Showing 1–41 of 41 results for author: Diaz, J

Thickness dependent rare earth segregation in magnetron deposited NdCo$_{4.6}$ thin films studied by Xray reflectivity and Hard Xray photoemission

Authors: J. Díaz, J. Rodríguez-Fernández, J. Rubio-Zuazo

Abstract: Magnetic anisotropy in disordered rare-earth-transition metals (RE-TM) compounds arises from RE atoms occupying asymmetric environments within the TM lattice. However, the underlying mechanism that promotes such environments remains not fully understood. In this study, we investigate amorphous NdCo$_{4.6}$ thin films deposited by magnetron sputtering, where the magnetic anisotropy evolves with thi… ▽ More Magnetic anisotropy in disordered rare-earth-transition metals (RE-TM) compounds arises from RE atoms occupying asymmetric environments within the TM lattice. However, the underlying mechanism that promotes such environments remains not fully understood. In this study, we investigate amorphous NdCo$_{4.6}$ thin films deposited by magnetron sputtering, where the magnetic anisotropy evolves with thickness from in-plane to out-of-plane orientation above 40 nm. X-ray reflectivity measurements revealed the progressive formation of an additional layer between the 3 nm Si capping layer and the NdCo film with increasing film thickness. To probe the composition and distribution of Co and Nd near the surface, Hard X-ray Photoemission Spectroscopy (HAXPES) was performed on films ranging from 5 nm to 65 nm in thickness using incident photon energies of 7, 10, and 13 keV. These correspond to inelastic electron mean free paths of 7.2-12.3 nm in cobalt. The cobalt atomic concentration, deduced from HAXPES at the Nd 3d and Co 2p excitations, was consistently below the nominal value and varied with both thickness and photon energy. This indicates segregation of RE atoms at the film surface, which becomes more pronounced with increasing thickness. Background analysis of the Co 2p and Nd 3d peaks supports this conclusion. The thickness of the Nd segregated layer is estimated in 2-3 nm. These findings demonstrate that incorporating neodymium into the cobalt lattice incurs an energy cost which is associated with strain due to the volume mismatch between the two elements. Reducing this strain energy promotes anisotropic atomic environments for Nd, thereby explaining the emergence of perpendicular anisotropy and its dependence on film thickness in these RE-TM compounds. △ Less

Submitted 22 January, 2026; v1 submitted 27 October, 2025; originally announced October 2025.

Comments: 13 pages, 14 figures, regular paper

Hydrodynamic Mechanism of Colloidal Propulsion through Momentum Exchange

Authors: Javier Diaz, Ignacio Pagonabarraga, Carles Calero

Abstract: Propulsion of colloidal particles due to momentum transfer from localized surface reactions is investigated by solving the exact unsteady Stokes equation. We model the effect of surface reactions as either a {\it force dipole} acting on the fluid or a {\it pair force} acting on both the colloid and the fluid. Our analysis reveals that after a single reaction event the colloid's velocity initially… ▽ More Propulsion of colloidal particles due to momentum transfer from localized surface reactions is investigated by solving the exact unsteady Stokes equation. We model the effect of surface reactions as either a {\it force dipole} acting on the fluid or a {\it pair force} acting on both the colloid and the fluid. Our analysis reveals that after a single reaction event the colloid's velocity initially decays as $\sim t^{-1/2}$, followed by a long-time tail decay $\sim t^{-5/2}$. This behavior is distinct from the $\sim t^{-3/2}$ decay seen for simple impulsively forced particles, a result of the force-free nature of the reaction mechanism. The velocity and transient dynamics are strongly controlled by the distance of the reaction from the colloid surface. For a colloid subject to periodic reactions, the theory predicts a steady-state velocity that is comparable to experimental results and previous simulations, suggesting that direct momentum transfer is a relevant mechanism for self-propulsion in systems like Janus particles. Finally, our study shows that fluid compressibility is not required for momentum transfer to produce colloidal propulsion. △ Less

Submitted 7 October, 2025; originally announced October 2025.

Localized Triplons and Site Stuffing in the Quantum Dimer Magnet BiYbGeO$_5$

Authors: Rachit Kapoor, J. Ramirez Diaz, D. Yahne, V. O. Garlea, G. Hester

Abstract: Thermodynamic and muon spin-relaxation measurements have recently highlighted BiYbGeO$_5$ as a new example of a rare-earth-based quantum dimer magnet with isolated Yb$^{3+}$ spin-$\frac{1}{2}$ dimers. However, direct spectroscopic evidence of the triplet excitations and measurements of the structural disorder are lacking. In this work, polycrystalline BiYbGeO$_5$ was synthesized using conventional… ▽ More Thermodynamic and muon spin-relaxation measurements have recently highlighted BiYbGeO$_5$ as a new example of a rare-earth-based quantum dimer magnet with isolated Yb$^{3+}$ spin-$\frac{1}{2}$ dimers. However, direct spectroscopic evidence of the triplet excitations and measurements of the structural disorder are lacking. In this work, polycrystalline BiYbGeO$_5$ was synthesized using conventional high-temperature solid-state methods and investigated via high-resolution neutron powder diffraction and inelastic neutron scattering. Diffraction measurements down to 58 mK reveal no signatures of magnetic order and indicate that nearly 20\% of Yb$^{3+}$ sites are replaced by non-magnetic Bi$^{3+}$, introducing significant structural disorder. Inelastic neutron scattering shows dispersionless triplon excitations, consistent with localized, non-interacting spin dimers. Fits to the triplet excitation spectrum identify an XXZ-type anisotropic exchange with $J_{XX}$ = 0.11(2) meV and $J_Z = 0.15(1)$ meV. These findings establish BiYbGeO$_5$ as a structurally disordered but magnetically well-isolated quantum dimer system, providing a model platform for studying the resilience of entangled spin states to site dilution. △ Less

Submitted 8 December, 2025; v1 submitted 28 May, 2025; originally announced May 2025.

Comments: 6 pages, 5 figures

Active droplets controlled by enzymatic reactions

Authors: Jacques Fries, Javier Diaz, Marie Jardat, Ignacio Pagonabarraga, Pierre Illien, Vincent Dahirel

Abstract: The formation of condensates is now considered as a major organization principle of eukaryotic cells. Several studies have recently shown that the properties of these condensates are affected by enzymatic reactions. We propose here a simple generic model to study the interplay between two enzyme populations and a two-state protein. In one state, the protein forms condensed droplets through attract… ▽ More The formation of condensates is now considered as a major organization principle of eukaryotic cells. Several studies have recently shown that the properties of these condensates are affected by enzymatic reactions. We propose here a simple generic model to study the interplay between two enzyme populations and a two-state protein. In one state, the protein forms condensed droplets through attractive interactions, while in the other state, the proteins remain dispersed. Each enzyme catalyzes the production of one of these two protein states only when reactants are in its vicinity. A key feature of our model is the explicit representation of enzyme trajectories, capturing the fluctuations in their local concentrations. The spatially dependent growth rate of droplets naturally arises from the stochastic motion of these explicitly modeled enzymes. Using two complementary numerical methods, (1) Brownian Dynamics simulations, and (2) a hybrid method combining Cahn-Hilliard-Cook diffusion equations with Brownian Dynamics for the enzymes, we investigate how enzyme concentration and dynamics influence the evolution with time, and the steady-state number and size of droplets. Our results show that the concentration and diffusion coefficient of enzymes govern the formation and size-selection of biocondensates. △ Less

Submitted 18 November, 2024; originally announced November 2024.

Comments: 14 pages, 9 figures

Active particles confined in deformable droplets

Authors: Javier Diaz, Ignacio Pagonabarraga

Abstract: Active particles under soft confinement such as droplets or vesicles present intriguing phenomena, as collective motion emerges alongside the deformation of the environment. A model is employed to systematically investigate droplet morphology and particle distribution in relation to activity and concentration,revealing that active particles have the capacity to induce enhanced shape fluctuations i… ▽ More Active particles under soft confinement such as droplets or vesicles present intriguing phenomena, as collective motion emerges alongside the deformation of the environment. A model is employed to systematically investigate droplet morphology and particle distribution in relation to activity and concentration,revealing that active particles have the capacity to induce enhanced shape fluctuations in the droplet interface with respect to the thermal fluctuations, aligning with recent experimental observations. A rich phase behaviour can be identified with two different mechanism of droplet breakage. △ Less

Submitted 24 July, 2024; originally announced July 2024.

