arXiv subjects

Rafael Almeida

Publications and source records attributed to Rafael Almeida.

9 recordsLinked to original sources

arXiv2026

Simulation Design for Velocity-Controlled Spatio-Temporal Drivers in Laser Wakefield Acceleration

Velocity-controlled spatio-temporal (ST) laser drivers offer a route to tailoring laser-plasma interactions by allowing the velocity of the intensity peak to be controlled independently of the envelope group velocity. In this work, we present a simulation-design workflow for PIC modelling of subluminal velocity-controlled ST pulses in OSIRIS based on a Maxwell-consistent spectral construction expressed as a superposition of exact vacuum solutions, and we describe its discrete k-space representation for numerical initialisation. We then examine wakefield excitation with velocity-controlled drivers, showing how the ST geometry couples the effective longitudinal extent of the high-intensity region to the transverse scale and deriving scaling guidelines for near-resonant excitation in the subluminal regime. Finally, we discuss the geometric constraints that make long-distance simulations costly, including focus-envelope slippage and strong transverse expansion, and we show that continuous wall injection can reproduce the intended vacuum propagation while substantially reducing the transverse domain size. Together, these results provide practical guidelines for accurate and computationally efficient PIC simulations of velocity-controlled ST drivers in wakefield-relevant regimes.

physics.plasm-ph↗

arXiv2026

Self-thermometry measurements of the adiabatic temperature change in first-order phase transition magnetocaloric materials

Accurately measuring the magnetocaloric effect is necessary to foster the development of magnetic refrigeration devices. However, current methods are inconvenient, requiring different instruments to measure each individual property or a custom-made setup. By measuring the time-varying magnetization in a commercially available VersaLab\textsuperscript{\textregistered} PPMS\textsuperscript{\textregistered} from Quantum Design, we have determined the adiabatic temperature change ($\Delta$T$_{\textrm{ad}}$) of the first-order phase transition material Gd$_5$Si$_2$Ge$_2$, for a magnetic field change of 0 to 1 T, under high vacuum ($<$ 0.1 mTorr). For each temperature and magnetic field, the equilibrium magnetization is used as the magnetization-to-temperature conversion curve, allowing us to extend the validity of a previously proposed technique to the first-order phase transition material Gd$_5$Si$_2$Ge$_2$, which exhibits significant hysteresis. Our method thus enables full characterization (magnetic entropy change, adiabatic temperature change, and heat capacity) of any magnetocaloric material, whether it has a first-order or a second-order phase transition, using a single instrument. Comparing to a directly measured $\Delta$T$_{\textrm{ad}}$, our method resulted in a peak $\Delta$T$_{\textrm{ad}}$ value of 4.47 K, within 1\% of the directly measured value for a sample of the same composition.

cond-mat.mtrl-sci↗

arXiv2026

Space-time beams with tunable orbital group velocity toward plasma superradiance

Light springs are space-time beams that have a helical wavepacket. Due to this special property, light springs result into a rotating pulse when intercepting a plane lying orthogonal to their propagation direction. Associated to this, we introduce here the orbital group velocity, an additional tunable property of light springs. The orbital group velocity quantifies the speed of the light spring intensity rotation, distinctly from the conventional longitudinal group velocity, which describes the motion of the wavepacket envelope along its propagation axis. We demonstrate experimentally by tunable Fourier synthesis that the orbital group velocity can assume sub- and superluminal values, thus becoming a new platform for synthetic motion studies and control of laser-matter interactions. Particularly, in the superluminal regime, when interacting with a thin overdense plasma, we reveal by particle-in-cell simulations that the light spring unlocks superradiant radiation, due to the coherent excitation of the electrons in the plasma acting as a quasiparticle. This superradiant source inherits the ultrafast temporal dynamics of the light springs while emitting in the terahertz region, thus creating a new source of terahertz radiation controlled by the properties of spatiotemporal coupling of the laser. Therefore, spatiotemporal tuning of light springs is at the frontier of controlling laser-matter interaction and generating new tunable sources of radiation.

physics.optics↗

arXiv2026

Rotating Magnetocaloric Effect in Sintered La(Fe,Mn,Si)$_{13}$H$_z$ Plates

La-Fe-Si-based alloys are among the most application-ready magnetocaloric materials for room-temperature magnetic refrigeration. Powder metallurgy methods have been previously demonstrated to successfully produce structures with sub-mm features for magnetic refrigerators in a scalable method. In this work, we explore the rotating magnetocaloric effect (RMCE) present in a 0.27 mm thin plate of sintered and hydrogenated La(Fe,Mn,Si)$_{13}$. The high aspect ratio ($\sim$50) of the thin plate leads to an anisotropic magnetocaloric effect (MCE), dependent on the relative orientation of the external magnetic field, and an RMCE when the external field is rotated. We find a maximum rotating adiabatic temperature change ($\Delta T_{ad}^{rot}$) of 1.17 K with the rotation of a 1 T magnetic field and 1.12 K when rotating a 0.6 T magnetic field, a reduction of only 4% for a 40% reduction in applied field strength. Magnetostatic computations revealed a considerable rotating isothermal entropy change ($\Delta S_{iso}^{rot}$), comparable to the conventional MCE of Gd for similar fields, reaching 3.97 J K$^{-1}$ kg$^{-1}$ for 1 T and 3.68 J K$^{-1}$ kg$^{-1}$ for 0.6 T (7% reduction), highlighting La-Fe-Mn-Si alloys as high potential candidates for a magnetic refrigerator based on the RMCE utilizing relatively low external magnetic field amplitudes, such as 0.6 T.

