Física

In this work, a study of the optical and structural properties of a phosphate glass (P2O5-Al2O3-Na2O-K2O) doped with cadmium sulfide (CdS) nanocrystals and co-doped with Nd3+ ions produced by the fusion method was carried out. This study was conducted by analyzing the Optical absorption, Photoluminescence (PL) spectra, Time-resolved PL decay curves, and Raman spectroscopy measurements. The growth of the CdS nanocrystal was confirmed by the optical absorption spectra, Transmission Electron Microscopy, and Raman spectroscopy, which also indicates the presence of surface defect states. After Nd doping, PL spectra show an energy transfer process between the CdS nanocrystal and the Nd3+ ions. This process decreases with the nanocrystal size and can be eliminated by tuning the excitation wavelength. In addition, the influence of the CdS nanocrystal size on the Nd emission lifetime and luminescence quantum efficiency was studied in two different excitations (405 and 532 nm). This kind of system has the potential to work as optoelectronic devices, such as laser active media, optical amplifiers, and planar waveguides due to near-infrared emission of the Nd3+ ions.
 

In this work, a study of the optical and structural properties of a phosphate glass (P2O5-Al2O3-Na2O-K2O) doped with cadmium sulfide (CdS) nanocrystals and co-doped with Nd3+ ions produced by the fusion method was carried out. This study was conducted by analyzing the Optical absorption, Photoluminescence (PL) spectra, Time-resolved PL decay curves, and Raman spectroscopy measurements. The growth of the CdS nanocrystal was confirmed by the optical absorption spectra, Transmission Electron Microscopy, and Raman spectroscopy, which also indicates the presence of surface defect states. After Nd doping, PL spectra show an energy transfer process between the CdS nanocrystal and the Nd3+ ions. This process decreases with the nanocrystal size and can be eliminated by tuning the excitation wavelength. In addition, the influence of the CdS nanocrystal size on the Nd emission lifetime and luminescence quantum efficiency was studied in two different excitations (405 and 532 nm). This kind of system has the potential to work as optoelectronic devices, such as laser active media, optical amplifiers, and planar waveguides due to near-infrared emission of the Nd3+ ions.
 

In this paper, it is demonstrated that copper selenide (Cu2−XSe) films onto polyester sheets may serve as transparent electrodes in inorganic–organic hybrid light emission devices (IOHLED), as possible replacement to indium tin oxide or fluorine-doped tin oxide. The Cu2−XSe film synthesized via
bath chemical deposition is electrically stable with a sheet resistance of 148 Ω sq−1 and optical bandgap of 2.3 eV. IOHLED are made with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as an organic layer for hole transport and poly[2-metoxy-5-(2-ethylhexyloxy)-1,4-henylenevinylene] (MEH-PPV) as electroluminescent semiconductor. The IOHLED emits in the visible range owing to the simultaneous emission from Cu2−XSe and MEH-PPV layers. The enhanced performance is explained by analyzing the charge transport mechanisms at the inorganic–organic interface, which for Cu2−XSe/PEDOT:PSS changes from Fowler-Nordheim to direct tunneling regardless of the device temperature (90–370 K). The onset voltage is 75% smaller than in the absence of the PEDOT:PSS layer due to a 27 meV decrease in the potential barrier, and the direct tunneling becomes more relevant to device performance than the sheet resistance of the Cu2−XSe layer. Upon adding transparency, mechanical flexibility, and covering large areas, the ultrathin Cu2−XSe films on polyester substrates permit new designs for electro-optical devices with inorganic–organic heterojunctions.

In this work, a study of the optical and structural properties of a phosphate glass (P2O5-Al2O3-Na2O-K2O) doped with cadmium sulfide (CdS) nanocrystals and co-doped with Nd3+ ions produced by the fusion method was carried out. This study was conducted by analyzing the Optical absorption, Photoluminescence (PL) spectra, Time-resolved PL decay curves, and Raman spectroscopy measurements. The growth of the CdS nanocrystal was confirmed by the optical absorption spectra, Transmission Electron Microscopy, and Raman spectroscopy, which also indicates the presence of surface defect states. After Nd doping, PL spectra show an energy transfer process between the CdS nanocrystal and the Nd3+ ions. This process decreases with the nanocrystal size and can be eliminated by tuning the excitation wavelength. In addition, the influence of the CdS nanocrystal size on the Nd emission lifetime and luminescence quantum efficiency was studied in two different excitations (405 and 532 nm). This kind of system has the potential to work as optoelectronic devices, such as laser active media, optical amplifiers, and planar waveguides due to near-infrared emission of the Nd3+ ions.
 

