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Raul O. Freitas

Publications and source records attributed to Raul O. Freitas.

10 recordsLinked to original sources
arXiv2026

Magnetic permeability and zone-folded phonons in layered NiPS$_3$

Nickel phosphorus trisulfide (NiPS$_3$) stands out as a Mott insulator exhibiting XY-type antiferromagnetic order. In this work, we investigate the electrodynamic response of single-crystalline NiPS$_3$ using Terahertz Time-Domain Spectroscopy (THz-TDS) as a function of temperature. In cases where the magnetic permeability cannot be approximated to unity, conventional THz transmission analysis is often restricted to the complex refractive index, hindering the distinction between magnetic and dielectric contributions. To overcome this limitation, we demonstrate an extraction methodology capable of decoupling the magnetic permeability from the electric permittivity. Assuming that the dielectric contribution remains invariant below the N'eel temperature (TN), we used data from the paramagnetic phase to isolate the intrinsic magnetic component at 13 K. The angular and thermal dependence of the spectra revealed a magnon mode near 1 THz (for 0$^\circ$ orientation) and, distinctively, a zone-folded phonon activated by symmetry breaking (at 90$^\circ$). The isolated magnetic permeability was successfully modeled using Drude-Lorentz oscillators, enabling the extraction of fundamental spin dynamics parameters in this material.

cond-mat.mtrl-sci↗
arXiv2025

Two-dimensional talc as a natural hyperbolic material

This study demonstrates that two-dimensional talc, a naturally abundant mineral, supports hyperbolic phonon-polaritons (HPhPs) at mid-infrared wavelengths, thus offering a low-cost alternative to synthetic polaritonic materials. Using scattering scanning near-field optical microscopy (s-SNOM) and synchrotron infrared nano spectroscopy (SINS), we reveal tunable HPhP modes in talc flakes of a long lifetime. These results highlight the potential of natural 2D talc crystals to constituting an effective platform for establishing scalable optoelectronic and photonic devices.

cond-mat.mes-hall↗
arXiv2024

Ultrathin natural biotite crystals as a dielectric layer for van der Waals heterostructure applications

Biotite, an iron-rich mineral belonging to the trioctahedral mica group, is a naturally abundant layered material (LM) exhibiting attractive electronic properties for application in nanodevices. Biotite stands out as a non-degradable LM under ambient conditions, featuring high-quality basal cleavage, a significant advantage for van der Waals heterostructure (vdWH) applications. In this work, we present the micro-mechanical exfoliation of biotite down to monolayers (1Ls), yielding ultrathin flakes with large areas and atomically flat surfaces. To identify and characterize the mineral, we conducted a multi-elemental analysis of biotite using energy-dispersive spectroscopy mapping. Additionally, synchrotron infrared nano-spectroscopy was employed to probe its vibrational signature in few-layer form, with sensitivity to the layer number. We have also observed good morphological and structural stability in time (up to 12 months) and no important changes in their physical properties after thermal annealing processes in ultrathin biotite flakes. Conductive atomic force microscopy evaluated its electrical capacity, revealing an electrical breakdown strength of approximately 1 V/nm. Finally, we explore the use of biotite as a substrate and encapsulating LM in vdWH applications. We have performed optical and magneto-optical measurements at low temperatures. We find that ultrathin biotite flakes work as a good substrate for 1L-MoSe2, comparable to hexagonal boron nitride flakes, but it induces a small change of the 1L-MoSe2 g-factor values, most likely due to natural impurities on its crystal structure. Furthermore, our results show that biotite flakes are useful systems to protect sensitive LMs such as black phosphorus from degradation for up to 60 days in ambient air. Our study introduces biotite as a promising, cost-effective LM for the advancement of future ultrathin nanotechnologies.

cond-mat.mtrl-sci↗
arXiv2023

Review on Infrared Nanospectroscopy of Natural 2D Phyllosilicates

Phyllosilicates emerge as a promising class of large bandgap lamellar insulators. Their applications have been explored from fabrication of graphene-based devices to 2D heterostructures based on transition metal dicalcogenides with enhanced optical and polaritonics properties. In this review, we provide an overview on the use of IR s-SNOM for studying nano-optics and local chemistry of a variety of 2D natural phyllosilicates. Finally, we bring a brief update on applications that combine natural lamellar minerals into multifunctional nanophotonic devices driven by electrical control.

cond-mat.mes-hall↗
arXiv2023

Raman and Far Infrared Synchrotron Nanospectroscopy of Layered Crystalline Talc: Vibrational Properties, Interlayer Coupling and Symmetry Crossover

Talc is an insulating layered material that is stable at ambient conditions and has high-quality basal cleavage, which is a major advantage for its use in van der Waals heterostructures. Here, we use near-field synchrotron infrared nanospectroscopy, Raman spectroscopy, and first-principles calculations to investigate the structural and vibrational properties of talc crystals, ranging from monolayer to bulk, in the 300-750 cm-1 and <60 cm-1 spectral windows. We observe a symmetry crossover from mono to bilayer talc samples, attributed to the stacking of adjacent layers. The in-plane lattice parameters and frequencies of intralayer modes of talc display weak dependence with the number of layers, consistent with a weak interlayer interaction. On the other hand, the low-frequency (<60 cm-1) rigid-layer (interlayer) modes of talc are suitable to identify the number of layers in ultrathin talc samples, besides revealing strong in-plane and out-of-plane anisotropy in the interlayer force constants and related elastic stiffnesses of single crystals. The shear and breathing force constants of talc are found to be 66% and 28%, respectively, lower than those of graphite, making talc an excellent lubricant that can be easily exfoliated. Our results broaden the understanding of the structural and vibrational properties of talc at the nanoscale regime and serve as a guide for future ultrathin heterostructures applications.

