arXiv subjects

E. Santos

Publications and source records attributed to E. Santos.

At least 19 recordsLinked to original sources

arXiv2026

Interfacial Spin-to-Charge Conversion in Sputtered MoTe2 Heterostructures Probed by Spin Pumping and Spin-Torque Ferromagnetic Resonance

Transition metal dichalcogenides (TMDs) and their Weyl semimetal phases, such as MoTe$_2$, have attracted significant attention for spin-orbit torque applications due to their efficient charge-to-spin conversion. However, whether this conversion originates predominantly from the bulk or the interface remains unclear. Here, we investigate spin-charge interconversion in MoTe$_2$ using spin-pumping and spin-torque ferromagnetic resonance (SP-FMR and ST-FMR). Thickness-dependent measurements reveal large spin-to-charge conversion and spin-torque efficiencies that are essentially independent of MoTe$_2$ thickness, indicating that the conversion is predominantly governed by the Rashba-Edelstein effect at the Py/MoTe$_2$ interface rather than by bulk spin transport. This behavior contrasts with the characteristic thickness dependence observed in Pt heterostructures and is further supported by bidirectional SP-FMR and interface-separation measurements. Our results highlight the dominant role of the Py/MoTe$_2$ interface in enabling efficient spin-charge conversion and spin-orbit torques in TMD-based spintronic devices. These findings highlight the potential of sputtered MoTe$_2$/Py heterostructures for low-power spintronic applications, including magnetic memory and logic devices.

cond-mat.mes-hall↗

arXiv2026

Phase-selective orbital-charge conversion in $\mathrm{MoTe_2}$

Two-dimensional transition metal dichalcogenides (TMDs) have emerged as promising materials for spin--orbitronics owing to their strong spin--orbit coupling and rich electronic phases. However, their orbital transport properties remain largely unexplored. Here, we demonstrate that the orbitronic response of $\mathrm{MoTe_2}$ is governed by a thickness-driven structural phase transition. RF-sputtered $\mathrm{MoTe_2}$ thin films exhibit a crossover at a critical thickness of approximately $4.5\,\mathrm{nm}$, stabilizing in the metallic $1T^\prime$ phase below this threshold and in the semiconducting $2H$ phase above it. Raman spectroscopy and scanning tunneling spectroscopy (STS) confirm the structural and electronic transition, revealing gapless behavior in ultrathin films and a finite band gap in thicker samples. Spin-pumping measurements detect an additional transverse charge-conversion signal exclusively in metallic $1T^\prime$-$\mathrm{MoTe_2}$, in agreement with first-principles calculations that identify a dominant orbital Rashba--Edelstein response as the underlying conversion mechanism.

cond-mat.mtrl-sci↗

arXiv2026

Investigating spin and orbital effects via spin-torque ferromagnetic resonance

In this work, we experimentally investigate spin and orbital torque phenomena using the spin-torque ferromagnetic resonance (ST-FMR) technique in a series of bilayer systems composed of different normal metal (NM) materials. Permalloy (Py) and Ni were employed as ferromagnetic (FM) layers to probe the spin and orbital torque responses, respectively. For the SiO$_2$/FM/NM bilayers, we extracted the damping-like and field-like torque components, as well as the damping-like torque efficiency for each sample, and compared our results with previously reported numerical and experimental data in the literature. Additionally, we experimentally demonstrate the presence of an out-of-plane torque component, which we attribute to interfacial mechanisms and associate with a spin-orbital polarized current along the $z$-direction. This interpretation is supported by the azimuthal angular dependence of the applied magnetic field. Our results provide compelling evidence of orbital torque associated with the orbital Hall effect (OHE) in several materials, thereby broadening the prospects for magnetization switching driven by orbital torque.

cond-mat.mes-hall↗

arXiv2025

Probing orbital currents through inverse orbital Hall and Rashba effects

We report a comprehensive experimental investigation of orbital-to-charge conversion in metallic and semiconductor materials, emphasizing the fundamental roles of the inverse orbital Hall effect (IOHE) and the inverse orbital Rashba effect. Using spin pumping driven by ferromagnetic resonance (SP-FMR) and the spin Seebeck effect (SSE), we demonstrate efficient orbital current generation and detection in YIG/Pt/NM structures, where NM is either a metal or a semiconductor. A central finding is the dominance of orbital contributions over spin-related effects, even in systems with weak spin-orbit coupling. In particular, a large enhancement of the SP-FMR and SSE signals is observed in the presence of naturally oxidized Cu in different heterostructures. Furthermore, we identify positive and negative IOHE signals in Ti and Ge, respectively, and extract orbital diffusion lengths in both systems using a diffusive model. Our results confirm the presence of orbital transport and offer valuable insights that may guide the further development of orbitronics.

