arXiv subjects

C. Santos

Publications and source records attributed to C. Santos.

At least 19 recordsLinked to original sources

arXiv2023

The pristine nature of SMSS 1605$-$1443 revealed by ESPRESSO

SMSS J160540.18$-$144323.1 is the carbon-enhanced metal-poor (CEMP) star with the lowest iron abundance ever measured, [Fe/H]=-6.2, which was first reported with the SkyMapper telescope. The carbon abundance is A(C)~6.1 in the low-C band, as the majority of the stars in this metallicity range. Yet, constraining the isotopic ratio of key species, such as carbon, sheds light on the properties and origin of these elusive stars. We performed high-resolution observations of SMSS1605$-$1443 with the ESPRESSO spectrograph to look for variations in the radial velocity ($v_{rad}$) with time. These data have been combined with older MIKE and UVES archival observations to enlarge the temporal baseline. The $^{12}$C/$^{13}$C isotopic ratio is also studied to explore the possibility of mass transfer from a binary companion. A cross-correlation function against a natural template was applied to detect $v_{rad}$ variability and a spectral synthesis technique was used to derive $^{12}$C/$^{13}$C in the stellar atmosphere. We confirm previous indications of binarity in SMSS1605$-$1443 and measured a lower limit $^{12}$C/$^{13}$C$>60$ at more than a 3$σ$ confidence level, proving that this system is chemically unmixed and that no mass transfer from the unseen companion has happened so far. Thus, we confirm the CEMP-no nature of SMSS1605$-$1443 and show that the pristine chemical composition of the cloud from which it formed is currently imprinted in its stellar atmosphere free of contamination.

astro-ph.SR↗

arXiv2022

Neutrino Physics with an Opaque Detector

In 1956 Reines & Cowan discovered the neutrino using a liquid scintillator detector. The neutrinos interacted with the scintillator, producing light that propagated across transparent volumes to surrounding photo-sensors. This approach has remained one of the most widespread and successful neutrino detection technologies used since. This article introduces a concept that breaks with the conventional paradigm of transparency by confining and collecting light near its creation point with an opaque scintillator and a dense array of optical fibres. This technique, called LiquidO, can provide high-resolution imaging to enable efficient identification of individual particles event-by-event. A natural affinity for adding dopants at high concentrations is provided by the use of an opaque medium. With these and other capabilities, the potential of our detector concept to unlock opportunities in neutrino physics is presented here, alongside the results of the first experimental validation.

physics.ins-det↗

arXiv2020

Understanding cooperative loading in carbon nanotube fibres through in-situ structural studies during stretching

Carbon nanotube (CNT) fibres are firmly established as a new high-performance fibre, but their tensile mechanical properties remain a relatively small fraction of those of the constituent CNTs. Clear structure-property relations and accurate mechanical models are pressing requirements to bridge this gap. In this work we analyse the structural evolution and molecular stress transfer in CNT fibres by performing in-situ synchrotron wide- and small-angle X-ray scattering and Raman spectroscopy during tensile deformation. The results show that CNT fibres can be accurately described as network of bundles that slide progressively according to the initial orientation distribution function of the material following a Weibull distribution. This model decouples the effects of CNT alignment and degree of cooperative loading, as demonstrated for fibres produced at different draw ratios. It also helps explain the unusually high toughness (fracture energy) of CNT fibres produced by the direct spinning method, a key property for impact resistance in structural materials, for example.

physics.app-ph↗

arXiv2019

Study of multi-neutron emission in the $β$-decay of $^{11}$Li

The kinematics of two-neutron emission following the $β$-decay of $^{11}$Li was investigated for the first time by detecting the two neutrons in coincidence and by measuring their angle and energy. An array of liquid-scintillator neutron detectors was used to reject cosmic-ray and $γ$-ray backgrounds by pulse-shape discrimination. Cross-talk events in which two detectors are fired by a single neutron were rejected using a filter tested on the $β$-1n emitter $^9$Li. A large cross-talk rejection rate is obtained ($> 95 \%$) over most of the energy range of interest. Application to $^{11}$Li data leads to a significant number of events interpreted as $β$-2n decay. A discrete neutron line at $\approx$ 2 MeV indicates sequential two-neutron emission, possibly from the unbound state at 10.6 MeV excitation energy in $^{11}$Be.