cond-mat.mtrl-sci↗
arXiv2022

High throughput investigation of an emergent and naturally abundant 2D material: Clinochlore

Phyllosilicate minerals, which form a class of naturally occurring layered materials (LMs), have been recently considered as a low-cost source of two-dimensional (2D) materials. Clinochlore [Mg5Al(AlSi3)O10(OH)8] is one of the most abundant phyllosilicate minerals in nature, exhibiting the capability to be mechanically exfoliated down to a few layers. An important characteristic clinochlore is the natural occurrence of defects and impurities which can strongly affect their optoelectronic properties, possibly in technologically interesting ways. In the present work, we carry out a thorough investigation of the clinochlore structure on both bulk and 2D exfoliated forms, discussing its optical features and the influence of the insertion of impurities on its macroscopic properties. Several experimental techniques are employed, followed by theoretical first-principles calculations considering several types of naturally-ocurring transition metal impurities in the mineral lattice and their effect on electronic and optical properties. We demonstrate the existence of requirements concerning surface quality and insulating properties of clinochlore that are mandatory for its suitable application in nanoelectronic devices. The results presented in this work provide important informations for clinochlore potential applications and establish a basis for further works that intend to optimize its properties to relevant 2D technological applications through defect engineering.

cond-mat.mtrl-sci↗
arXiv2021

Ultrabroadband nanocavity of hyperbolic phonon polaritons in 1D-like α-MoO3

The exploitation of phonon-polaritons in nanostructured materials offers a pathway to manipulate infrared (IR) light for nanophotonic applications. Notably, hyperbolic phonons polaritons (HP2) in polar bidimensional crystals have been used to demonstrate strong electromagnetic field confinement, ultraslow group velocities, and long lifetimes (~ up to 8 ps). Here we present nanobelts of α-phase molybdenum trioxide (α-MoO3) as a low-dimensional medium supporting HP2 modes in the mid- and far-IR ranges. By real-space nanoimaging, with IR illuminations provided by synchrotron and tunable lasers, we observe that such HP2 response happens via formation of Fabry-Perot resonances. We remark an anisotropic propagation which critically depends on the frequency range. Our findings are supported by the convergence of experiment, theory, and numerical simulations. Our work shows that the low dimensionality of natural nanostructured crystals, like α-MoO3 nanobelts, provides an attractive platform to study polaritonic light-matter interactions and offer appealing cavity properties that could be harnessed in future designs of compact nanophotonic devices.

cond-mat.mtrl-sci↗
arXiv2018

Observation of plasmon-phonon coupling in natural 2D graphene-talc heterostructures

Two-dimensional (2D) materials occupy noteworthy place in nanophotonics providing for subwavelength light confinement and optical phenomena dissimilar to those of their bulk counterparts. In the mid-infrared, graphene-based heterostructures and van der Waals crystals of hexagonal boron nitride (hBN) overwhelmingly concentrate the attention by exhibiting real-space nano-optics from plasmons, phonon-polaritons and hybrid plasmon phonon-polaritons quasiparticles. Here we present the mid-infrared nanophotonics of talc, a natural atomically flat layered material, and graphene-talc (G-talc) heterostructures using broadband synchrotron infrared nano-spectroscopy. We achieve wavelength tuning of the talc resonances, assigned to in- and out-of-plane vibrations by changing the thickness of the crystals, which serves as its infrared fingerprints. Moreover, we encounter coupling of the graphene plasmons polaritons with surface optical phonons of talc. As in the case of the G-hBN heterostructures, this coupling configures hybrid surface plasmon phonon-polariton modes causing 30 % increase in intensity for the out-of-plane mode, blue-shift for the in-plane mode and we have succeeded in altering the amplitude of such hybridization by varying the gate voltage. Therefore, our results promote talc and G-talc heterostructures as appealing materials for nanophotonics, like hBN and G-hBN, with potential applications for controllably manipulating infrared electromagnetic radiation at the subdiffraction scale.

cond-mat.mes-hall↗
arXiv2017

Observation of Diode Behavior and Gate Voltage Control of Hybrid Plasmon-Phonon Polaritons in Graphene-Hexagonal Boron Nitride Heterostructures

Light-matter interaction in two-dimension photonic materials allows for confinement and control of free-space radiation on sub-wavelength scales. Most notably, the van der Waals heterostructure obtained by stacking graphene (G) and hexagonal Boron Nitride (hBN) can provide for hybrid hyperbolic plasmon phonon-polaritons (HP3). Here, we present a polariton diode effect and low-bias control of HP3 modes confined in G-hBN. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP3 waves over both hBN Reststrahlen bands and observe the unidirectional propagation of HP3 modes at in-plane heterointerfaces associated with the transition between different substrate dielectrics. By electric gating we further control the HP3 hybridization modifying the coupling between the continuum graphene plasmons and the discrete hyperbolic phonon polaritons of hBN as described by an extended Fano model. This is the first demonstration of unidirectional control of polariton propagation, with break in reflection/transmission symmetry for HP3 modes. G-hBN and related hyperbolic metamaterial nanostructures can therefore provide the basis for novel logic devices of on-chip nano-optics communication and computing.

cond-mat.mes-hall↗