arXiv subjects

A. Nonato

Publications and source records attributed to A. Nonato.

7 recordsLinked to original sources

arXiv2026

Coordination-Induced Tuning of Ligand-Centered Red Emission in a cis-[Cd(Tz)2(py)2] Complex for Light-Emitting Diodes

Organic--inorganic complexes are promising materials for light-emitting applications. Here, we report a new organometallic complex, cis-[Cd(Tz)$_2$(py)$_2$], featuring a distorted octahedral Cd(II) coordination environment. IR and Raman spectroscopy reveal pronounced coordination-induced changes, particularly in the Raman response of the triazene moiety, indicating electronic and structural perturbation upon Cd(II) complexation. Hirshfeld surface analysis shows that the crystal packing is mainly governed by H$\cdots$H, O$\cdots$H/H$\cdots$O, and C$\cdots$H/H$\cdots$C contacts, whereas $\pi$--$\pi$ stacking interactions contribute modestly. Solid-state UV--Vis spectroscopy reveals broad absorption from $\sim$700 to 200 nm and a direct optical band gap of 1.83 eV, indicating semiconductor-like behavior. Photoluminescence measurements show a broad emission band at 500--850 nm with enhanced red contribution upon coordination. The emission is mainly assigned to ligand-centered transitions ($\pi \rightarrow \pi^*$ and $n \rightarrow \pi^*$), consistent with the $d^{10}$ configuration of Cd(II), which suppresses metal-centered and charge-transfer processes. The CIE chromaticity coordinates confirm warm emission, highlighting the potential of cis-[Cd(Tz)$_2$(py)$_2$] for red-emitting optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Bi-isotropic effects on hybrid surface polaritons in bilayer configurations

In this work, we investigate the bi-isotropic effects in the formation and tunability of hybrid surface polaritons in bilayer configurations. We consider a heterostructure composed of a medium with bi-isotropic constitutive relations and an AFM layer. Using the transfer matrix formalism, we derive general expressions for the dispersion relations of surface polaritonic modes, including the dependence on the bi-isotropic parameter, and analyze their coupling to bulk magnon-polaritons. As an illustration of application, we consider a heterostructure formed with Bi$_{2}$Se$_{3}$ interfaced with antiferromagnetic (AFM) materials that support terahertz-frequency magnons, specifically Cr$_{2}$O$_{3}$ and FeF$_{2}$. In the strong bi-isotropic coupling regime, the surface Dirac plasmon--phonon--magnon polariton (DPPMP) dispersion undergoes a pronounced redshift, accompanied by suppression of the characteristic anticrossing between the Dirac plasmon and the phonon. This effect, observed in all AFM materials considered, suggests a weakening of the hybrid interaction, possibly due to saturation or detuning mechanisms induced by increased $\alpha$. Furthermore, increasing the Fermi energy of the topological insulator enhances the surface plasmon and phonon contributions, inducing a blueshift of the DPPP branches and bringing them closer to resonance with the magnon mode, thereby increasing the hybridization strength. Intriguingly, this redshift partially compensates the blueshift induced by a higher Fermi level, restoring the system to a weak-coupling regime analogous to that observed at lower Fermi energies. Our findings reveal that both the Fermi level and the bi-isotropic response offer independent and complementary control parameters for tuning the strength of light--magnon coupling in TI/AFM heterostructures, with potential implications for reconfigurable THz spintronic and photonic devices.

cond-mat.other↗

arXiv2025

About the Strain-Coupled Molecular Dynamics in the Ferroelastic Phase Transition of TMACd(N$_3$)$_3$

Tetramethylammonium (TMA) cadmium azide, is a new perovskite-like compound which undergoes a series of first-order phase transitions, including a ferroelastic transition above room temperature. Understanding the order-disorder structural phase transition (SPT) mechanism in hybrid organic--inorganic perovskites (HOIPs) is crucial for designing new compounds with enhanced barocaloric efficiency, as well as unlocking other multifunctional properties. In this paper, we employed the energy fluctuation (EF) model to investigate the experimental linewidth of Raman modes in TMACd(N$_3$)$_3$ near the critical phase transition temperature ($T_C = {322}{K}$), aiming to gain insights into the molecular dynamics around the SPT. The temperature dependence of the strain, used as an order parameter, was obtained using the appropriate thermodynamic potential for the first-order phase transition in TMACd(N$_3$)$_3$, expressed through a Landau expansion, which can be successfully employed to model first-order ferroelastic phase transitions. We show that the EF model suitably captures the behavior of the Raman linewidths in the vicinity of the structural phase transition in TMACd(N$_3$)$_3$. The activation energies obtained for TMACd(N$_3$)$_3$ are comparable to those of DMACd(N$_3$)$_3$, as well as to $k_B T_C$. Additionally, the temperature dependence of the relaxation reveals that the torsional and librational modes require longer to renormalize after the phase transition in TMACd(N$_3$)$_3$ when compared with DMACd(N$_3$)$_3$. The discussion based on these new parameters provides a new perspective for understanding molecular dynamics in systems undergoing order-disorder phase transitions, particularly in ferroelastic transitions, where order-disorder mechanisms are coupled to symmetry-breaking lattice distortions.

cond-mat.mtrl-sci↗

arXiv2024

Resonant Raman Scattering and Optical Absorption Studies of Zn(II) Impurities in L-Alanine Single Crystal