cond-mat.mtrl-sci↗

arXiv2026

Rotating Magnetocaloric Effect in First-order Phase Transition Material Gd5Si2Ge2

The rotating magnetocaloric effect (RMCE) induced by self-demagnetization has been investigated in the giant magnetocaloric effect (GMCE) material Gd$_5$Si$_2$Ge$_2$. This shape-dependent effect had thus far only been reported in pure Gd, marking this as the first analysis of the effect in a sample with a magnetostructural first-order phase transition. By rotating the applied magnetic field vector while keeping its intensity constant, the demagnetizing field within a high-aspect ratio sample changes significantly, resulting in a RMCE. We characterize RMCE by determining the adiabatic temperature change ($\Delta T_{ad}^{rot}$) directly through temperature measurements, and the isothermal entropy change ($\Delta S_M^{rot}$) via magnetometry and magnetostatic simulations. We obtain a remarkable maximum $\Delta T_{ad}^{rot}$ of 1.77 K for a constant external field of 0.8 T, higher than that obtained under 1.0 T. The magnetostatic simulations not only corroborate the highly non-monotonous field-dependence of $|\Delta S_{M}^{rot}|$, which reaches 95\% of its maximum value at 0.8 T, 6.12 J K$^{-1}$ kg$^{-1}$ for the experimentally measured shape, but also estimate a 35\% increase in the maximum $|\Delta S_{M}^{rot}|$ up to 8.67 J K$^{-1}$ kg$^{-1}$ in a simulated shape with higher aspect ratio.

cond-mat.mtrl-sci↗

arXiv2025

Plasma Accelerator For Decaying Particles

We introduce a plasma wakefield acceleration scheme capable of boosting initially subrelativistic particles to relativistic velocities within millimeter-scale distances. A subluminal light pulse drives a wake whose velocity is continuously matched to the beam speed through a tailored plasma density, thereby extending the dephasing length. We develop a theoretical model that is generalizable across particle mass, initial velocity, and the particular accelerating bucket being used, and we verify its accuracy with particle-in-cell simulations using laser drivers with energies in the Joule range.

physics.plasm-ph↗

arXiv2018

A numerical calculation of the electronic specific heat for the compound Sr$_2$RuO$_4$ below its superconducting transition temperature

In this work, a numerical study of the superconducting specific heat of the unconventional multiband superconductor Strontium Ruthenate is performed. Two band gaps models are employed, and the results rendered for each of them are compared. One of the models, previously proposed by one of the authors to explain the experimental temperature behavior of the ultrasound attenuation, considers two gaps with point nodes of different magnitude on different gap surface sheets, while the other one is an isotropic and line node model, reported in the literature for describing quantitatively experimental specific heat data. The superconducting density of states DOS is computed by employing these two models and then, a detailed numerical study of the electronic specific heat, that includes the contribution from the different Fermi sheets, is carried out. It is found that the calculated point node model specific heat temperature behavior shows an excellent agreement with the existent Sr$_2$RuO$_4$ experimental data at zero field, particularly, it is obtained that the observed specific heat jump at T$_c$ is precisely reproduced. Also, it is found that the sum of the contributions from the different bands fits quantitatively the measured specific heat data. The results in this work evidence that the Sr$_2$RuO$_4$ superconducting states are of unconventional nature, corresponding to those of a point node superconductor, and show the importance of taking into account the multiband nature of the material when calculating thermodynamic superconducting quantities.

cond-mat.supr-con↗

arXiv2018

Symmetry Field Breaking Effects in Sr$_2$RuO$_4$

In this work, after reviewing the theory of the elastic properties of Strontium ruthenate, an extension suitable to explain the sound speed experiments of Lupien et. al. \protect\cite{lup2} and Clifford et. al. \protect\cite{clif1} is carried out. It is found that the discontinuity in the elastic constant C$_{66}$ gives unambiguous experimental evidence that the \sr superconducting order parameter $Ψ$ has two components and shows a broken time-reversal symmetry state. A detailed study of the elastic behavior is performed by means of a phenomenological theory employing the Ginzburg-Landau formalism.

cond-mat.supr-con↗

arXiv2018

On the geometric and magnetic properties of the monomer, dimer and trimer of NiFe2O4

In this work, by employing Density Functional Theory, we compute and discuss some geometric and magnetic properties of the monomer, dimer and trimer of NiFe2 O4 . The calculations are performed at the UDFT/ B3LYP level of calculation, by employing the LANL2DZ effective pseudo potential. The results of the Mulliken spin densities and the spin polarization will be presented. Finally the outcome of the system density of states is considered.

cond-mat.mtrl-sci↗