cond-mat.mes-hall↗

arXiv2025

Anisotropy-Driven Anomalous Inverse Orbital Hall Effect in Fe Films

This study investigates the anomalous orbital effects in iron (Fe) films with strong uniaxial anisotropy, highlighting the interactions between spin and orbital currents. Using heterogeneous YIG/Fe and YIG/Pt/Fe structures, fabricated by oblique deposition in a magnetic field, spin pumping ferromagnetic resonance (SP-FMR) measurements were performed. It was observed that the uniaxial anisotropy enables the emergence of spin-to-charge (AISHE) and orbital-to-charge (AIOHE) conversion signals in out-of-plane configurations, where the spin polarization is parallel to the direction of the spin current. Experimental analysis revealed that orbital dynamics, mediated by orbital Hall conductivity, are more prominent in Fe films due to the low spin-orbit interaction (SOC) and high orbital response. These findings provide fundamental insights for the advancement of orbitronics devices, indicating the potential for controlling orbital and spin currents through magnetic and anisotropic parameters.

cond-mat.mes-hall↗

arXiv2025

Revealing the Dominance of the Orbital Hall Effect over Spin in Transition Metal Heterostructures

We study inverse spin and orbital Hall effects in 19 transition metals using spin-pumping driven by ferromagnetic resonance. Spin-to-charge conversion was measured in YIG/X(5), while orbital-to-charge conversion was probed in YIG/Pt(2)/X(5) heterostructures. Here, X represents the different transition metals. Surprisingly, the orbital contribution overwhelmingly dominates over the spin response, clarifying the challenge of disentangling these effects. Our results largely agree with first-principles predictions for spin and orbital Hall conductivities but reveal discrepancies in select materials. These findings emphasize the fundamental role of the orbital Hall effect, and position orbitronics as a pivotal frontier in condensed matter physics.

cond-mat.mes-hall↗

arXiv2024

Auger Open Data and the Pierre Auger Observatory International Masterclasses

The Pierre Auger Observatory has a public data policy following the FAIR principles (Findable, Accessible, Interoperable, and Reusable). We aim to share the data with the scientific community as part of the multi-messenger effort at different levels and for educational activities to engage the general public. Following the first portal created in 2007, a new portal hosted at \url{https://opendata.auger.org} was established in February 2021. The portal is regularly updated and comprises 10\% of the recorded cosmic-ray data organized in various datasets, each with a specific DOI provided by Zenodo. Moreover, a catalog with the 100 most energetic events is available. The portal adopts a ``dual'' concept, offering not only the download of public data but also a series of Jupyter notebooks. These notebooks allow the general public to reproduce some of the most important results obtained by the Pierre Auger Collaboration and understand the main mechanisms governing the development of the extensive air showers produced by the interaction of cosmic rays in the Earth's atmosphere. In 2023, the Pierre Auger Observatory joined the International Particle Physics Outreach Group (IPPOG). The successful debut enrolled 550 high-school students at 12 research institutions from 5 countries and was repeated this year, embracing yet more students and countries worldwide. During this day, the participants attend seminars about cosmic rays and are asked to reconstruct subsets of public data events using an Auger 3-D event display. Finally, they participate in a Zoom session with scientists at the Auger site.

astro-ph.IM↗

arXiv2024

Anomalous Spin and Orbital Hall Phenomena in Antiferromagnetic Systems

We investigate anomalous spin and orbital Hall phenomena in antiferromagnetic (AF) materials via orbital pumping experiments. Conducting spin and orbital pumping experiments on YIG/Pt/Ir20Mn80 heterostructures, we unexpectedly observe strong spin and orbital anomalous signals in an out-of-plane configuration. We report a sevenfold increase in the signal of the anomalous inverse orbital Hall effect (AIOHE) compared to conventional effects. Our study suggests expanding the Orbital Hall angle ({\theta}_OH) to a rank 3 tensor, akin to the Spin Hall angle ({\theta}_SH), to explain AIOHE. This work pioneers converting spin-orbital currents into charge current, advancing the spin-orbitronics domain in AF materials.

cond-mat.mes-hall↗

arXiv2024

Negative orbital Hall effect in Germanium

Our investigation reveals a groundbreaking discovery of a negative inverse orbital Hall effect (IOHE) in Ge thin films. We employed the innovative orbital pumping technique where spin-orbital coupled current is injected into Ge films using YIG/Pt(2)/Ge($t_{Ge}$) and YIG/W(2)/Ge($t_{Ge}$) heterostructures. Through comprehensive analysis, we observe significant reductions in the signals generated by coherent (RF-driven) and incoherent (thermal-driven) spin-orbital pumping techniques. These reductions are attributed to the presence of a remarkable strong negative IOHE in Ge, showing its magnitude comparable to the spin-to-charge signal in Pt. Our findings reveal that although the spin-to-charge conversion in Ge is negligible, the orbital-to-charge conversion exhibits large magnitude. Our results are innovative and pioneering in the investigation of negative IOHE by the injection of spin-orbital currents.