A Free Energy Model for the Plateau Shear Modulus in Thermosensitive Microgel Suspensions

Authors: Maxime Bergman, Yixuan Xu, José Muñéton Díaz, Zhang Chi, Thomas G. Mason, Frank Scheffold

Abstract: Polymer microgels exhibit intriguing macroscopic flow properties arising from their unique microscopic structure. Microgel colloids usually comprise a crosslinked polymer network with a radially decaying density profile, resulting in a dense core surrounded by a fuzzy corona. Notably, microgels synthesized from poly(N-isopropyl acrylamide) (PNIPAM) are thermoresponsive, capable of adjusting their… ▽ More Polymer microgels exhibit intriguing macroscopic flow properties arising from their unique microscopic structure. Microgel colloids usually comprise a crosslinked polymer network with a radially decaying density profile, resulting in a dense core surrounded by a fuzzy corona. Notably, microgels synthesized from poly(N-isopropyl acrylamide) (PNIPAM) are thermoresponsive, capable of adjusting their size and density profile based on temperature. Above the lower critical solution temperature ($T_\text{LCST} \sim 33$ $^\circ$C), the microgel's polymer network collapses, expulsing water through a reversible process. Conversely, below $33$ $^\circ$C, the microgel's network swells, becoming highly compressible and allowing overpacking to effective volume fractions exceeding one. Under conditions of dense packing, microgels undergo deformation in distinct stages: corona compression and faceting, interpenetration, and finally, isotropic compression. Each stage exhibits a characteristic signature in the dense microgel suspensions' yield stress and elastic modulus. Here, we introduce a model for the linear elastic shear modulus by minimizing a quasi-equilibrium free energy, encompassing all relevant energetic contributions. We validate our model by comparing its predictions to experimental results from oscillatory shear rheology tests on microgel suspensions at different densities and temperatures. Our findings demonstrate that combining macroscopic rheological measurements with the model allows for temperature-dependent characterization of polymer interaction parameters. △ Less

Submitted 7 February, 2025; v1 submitted 12 March, 2024; originally announced March 2024.

Comments: 24 pages, 4 figures. Langmuir (online, February 3, 2025)

Activity-driven emulsification of phase-separating binary mixtures

Authors: Javier Diaz, Ignacio Pagonabarraga

Abstract: Systems containing active components are intrinsically out of equilibrium, while binary mixtures reach their equilibrium configuration when complete phase separation is achieved. Active particles are found to stabilise non-equilibrium morphologies in phase separating binary mixtures by arresting coarsening by exerting active pressure that competes with surface tension driving forces. For moderate… ▽ More Systems containing active components are intrinsically out of equilibrium, while binary mixtures reach their equilibrium configuration when complete phase separation is achieved. Active particles are found to stabilise non-equilibrium morphologies in phase separating binary mixtures by arresting coarsening by exerting active pressure that competes with surface tension driving forces. For moderate activities, an emulsion morphology is stabilised, where the droplet size is well-defined and controlled by activity. Conversely, the ability of active particles to drive phase-separated mixtures away from their equilibrium configuration is shown. A rich co-assembly behaviour is shown due to the competing energy scales involved in the system. △ Less

Submitted 22 October, 2023; originally announced October 2023.

Experimental and theoretical assessment of native oxide in the superconducting TaN

Authors: V. Quintanar-Zamora, M. Cedillo-Rosillo, O. Contreras-López, C. Corona-García, A. Reyes-Serrato, R. Ponce-Pérez, J. Guerrero-Sánchez, J. A. Díaz

Abstract: In this manuscript, we show through an experimental-computational proof of concept the native oxide formation into superconducting TaN films. First, TaN was synthesized at an ultra-high vacuum system by reactive pulsed laser deposition and characterized in situ by X-ray photoelectron spectroscopy. The material was also characterized ex situ by X-ray diffraction, transmission electron microscopy, a… ▽ More In this manuscript, we show through an experimental-computational proof of concept the native oxide formation into superconducting TaN films. First, TaN was synthesized at an ultra-high vacuum system by reactive pulsed laser deposition and characterized in situ by X-ray photoelectron spectroscopy. The material was also characterized ex situ by X-ray diffraction, transmission electron microscopy, and the four-point probe method. It was detected that TaN contained considerable oxygen impurities (up to 26 %O) even though it was grown in an ultra-high vacuum chamber. Furthermore, the impurified TaN evidence a face-centered cubic crystalline structure only and exhibits superconductivity at 2.99 K. To understand the feasibility of the native oxide in TaN, we study the effect of incorporating different amounts of O atoms in TaN using ab-initio calculations. A thermodynamic stability analysis shows that a TaOxN1-x model increases its stability as oxygen is added, demonstrating that oxygen may always be present in TaN, even when obtained at ultra-high vacuum conditions. All analyzed models exhibit metallic behavior. Charge density difference maps reveal that N and O atoms have a higher charge density redistribution than Ta atoms. The electron localization function maps and line profiles indicate that Ta-O and Ta-N bonds are mainly ionic. As expected, stronger ionic behavior is observed in the Ta-O bonds due to the electronegativity difference between O and N atoms. Recent evidence points to superconductivity in bulk TaO, confirming the asseverations of superconductivity in our samples. The results discussed here highlight the importance of considering native oxide when reporting superconductivity in TaN films since the TaO regions formed in the compound may be key to understanding the different critical temperatures reported in the literature. △ Less

Submitted 21 September, 2023; originally announced September 2023.

Comments: 23 pages, 7 figures

Morphology and Magnetic vortex chiral symmetry of 2D arrays of magnetic trilayer disks with magnetostatic interlayer coupling determined by X ray resonant magnetic scattering

Authors: J. Díaz, L. M. Álvarez-Prado, S. M. Valvidares, I. Montoya, C. Redondo, R. Morales, M. Vélez

Abstract: X ray resonant magnetic scattering (XRMS) was used to characterize the magnetization of 2D arrays of trilayer submicron magnets. The interpretation of the data required the understanding of the morphology of the magnets which was also deduced from the scattered intensity. The magnets consisted of two magnetostatically coupled ferromagnetic layers separated by a non-magnetic spacer. The scattered i… ▽ More X ray resonant magnetic scattering (XRMS) was used to characterize the magnetization of 2D arrays of trilayer submicron magnets. The interpretation of the data required the understanding of the morphology of the magnets which was also deduced from the scattered intensity. The magnets consisted of two magnetostatically coupled ferromagnetic layers separated by a non-magnetic spacer. The scattered intensity from the disks resulted to be dependent on the disks surface curvature. This made the collected intensity at each Bragg reflection (BR) to be correlated to the reflected light from locations of the disk with the same angle of curvature. Due to this, quantitative information was obtained, averaged over the disks illuminated by x rays, of the variations in thickness and magnetization across the entire area of the disks. This averaged magnetization mapping of the disks served to study their vortex configuration in each of their magnetic layers, determining the average location of the vortex, the chiral symmetry of its magnetic circulation, and the specific locations where the vortex nucleation starts within the disks. Chiral asymmetry appeared in the disks when the field was oriented at an oblique angle with respect to the easy axis of the array. The local magnetic sensitivity of the technique allowed to identify a non-centrosymmetric distribution of the magnetization of the disks that explains the observed chiral asymmetry. Unexpectedly, the magnetic circulation sense of the vortex was the same in both ferromagnetic layers. In addition, the magnetization of the buried layer was different in the descent branch than in the ascent branch of its hysteresis loops. △ Less

Submitted 27 January, 2023; originally announced January 2023.

Comments: 13 pages, 16 figures

Phase behaviour of active particles in block copolymer melts

Authors: Javier Diaz, Ignacio Pagonabarraga

Abstract: Block copolymer melts offer a unique template to control the position and alignment of nanoparticles due to their ability to self-assemble into periodic ordered structures. Active Brownian particles are shown to co-assemble with block copolymers leading to emergent organised structures. The block copolymer acts as a soft confinement that can control the self-propulsion of the active Brownian parti… ▽ More Block copolymer melts offer a unique template to control the position and alignment of nanoparticles due to their ability to self-assemble into periodic ordered structures. Active Brownian particles are shown to co-assemble with block copolymers leading to emergent organised structures. The block copolymer acts as a soft confinement that can control the self-propulsion of the active Brownian particles, both for interface-segregated and selective nanoparticles. At moderate activity regimes, active Brownian particles can form organised structures such as polarised trains or rotating vortexes. At high activity, the contrast in the polymeric and colloidal time scales can lead to particle swarms with distorted block copolymer morphology, due to the competition between the polymeric self-assembly and the active Brownian self-propulsion. △ Less

Submitted 15 March, 2022; v1 submitted 12 March, 2022; originally announced March 2022.

