Single-particle excitations and many-particle interactions in quantum wires and dots

Abstract

We have investigated electronic excitations in GaAs-Al x Ga 1Ϫx As quantum wires, and dots by resonance Raman spectroscopy. We find for quantum dots and quantum wires three types of excitations; single-particle, spin-density, and charge-density excitations ͑SPE's, SDE's, and CDE's, respectively͒. Our experiments show that SPE's are enhanced under conditions of extreme resonance, which indicates that energy-density fluctuations are responsible for the excitation of unscreened SPE's. The high intensity of the SPE's allows a detailed analysis of collective effects in the zero-and one-dimensional electron systems. ͓S0163-1829͑96͒51848-5͔In high-quality modulation-doped GaAs-Al x Ga 1Ϫx As heterostructures nearly perfect two-dimensional ͑2D͒ electron systems with quantized energy levels can be realized. With sophisticated technology it is possible to produce lateral structures and reduce the dimensionality even further and fabricate one-dimensional ͑1D͒ quantum wires and zerodimensional ͑0D͒ quantum dots, the latter with a totally discrete energy spectrum. 1 These systems have attracted great interest since they allow, in specially tailored structures, a detailed investigation of quantum and, in particular, manybody effects. Raman spectroscopy is an ideal tool to study these many-body effects since, with polarization-dependent selection rules, one can map out different many-body effects. 2 For a parallel polarization of incoming and Ramanscattered photons one measures so called charge-density excitations ͑CDE's͒, which reflect the collective excited states of the electron system and which are renormalized due to the dynamic charge and spin redistribution. In highly symmetric systems CDE's are the only excitations observed in direct far-infrared absorption. 1 In Raman spectroscopy one can additionally measure in crossed polarization collective spindensity excitations, which are affected only by the dynamic spin redistribution. It was very surprising with respect to the so far accepted theory 3 that Pinczuk et al. 4 observed on high-quality two-dimensional systems, in addition to CDE's and SDE's, an excitation that exhibited all features of an unrenormalized single-particle excitation. This excitation is observed in both polarizations. These results have been confirmed by a number of authors 5 -we see it in our 2D samples, too-however, the scattering mechanism was so far not clear. There are also very interesting Raman experiments on quantum wires and dots. 6-13 Strenz et al. reported on SPE in quantum wires and observed SPE in dots. 11 Onedimensional electron systems which are nearly in the 1D quantum limit have been investigated by Schmeller et al. 9 They found that the relative energy renormalizations of the 1D intersubband SDE and CDE with respect to the simultaneously observed corresponding SPE are quite similar to those found for 2D systems. 4 However, the mechanism and conditions for the excitation and strength of collective effects for quantum dots and wires are even less clear. Very recently, an interesting pap...

Raman spectroscopy of wurtzite and zinc-blende GaAs nanowires: Polarization dependence, selection rules, and strain effects

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Collective electronic excitation in a trapped ensemble of photogenerated dipolar excitons and free holes revealed by inelastic light scattering

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Raman Spectroscopy on Semiconductor Nanowires

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