster algorithms for the large-scale production of the expected Euclid data products. In this paper, we present the Euclid SGS simulation framework as applied in a large-scale end-to-end simulation exercise named Science Challenge 8. Our simulation pipeline enables the swift production of detailed image simulations for the construction and validation of the Euclid mission during its qualification phase and will serve as a reference throughout operations. Our end-to-end simulation framework starts with the production of a large cosmological N-body & mock galaxy catalogue simulation. We perform a selection of galaxies down to I_E=26 and 28 mag, respectively, for a Euclid Wide Survey spanning 165 deg^2 and a 1 deg^2 Euclid Deep Survey. We build realistic stellar density catalogues containing Milky Way-like stars down to H<26. Using the latest instrumental models for both the Euclid instruments and spacecraft as well as Euclid-like observing sequences, we emulate with high fidelity Euclid satellite imaging throughout the mission's lifetime. We present the SC8 data set consisting of overlapping visible and near-infrared Euclid Wide Survey and Euclid Deep Survey imaging and low-resolution spectroscopy along with ground-based. This extensive data set enables end-to-end testing of the entire ground segment data reduction and science analysis pipeline as well as the Euclid mission infrastructure, paving the way to future scientific and technical developments and enhancements.

astro-ph.CO↗

arXiv2018

Compelling experimental evidence of a Dirac cone in the electronic structure of a 2D Silicon layer

The remarkable properties of graphene stem from its two-dimensional (2D) structure, with a linear dispersion of the electronic states at the corners of the Brillouin zone (BZ) forming a Dirac cone. Since then, other 2D materials have been suggested based on boron, silicon, germanium, phosphorus, tin, and metal di-chalcogenides. Here, we present an experimental investigation of a single silicon layer on Au(111) using low energy electron diffraction (LEED), high resolution angle-resolved photoemission spectroscopy (HR-ARPES), and scanning tunneling microscopy (STM). The HR-ARPES data show compelling evidence that the silicon based 2D overlayer is responsible for the observed linear dispersed feature in the valence band, with a Fermi velocity of v_F ~10^(+6) m.s^(-1) comparable to that of graphene. The STM images show extended and homogeneous domains, offering a viable route to the fabrication of silicene-based opto-electronic devices.

cond-mat.mtrl-sci↗

arXiv2013

Thermal stability of standalone silicene sheet

Extensive Monte Carlo simulations are carried out to study thermal stability of an infinite standalone silicon sheet. We used the Tersoff potential that has been used with success for silicon at low temperatures. However, the melting temperature $T_m$ calculated with the original parameters provided by Tersoff is too high with respect to the experimental one. Agrawal, Raff and Komanduri have proposed a modified set of parameters to reduce $T_m$. For comparison, we have used these two sets of parameters to study the stability and the melting of a standalone 2D sheet of silicon called "silicene", by analogy with graphene for the carbon sheet. We find that 2D crystalline silicene is stable up to a high temperature unlike in 2D systems with isotropic potentials such as Lennard-Jones. The differences in the obtained results using two sets of parameters are striking.

cond-mat.mtrl-sci↗

arXiv2010

Island Size Selectivity during 2D Ag Island Coarsening on Ag (111)

We report on early stages of submonolayer Ag island coarsening on Ag(111) surface at room temperature ($300$ K) carried out using realistic kinetic Monte Carlo (KMC) simulations. We find that during early stages, coarsening proceeds as a sequence of selected island sizes creating peaks and valleys in the island size distribution. We find that island-size selectivity is due to formation of kinetically stable islands for certain sizes because of adatom detachment/attachment processes and large activation barrier for kink detachment. In addition, we find that the ratio of number of adatom attachment to detachment processes to be independent of parameters of initial configuration and also on the initial shapes of the islands confirming that island-size selectivity is independent of initial conditions.These simulations were carried out using a very large database of processes identified by their local environment and whose activation barriers were calculated using the embedded-atom method.

cond-mat.mtrl-sci↗

arXiv2008

Physics of Silicene Stripes

Silicene, a monolayer of silicon atoms tightly packed into a two-dimensional honeycomb lattice, is the challenging hypothetical reflection in the silicon realm of graphene, a one-atom thick graphite sheet, presently the hottest material in condensed matter physics. If existing, it would also reveal a cornucopia of new physics and potential applications. Here, we reveal the epitaxial growth of silicene stripes self-aligned in a massively parallel array on the anisotropic silver (110) surface. This crucial step in the silicene gold rush could give a new kick to silicon on the electronics road-map and opens the most promising route towards wide-ranging applications. A hint of superconductivity in these silicene stripes poses intriguing questions related to the delicate interplay between paired correlated fermions, massless Dirac fermions and bosonic quasi-particules in low dimensions.

cond-mat.mtrl-sci↗

arXiv2008

Anomalous Behavior of the Diffusion Coefficient in Interacting Adsorbates

Langevin simulations provide an effective way to study collective effects of Brownian particles immersed in a two-dimensional periodic potential. In this paper, we concentrate essentially on the behaviour of the tracer (DTr) and bulk (DB) diffusion coefficients as function of friction. Our simulations show that in the high friction limit, the two physical quantities DTr and DB present qualitatively the same behaviour, for both coupled and decoupled substrate potentials. However, for the low friction regime, and especially for the coupled potential case, an anomalous diffusion behaviour is found. We also found that in the case of weak dynamical coupling between the ad-particles and the substrate, the exponents are not universal and rather depend on the potentials. Moreover, changes in the inter-particle potentials may reverse the behaviour to a normal one.

cond-mat.stat-mech↗

arXiv2008

A comparative study of CO adsorption on flat, stepped and kinked Au surfaces using density functional theory

Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (211), and kinked (532) gold surfaces show a strong dependence on local coordination with a reduction in Au atom coordination leading to higher binding energies. We find trends in adsorption energies to be similar to those reported in experiments and calculations for other metal surfaces. The (532) surface provides insights into these trends because of the availability of a large number of kink sites which naturally have the lowest coordination (6). We also find that, for all surfaces, an increase in CO coverage triggers a decrease in the adsorption energy. Changes in the work-function upon CO adsorption, as well as the frequencies of the CO vibrational modes are calculated, and their coverage dependence is reported.

cond-mat.mtrl-sci↗

arXiv2006

Atom-by-atom extraction using scanning tunneling microscope tip-cluster interaction

We investigate atomistic details of a single atom extraction process realized by using scanning tunneling microscope (STM) tip-cluster interaction on a Ag(111) surface at 6 K. Single atoms are extracted from a silver cluster one atom at a time using small tunneling biases less than 35 mV and a threshold tunneling resistance of 47 kOhm. A combination of total energy calculations and molecular dynamics simulations shows a lowering of the atom extraction barrier upon approaching the tip to the cluster. Thus, a mere tuning of the proximity between the tip and the cluster governs the extraction process and is sufficient to extract an atom. The atomically precise control and reproducibility of the process are demonstrated by repeatedly extracting single atoms from a silver cluster on an atom-by-atom basis.

cond-mat.mtrl-sci↗