Ba$^{2+}$ ion trapping by organic submonolayer: towards an ultra-low background neutrinoless double beta decay detector

Authors: P. Herrero-Gómez, J. P. Calupitan, M. Ilyn, A. Berdonces-Layunta, T. Wang, D. G. de Oteyza, M. Corso, R. González-Moreno, I. Rivilla, B. Aparicio, A. I. Aranburu, Z. Freixa, F. Monrabal, F. P. Cossío, J. J. Gómez-Cadenas, C. Rogero, C. Adams, H. Almazán, V. Alvarez, L. Arazi, I. J. Arnquist, S. Ayet, C. D. R. Azevedo, K. Bailey, F. Ballester , et al. (90 additional authors not shown)

Abstract: If neutrinos are their own antiparticles, the otherwise-forbidden nuclear reaction known as neutrinoless double beta decay ($ββ0ν$) can occur, with a characteristic lifetime which is expected to be very long, making the suppression of backgrounds a daunting task. It has been shown that detecting (``tagging'') the Ba$^{+2}$ dication produced in the double beta decay… ▽ More If neutrinos are their own antiparticles, the otherwise-forbidden nuclear reaction known as neutrinoless double beta decay ($ββ0ν$) can occur, with a characteristic lifetime which is expected to be very long, making the suppression of backgrounds a daunting task. It has been shown that detecting (``tagging'') the Ba$^{+2}$ dication produced in the double beta decay ${}^{136}\mathrm{Xe} \rightarrow {}^{136}$Ba$^{+2}+ 2 e + (2 ν)$ in a high pressure gas experiment, could lead to a virtually background free experiment. To identify these \Bapp, chemical sensors are being explored as a key tool by the NEXT collaboration . Although used in many fields, the application of such chemosensors to the field of particle physics is totally novel and requires experimental demonstration of their suitability in the ultra-dry environment of a xenon gas chamber. Here we use a combination of complementary surface science techniques to unambiguously show that Ba$^{+2}$ ions can be trapped (chelated) in vacuum by an organic molecule, the so-called fluorescent bicolour indicator (FBI) (one of the chemosensors developed by NEXT), immobilized on a surface. We unravel the ion capture mechanism once the molecules are immobilised on Au(111) surface and explain the origin of the emission fluorescence shift associated to the trapping of different ions. Moreover, we prove that chelation also takes place on a technologically relevant substrate, as such, demonstrating the feasibility of using FBI indicators as building blocks of a Ba$^{+2}$ detector. △ Less

Submitted 22 January, 2022; originally announced January 2022.

Review of Damage Models for Environmentally Assisted Cracking on Metals

Authors: Jorge G. Diaz

Abstract: Stress corrosion cracking (SCC) is a form of failure in metals that is present mostly in systems where humidity contact and mechanical load play a key role. Therefore, prediction of the combined effect is desired. There are models based on path dissolution and film rupture, Hydrogen-embrittlement, based on empirical observations, and the unified approach which recognizes not the maximum applied st… ▽ More Stress corrosion cracking (SCC) is a form of failure in metals that is present mostly in systems where humidity contact and mechanical load play a key role. Therefore, prediction of the combined effect is desired. There are models based on path dissolution and film rupture, Hydrogen-embrittlement, based on empirical observations, and the unified approach which recognizes not the maximum applied stress intensity factor (SIF) but the SIF range as the fatigue driving force. So, in the later one, the contribution of corrosion to crack growth is recognized by adding a corrosion SIF. △ Less

Submitted 5 November, 2021; originally announced November 2021.

Quasi-symmetry protected topology in a semi-metal

Authors: Chunyu Guo, Lunhui Hu, Carsten Putzke, Jonas Diaz, Xiangwei Huang, Kaustuv Manna, Feng-Ren Fan, Chandra Shekhar, Yan Sun, Claudia Felser, Chaoxing Liu, B. Andrei Bernevig, Philip J. W. Moll

Abstract: The crystal symmetry of a material dictates the type of topological band structures it may host, and therefore symmetry is the guiding principle to find topological materials. Here we introduce an alternative guiding principle, which we call 'quasi-symmetry'. This is the situation where a Hamiltonian has an exact symmetry at lower-order that is broken by higher-order perturbation terms. This enfor… ▽ More The crystal symmetry of a material dictates the type of topological band structures it may host, and therefore symmetry is the guiding principle to find topological materials. Here we introduce an alternative guiding principle, which we call 'quasi-symmetry'. This is the situation where a Hamiltonian has an exact symmetry at lower-order that is broken by higher-order perturbation terms. This enforces finite but parametrically small gaps at some low-symmetry points in momentum space. Untethered from the restraints of symmetry, quasi-symmetries eliminate the need for fine-tuning as they enforce that sources of large Berry curvature will occur at arbitrary chemical potentials. We demonstrate that a quasi-symmetry in the semi-metal CoSi stabilizes gaps below 2 meV over a large near-degenerate plane that can be measured in the quantum oscillation spectrum. The application of in-plane strain breaks the crystal symmetry and gaps the degenerate point, observable by new magnetic breakdown orbits. The quasi-symmetry, however, does not depend on spatial symmetries and hence transmission remains fully coherent. These results demonstrate a class of topological materials with increased resilience to perturbations such as strain-induced crystalline symmetry breaking, which may lead to robust topological applications as well as unexpected topology beyond the usual space group classifications. △ Less

Submitted 31 May, 2022; v1 submitted 4 August, 2021; originally announced August 2021.

Comments: Nat. Phys. (2022)

Ultrasonic chaining of emulsion droplets

Authors: Mohammed A. Abdelaziz, Jairo A. Diaz, Jean-Luc Aider, David J. Pine, David G. Grier, Mauricio Hoyos

Abstract: Emulsion droplets trapped in an ultrasonic levitator behave in two ways that solid spheres do not: (1) Individual droplets spin rapidly about an axis parallel to the trapping plane, and (2) coaxially spinning droplets form long chains aligned with their common axis of rotation. Acoustically-organized chains interact hydrodynamically, either to merge into longer chains or to form three-dimensional… ▽ More Emulsion droplets trapped in an ultrasonic levitator behave in two ways that solid spheres do not: (1) Individual droplets spin rapidly about an axis parallel to the trapping plane, and (2) coaxially spinning droplets form long chains aligned with their common axis of rotation. Acoustically-organized chains interact hydrodynamically, either to merge into longer chains or to form three-dimensional bundles of chains. Solid spheres, by contrast, form close-packed planar crystals drawn together by the sound-mediated secondary Bjerknes interaction. We demonstrate the chain-to-crystal transition with a model system in which fluid emulsion droplets can be photopolymerized into solid spheres without significantly changing other material properties. The behavior of this experimental system is quantitatively consistent with an acoustohydrodynamic model for spinning spheres in an acoustic levitator. This study therefore introduces acoustically-driven spinning as a mechanism for guiding self-organization of acoustically levitated matter. △ Less

Submitted 1 July, 2021; originally announced July 2021.