We present a study of low-concentration Zn (II) impurities in L-Alanine Single Crystal (ZNLA) employing resonant Raman scattering and optical absorption spectroscopy. By analyzing the relative integrated intensity of the selective vibrational modes, we observe a resonant enhancement of Raman-active modes near the UV absorption band, associated with vibrations of the amino and carboxyl groups in ZNLA. These findings suggest that zinc cations occupy an interstitial position coordinated by amino and carboxyl groups. Also, ZNLA single crystal shows a strong antiresonance behavior for some modes associated with amino and carboxyl groups. The antiresonance model in the cross-section allowed the estimation of the bandgap to be 3.8 eV, which perfectly agrees with the direct bandgap obtained from the absorption spectrum (3.75 eV). Finally, our approach proves to be useful for detecting interactions between transition metal ion impurities and intramolecular structures in other transition metal-doped organic amino acid systems.

cond-mat.mtrl-sci↗

arXiv2024

Tuning Electric Polarization via Exchange Striction Interaction in CaMn$_7$O$_{12}$ by Sr-Doping

Magnetoelectric (ME) materials displaying strong magnetically induced polarization have attracted considerable interest due to their potential applications in spintronics and various fast electrically controlled magnetic devices. CaMn$_7$O$_{12}$ (CMO) stands out for its giant spin-induced ferroelectric polarization. However, the origin of the induced electric polarization in CMO remains highly controversial and continues to be a subject of ongoing debate. In this paper, through room temperature X-ray powder diffraction (XRPD), temperature-dependent magnetic susceptibility, and thermally stimulated depolarizing current (TSDC) measurements, we provide experimental evidence for a route to tune the magnetically induced polarization by modifying the exchange-striction in CMO via Sr-doping. Our findings demonstrate that the large and broad current peaks observed near the first magnetic phase transition ($T_N1 \sim 90$ K) indicate contributions to the TSDC density from both extrinsic thermally stimulated depolarization processes and intrinsic pyroelectric current arising from magnetically induced polarization changes. We suggest that this reduction in induced electric polarization in CMO originates from the increase in the Mn$^{3+}$ -- O -- Mn$^{4+}$ bond angle due to Sr$^{2+}$ doping, weakening the exchange-striction interaction. Meanwhile, the Dzyaloshinskii-Moriya (DM) effect determines the direction of the induced electric polarization. Our result sheds light on understanding the intriguing giant-induced polarization in CMO and similar compounds with complex magnetic structures.

cond-mat.mtrl-sci↗

arXiv2024

Strong Electron-Phonon Coupling and Lattice Dynamics in One-Dimensional [(CH3)2NH2]PbI3 Hybrid Perovskite

Hybrid halide perovskites (HHPs) have attracted significant attention due to their remarkable optoelectronic properties that combine the advantages of low cost-effective fabrication methods of organic-inorganic materials. Notably, low-dimensional hybrid halide perovskites including two-dimensional (2D) layers and one-dimensional (1D) chains, are recognized for their superior stability and moisture resistance, making them highly appealing for practical applications. Particularly, DMAPbI3 has attracted attention due to other interesting behaviors and properties, such as thermally induced order-disorder processes, dielectric transition, and cooperative electric ordering of DMA dipole moments. In this paper, we investigated the interplay between low-temperature SPT undergone by the low-dimensional (1D) hybrid halide perovskite-like material DMAPbI3 and its optoelectronic properties. Our approach combines synchrotron X-ray powder diffraction, Raman spectroscopy, thermo-microscopy, differential scanning calorimetry (DSC), and photoluminescence (PL) techniques. Temperature-dependent Synchrotron powder diffraction and Raman Spectroscopy reveal that the modes associated with I-Pb-I and DMA+ ion play a crucial role in the order-disorder SPT in DMAPbI3. The reversible SPT modifies its optoelectronic properties, notably affecting its thermochromic behavior and PL emission. The origin of the PL phenomenon is associated to self-trapped excitons (STEs), which are allowed due to a strong electron-phonon coupling quantified by the Huang-Rhys factor (S = 97+-1). Notably, we identify the longitudinal optical (LO) phonon mode at 84 cm-1 which plays a significant role in electron-phonon interaction. Our results show these STEs not only intensify the PL spectra at lower temperatures but also induce a shift in the color emission, transforming it from a light orange-red to an intense bright strong red.

cond-mat.mtrl-sci↗

arXiv2018

High-temperature structural phase transition and infrared dielectric features of La2CoMnO6

Temperature-dependent FAR-infrared reflectivity spectra of partially ordered magnetodielectric La2CoMnO6 is presented, from room temperature up to 675 K. A clear first-ordered structural phase transition (SPT) from a monoclinic structure with P2_1/n symmetry to a rhombohedral phase with R-3 symmetry was evidenced from the behaviour of polar phonon modes at TC~590 K. The temperature dependences of the transversal and longitudinal phonon branches, dielectric strengths, and damping of the strongest dielectric modes confirm the significant contribution of the phonon modes on the SPT, and revealed an important lattice anharmonicity, particularly for the low frequency modes. In addition, these investigations showed that structural ordering does not inhibit the SPT, and provided valuable information towards the polar phonons, their implications on intrinsic dielectric constant in double perovskites and in related compounds.

cond-mat.mtrl-sci↗