The ability to construct 2D systems, beyond materials’ natural formation, enriches the search and control capability of new phenomena, for instance, the synthesis of topological lattices of vacancies on metal surfaces through scanning tunneling microscopy. In the present study, we demonstrate that metal atoms encaged in a silicate adlayer on silicon carbide is an interesting platform for lattice design, providing a ground to experimentally construct tight-binding models on an insulating substrate. Based on the density functional theory, we have characterized the energetic and electronic properties of 2D metal
lattices embedded in the silica adlayer. We show that the characteristic band structures of those lattices are ruled by surface states induced by the metal-s orbitals coupled by the host-pxy states, giving rise to spxy Dirac bands neatly lying within the energy gap of the semiconductor substrate.

We have investigated the Kondo physics of a single magnetic impurity embedded in multi-Dirac (Weyl) node fermionic systems. By using a generic effective model for the host material and employing a numerical renormalization group approach we access the low temperature behavior of the system, identifying the existence of Kondo screening in single-, double-, and triple-Dirac (Weyl) node models. We find that in any multi-Dirac node systems the low-energy regime lies within one of the known classes of pseudogap Kondo problem, extensively studied in the literature. Kondo screening is also observed for time reversal symmetry broken Weyl systems. This is, however, possible only in the particle-hole symmetry broken regime obtained for finite chemical potential μ. Although weakly, breaking time-reversal symmetry suppresses the Kondo resonance, especially in the single-node Weyl semimetals. More interesting Kondo screening regimes are obtained for inversion symmetry broken multi-Weyl fermions. In these systems the Kondo regimes of double- and triple-Weyl node models are much richer than in the single-Weyl node model. While in the single-Weyl node model the Kondo temperature increases monotonically with |μ| regardless the value of the inversion symmetry breaking parameter Q0, in double- and triple-Weyl node models there are two distinct regimes: (i) For Q0<|μ| the Kondo temperature depends strongly on μ, while (ii) for Q0>|μ| the Kondo temperature depends very weakly on μ, resembling the flat-band single impurity Anderson model.

We have investigated the Kondo physics of a single magnetic impurity embedded in multi-Dirac (Weyl) node fermionic systems. By using a generic effective model for the host material and employing a numerical renormalization group approach we access the low temperature behavior of the system, identifying the existence of Kondo screening in single-, double-, and triple-Dirac (Weyl) node models. We find that in any multi-Dirac node systems the low-energy regime lies within one of the known classes of pseudogap Kondo problem, extensively studied in the literature. Kondo screening is also observed for time reversal symmetry broken Weyl systems. This is, however, possible only in the particle-hole symmetry broken regime obtained for finite chemical potential μ. Although weakly, breaking time-reversal symmetry suppresses the Kondo resonance, especially in the single-node Weyl semimetals. More interesting Kondo screening regimes are obtained for inversion symmetry broken multi-Weyl fermions. In these systems the Kondo regimes of double- and triple-Weyl node models are much richer than in the single-Weyl node model. While in the single-Weyl node model the Kondo temperature increases monotonically with |μ| regardless the value of the inversion symmetry breaking parameter Q0, in double- and triple-Weyl node models there are two distinct regimes: (i) For Q0<|μ| the Kondo temperature depends strongly on μ, while (ii) for Q0>|μ| the Kondo temperature depends very weakly on μ, resembling the flat-band single impurity Anderson model.

Controlling the spin dynamics and spin lifetimes is one of the main challenges in spintronics. To this end, the study of the spin diffusion in two-dimensional electron gases (2DEGs) shows that when the Rashba and Dresselhaus spin-orbit couplings (SOC) are balanced, a persistent spin helix regime arises. There, a striped spin pattern shows a long lifetime, limited only by the cubic Dresselhaus SOC, and its dynamics can be controlled by in-plane drift fields. Here, we derive a spin diffusion equation for non-degenerate two-subbands 2DEGs. We show that the intersubband scattering rate, which is defined by the overlap of the subband densities, enters as a new nob to control the spin dynamics, and can be controlled by electric fields, being maximum for symmetric quantum wells. We find that for large intersubband couplings the dynamics follow an effective diffusion matrix given by approximately half of the subband-averaged matrices. This extra 1/2 factor arises from Matthiessen's rule summing over the intra- and intersubband scattering rates, and leads to a reduced diffusion constant and larger spin lifetimes. We illustrate our findings with numerical solutions of the diffusion equation with parameters extracted from realistic Schrödinger-Poisson calculations.