Comments: 10 pages, 4 figures

Magnetic moment orientation and in depth distribution of dysprosium near the surface of DyCo$_{4.6}$ thin films determined by the analysis of their X ray circularly polarized absorption Dy $M_{5}$ spectra

Authors: Javier Díaz, Cristina Blanco-Roldán

Abstract: We have investigated the RE atomic distribution and its magnetic moment orientation at the region near the surface of DyCo$_{4.4 \& 4.6}$ ferrimagnetic amorphous films with perpendicular magnetic anisotropy (PMA). XMCD spectroscopy of the films at the Dy $M_{4,5}$ and Co $L_{2,3}$ edges using total electron yield (TEY) detection was performed at 2 K and 300 K temperatures, and at sample orientatio… ▽ More We have investigated the RE atomic distribution and its magnetic moment orientation at the region near the surface of DyCo$_{4.4 \& 4.6}$ ferrimagnetic amorphous films with perpendicular magnetic anisotropy (PMA). XMCD spectroscopy of the films at the Dy $M_{4,5}$ and Co $L_{2,3}$ edges using total electron yield (TEY) detection was performed at 2 K and 300 K temperatures, and at sample orientations ranged from 0$^{\circ}$ to 70$^{\circ}$ with respect to the magnetic easy axis. The measurements showed an apparent partial decoupling between the cobalt and dysprosium magnetic sublattices. At RT, the magnetic moment per atom of dysprosium was below the minimum value expected if all dysprosium moments were AF coupled to cobalt. At 2 K, the cobalt sublattice presented a stronger magnetic anisotropic behavior than the dysprosium sublattice. A detailed analysis of the circularly polarized spectra of the Dy $M_{5}$ edge, based on the deconvolution of the spectra in their related parallel, antiparallel and transverse to $J_{z}$ spectral components, demonstrates that the spectra are composed by dysprosium with different magnetic moment distributions. The fit of the Dy $M_{5}$ spectra using the $J_{z}$ spectral components evidenced a gradation of dysprosium concentration due to segregation at the region probed by TEY. The topmost layer was magnetically uncoupled from cobalt. At RT, 25$\%$ of the dysprosium magnetic moments in the under layer were found averaged oriented in the same direction as cobalt. The expected weak magnetic coupling of these dysprosium atoms to cobalt would explain the surprisingly lower magnetic anisotropy of the dysprosium sublattice compared to that of cobalt probed by TEY at 2 K. △ Less

Submitted 18 March, 2021; originally announced March 2021.

Comments: 12 figures, 4 tables, 1 appendix, 19 pages

Journal ref: Phys. Rev. B 104, 054439 (2021)

Sondheimer oscillations as a probe of non-ohmic flow in type-II Weyl semimetal WP$_2$

Authors: Maarten R. van Delft, Yaxian Wang, Carsten Putzke, Jacopo Oswald, Georgios Varnavides, Christina A. C. Garcia, Chunyu Guo, Heinz Schmid, Vicky Süss, Horst Borrmann, Jonas Diaz, Yan Sun, Claudia Felser, Bernd Gotsmann, Prineha Narang, Philip J. W. Moll

Abstract: As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm's law. Depending on the length scales of momentum conserving ($l_{MC}$) and relaxing ($l_{MR}$) electron scattering, and the device size ($d$), current flows may shift from ohmic to ballistic to hydrodynamic regimes and more exotic mixtures thereof. So far, an in situ, in-operando m… ▽ More As conductors in electronic applications shrink, microscopic conduction processes lead to strong deviations from Ohm's law. Depending on the length scales of momentum conserving ($l_{MC}$) and relaxing ($l_{MR}$) electron scattering, and the device size ($d$), current flows may shift from ohmic to ballistic to hydrodynamic regimes and more exotic mixtures thereof. So far, an in situ, in-operando methodology to obtain these parameters self-consistently within a micro/nanodevice, and thereby identify its conduction regime, is critically lacking. In this context, we exploit Sondheimer oscillations, semi-classical magnetoresistance oscillations due to helical electronic motion, as a method to obtain $l_{MR}$ in micro-devices even when $l_{MR}\gg d$. This gives information on the bulk $l_{MR}$ complementary to quantum oscillations, which are sensitive to all scattering processes. We extract $l_{MR}$ from the Sondheimer amplitude in the topological semi-metal WP$_2$, at elevated temperatures up to $T\sim 50$~K, in a range most relevant for hydrodynamic transport phenomena. Our data on micrometer-sized devices are in excellent agreement with experimental reports of the large bulk $l_{MR}$ and thus confirm that WP$_2$ can be microfabricated without degradation. Indeed, the measured scattering rates match well with those of theoretically predicted electron-phonon scattering, thus supporting the notion of strong momentum exchange between electrons and phonons in WP$_2$ at these temperatures. These results conclusively establish Sondheimer oscillations as a quantitative probe of $l_{MR}$ in micro-devices in studying non-ohmic electron flow. △ Less

Submitted 15 December, 2020; originally announced December 2020.

Comments: 14 pages with 4 figures

Tuning the structural and antiferromagnetic phase transitions in UCr$_{2}$Si$_2$: hydrostatic pressure and chemical substitution

Authors: Y. Lai, K. Wei, G. Chappell, J. Diaz, T. Siegrist, P. J. W. Moll, D. Graf, R. E. Baumbach

Abstract: Structural phase transitions in $f$-electron materials have attracted sustained attention both for practical and basic science reasons, including that they offer an environment to directly investigate relationships between structure and the $f$-state. Here we present results for UCr$_2$Si$_2$, where structural (tetragonal $\rightarrow$ monoclinic) and antiferromagnetic phase transitions are seen a… ▽ More Structural phase transitions in $f$-electron materials have attracted sustained attention both for practical and basic science reasons, including that they offer an environment to directly investigate relationships between structure and the $f$-state. Here we present results for UCr$_2$Si$_2$, where structural (tetragonal $\rightarrow$ monoclinic) and antiferromagnetic phase transitions are seen at $T_{\rm{S}}$ $=$ 205 K and $T_{\rm{N}}$ $=$ 25 K, respectively. We also provide evidence for an additional second order phase transition at $T_{\rm{X}}$ = 280 K. We show that $T_{\rm{X}}$, $T_{\rm{S}}$, and $T_{\rm{N}}$ respond in distinct ways to the application of hydrostatic pressure and Cr $\rightarrow$ Ru chemical substitution. In particular, hydrostatic compression increases the structural ordering temperature, eventually causes it to merge with $T_{\rm{X}}$ and destroys the antiferromagnetism. In contrast, chemical substitution in the series UCr$_{2-x}$Ru$_x$Si$_2$ suppresses both $T_{\rm{S}}$ and $T_{\rm{N}}$, causing them to approach zero temperature near $x$ $\approx$ 0.16 and 0.08, respectively. The distinct $T-P$ and $T-x$ phase diagrams are related to the evolution of the rigid Cr-Si and Si-Si substructures, where applied pressure semi-uniformly compresses the unit cell and Cr $\rightarrow$ Ru substitution results in uniaxial lattice compression along the tetragonal $c$-axis and an expansion in the $ab$-plane. These results provide insights into an interesting class of strongly correlated quantum materials where degrees of freedom associated with $f$-electron magnetism, strong electronic correlations, and structural instabilities are readily controlled. △ Less

Submitted 19 June, 2020; originally announced June 2020.

Comments: Main text 9 pages, 7 figures. Supplementary materials included at the end

Journal ref: Phys. Rev. Materials 4, 075003 (2020)

Effective Hamiltonian for Stokes--anti-Stokes pair generation with pump and probe polarized modes

Authors: R. Acosta Diaz, C. H. Monken, A. Jorio, Marcelo F. Santos

Abstract: In the correlated Stokes--anti-Stokes scattering (SAS) an incident photon interacts with a Raman-active material, creating a Stokes photon and exciting a quantum vibrational mode in the medium, which is posteriorly annihilated on contact with a second incident photon, producing in turn an anti-Stokes photon. This can be accomplished by real and virtual processes. In real process the quantum mode s… ▽ More In the correlated Stokes--anti-Stokes scattering (SAS) an incident photon interacts with a Raman-active material, creating a Stokes photon and exciting a quantum vibrational mode in the medium, which is posteriorly annihilated on contact with a second incident photon, producing in turn an anti-Stokes photon. This can be accomplished by real and virtual processes. In real process the quantum mode shared between the Stokes and anti-Stokes events is a real particle, whereas in virtual processes the pair formation is mediated by the exchange of virtual particles. Here, we introduce a Hamiltonian to describe the pair production in SAS scattering, for both types of process, when stimulated by two orthogonally polarized laser pulses in a pump-and-probe configuration. We also model the effect of the natural decay of the vibration created in the Stokes event and compute the probability of producing SAS pairs. Additionally, we follow the dynamics of the vibration by considering the Stokes and anti-Stokes fields as external reservoirs, obtaining thus a master equation for the reduced density matrix for the vibrational population. Finally, we compare our theoretical results with recently published experimental data. △ Less

Submitted 22 October, 2020; v1 submitted 13 June, 2020; originally announced June 2020.

