Resonant Raman Scattering by Elementary Electronic Excitations in Semiconductor Structures

Abstract

We explain quantitatively why resonant Raman scattering spectroscopy, an extensively used experimental tool in studying elementary electronic excitations in doped low-dimensional semiconductor nanostructures, always produces an observable peak at the so-called "single particle" excitation although the standard theory predicts that there should be no such single particle peak in the Raman spectra. We have thus resolved an experimental puzzle which dates back more than 25 years. PACS numbers: 73.20.Mf, 78.30.Fs Resonant Raman scattering (RRS) involving inelastic scattering of light by electrons has long been a powerful and versatile spectroscopic tool for studying [1][2][3][4][5][6][7][8][9][10] elementary excitations in doped low-dimensional semiconductor structures such as quantum wells, superlattices, and, more recently, one-dimensional quantum wire systems. RRS has been extensively used in experimentally studying the collective charge density excitation (CDE) dispersion in semiconductor quantum wells, quantum wires, and superlattices for both intrasubband and intersubband transitions. In the standard theory [11,12], which ignores the role of the valence band and simplistically assumes the photon to be interacting entirely with conduction band electrons, RRS intensity is proportional to the dynamical structure factor [13] of the conduction band electron system and therefore has peaks at the collective mode frequencies at the appropriate wave vectors defined by the experimental geometry. Restricting to the polarized RRS geometry [12], where the incident and scattered photons have the same polarization vectors indicating the absence of spin flips in the electronic excitations, the dynamical structure factor peaks should correspond to the poles of the reducible density response function, which are given by the collective CDEs of the system. In particular, the single particle electron-hole excitations (SPE), which are at the poles of the corresponding irreducible response function, carry no long wavelength spectral weight in the density response function and should not, as a matter of principle, show up in the polarized RRS spectra. The remarkable experimental fact, however, is that there is always a relatively weak (but quite distinct) low energy SPE peak in the observed RRS spectra (near resonance) in addition to the expected strong CDE peak at higher energy. This observed SPE peak in the polarized RRS spectra is a factor of 10 3 10 4 times stronger than that given by the calculated dynamical structure factor in the standard theory. This puzzling feature of an ubiquitous anomalous SPE peak (in addition to the expected CDE peak) in the observed RRS spectra occurs in one-dimensional GaAs-AlGaAs quantum wires, twodimensional GaAs quantum wells, and even in the doped three-dimensional bulk GaAs systems [1]. It exists in the low-dimensional structures both for intrasubband and intersubband (i.e., transitions along and perpendicular to the confining quantization direction) excitations, and in zero and finite magnetic...

Symmetries in the collective excitations of an electron gas in core-shell nanowires

“…Indeed, ILS has been used for many years to study the collective excitations of excess carriers in semiconductor heterostructures [24,25], as it enables the detection of charge-density excitations (CDEs) and spin-density excitations (SDEs) separately, and, under strong resonant conditions, unscreened single-particle excitations (SPEs) [26][27][28][29][30][31][32]. The comparison of ILS with theoretical models allows one to obtain the subband structure, electron density, mobilities, many-body interactions, etc.…”

Raman Light Scattering in Pseudospin-Electron Model at Strong Pseudospin-Electron Interaction

“…Many investigations (both theoretical and experimental) were devoted to the study of Raman spectra in high-temperature superconductors (HTSC) (see review [1]) in order to better understand the nature of superconducting pairing mechanism and to explain the reason of complicated Raman spectra in HTSC. Raman spectra are also investigated for normal (nonsuperconducting) state for the cases of metals and dielectrics [2][3][4]. Resonance, nonresonance and mixed contributions to Raman spectra are studied [3][4][5].…”

“…Raman spectra are also investigated for normal (nonsuperconducting) state for the cases of metals and dielectrics [2][3][4]. Resonance, nonresonance and mixed contributions to Raman spectra are studied [3][4][5].…”

“…In this work we calculate Raman spectra for the system described by the pseudospin-electron model (PEM) using the method which is based on the construction of a polarizability operatorP in the framework of a microscopic approach; the method was developed in [6][7][8][9] and is an alternative to standard ones [2][3][4][5]. PEM describes the systems with locally anharmonic elements (anharmonic vibrations of apical oxygen O4 in HTSC of YBaCuO-type, the systems with hydrogen bonds, etc.).…”

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

“…We note that if the polarization selection rules are strictly fulfilled, plasmon excitations should be only observable in the polarized scattering geometry with incoming and scattered light copolarized [66,67] . In our experiments, however, the selection rules are less strict due to the combined effects of excitations under extreme resonance conditions [68] , valence band mixing of heavy and light hole bands resulting in a band non-parabolicity [69] and an applied electric field perpendicular to the QW plane.…”