Física

Cellulose disassembly is an important issue in designing nanostructures using cellulose-based materials. In this work, we present a combination of experimental and theoretical study addressing the disassembly of cellulose nanofibrils. Through 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidation processes, combined with atomic force microscopy results, we show the formation of nanofibers with diameters corresponding to that of a single-cellulose polymer chain. The formation of these polymer chains is controlled by repulsive electrostatic interactions between the oxidized chains. Further, first-principles calculations have been performed in order to provide an atomistic understanding of the cellulose disassembling processes, focusing on the balance between the interchain (IC) and intersheet (IS) interactions upon oxidation. First, we analyze these interactions in pristine systems, where we found the IS interaction to be stronger than the IC interaction. In the oxidized systems, we have considered the formation of (charged) carboxylate groups along the inner sites of elementary fibrils. We show a net charge concentration on the carboxylate groups, supporting the emergence of repulsive electrostatic interactions between the cellulose nanofibers. Indeed, our total energy results show that the weakening of the binding strength between the fibrils is proportional to the concentration and net charge density of the carboxylate group. Moreover, by comparing the IC and IS binding energies, we found that most of the disassembly processes should take place by breaking the IC O−H···O hydrogen bond interactions and thus supporting the experimental observation of single- and double-cellulose polymer chains.

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.

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.

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.
 

A systematic investigation of both electrocaloric effect (ECE) and energy storage properties is presented in the Pb0.88La0.08(Zr0.60Ti0.40)O3 relaxor system. The ECE, studied from direct and indirect methods, revealed an electrocaloric temperature change (ΔT) around 1.6 K, under an applied electric field of 60 kV/cm at room temperature, with a corresponding electrocaloric strength (ΔT/ΔE) around 0.03 K cm/kV. The temperature dependence of ΔT displayed its maximum value (∼2.32 K) around the freezing temperature (Tf), which is known as a critical temperature for relaxor ferroelectrics, thus suggesting an important contribution of the polar nanoregions to the electrocaloric properties. This result, together with the observed recoverable energy density (Jr) value around 0.43 J/cm3 and a relatively high discharge energy efficiency, reveals the studied system as a promising material for application in multifunctional devices based on high ECE materials combined with excellent energy storage performance.

We consider the problem of a semiclassical description of quantum chaotic transport, when a tunnel barrier is present in one of the leads. Using a semiclassical approach formulated in terms of a matrix model, we obtain transport moments as power series in the reflection probability of the barrier, whose coefficients are rational functions of the number of open channels M. Our results are therefore valid in the quantum regime and not only when M  1. The expressions we arrive at are not identical with the corresponding predictions from random matrix theory, but are in fact much simpler. Both theories agree as far as we can test.

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.
 

We consider the problem of a semiclassical description of quantum chaotic transport, when a tunnel barrier is present in one of the leads. Using a semiclassical approach formulated in terms of a matrix model, we obtain transport moments as power series in the reflection probability of the barrier, whose coefficients are rational functions of the number of open channels M. Our results are therefore valid in the quantum regime and not only when M  1. The expressions we arrive at are not identical with the corresponding predictions from random matrix theory, but are in fact much simpler. Both theories agree as far as we can test.

Cellulose disassembly is an important issue in designing nanostructures using cellulose-based materials. In this work, we present a combination of experimental and theoretical study addressing the disassembly of cellulose nanofibrils. Through 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidation processes, combined with atomic force microscopy results, we show the formation of nanofibers with diameters corresponding to that of a single-cellulose polymer chain. The formation of these polymer chains is controlled by repulsive electrostatic interactions between the oxidized chains. Further, first-principles calculations have been performed in order to provide an atomistic understanding of the cellulose disassembling processes, focusing on the balance between the interchain (IC) and intersheet (IS) interactions upon oxidation. First, we analyze these interactions in pristine systems, where we found the IS interaction to be stronger than the IC interaction. In the oxidized systems, we have considered the formation of (charged) carboxylate groups along the inner sites of elementary fibrils. We show a net charge concentration on the carboxylate groups, supporting the emergence of repulsive electrostatic interactions between the cellulose nanofibers. Indeed, our total energy results show that the weakening of the binding strength between the fibrils is proportional to the concentration and net charge density of the carboxylate group. Moreover, by comparing the IC and IS binding energies, we found that most of the disassembly processes should take place by breaking the IC O−H···O hydrogen bond interactions and thus supporting the experimental observation of single- and double-cellulose polymer chains.