cond-mat.mes-hall↗

arXiv2024

Exploring orbital-charge conversion mediated by interfaces with copper through spin-orbital pumping

We investigated how different materials affect the orbital-charge conversion in heterostructures with the naturally oxidized cooper capping layer. When we added a thin layer of $CuOx(3nm)$ onto yttrium iron garnet $(YIG)/W$ stacks, we observed a significant reduction in the charge current signal measured by means the spin pumping effect technique. This finding contrasts with the results of a prior study conducted on YIG/Pt/CuOx, which reported the opposite effect. On the other hand, when we added the same $CuOx(3nm)$ layer to $YIG/Ti(4nm)$ structures, there was not much change in the spin pumping signal. This occurred because Ti does not generate much orbital current at the $Ti/CuOx$ interface, unlike Pt, due to its weaker spin-orbit coupling. Interestingly, when we added the $CuOx(3nm)$ layer to $SiO_{2}/Py(5nm)/Pt(4nm)$ structures, the spin pumping signal increased. However, in $SiO_{2}/CuOx(3nm)/Pt(4nm)/Py(5nm)$ structures, the signal decreased. Finally, we delve into a theoretical analysis of the spin (orbital) Hall effect in YIG/Heavy-metal systems. These findings have the potential to advance research in the innovative field of orbitronics and contribute to the development of new technologies based on spin-orbital conversion.

cond-mat.mes-hall↗

arXiv2023

Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers

The present white paper is submitted as part of the "Snowmass" process to help inform the long-term plans of the United States Department of Energy and the National Science Foundation for high-energy physics. It summarizes the science questions driving the Ultra-High-Energy Cosmic-Ray (UHECR) community and provides recommendations on the strategy to answer them in the next two decades.

astro-ph.HE↗

arXiv2022

Inverse Orbital Torque via Spin-Orbital Entangled States

While current-induced torque by orbital current has been experimentally found in various structures, evidence for its reciprocity has been missing so far. Here, we report experimental evidence of strong inverse orbital torque in YIG/Pt/CuOx (YIG = Y3Fe5O12) mediated by spin-orbital entangled electronic states in Pt. By injecting spin current from YIG to Pt by the spin pumping via ferromagnetic resonance and by the spin Seebeck effect, we find a pronounced inverse spin Hall effect-like signal. While a part of the signal is explained as due to the inverse spin-orbital Hall effect in Pt, we also find substantial increase of the signal in YIG/Pt/CuOx structures compared to the signal in YIG/Pt. We attribute this to the inverse orbital Rashba-Edelstein effect at Pt/CuOx interface mediated by the spin-orbital entangled states in Pt. Our work paves the way toward understanding of spin-orbital entangled physics in nonequilibrium and provides a way for electrical detection of the orbital current in orbitronic device applications.

cond-mat.mes-hall↗

arXiv2020

Magnon-mediated spin currents in Tm3Fe5O12/Pt with perpendicular magnetic anisotropy

The control of pure spin currents carried by magnons in magnetic insulator (MI) garnet films with a robust perpendicular magnetic anisotropy (PMA) is of great interest to spintronic technology as they can be used to carry, transport and process information. Garnet films with PMA present labyrinth domain magnetic structures that enrich the magnetization dynamics, and could be employed in more efficient wave-based logic and memory computing devices. In MI/NM bilayers, where NM being a normal metal providing a strong spin-orbit coupling, the PMA benefits the spin-orbit torque (SOT) driven magnetization's switching by lowering the needed current and rendering the process faster, crucial for developing magnetic random-access memories (SOT-MRAM). In this work, we investigated the magnetic anisotropies in thulium iron garnet (TIG) films with PMA via ferromagnetic resonance measurements, followed by the excitation and detection of magnon-mediated pure spin currents in TIG/Pt driven by microwaves and heat currents. TIG films presented a Gilbert damping constant α~0.01, with resonance fields above 3.5 kOe and half linewidths broader than 60 Oe, at 300 K and 9.5 GHz. The spin-to-charge current conversion through TIG/Pt was observed as a micro-voltage generated at the edges of the Pt film. The obtained spin Seebeck coefficient was 0.54 μV/K, confirming also the high interfacial spin transparency.

physics.app-ph↗

arXiv2019

A Measurement of the Branching Ratio of $π^0$ Dalitz Decay using $K_L \rightarrow π^0π^0π^0$ Decays

