arXiv subjects

Salvatore Macis

Publications and source records attributed to Salvatore Macis.

7 recordsLinked to original sources

arXiv2026

Phase-Transition-Driven Hyperbolic Optical Response and Directional Polaritons in Epitaxial VO2 Thin Films

Optical anisotropy in crystalline solids enables direction-dependent light-matter interactions and underpins a variety of advanced photonic functionalities. In this context, Vanadium dioxide (VO2) represents a prototypical material that undergoes a reversible MIT near 67°C, accompanied by pronounced electronic, structural, and optical modifications. The MIT not only dramatically modifies the VO2 electrical conductivity but also reshapes its anisotropic optical response, making VO2 an exceptional platform for dynamically tunable photonic and optoelectronic devices. In this work, we investigate how the intrinsic crystalline anisotropy of VO2 induces a hyperbolic optical behavior in the metallic rutile phase. We study two epitaxial VO2 thin films of different thicknesses grown on (110) oriented MgF2 substrates. Broadband polarized spectroscopic measurements, spanning the infrared to UV spectral range, are employed to independently investigate the optical response in both the monoclinic and rutile phases. From these measurements, we extract the optical conductivity and the dielectric function, revealing a pronounced anisotropy in the rutile metallic phase, with an enhanced free-carrier response along the rutile c axis. Our data show that, within a narrow near-infrared spectral window, the real parts of the dielectric tensor components along the two principal axes acquire opposite signs, indicating the emergence of a hyperbolic type-II dispersion. The hyperbolic response is quantitatively evaluated through the quality factor and the degree of dielectric anisotropy, enabling a systematic assessment of VO2 as a thermally switchable, hyperbolic optical medium. These findings expand the understanding of anisotropy-driven optical phenomena in phase-change materials and highlight VO2 thin films as a promising platform for tunable and reconfigurable photonic applications.

physics.optics↗

arXiv2026

Enhanced third harmonic response of the PtTe$_2$ transition metal dichalcogenide

We investigate the third harmonic response of platinum ditelluride (PtTe$_2$), a Dirac semimetal belonging to the transition metal dichalcogenides class. Due to its topological properties, this material has drawn a lot of attention, particularly because it hosts type-II (super-critically tilted) Dirac fermions in the $\rm A-Γ-\rm A$ high symmetry direction. Adopting a low-energy model fitted directly from density functional theory band structure simulations, we calculate analytically the nonlinear conductivity. We observe that third-order optical nonlinearities are efficiently modulated by the cones tilting, which produces a significant enhancement of the nonlinear susceptibility. Our results, besides shedding light on topological transitions of platinum ditelluride, are relevant for future nanophotonic devices exploiting the tunable nonlinear properties of type-II Dirac fermions.

physics.optics↗

arXiv2025

Search for structural differences in spike glycoprotein variants of SARS-CoV-2: Infrared Spectroscopy, Circular Dichroism and Computational Analysis

The SARS-CoV-2 pandemic has led to a significant emergence of highly mutated forms of viruses with a great ability to adapt to the human host. Some mutations resulted in changes in the amino acid sequences of viral proteins, including the Spike glycoproteins, affecting protein physico-chemical properties and functionalities. Here, we propose, for the first time to the best of our knowledge, a systematic and comparative study of the monomeric spike protein subunits 1 of three SARS-CoV-2 variants at pH 7.4, combining both an experimental approach, taking advantage of Attenuated Total Reflection Infrared and Circular Dichroism spectroscopies, and a computational approach via Molecular Dynamics simulations. Experimental data in combination with Molecular Dynamics and Surface polarity calculations provide a comprehensive understanding of variants proteins in terms of their secondary structure content, 3D conformational structure and order and interaction with the solvent. The present structural investigation clarifies which kind of changes in conformation and functionalities occurred as long as mutations appeared in amino acids sequences. This information is essential for preventive targeted actions, drug design, and biosensing applications.

physics.bio-ph↗

arXiv2025

Dual Optical Hyperbolicity of PdCoO$_2$ and PdCrO$_2$ Delafossite Single Crystals

