Experimental Evidence of Delocalized States in Random Dimer Superlattices

Abstract

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...

Resistance of a 1D random chain: Hamiltonian version of the transfer matrix approach

“…As one can see, the correlator R 2 changes from −1/2 in the ballistic regime, λ n ≪ 1, to 1/2 in the localized regime, λ n ≫ 1. Taking into account that sin 2 θ n = 1/2, the latter means synchronization of the phases in the localized regime, θ (1) n ≈ θ (2) n , see details in [20]. In the ballistic regime the phases are anti-synchronized, θ (1) n ≈ −θ (2) n , however, this holds only for very small values of λ as one can see from the graph in Fig.…”

“…As was mentioned, in this regime the phases fluctuate coherently, i.e. θ (1) n ≈ θ (2) n . This property helps to evaluate the mean value S P , see Eq.…”

“…These correlations is the main problem for further analytical treatment. It turns out that the correlations between phases θ (1) n and θ (2) n are very different in the ballistic (λ ≪ 1) and localized (λ ≫ 1) regimes.…”

“…(1) or Eq. (2). Such a representation allows one to determine the Lyapunov exponent Λ by running the trajectory of the map Eq.…”

Universal scheme to generate metal–insulator transition in disordered systems

“…A band of extended states will emerge in the binary alloy when the site energies are long-range correlated [23]. In addition, other theoretical models have been suggested to produce conducting states in low-dimensional disordered systems [24][25][26][27][28][29][30][31][32][33], and some of them have been corroborated in GaAs-AlGaAs superlattices [34] and Bose-Einstein condensate (BEC) [14]. Nevertheless, we notice that all electronic states become localized in these correlated disordered systems when the disorder degree is extremely large.…”