Comments: 9 pages, 3 figures

Journal ref: Phys. Rev. B 102, 134304 (2020)

Block copolymer-nanorod co-assembly in thin films: effects of rod-rod interaction and confinement

Authors: Javier Diaz, Marco Pinna, Andrei V. Zvelindovsky, Ignacio Pagonabarraga, Roy Shenhar

Abstract: Simulations and experiments of nanorods (NRs) show that co-assembly with block copolymer (BCP) melts leads to the formation of a superstructure of side-to-side NRs perpendicular to the lamellar axis. A mesoscopic model is validated against scanning electron microscopy (SEM) images of CdSe NRs mixed with polystyrene-block-poly(methyl methacrylate). It is then used to study the co-assembly of anisot… ▽ More Simulations and experiments of nanorods (NRs) show that co-assembly with block copolymer (BCP) melts leads to the formation of a superstructure of side-to-side NRs perpendicular to the lamellar axis. A mesoscopic model is validated against scanning electron microscopy (SEM) images of CdSe NRs mixed with polystyrene-block-poly(methyl methacrylate). It is then used to study the co-assembly of anisotropic nanoparticles (NPs) with a length in the same order of magnitude as the lamellar spacing. The phase diagram of BCP/NP is explored as well as the time evolution of the NR. NRs that are slightly larger than the lamellar spacing are found to rotate and organise side-to-side with a tilted orientation with respect to the interface. Strongly interacting NPs are found to dominate the co-assembly while weakly interacting nanoparticles are less prone to form aggregates and tend to form well-ordered configurations. △ Less

Submitted 13 April, 2020; originally announced April 2020.

Magnetic electron collimation in three-dimensional semi-metals

Authors: Xiangwei Huang, Carsten Putzke, Chunyu Guo, Jonas Diaz, Markus König, Horst Borrmann, Nityan L. Nair, James G. Analytis, Philip J. W. Moll

Abstract: While electrons moving perpendicular to a magnetic field are confined to cyclotron orbits, they can move freely parallel to the field. This simple fact leads to complex current flow in clean, low carrier density semi-metals, such as long-ranged current jets forming along the magnetic field when currents pass through point-like constrictions. Occurring accidentally at imperfect current injection co… ▽ More While electrons moving perpendicular to a magnetic field are confined to cyclotron orbits, they can move freely parallel to the field. This simple fact leads to complex current flow in clean, low carrier density semi-metals, such as long-ranged current jets forming along the magnetic field when currents pass through point-like constrictions. Occurring accidentally at imperfect current injection contacts, the phenomenon of "current jetting" plagues the research of longitudinal magneto-resistance which is particularly important in topological conductors. Here we demonstrate the controlled generation of tightly focused electron beams in a new class of micro-devices machined from crystals of the Dirac semi-metal Cd3As2. The current beams can be guided by tilting a magnetic field and their range tuned by the field strength. Finite element simulations quantitatively capture the voltage induced at faraway contacts when the beams are steered towards them, supporting the picture of controlled electron jets. These experiments demonstrate the first direct control over the highly nonlocal signal propagation unique to 3D semi-metals in the current jetting regime, and may lead to novel applications akin to electron optics in free space. △ Less

Submitted 9 January, 2020; originally announced January 2020.

Comments: 18 pages, 11 figures

Large scale three dimensional simulations of hybrid block copolymer/nanoparticle systems

Authors: Javier Diaz, Marco Pinna, Andrei V. Zvelindovsky, Ignacio Pagonabarraga

Abstract: Block copolymer melts self-assemble in the bulk into a variety of nanostructures, making them perfect candidates to template the position of nanoparticles. The morphological changes of block copolymers are studied in the presence of a considerable filling fraction of colloids. Furthermore, colloids can be found to assemble into ordered hexagonally close-packed structures in a defined number of lay… ▽ More Block copolymer melts self-assemble in the bulk into a variety of nanostructures, making them perfect candidates to template the position of nanoparticles. The morphological changes of block copolymers are studied in the presence of a considerable filling fraction of colloids. Furthermore, colloids can be found to assemble into ordered hexagonally close-packed structures in a defined number of layers when softly confined within the phase-separated block copolymer. A high concentration of interface-compatible nanoparticles leads to complex block copolymer morphologies depending on the polymeric composition. Macrophase separation between the colloids and the block copolymer can be induced if colloids are unsolvable within the matrix. This leads to the formation of ellipsoid-shaped polymer-rich domains elongated along the direction perpendicular to the interface between block copolymer domains. △ Less

Submitted 30 October, 2019; originally announced October 2019.

Phase Behavior of Block Copolymer Nanocomposite Systems

Authors: Javier Diaz, Marco Pinna, Andrei V. Zvelindovsky, Ignacio Pagonabarraga

Abstract: Nanocomposite materials made of block copolymer and nanoparticles display properties which can be different from the purely polymeric matrix. The resulting material is a crossover of the original properties of the block copolymer and the presence of the assembled nanoparticles. We propose a mesoscopic study via cell dynamic simulations to quantitatively assert the properties of such hybrid materia… ▽ More Nanocomposite materials made of block copolymer and nanoparticles display properties which can be different from the purely polymeric matrix. The resulting material is a crossover of the original properties of the block copolymer and the presence of the assembled nanoparticles. We propose a mesoscopic study via cell dynamic simulations to quantitatively assert the properties of such hybrid materials. The most relevant parameters are identified to be the fraction of nanoparticles in the system and its chemical affinity, while the nanoparticle size with respect to the block copolymer length scales plays a role in the assembly. The morphological phase diagram of the block copolymer is constructed for nanoparticles with chemical affinity ranging from A-compatible to B-compatible for a symmetric A-B diblock copolymer. Block-compatible nanoparticles is found to induce a phase transition due to changes in the effective concentration of the hosting phase, while interface-compatible particles induces the appearance of two new phases due to the saturation of the diblock copolymer interface. △ Less

Submitted 1 November, 2019; v1 submitted 25 October, 2019; originally announced October 2019.

Journal ref: Advanced Theory and Simulations. 2018 Sep;1(9):1800066

Non-spherical nanoparticles in block copolymer composites: nanosquares, nanorods and diamonds

Authors: Javier Diaz, Marco Pinna, Andrei V. Zvelindovsky, Ignacio Pagonabarraga

Abstract: A hybrid block copolymer(BCP) nanocomposite computational model is proposed to study nanoparticles(NPs) with a generalised shape including squares, rectangles and rhombus. Simulations are used to study the role of anisotropy in the assembly of colloids within BCPs, ranging from NPs that are compatible with one phase, to neutral NPs. The ordering of square-like NPs into grid configurations within a… ▽ More A hybrid block copolymer(BCP) nanocomposite computational model is proposed to study nanoparticles(NPs) with a generalised shape including squares, rectangles and rhombus. Simulations are used to study the role of anisotropy in the assembly of colloids within BCPs, ranging from NPs that are compatible with one phase, to neutral NPs. The ordering of square-like NPs into grid configurations within a minority BCP domain was investigated, as well as the alignment of nanorods in a lamellar-forming BCP, comparing the simulation results with experiments of mixtures of nanoplates and PS-\textit{b}-PMMA BCP. The assembly of rectangular NPs at the interface between domains resulted in alignment along the interface. The aspect ratio is found to play a key role on the aggregation of colloids at the interface, which leads to a distinct co-assembly behaviour for low and high aspect ratio NPs. △ Less

Submitted 25 October, 2019; originally announced October 2019.

