We study the electronic properties of GaAs-AlGaAs superlattices with intentional correlated disorder by means of photoluminescence and vertical dc resistance. The results are compared to those obtained in ordered and uncorrelated disordered superlattices. We report the first experimental evidence that spatial correlations inhibit localization of states in disordered low-dimensional systems, as our previous theoretical calculations suggested, in contrast to the earlier belief that all eigenstates are localized. [S0031-9007(99) PACS numbers: 73.20.Dx, 73.20.Jc, In recent years, a number of tight-binding [1][2][3] and continuous [4] models of disordered one-dimensional (1D) systems have predicted the existence of sets of extended states, in contrast to the earlier belief that all the eigenstates are localized in 1D disordered systems. These systems are characterized by the key ingredient that structural disorder is short-range correlated. Because of the lack of experimental confirmations, there are still some controversies as to the relevance of these results and their implications on physical properties. In this context, some authors have proposed finding physically realizable systems that allow for a clear cut validation of the above-mentioned purely theoretical prediction [5][6][7][8]. Given that semiconductor superlattices (SL's) have been already used successfully to observe electron localization due to disorder [9][10][11][12][13][14], these authors have suggested SL's as ideal candidates for controllable experiments on localization or delocalization and related electronic properties [5][6][7][8].To the best of our knowledge, up to now there is no experimental verification of this theoretical prediction owing to the difficulty in building nanoscale materials with intentional and short-range correlated disorder. However, the confirmation of this phenomenon is important both from the fundamental point of view and for the possibility to develop new devices based on these peculiar properties. In this work we present an experimental verification of this phenomenon in semiconductor nanoscale materials, taking advantage of the molecular beam epitaxy growth technique, which allows the fabrication of semiconductor nanostructures with monolayer controlled perfection.We grew several GaAs-Al 0.35 Ga 0.65 As SL's and we studied their electronic properties by photoluminescence (PL) at low temperature and dc vertical transport in the dark. Indeed PL has been proven to be a good technique to study the electronic properties of disordered SL's [9-11], giving transition energies comparable with theoretical calculations of the electronic levels. The electronic states were calculated using a Kronig-Penney model that has been shown to hold in this range of well and barrier thicknesses, with precise results [15]. This allows the analysis of the experimental transition energies for PL and the ascertainment of the localization and delocalization properties of the SL's. The details of the calculations and a schematic view of the cond...

We investigated the inhomogeneities in the charge density of unintentionally doped graphene on SiO2 prepared by mechanical exfoliation. From the analysis of the G, D, and 2D phonon modes of the Raman spectra after displacing contaminants on graphene surface, and measuring the separation monolayer-substrate distance among zones with different doping levels, we deduce that the interaction with the substrate is the main cause of doping in graphene rather than particle contamination. In particular, we show how graphene doping levels vary within the same flake depending on the distance between graphene and the substrate.

Films of a few layers in thickness of reduced graphite oxide (RGO) sheets functionalized by the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDPS) are obtained by using the Langmuir-Blodgett method. The quality of the RGO sheets is checked by analyzing the degrees of reduction and defect repair by means of X-ray photoelectron spectroscopy, atomic force microscopy (AFM), field-emission scanning electron microscopy (SEM), micro-Raman spectroscopy, and electrical conductivity measurements. A modified Hummers method is used to obtain highly oxidized graphite oxide (GO) together with a centrifugation-based method to improve the quality of GO. The GO samples are reduced by hydrazine or vitamin C. Functionalization of RGO with the zwitterionic surfactant improves the degrees of reduction and defect repair of the two reducing agents and significantly increases the electrical conductivity of paperlike films compared with those prepared from unfunctionalized RGO.

Abstract-We study disordered quantum-well-based semiconductor superlattices where the disorder is intentional and short-range correlated. Such systems consist of quantum wells of two different thicknesses randomly distributed along the growth direction, with the additional constraint that wells of one kind always appears in pairs. Imperfections due to interface roughness are considered by allowing the quantum-well thicknesses to fluctuate around their ideal values. As particular examples, we consider wide-gap (GaAs-Gal-,AI,As) and narrow-gap (InAs-GaSb) superlattices. We show the existence of a band of extended states in perfect correlated disordered superlattices, giving rise to a strong enhancement of their finite-temperature dc conductance as compared to usual random ones whenever the Fermi level matches this band. This feature is seen to survive even if interface roughness is taken into account. Our predictions can be used to demonstrate experimentally that structural correlations inhibit the localization effects of disorder, even in the presence of imperfections. This effect might be the basis of new, filter-like or other specific-purpose electronic devices.

We study the electron transmission probability in semiconductor superlattices where the height of the barriers is modulated by a Gaussian profile. Such structures act as efficient energy band-pass filters and, contrary to previous designs, it is expected to present a lower number of unintentional defects and, consequently, better performance. The j–V characteristic presents negative differential resistance with peak-to-valley ratios much greater than in conventional semiconductor superlattices.

Raman spectroscopy is a technique widely used to detect defects in semiconductors because it provides information of structural or chemical defects produced in its structure. In the case of graphene monolayer, the Raman spectrum presents two bands centered at 1582 cm−1 (G band) and 2700 cm−1 (2D band). However, when the periodic lattice of graphene is broken by different types of defects, new bands appear. This is the situation for the Raman spectrum of graphene oxide. It is well established that the existence of these bands, the position and the intensity or width of peaks can provide information about the origin of defects. However, in the case of the graphene oxide spectrum, we can find in the literature several discrepant results, probably due to differences in chemical composition and the type of defects of the graphene oxide used in these studies. Besides, theoretical calculations proved that the shape of bands, intensity and width, and the position of graphene oxide Raman spectrum depend on the atomic configuration. In the current work, we will summarize our current understanding of the effect of the chemical composition on the Raman spectrum of graphene oxide. Finally, we apply all this information to analyze the evolution of the structure of graphene oxide during the thermal annealing of the heterostructures formed by graphene oxide sandwiches in a hexagonal boron nitride.

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It is widely admitted that the helical conformation of certain chiral molecules may induce a sizable spin selectivity observed in experiments. Spin selectivity arises as a result of the interplay between a helicity-induced spin-orbit coupling (SOC) and electric dipole fields in the molecule. From the theoretical point of view, different phenomena might affect the spin dynamics in helical molecules, such as quantum dephasing, dissipation and the role of metallic contacts. With a few exceptions, previous studies usually neglect the local deformation of the molecule about the carrier, but this assumption seems unrealistic to describe charge transport in molecular systems. We introduce an effective model describing the electron spin dynamics in a deformable helical molecule with weak SOC. We find that the electron-lattice interaction allows the formation of stable solitons such as bright solitons with well defined spin projection onto the molecule axis. We present a thorough study of these bright solitons and analyze their possible impact on the spin dynamics in deformable helical molecules.