arXiv subjects

Flávio H. Feres

Publications and source records attributed to Flávio H. Feres.

4 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↗

arXiv2023

Terahertz Twistoptics -- engineering canalized phonon polaritons

The terahertz (THz) frequency range is key to study collective excitations in many crystals and organic molecules. However, due to the large wavelength of THz radiation, the local probing of these excitations in smaller crystalline structures or few-molecular arrangements, requires sophisticated methods to confine THz light down to the nanometer length scale, as well as to manipulate such a confined radiation. For this purpose, in recent years, taking advantage of hyperbolic phonon polaritons (HPhP) in highly anisotropic van der Waals (vdW) materials has emerged as a promising approach, offering a multitude of manipulation options such as control over the wavefront shape and propagation direction. Here, we demonstrate the first THz application of twist-angle-induced HPhP manipulation, designing the propagation of confined THz radiation between 8.39 and 8.98 THz in the vdW material alpha-molybdenum trioxide ($α-MoO_{3}$), hence extending twistoptics to this intriguing frequency range. Our images, recorded by near-field optical microscopy, show the frequency- and twist-angle-dependent change between hyperbolic and elliptic polariton propagation, revealing a polaritonic transition at THz frequencies. As a result, we are able to allocate canalization (highly collimated propagation) of confined THz radiation by carefully adjusting these two parameters, i.e. frequency and twist angle. Specifically, we report polariton canalization in $α-MoO_{3}$ at 8.67 THz for a twist angle of 50°. Our results demonstrate an unprecedented control and the manipulation of highly-confined collective excitations at THz frequencies, offering novel possibilities for nanophotonic applications.

physics.optics↗

arXiv2019

Group velocity modulation and wavelength tuning of phonon-polaritons by engineering dielectric-metallic substrates

In analogy to the observed for single plasmon-polaritons, we show that subdiffractional hyperbolic phonon-polariton (HP2) modes confined in hexagonal boron nitride (hBN) nanocrystals feature wave-particle duality. First, we use Synchrotron Infrared Nanospectroscopy to demonstrate modulation of the HP2 frequency-momentum dispersion relation and group velocity by varying the thickness of the SiO2 layer in the heterostructure hBN/SiO2/Au. These modulations are, then, exploited for the hBN crystal lying on a SiO2 wedge, with a gradient of thickness of such a dielectric medium, built into the Au substrate. Simulations show that a phonon-polariton pulse accelerates as the thickness of the wedge increases. This is explained by a parameter-free semi-classical approach considering the pulse as free quantum particle. Within this picture, an estimated average acceleration value of ~ 1.45 10^18 m.s^(-2) is determined using experimental inputs. This estimation is in good agreement with the value of 2.0 10^18 m.s^(-2) obtained from the theory directly.

physics.optics↗