We present a measurement of $B(π^0 \rightarrow e^+e^- γ)/B(π^0 \rightarrow γγ)$, the Dalitz branching ratio, using data taken in 1999 by the E832 KTeV experiment at Fermi National Accelerator Laboratory. We use neutral pions from fully reconstructed $K_L$ decays in flight; the measurement is based on about 60 thousand $K_L \rightarrow π^0π^0π^0 \rightarrow γγ~γγ~e^+e^-γ$ decays. We normalize to $K_L \rightarrow π^0π^0π^0 \rightarrow 6γ$ decays. We find $B(π^0 \rightarrow e^+e^- γ)/B(π^0 \rightarrow γγ)$ $(m_{e^+e^-}$ > 15 MeV/$c^2)$ = $[3.920 \pm 0.016(stat) \pm 0.036 (syst)] \times 10^{-3}$. Using the Mikaelian and Smith prediction for the $e^+e^-$ mass spectrum, we correct the result to the full $e^+e^-$ mass range. The corrected result is $B(π^0 \rightarrow e^+e^- γ)/B(π^0 \rightarrow γγ) = [1.1559 \pm 0.0047(stat) \pm 0.0106 (syst)]$%. This result is consistent with previous measurements and the uncertainty is a factor of three smaller than any previous measurement.

hep-ex↗

arXiv2019

Contributions of the LAGO Collaboration to the 36th ICRC

The LAGO (Latin American Giant Observatory) observatory is an experiment that spans over Latin America in a wide range of latitudes that gives different rigidity cut offs for the enter of cosmic rays in the atmosphere. The motivation of the Observatory is to study atmospheric radiation and space weather through the measurement of the secondary emission of low energy cosmic rays at ground level using Water Cherenkov Detectors (WCD). This work presents the contributions of the LAGO collaboration to the 2019 36th ICRC.

hep-ex↗

arXiv2019

MAUS: The MICE Analysis User Software

The Muon Ionization Cooling Experiment (MICE) collaboration has developed the MICE Analysis User Software (MAUS) to simulate and analyze experimental data. It serves as the primary codebase for the experiment, providing for offline batch simulation and reconstruction as well as online data quality checks. The software provides both traditional particle-physics functionalities such as track reconstruction and particle identification, and accelerator physics functions, such as calculating transfer matrices and emittances. The code design is object orientated, but has a top-level structure based on the Map-Reduce model. This allows for parallelization to support live data reconstruction during data-taking operations. MAUS allows users to develop in either Python or C++ and provides APIs for both. Various software engineering practices from industry are also used to ensure correct and maintainable code, including style, unit and integration tests, continuous integration and load testing, code reviews, and distributed version control. The software framework and the simulation and reconstruction capabilities are described.

physics.comp-ph↗

arXiv2017

Magnetism in Semiconducting Molybdenum Dichalcogenides

Transition metal dichalcogenides (TMDs) are interesting for understanding fundamental physics of two-dimensional materials (2D) as well as for many emerging technologies, including spin electronics. Here, we report the discovery of long-range magnetic order below TM = 40 K and 100 K in bulk semiconducting TMDs 2H-MoTe2 and 2H-MoSe2, respectively, by means of muon spin-rotation (muSR), scanning tunneling microscopy (STM), as well as density functional theory (DFT) calculations. The muon spin rotation measurements show the presence of a large and homogeneous internal magnetic fields at low temperatures in both compounds indicative of long-range magnetic order. DFT calculations show that this magnetism is promoted by the presence of defects in the crystal. The STM measurements show that the vast majority of defects in these materials are metal vacancies and chalcogen-metal antisites which are randomly distributed in the lattice at the sub-percent level. DFT indicates that the antisite defects are magnetic with a magnetic moment in the range of 0.9-2.8 mu_B. Further, we find that the magnetic order stabilized in 2H-MoTe2 and 2H-MoSe2 is highly sensitive to hydrostatic pressure. These observations establish 2H-MoTe2 and 2H-MoSe2 as a new class of magnetic semiconductors and opens a path to studying the interplay of 2D physics and magnetism in these interesting semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

The reconstruction software for the MICE scintillating fibre trackers

The Muon Ionization Cooling Experiment (MICE) will demonstrate the principle of muon beam phase-space reduction via ionization cooling. Muon beam cooling will be required for the proposed Neutrino Factory or Muon Collider. The phase-space before and after the cooling cell must be measured precisely. This is achieved using two scintillating-fibre trackers, each placed in a solenoidal magnetic field. This paper describes the software reconstruction for the fibre trackers: the GEANT4 based simulation; the implementation of the geometry; digitisation; space-point reconstruction; pattern recognition; and the final track fit based on a Kalman filter. The performance of the software is evaluated by means of Monte Carlo studies and the precision of the final track reconstruction is evaluated.

physics.ins-det↗