nucl-ex↗

arXiv2010

The Thick-COBRA: a New Gaseous Electron Multiplier for Radiation Detectors

The operation principle and preliminary results of a novel gas-avalanche patterned hole electron multiplier, the Thick-COBRA (THCOBRA), are presented. This micro-hole structure is derived from the THGEM and MHSP. Sub-millimeter diameter holes are mechanically drilled in a thin G10 plate, Cu-clad on both faces; on one of the faces the Cu is etched to produce additional anode strips winding between circular cathode strips. Primary avalanches occurring within the holes are followed by additional ones at the anode-strips vicinity. Gains in excess of 5*104 were reached with 22.1 x-rays in Ar, Ne and Ar-10%CH4, with 12.2 % FWHM energy resolution in Ar-10%CH4. Higher gains were measured with single photoelectrons. This robust multiplier may have numerous potential applications.

physics.ins-det↗

arXiv2010

Development of ultra-light pixelated ladders for an ILC vertex detector

The development of ultra-light pixelated ladders is motivated by the requirements of the ILD vertex detector at ILC. This paper summarizes three projects related to system integration. The PLUME project tackles the issue of assembling double-sided ladders. The SERWIETE project deals with a more innovative concept and consists in making single-sided unsupported ladders embedded in an extra thin plastic enveloppe. AIDA, the last project, aims at building a framework reproducing the experimental running conditions where sets of ladders could be tested.

physics.ins-det↗

arXiv2006

Gravitational Effects of Varying Alpha Strings

We study spatial variations of the fine-structure constant in the presence of static straight cosmic strings in the weak-field approximation in Einstein gravity. We work in the context of a generic Bekenstein-type model and consider a gauge kinetic function linear in the scalar field. We determine an analytical form for the scalar field and the string metric at large distances from the core. We show that the gravitational effects of $α$-varying strings can be seen as a combination of the gravitational effects of global and local strings. We also verify that at large distances to the core the space-time metric is similar to that of a global string. We study the motion of test particles approaching from infinity and show that photons are scattered to infinity while massive particles are trapped in bounded trajectories. We also calculate an overall limit on the magnitude of the variation of $α$ for a GUT string, by considering suitable cosmological constraints coming from the Equivalence Principle.

gr-qc↗

arXiv2005

Evolution of the fine-structure constant in the non-linear regime

We study the evolution of the fine-structure constant, $α$, induced by non-linear density perturbations in the context of the simplest class of quintessence models with a non-minimal coupling to the electromagnetic field, in which the two available free functions (potential and gauge kinetic function) are Taylor-expanded up to linear order. We show that the results obtained using the spherical infall model for an infinite wavelength inhomogeneity are inconsistent with the results of a local linearized gravity study and we argue in favour of the second approach. We also discuss recent claims that the value of $α$ inside virialised regions could be significantly different from the background one on the basis of these findings.

astro-ph↗

arXiv2005

Varying Alpha Monopoles

We study static magnetic monopoles in the context of varying alpha theories and show that there is a group of models for which the t'Hooft-Polyakov solution is still valid. Nevertheless, in general static magnetic monopole solutions in varying alpha theories depart from the classical t'Hooft-Polyakov solution with the electromagnetic energy concentrated inside the core seeding spatial variations of the fine structure constant. We show that Equivalence Principle constraints impose tight limits on the allowed variations of alpha induced by magnetic monopoles which confirms the difficulty to generate significant large-scale spatial variation of the fine structure constant found in previous works. This is true even in the most favorable case where magnetic monopoles are the source for these variations.