Co-assembly of Janus nanoparticles in block copolymer systems

Authors: Javier Diaz, Marco Pinna, Andrei Zvelindovsky, Ignacio Pagonabarraga

Abstract: Block copolymer are ideal matrices to control the localisation of colloids. Furthermore, anisotropic nanoparticles such as Janus nanoparticles possess an additional orientational degree of freedom that can play a crucial role in the formation of highly ordered materials made of block copolymers. This work presents a mesoscopic simulation method to assert the co-assembly of Janus nanoparticles in a… ▽ More Block copolymer are ideal matrices to control the localisation of colloids. Furthermore, anisotropic nanoparticles such as Janus nanoparticles possess an additional orientational degree of freedom that can play a crucial role in the formation of highly ordered materials made of block copolymers. This work presents a mesoscopic simulation method to assert the co-assembly of Janus nanoparticles in a block copolymer mixture, finding numerous instances of aggregation and formation of ordered configurations which can be related to dispersions of pure Janus colloids. Comparison with chemically homogeneous neutral nanoparticles determines that Janus nanoparticles are less prone to induce bridging along lamellar domains, thus being a less destructive way to segregate nanoparticles at interfaces. The combination of asymmetric block copolymer and asymmetric Janus nanoparticles can result in assembly of colloids in an even number of layers within one of the block domains. △ Less

Submitted 12 August, 2019; originally announced August 2019.

Comments: Full Paper

Journal ref: Soft Matter, 2019,15, 6400-6410

Chiral asymmetry detected in a 2D array of permalloy square nanomagnets using circularly polarized X ray Resonant Magnetic Scattering

Authors: J. Díaz, P. Gargiani, C. Quirós, C. Redondo, R. Morales, L. M. Álvarez-Prado, J. I. Martín, A. Scholl, S. Ferrer, M. Vélez, S. M. Valvidares

Abstract: The sensitivity of Circularly polarized X ray Resonant Magnetic Scattering (CXRMS) to chiral asymmetry has been demonstrated. The study was performed on a 2D array of Permalloy (Py) square nanomagnets of 700 nm lateral size arranged in a chess lattice of 1000 nm lattice parameter. Previous X ray Magnetic Circular Dichroism Photoemission Electron microscopy (XMCD-PEEM) images on this sample showed… ▽ More The sensitivity of Circularly polarized X ray Resonant Magnetic Scattering (CXRMS) to chiral asymmetry has been demonstrated. The study was performed on a 2D array of Permalloy (Py) square nanomagnets of 700 nm lateral size arranged in a chess lattice of 1000 nm lattice parameter. Previous X ray Magnetic Circular Dichroism Photoemission Electron microscopy (XMCD-PEEM) images on this sample showed the formation of vortices at remanence and a preference in their chiral state. The magnetic hysteresis loops of the array along the diagonal axis of the squares indicate a non-negligible and anisotropic interaction between vortices. The intensity of the magnetic scattering using circularly polarized light along one of the diagonal axes of the square magnets becomes asymmetric in intensity in the direction transversal to the incident plane at fields where the vortex states are formed. The asymmetry sign is inverted when the direction of the applied magnetic field is inverted. The result is the expected in the presence of an unbalanced chiral distribution. The effect is observed by CXRMS due to the interference between the charge scattering and the magnetic scattering. △ Less

Submitted 16 September, 2019; v1 submitted 3 July, 2019; originally announced July 2019.

Comments: The new revised version corrected some grammatical and orthographic mistakes like "cero" instead of zero and "core vortex" instead of vortex core. Bibliographic entries were modified to have all the same format

Cell Dynamic Simulations of Diblock Copolymer/Colloid Systems

Authors: Javier Diaz, Marco Pinna, Andrei Zvelindovsky, Adelchi Asta, Ignacio Pagonabarraga

Abstract: The presence of nanoparticles in a diblock copolymer leads to changes in the morphology and properties of the matrix and can produce highly organized hybrid materials. The resulting material properties depend not only on the polymer composition but also on the size, shape, and surface properties of the colloids. The dynamics of this kind of systems using a hybrid mesoscopic approach has been studi… ▽ More The presence of nanoparticles in a diblock copolymer leads to changes in the morphology and properties of the matrix and can produce highly organized hybrid materials. The resulting material properties depend not only on the polymer composition but also on the size, shape, and surface properties of the colloids. The dynamics of this kind of systems using a hybrid mesoscopic approach has been studied in this work. A continuum description for the polymer is used, while colloids are individually resolved. The method allows for a variable preference of the colloids, which can have different sizes, to the different components the block copolymer is made of. The impact that the nanoparticle preference for either, both, or none of the blocks has on the collective properties of nanoparticle-block copolymer composites can be analyzed. Several experimental results are reproduced covering colloid-induced phase transition, particles' placement within the matrix, and the role of incompatibilities between colloids and monomers. △ Less

Submitted 29 May, 2018; originally announced May 2018.

Comments: Article

Journal ref: Diaz, J., Pinna, M., Zvelindovsky, A.V., Asta, A. and Pagonabarraga, I., 2017. Cell dynamic simulations of diblock copolymer/colloid systems. Macromolecular Theory and Simulations, 26(1)

Disordered $λ\varphi^{4}+ρ\varphi^{6}$ Landau-Ginzburg model

Authors: R. Acosta Diaz, G. Krein, N. F. Svaiter, C. A. D. Zarro

Abstract: We discuss a disordered $λ\varphi^{4}+ρ\varphi^{6}$ Landau-Ginzburg model defined in a d-dimensional space. First we adopt the standard procedure of averaging the disorder dependent free energy of the model. The dominant contribution to this quantity is represented by a series of the replica partition functions of the system. Next, using the replica symmetry ansatz in the saddle-point equations, w… ▽ More We discuss a disordered $λ\varphi^{4}+ρ\varphi^{6}$ Landau-Ginzburg model defined in a d-dimensional space. First we adopt the standard procedure of averaging the disorder dependent free energy of the model. The dominant contribution to this quantity is represented by a series of the replica partition functions of the system. Next, using the replica symmetry ansatz in the saddle-point equations, we prove that the average free energy represents a system with multiple ground states with different order parameters. For low temperatures we show the presence of metastable equilibrium states for some replica fields for a range of values of the physical parameters. Finally, going beyond the mean-field approximation, the one-loop renormalization of this model is performed, in the leading order replica partition function. △ Less

Submitted 21 December, 2017; originally announced December 2017.

Comments: 10 pages

Journal ref: Phys. Rev. D 97, 065017 (2018)

Compensation temperature in spin-$1/2$ Ising trilayers: A Monte Carlo study

Authors: Ian Jordy Lopez Diaz, Nilton Silva Branco

Abstract: We study the magnetic and thermodynamic properties of a spin-$1/2$ Ising system containing three layers, each of which is composed exclusively of one out of two possible types of atoms, \textbf{A} or \textbf{B}. The \textbf{A-A} and \textbf{B-B} bonds are ferromagnetic while the \textbf{A-B} bonds are antiferromagnetic. The study is performed through Monte Carlo simulations using the Wolff algorit… ▽ More We study the magnetic and thermodynamic properties of a spin-$1/2$ Ising system containing three layers, each of which is composed exclusively of one out of two possible types of atoms, \textbf{A} or \textbf{B}. The \textbf{A-A} and \textbf{B-B} bonds are ferromagnetic while the \textbf{A-B} bonds are antiferromagnetic. The study is performed through Monte Carlo simulations using the Wolff algorithm and the data are analyzed with the aid of the multiple-histogram reweighting technique and finite-size scaling tools. We verify the occurrence of a compensation phenomenon and obtain the compensation and critical temperatures of the model as functions of the Hamiltonian parameters. The influence of each parameter on the overall behavior of the system is discussed in detail and we present our results in the form of phase diagrams dividing the parameter space in regions where the compensation phenomenon is present or absent. Our results may provide invaluable information for experimentalists seeking to build materials with desired characteristics. △ Less

Submitted 2 October, 2019; v1 submitted 27 November, 2017; originally announced November 2017.

Comments: 8 pages of text; 20 figures. arXiv admin note: substantial text overlap with arXiv:1705.10192, arXiv:1710.10298

Bubble nucleation in disordered Landau-Ginzburg model

Authors: R. Acosta Diaz, N. F. Svaiter, C. A. D. Zarro

Abstract: In this paper we investigate bubble nucleation in a disordered Landau-Ginzburg model. First we adopt the standard procedure to average over the disordered free energy. This quantity is represented as a series of the replica partition functions of the system. Using the saddle-point equations in each replica partition function, we discuss the presence of a spontaneous symmetry breaking mechanism. Th… ▽ More In this paper we investigate bubble nucleation in a disordered Landau-Ginzburg model. First we adopt the standard procedure to average over the disordered free energy. This quantity is represented as a series of the replica partition functions of the system. Using the saddle-point equations in each replica partition function, we discuss the presence of a spontaneous symmetry breaking mechanism. The leading term of the series is given by a large-N Euclidean replica field theory. Next, we consider finite temperature effects. Below some critical temperature, there are N real instantons-like solutions in the model. The transition from the false to the true vacuum for each replica field is given by the nucleation of a bubble of the true vacuum. In order to describe these irreversible processes of multiple nucleation, going beyond the diluted instanton approximation, an effective model is constructed, with one single mode of a bosonic field interacting with a reservoir of N identical two-level systems. △ Less

Submitted 9 November, 2017; originally announced November 2017.