Hyperbolic materials exhibit a very peculiar optical anisotropy with simultaneously different signs of the dielectric tensor components. This anisotropy allows the propagation of exotic surface-wave excitations like hyperbolic phonons and plasmon polaritons. While hyperbolic materials hold promise for applications in subwavelength photonics and enhanced light-matter interactions, their natural occurrence is limited to few materials, often accompanied by significant dielectric losses and limited hyperbolic spectral bandwidth. Focusing on PdCoO$_2$ and PdCrO$_2$ delafossite transition-metal oxides, in this paper we demonstrate their unique dual hyperbolic regimes: one localized around a phonon absorption in the mid-infrared spectral region, and the other extending into the visible range. Both hyperbolic regimes show exceptional properties including low dissipation and high hyperbolic quality factors. These results pave the way for innovative applications of delafossite layered metals in subwavelength photonics, imaging, and sensing.

cond-mat.mtrl-sci↗

arXiv2023

Optical Properties of Superconducting Nd0.8Sr0.2NiO2 Nickelate

The intensive search for alternative non-cuprate high-transition-temperature ($T_c$) superconductors has taken a positive turn recently with the discovery of superconductivity in infinite layer nickelates. This discovery is expected to be the basis for disentangling the puzzle behind the physics of high $T_c$ in oxides. In the unsolved quest for the physical conditions necessary for inducing superconductivity, we report an optical study of a Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ film measured using optical spectroscopy, at temperatures above and below the critical temperature $T_c\sim 13$ K. The normal-state electrodynamics of Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, is described by the Drude model characterized by a scattering time just above $T_c$ ($τ\sim 1.7\times 10^{-14}$ s) and a plasma frequency $ω_p = 8500$ cm$^{-1}$ in combination with an absorption band in the Mid-Infrared (MIR) around $ω_0 \sim 4000$ cm$^{-1}$. The MIR absorption indicates the presence of strong electronic correlation effect in the NiO$_2$ plane similarly to cuprates. Below $T_c$, a superconducting energy gap ($2Δ$) of $\sim 3.2$ meV is extracted from the Terahertz reflectivity using the the Mattis-Bardeen model. From the Ferrel-Glover-Thinkam Rule applied to the real part of the optical conductivity, we also estimate a London penetration depth of about 490 nm, in agreement with a type-II superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ Nickelate.

cond-mat.supr-con↗

arXiv2023

Optical properties of two-dimensional tin nanosheets epitaxially grown on graphene

Heterostacks formed by combining two-dimensional materials show novel properties which are of great interest for new applications in electronics, photonics and even twistronics, the new emerging field born after the outstanding discoveries on twisted graphene. Here, we report the direct growth of tin nanosheets at the two-dimensional limit via molecular beam epitaxy on chemical vapor deposited graphene on Al2O3(0001). The mutual interaction between the tin nanosheets and graphene is evidenced by structural and chemical investigations. On the one hand, Raman spectroscopy indicates that graphene undergoes compressive strain after the tin growth, while no charge transfer is observed. On the other hand, chemical analysis shows that tin nanosheets interaction with sapphire is mediated by graphene avoiding the tin oxidation occurring in the direct growth on this substrate. Remarkably, optical measurements show that the absorption of tin nanosheets show a graphene-like behavior with a strong absorption in the ultraviolet photon energy range, therein resulting in a different optical response compared to tin nanosheets on bare sapphire. The optical properties of tin nanosheets therefore represent an open and flexible playground for the absorption of light in a broad range of the electromagnetic spectrum and technologically relevant applications for photon harvesting and sensors.

cond-mat.mtrl-sci↗

arXiv2020

Interplay among Work Function, electronic structure and stoichiometry in nanostructured vanadium oxides films

The work function is the parameter of greatest interest in many technological applications involving charge exchange mechanisms at the interface. The possibility to produce samples with a controlled work function is then particularly interesting, albeit challenging. We synthetized nanostructured vanadium oxides films by a room temperature Supersonic Cluster Beam Deposition method, obtaining samples with tunable stoichiometry and work function (3.7-7 eV). We present an investigation of the electronic structure of several vanadium oxides films as a function of the oxygen content via in-situ Auger, valence-band photoemission spectroscopy and work function measurements. The experiments probed the partial 3d density of states, highlighting the presence of strong V3d-O2p and V3d-V4s hybridization which influence 3d occupation. We show how controlling the stoichiometry of the sample implies a control over work function, and that the access to nanoscale quantum confinement can be exploited to increase the work function of the sample relative to the bulk analogue. In general, the knowledge of the interplay among work function, electronic structure, and stoichiometry is strategic to match nanostructured oxides to their target applications.

cond-mat.mtrl-sci↗