hep-ph↗

arXiv2005

Cosmic strings in Bekenstein-type models

We study static cosmic string solutions in the context of Bekenstein-type models. We show that there is a class of models of this type for which the classical Nielsen-Olesen vortex is still a valid solution. However, in general static string solutions in Bekenstein-type models strongly depart from the standard Nielsen-Olesen solution with the electromagnetic energy concentrated along the string core seeding spatial variations of the fine structure constant, $α$. We consider models with a generic gauge kinetic function and show that equivalence principle constraints impose tight limits on the allowed variations of $α$ induced by string networks on cosmological scales.

astro-ph↗

arXiv2004

Ferromagnetism and temperature-dependent Electronic Structure of hcp Gadolinium

We use a combination of a many-body model analysis with an ab initio band structure calculation to derive the temperature dependent electronic quasiparticle structure of the rare-earth metal Gadolinium. As a local-moment system Gd is properly represented by the ferromagnetic (multiband) Kondo-lattice model (s-f (d-f) model). The single-particle part of the model-Hamiltonian is taken from an augmented spherical wave (ASW) band calculation. The proposed method avoids the double counting of relevant interactions by exploiting an exact limiting case of the model and takes into account the correct symmetry of atomic orbitals. The a priori only weakly correlated 5d conduction bands get via interband exchange coupling to the localized 4f levels a distinct temperature dependence which explains by a Rudermann-Kittel-Kasuya-Yosida (RKKY) -type mechanism the ferromagnetism of Gd. We get a self-consistently derived Curie temperature of 294.1 K and a T=0-moment of 7.71 $μ_{\rm B}$, surprisingly close to the experimental values. The striking induced temperature-dependence of the 5d conduction bands explains respective photoemission data. The only parameter of the theory (interband exchange coupling J) is uniquely fixed by the band calculation.

cond-mat.str-el↗

arXiv2003

Topological defects: A problem for cyclic universes?

We study the behaviour of cosmic string networks in contracting universes, and discuss some of their possible consequences. We note that there is a fundamental time asymmetry between defect network evolution for an expanding universe and a contracting universe. A string network with negligible loop production and small-scale structure will asymptotically behave during the collapse phase as a radiation fluid. In realistic networks these two effects are important, making this solution only approximate. We derive new scaling solutions describing this effect, and test them against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation background it has generated, can significantly affect the dynamics of the universe both locally and globally. The network can be an important source of radiation, entropy and inhomogeneity. We discuss the possible implications of these findings for bouncing and cyclic cosmological models.

astro-ph↗

arXiv2003

The Correlated Kondo-lattice Model

We investigate the ferromagnetic Kondo-lattice model (FKLM) with a correlated conduction band. A moment conserving approach is proposed to determine the electronic self-energy. Mapping the interaction onto an effective Heisenberg model we calculate the ordering of the localized spin system self-consistently. Quasiparticle densities of states (QDOS) and the Curie temperature are calculated. The band interaction leads to an upper Hubbard peak and modifies the magnetic stability of the FKLM.

cond-mat.str-el↗

arXiv2003

Ferromagnetic Kondo-Lattice Model

We present a many-body approach to the electronic and magnetic properties of the (multiband) Kondo-lattice model with ferromagnetic interband exchange. The coupling between itinerant conduction electrons and localized magnetic moments leads, on the one hand, to a distinct temperature-dependence of the electronic quasiparticle spectrum and, on the other hand, to magnetic properties, as e.~g.the Curie temperature T_C or the magnon dispersion, which are strongly influenced by the band electron selfenergy and therewith in particular by the carrier density. We present results for the single-band Kondo-lattice model in terms of quasiparticle densities of states and quasiparticle band structures and demonstrate the density-dependence of the self-consistently derived Curie temperature. The transition from weak-coupling (RKKY) to strong-coupling (double exchange) behaviour is worked out. The multiband model is combined with a tight-binding-LMTO bandstructure calculation to describe real magnetic materials. As an example we present results for the archetypal ferromagnetic local-moment systems EuO and EuS. The proposed method avoids the double counting of relevant interactions and takes into account the correct symmetry of atomic orbitals.