Ferrimagnetism and compensation temperature in spin-$1/2$ Ising trilayers

Authors: I J L Diaz, N S Branco

Abstract: The mean-field and effective-field approximations are applied in the study of magnetic and thermodynamic properties of a spin-$1/2$ Ising system containing three layers, each of which is composed exclusively of one out of two possible types of atoms, \textbf{A} or \textbf{B}. The \textbf{A-A} and \textbf{B-B} bonds are ferromagnetic while the \textbf{A-B} bonds are antiferromagnetic. The occurrenc… ▽ More The mean-field and effective-field approximations are applied in the study of magnetic and thermodynamic properties of a spin-$1/2$ Ising system containing three layers, each of which is composed exclusively of one out of two possible types of atoms, \textbf{A} or \textbf{B}. The \textbf{A-A} and \textbf{B-B} bonds are ferromagnetic while the \textbf{A-B} bonds are antiferromagnetic. The occurrence of a compensation phenomenon is verified and the compensation and critical temperatures are obtained as functions of the Hamiltonian parameters. We present phase diagrams dividing the parameter space in regions where the compensation phenomenon is present or absent and a detailed discussion about the influence of each parameter on the overall behavior of the system is made. △ Less

Submitted 27 October, 2017; originally announced October 2017.

Influence of substrate on structural and transport properties of LaNiO3 thin films prepared by pulsed laser deposition

Authors: L. Cichetto Jr, S. Sergeenkov, J. C. C. A. Diaz, E. Longo, F. M. Araujo-Moreira

Abstract: We report the structural and transport properties of LaNiO3 thin films prepared by pulsed laser deposition technique. To understand the effects of film thickness, lattice mismatch and grain size on transport properties, various oriented substrates were used for deposition, including single-crystalline SrLaAlO4 (001), SrTiO3 (100) and LaAlO3 (100). To achieve a high quality LaNiO3 thin films, the v… ▽ More We report the structural and transport properties of LaNiO3 thin films prepared by pulsed laser deposition technique. To understand the effects of film thickness, lattice mismatch and grain size on transport properties, various oriented substrates were used for deposition, including single-crystalline SrLaAlO4 (001), SrTiO3 (100) and LaAlO3 (100). To achieve a high quality LaNiO3 thin films, the vital parameters (such as laser fluence, substrate temperature, oxygen pressure, and deposition time) were optimized. The best quality films are found to be well textured samples with good crystalline properties. △ Less

Submitted 28 August, 2017; originally announced August 2017.

Journal ref: AIP Advances 7, 025005 (2017)

Monte Carlo study of an anisotropic Ising multilayer with antiferromagnetic interlayer couplings

Authors: Ian Jordy Lopez Diaz, Nilton da Silva Branco

Abstract: We present a Monte Carlo study of the magnetic properties of an Ising multilayer ferrimagnet. The system consists of two kinds of non-equivalent planes, one of which is site-diluted. All intralayer couplings are ferromagnetic. The different kinds of planes are stacked alternately and the interlayer couplings are antiferromagnetic. We perform the simulations using the Wolff algorithm and employ mul… ▽ More We present a Monte Carlo study of the magnetic properties of an Ising multilayer ferrimagnet. The system consists of two kinds of non-equivalent planes, one of which is site-diluted. All intralayer couplings are ferromagnetic. The different kinds of planes are stacked alternately and the interlayer couplings are antiferromagnetic. We perform the simulations using the Wolff algorithm and employ multiple histogram reweighting and finite-size scaling methods to analyze the data with special emphasis on the study of compensation phenomena. Compensation and critical temperatures of the system are obtained as functions of the Hamiltonian parameters and we present a detailed discussion about the contribution of each parameter to the presence or absence of the compensation effect. A comparison is presented between our results and those reported in the literature for the same model using the pair approximation. We also compare our results with those obtained through both the pair approximation and Monte Carlo simulations for the bilayer system. △ Less

Submitted 8 September, 2017; v1 submitted 25 May, 2017; originally announced May 2017.

Comments: 13 figures. arXiv admin note: text overlap with arXiv:1607.05221

Directed Polymers and Interfaces in Disordered Media

Authors: Róbinson J. Acosta Diaz, Christian D. Rodríguez-Camargo, Nami F. Svaiter

Abstract: We consider field theory formulation for directed polymers and interfaces in the presence of quenched disorder. We write a series representation for the averaged free energy, where all the integer moments of the partition function of the model contribute. The structure of field space is analysed for polymers and interfaces at finite temperature using the saddle-point equations derived from each in… ▽ More We consider field theory formulation for directed polymers and interfaces in the presence of quenched disorder. We write a series representation for the averaged free energy, where all the integer moments of the partition function of the model contribute. The structure of field space is analysed for polymers and interfaces at finite temperature using the saddle-point equations derived from each integer moments of the partition function. For the case of an interface we obtain the wandering exponent $ξ= (4-d)/2$, also obtained by the conventional replica method for the replica symmetric scenario. △ Less

Submitted 7 May, 2020; v1 submitted 22 September, 2016; originally announced September 2016.

Comments: 17 pages. arXiv admin note: text overlap with arXiv:1603.05919

Journal ref: Polymers 2020, 12(5), 1066

Monte Carlo simulations of an Ising bilayer with non-equivalent planes

Authors: Ian Jordy Lopez Diaz, Nilton da Silva Branco

Abstract: We study the thermodynamic and magnetic properties of an Ising bilayer ferrimagnet. The system is composed of two interacting non-equivalent planes in which the intralayer couplings are ferromagnetic while the interlayer interactions are antiferromagnetic. Moreover, one of the planes is randomly diluted. The study is carried out within a Monte Carlo approach employing the multiple histogram reweig… ▽ More We study the thermodynamic and magnetic properties of an Ising bilayer ferrimagnet. The system is composed of two interacting non-equivalent planes in which the intralayer couplings are ferromagnetic while the interlayer interactions are antiferromagnetic. Moreover, one of the planes is randomly diluted. The study is carried out within a Monte Carlo approach employing the multiple histogram reweighting method and finite-size scaling tools. The occurrence of a compensation phenomenon is verified and the compensation temperature, as well as the critical temperature for the model, are obtained as functions of the Hamiltonian parameters. We present a detailed discussion of the regions of the parameter space where the compensation effect is present or absent. Our results are then compared to a mean-field-like approximation applied to the same model by Balcerzak and Szałowski (2014). Although the Monte Carlo and mean-field results agree qualitatively, our quantitative results are significantly different. △ Less

Submitted 13 September, 2016; v1 submitted 18 July, 2016; originally announced July 2016.

Finite-Size Effects in Disordered $λφ^{4}$ Model

Authors: R. Acosta Diaz, N. F. Svaiter

Abstract: We discuss finite-size effects in one disordered $λφ^{4}$ model defined in a $d$-dimensional Euclidean space. We consider that the scalar field satisfies periodic boundary conditions in one dimension and it is coupled with a quenched random field. In order to obtain the average value of the free energy of the system we use the replica method. We first discuss finite-size effects in the one-loop ap… ▽ More We discuss finite-size effects in one disordered $λφ^{4}$ model defined in a $d$-dimensional Euclidean space. We consider that the scalar field satisfies periodic boundary conditions in one dimension and it is coupled with a quenched random field. In order to obtain the average value of the free energy of the system we use the replica method. We first discuss finite-size effects in the one-loop approximation in $d=3$ and $d=4$. We show that in both cases there is a critical length where the system develop a second-order phase transition, when the system presents long-range correlations with power-law decay. Next, we improve the above result studying the gap equation for the size- dependent squared mass, using the composite field operator method. We obtain again, that the system present a second order phase transition with long-range correlation with power-law decay. △ Less

Submitted 19 February, 2016; v1 submitted 7 October, 2015; originally announced October 2015.

