Spatial Mapping of Local Density Variations in Two-dimensional Electron Systems Using Scanning Photoluminescence

Abstract

We have developed a scanning photoluminescence technique that can directly map out the local two-dimensional electron density with a relative accuracy of ∼2.2 × 10 8 cm −2 . The validity of this approach is confirmed by the observation of the expected density gradient in a high-quality GaAs quantum well sample that was not rotated during the molecular beam epitaxy of its spacer layer. In addition to this global variation in electron density, we observe local density fluctuations across the sample. These random density fluctuations are also seen in samples that were continuously rotated during growth, and we attribute them to residual space charges at the substrate−epitaxy interface. This is corroborated by the fact that the average magnitude of density fluctuations is increased to ∼9 × 10 9 cm −2 from ∼1.2 × 10 9 cm −2 when the buffer layer between the substrate and the quantum well is decreased by a factor of 7. Our data provide direct evidence for local density inhomogeneities even in very high-quality two-dimensional carrier systems.

Observation of Flat Bands in Gated Semiconductor Artificial Graphene

“…Onset 0 in the three regions is assigned to optical transitions from c0 band electrons to VB holes near the  point of the BZ. The broad PL band in region QU is similar to that from uniform 2DES in unpatterned QWs, whose line shape yields an accurate estimation of EF [15][16][17]. The evaluation of EF as a function of Vb in Fig.…”

“…PL spectra are from optical recombination transitions between mobile electrons in conduction bands (CB) and weakly photoexcited holes in valence bands (VB). Holes in GaAs AG have nearly dispersionless VB, so that the line shapes of PL spectra offer direct insights on the electron DOS [15][16][17]. The evolution of PL spectra as a function of EF, enabled by gating semiconductor AG, would distinguish emerging optical characteristics.…”

Anisotropy and quench dynamics of quasiholes in fractional quantum Hall liquids

“…On the experimental side, the intrinsic metric of a gapless composite fermion liquid has been successfully measured from the anisotropy of the composite-fermion Fermi surface [66,67]. As it is possible to image FQH quasiholes in experiments [68,70], the correspondence between the quasihole anisotropy and the intrinsic metric of the quasihole state, which we observe in our numerical results for the Laughlin state, suggests that the spatial extent of a localized quasihole may be used to experimentally estimate the intrinsic metric of a gapped FQH state.…”

“…Our theoretical results suggest a new experimental method to measure the intrinsic metric of an FQH state. Because the delta impurity used in our calculations can approximate the potential of an STM tip [13,15,16], it would be feasible to use STM to localize a quasihole and then measure its anisotropy by some imaging techniques [68][69][70], from which the intrinsic metric of the underlying FQH state can be estimated.…”

Exploring Quantum Hall Physics at Ultra-Low Temperatures and at High Pressures

“…Density inhomogeneity in high quality GaAs/AlGaAs samples may be estimated from the widths of sharp magnetoresistance peaks, 72 from an analysis of quantum lifetime measurements, 73 and it is also accessible with the micro-photoluminescence technique on lengthscales larger than the laser spotsize. 74,75 We believe that the improved sample homogeneity results from a combination of improved sample growth and sample illumination techniques.…”