cond-mat.str-el↗

arXiv2002

Topological Defects in Contracting Universes

We study the behaviour and consequences of cosmic string networks in contracting universes. They approximately behave during the collapse phase as a radiation fluids. Scaling solutions describing this are derived and tested against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation it generates, can affect the dynamics of the universe both locally and globally, and be an important source of radiation, entropy and inhomogeneity. We discuss possible implications for bouncing and cyclic models.

astro-ph↗

arXiv2002

Ferromagnetism in the Kondo-lattice model

We propose a modified RKKY-technique to evaluate the magnetic properties of the ferromagnetic Kondo-lattice model. Together with a previously developed selfenergy approach to the conduction electron part of the model we get a closed system of equations which can be solved self-consistently. Ferromagnetism appears for relatively low electron (hole) densities, while it is excluded around half-filling ($n=1$). For small $J$ conventional RKKY ($\sim J^2$) is reproduced; however, with strong deviations already for very moderate exchange couplings. For not too small $n$ a critical $J_c$ is needed to produce ferromagnetism with a finite Curie temperature $T_{\textrm{C}}$, which increases with $J$, then running into a kind of saturation, in order to fall off again and disappearing above an upper critical exchange $J$.

cond-mat.str-el↗

arXiv2002

Temperature-dependent electronic structure and magnetic stability of thin ferromagnetic films

We study correlation effects and temperature dependencies in the electronic structure of thin ferromagnetic local-moment films. In a first step the Kondo-lattice model is investigated as a candidate for a proper representation of local-moment ferromagnets. Magnetic and electronic key-quantities as the Curie-temperature and the quasiparticle density of states are derived with previously tested many-body procedures. It is shown that the magnetic properties can be interpreted exclusively in terms of the temperature-dependent electronic quasiparticle structure. An extended RKKY theory leads to effective Heisenberg exchange integrals, which turn out to be functionals of the conduction electron selfenergy, getting therewith a remarkable temperature and band occupation dependence. In a second step the model studies are combined with tight binding-LMTO bandstructure calculations in order to get for real ferromagnetic films quasiparticle densities of states and quasiparticle bandstructures. The proposed method avoids the double-counting of relevant interactions and takes into account the correct symmetry of the atomic orbitals. Special results are given for thin ferromagnetic EuO (100) films. The Curie temperature $T_{\textrm{C}}$ of the EuO film turns out to be strongly thickness-dependent, starting from a very low value ($\simeq 15K$) for the monolayer and reaching the bulk value at about 30 layers. For a 20-layer film we predict the existence of a surface state, the temperature-behaviour of which can lead to a surface halfmetal-insulator transition.

cond-mat.str-el↗

arXiv2001

Quantum effects in the quasiparticle structure of the ferromagnetic Kondo lattice model

A new ``Dynamical Mean-field theory'' based approach for the Kondo lattice model with quantum spins is introduced. The inspection of exactly solvable limiting cases and several known approximation methods, namely the second-order perturbation theory, the self-consistent CPA and finally a moment-conserving decoupling of the equations of motion help in evaluating the new approach. This comprehensive investigation gives some certainty to our results: Whereas our method is somewhat limited in the investigation of the J<0-model, the results for J>0 reveal important aspects of the physics of the model: The energetically lowest states are not completely spin-polarized.A band splitting, which occurs already for relatively low interaction strengths, can be related to distinct elementary excitations, namely magnon emission (absorption) and the formation of magnetic polarons. We demonstrate the properties of the ferromagnetic Kondo lattice model in terms of spectral densities and quasiparticle densities of states.

cond-mat.str-el↗