Comments: 19 pages, 4 figures

Maximum entropy method: sampling bias

Authors: Jorge Fernandez-de-Cossio, Jorge Fernandez-de-Cossio Diaz

Abstract: Maximum entropy method is a constructive criterion for setting up a probability distribution maximally non-committal to missing information on the basis of partial knowledge, usually stated as constrains on expectation values of some functions. In connection with experiments sample average of those functions are used as surrogate of the expectation values. We address sampling bias in maximum entro… ▽ More Maximum entropy method is a constructive criterion for setting up a probability distribution maximally non-committal to missing information on the basis of partial knowledge, usually stated as constrains on expectation values of some functions. In connection with experiments sample average of those functions are used as surrogate of the expectation values. We address sampling bias in maximum entropy approaches with finite data sets without forcedly equating expectation values to corresponding experimental average values. Though we rise the approach in a general formulation, the equations are unfortunately complicated. We bring simple case examples, hopping clear but sufficient illustration of the concepts. △ Less

Submitted 16 July, 2015; originally announced July 2015.

Graphene Based Plasmonic Tunable Low Pass Filters in the THz Band

Authors: D. Correas Serrano, J. S. Gomez Diaz, J. Perruisseau-Carrier, A. Alvarez Melcon

Abstract: We propose the concept, synthesis, analysis, and design of graphene-based plasmonic tunable low-pass filters operating in the THz band. The proposed structure is composed of a graphene strip transferred onto a dielectric and a set of polysilicon DC gating pads located beneath it. This structure implements a stepped impedance low-pass filter for the propagating surface plasmons by adequately contro… ▽ More We propose the concept, synthesis, analysis, and design of graphene-based plasmonic tunable low-pass filters operating in the THz band. The proposed structure is composed of a graphene strip transferred onto a dielectric and a set of polysilicon DC gating pads located beneath it. This structure implements a stepped impedance low-pass filter for the propagating surface plasmons by adequately controlling the guiding properties of each strip section through graphene's field effect. A synthesis procedure is presented to design filters with desired specifications in terms of cut-off frequency, in-band performance, and rejection characteristics. The electromagnetic modeling of the structure is efficiently performed by combining an electrostatic scaling law to compute the guiding features of each strip section with a transmission line and transfer-matrix framework, approach further validated via full wave simulations. The performance of the proposed filters is evaluated in practical scenarios, taking into account the presence of the gating bias and the influence of graphene's losses. These results, together with the high miniaturization associated with plasmonic propagation, are very promising for the future use and integration of the proposed filters with other graphene and silicon-based elements in innovative THz communication systems. △ Less

Submitted 25 October, 2014; v1 submitted 23 April, 2013; originally announced April 2013.

Comments: 14 pages, 15 figures, 4 tables, IEEE Transactions on Nanotechnology, 2014

Reconfigurable THz Plasmonic Antenna Concept Using a Graphene Stack

Authors: Michele Tamagnone, Juan Sebastian Gomez Diaz, Juan Ramon Mosig, Julien Perruisseau-Carrier

Abstract: The concept and analysis of a Terahertz (THz) frequency-reconfigurable antenna using graphene are presented. The antenna exploits dipole-like plasmonic resonances that can be frequency-tuned on large range via the electric field effect in a graphene stack. In addition to efficient dynamic control, the proposed approach allows high miniaturization and good direct matching with continuous wave THz s… ▽ More The concept and analysis of a Terahertz (THz) frequency-reconfigurable antenna using graphene are presented. The antenna exploits dipole-like plasmonic resonances that can be frequency-tuned on large range via the electric field effect in a graphene stack. In addition to efficient dynamic control, the proposed approach allows high miniaturization and good direct matching with continuous wave THz sources. A qualitative model is used to explain the excellent impedance stability under reconfiguration. These initial results are very promising for future all-graphene THz transceivers and sensors. Keywords: Reconfigurable antenna, Graphene, Plasmons, Terahertz, frequency-tuning. △ Less

Submitted 6 February, 2014; v1 submitted 30 October, 2012; originally announced October 2012.

Comments: 4 pages, 7 Figures, Equation (7) modified in version 2. Published in Applied Physics Letters

Journal ref: Appl. Phys. Lett. 101 , 214102 (2012)

Optimally designed quantum transport across disordered networks

Authors: Mattia Walschaers, Jorge Fernandez-de-Cossio Diaz, Roberto Mulet, Andreas Buchleitner

Abstract: We establish a general mechanism for highly efficient quantum transport through finite, disordered 3D networks. It relies on the interplay of disorder with centro-symmetry and a dominant doublet spectral structure, and can be controlled by proper tuning of only coarse-grained quantities. Photosynthetic light harvesting complexes are discussed as potential biological incarnations of this design pri… ▽ More We establish a general mechanism for highly efficient quantum transport through finite, disordered 3D networks. It relies on the interplay of disorder with centro-symmetry and a dominant doublet spectral structure, and can be controlled by proper tuning of only coarse-grained quantities. Photosynthetic light harvesting complexes are discussed as potential biological incarnations of this design principle. △ Less

Submitted 27 June, 2013; v1 submitted 17 July, 2012; originally announced July 2012.

Comments: 7 pages (incl. 2 pages of suppl. mat.), 3 figures

Journal ref: Phys. Rev. Lett. 111, 180601 (2013)

Magnetic properties and critical behavior of disordered Fe_{1-x}Ru_x alloys: a Monte Carlo approach

Authors: I. J. L. Diaz, N. S. Branco

Abstract: We study the critical behavior of a quenched random-exchange Ising model with competing interactions on a bcc lattice. This model was introduced in the study of the magnetic behavior of Fe_{1-x}Ru_x alloys for ruthenium concentrations x=0%, x=4%, x=6%, and x=8%. Our study is carried out within a Monte Carlo approach, with the aid of a re-weighting multiple histogram technique. By means of a finite… ▽ More We study the critical behavior of a quenched random-exchange Ising model with competing interactions on a bcc lattice. This model was introduced in the study of the magnetic behavior of Fe_{1-x}Ru_x alloys for ruthenium concentrations x=0%, x=4%, x=6%, and x=8%. Our study is carried out within a Monte Carlo approach, with the aid of a re-weighting multiple histogram technique. By means of a finite-size scaling analysis of several thermodynamic quantities, taking into account up to the leading irrelevant scaling field term, we find estimates of the critical exponents α, β, γ, and ν, and of the critical temperatures of the model. Our results for x=0% are in excellent agreement with those for the three-dimensional pure Ising model in the literature. We also show that our critical exponent estimates for the disordered cases are consistent with those reported for the transition line between paramagnetic and ferromagnetic phases of both randomly dilute and $\pm J$ Ising models. We compare the behavior of the magnetization as a function of temperature with that obtained by Paduani and Branco (2008), qualitatively confirming the mean-field result. However, the comparison of the critical temperatures obtained in this work with experimental measurements suggest that the model (initially obtained in a mean-field approach) needs to be modified. △ Less

Submitted 9 February, 2012; originally announced February 2012.

Beryllium nitride thin film grown by reactive laser ablation

Authors: G. Soto, J. A. Diaz, R. Machorro, A. Reyes-Serrato, W. de la Cruz

Abstract: Beryllium nitride thin films were grown on silicon substrates by laser ablating a beryllium foil in molecular nitrogen ambient. The composition and chemical state were determined with Auger (AES), X-Ray photoelectron (XPS) and energy loss (EELS) spectroscopies. A low absorption coefficient in the visible region, and an optical bandgap of 3.8 eV, determined by reflectance ellipsometry, were obtai… ▽ More Beryllium nitride thin films were grown on silicon substrates by laser ablating a beryllium foil in molecular nitrogen ambient. The composition and chemical state were determined with Auger (AES), X-Ray photoelectron (XPS) and energy loss (EELS) spectroscopies. A low absorption coefficient in the visible region, and an optical bandgap of 3.8 eV, determined by reflectance ellipsometry, were obtained for films grown at nitrogen pressures higher than 25 mTorr. The results show that the reaction of beryllium with nitrogen is very effective using this preparation method, producing high quality films. △ Less

Submitted 23 March, 2001; originally announced March 2001.

Comments: 13 pages, 5 figures. Submitted to Materials Letters