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Annalen der Physik

Luttinger-liquid features in the resonant Raman spectra of quantum wires

Maura Sassetti, B. Krämer

Published Nov 1, 1998• 4 citations
Publisher

Abstract

Using the bosonization technique, we develop a theory for the Raman scattering of the interacting electrons in quantum wires. The energy spectra of the collective excitations in a two-subband model are given. The density correlation functions for the Raman cross section are calculated. The charge and spin density excitations observed in recent Raman experiments are identified. Hitherto unexplained “single particle excitations” which appear near resonance and are independent of the relative polarization of incoming and scattered light are shown to originate in higher order spin density excitations, which can be evaluated due to the bosonization. For the intraband excitations, the peak strengths in the Raman cross section as a function of the frequency of the incoming light is explicitely calculated. Power law behaviors depending on the strength of the interaction are predicted.

DOI
10.1002/(sici)1521-3889(199811)7:5/6<508::aid-andp508>3.0.co;2-1

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Using the bosonization technique, we develop a theory for the Raman scattering of the interacting electrons in quantum wires. The energy spectra of the collective excitations in a two-subband model are given. The density correlation functions for the Raman cross section are calculated. The charge and spin density excitations observed in recent Raman experiments are identified. Hitherto unexplained “single particle excitations” which appear near resonance and are independent of the relative polarization of incoming and scattered light are shown to originate in higher order spin density excitations, which can be evaluated due to the bosonization. For the intraband excitations, the peak strengths in the Raman cross section as a function of the frequency of the incoming light is explicitely calculated. Power law behaviors depending on the strength of the interaction are predicted.
The bosonisation technique is used to calculate the resonant Raman spectrum of a quantum wire with two electronic sub-bands occupied. Close to resonance, the cross-section at frequencies in the region of the inter–sub-band transitions shows distinct peaks in parallel polarisation of the incident and scattered light that are signature of collective higher-order spin density excitations . This is in striking contrast to the conventional selection rule for non-resonant Raman scattering according to which spin modes can appear only in perpendicular polarisation. We predict a new selection rule for the excitations observed near resonance, namely that, apart from charge density excitations, only spin modes with positive group velocities can appear as peaks in the spectra in parallel configuration close to resonance. The results are consistent with all of the presently available experimental data.
A theory of resonant Raman scattering spectroscopy of one-dimensional electronic systems is developed on the assumptions that (i) the excitations of the one-dimensional electronic system are described by the Luttinger-liquid model, (ii) Raman processes involve virtual excitations from a filled valence band to an empty state of the one-dimensional electronic system, and (iii) excitonic interactions between the valence and conduction bands may be neglected. Closed form analytic expressions are obtained for the Raman scattering cross sections, and are evaluated analytically and numerically for scattering in the polarized channel, revealing a ``double-peak'' structure with the lower peak involving multispinon excitations with total spin S=0S=0S=0 and the higher peak being the conventional plasmon. A key feature of our results is a nontrivial power-law dependence, involving the Luttinger-liquid exponents, of the dependence of the Raman cross sections on the difference of the laser frequency from resonance. We find that near resonance the calculated ratio of intensity in the lower energy feature to the intensity in the higher energy feature saturates at a value of the order of unity (times a factor of the ratio of the velocities of the two modes). We explicate the differences between the ``Luttinger-liquid'' and ``Fermi-liquid'' calculations of resonant Raman spectroscopy (RRS) spectra and argue that excitonic effects, neglected in all treatments so far, are essential for explaining the intensity ratios observed in quantum wires. We also discuss other Luttinger-liquid features which may be observed in future RRS experiments.
We systematically investigate the mode dispersion and spectral weight of the elementary excitation spectra in one-dimensional continuum and lattice electron systems by using the RPA, the Luttinger liquid model, and the Hubbard model. Both charge and spin excitations are studied in detail and compared among the theoretical models. For the lattice Hubbard model we use both Bethe-ansatz equations and Lanczos-Gagliano method to calculate dispersion and spectral weight separately. We discuss the theoretically calculated elementary excitation spectra in terms of the experimental inelastic light (Raman) scattering spectroscopy of one-dimensional (1D) semiconductor quantum wire systems. Our results show that in the polarized (i.e., non-spin-flip) Raman-scattering spectroscopy, only the 1D charge density excitations should show up with observable spectral weight with the single-particle excitations (in random-phase approximation) or singlet spin excitations (in the Luttinger model and the Hubbard model) having negligible spectral weight. The depolarized (spin-flip) Raman-scattering spectra manifest the spin density or the triplet spin excitations. We also provide a qualitative comparison between the continuum and the lattice 1D elementary excitation spectra.
The natural excitations of an interacting one-dimensional system at low energy are the hydrodynamic modes of a Luttinger liquid, protected by the Lorentz invariance of the linear dispersion. We show that beyond low energies, where the quadratic dispersion reduces the symmetry to Galilean, the main character of the many-body excitations changes into a hierarchy: calculations of dynamic correlation functions for fermions (without spin) show that the spectral weights of the excitations are proportional to powers of R2/L2{\mathcal{R}}^{2}/{L}^{2}R2/L2, where R\mathcal{R}R is a length-scale related to interactions and LLL is the system length. Thus only small numbers of excitations carry the principal spectral power in representative regions on the energy-momentum planes. We have analyzed the spectral function in detail and have shown that the first-level (strongest) excitations form a mode with parabolic dispersion, like that of a renormalized single particle. The second-level excitations produce a singular power-law line shape to the first-level mode and multiple power laws at the spectral edge. We have illustrated a crossover to a Luttinger liquid at low energy by calculating the local density of states through all energy scales: from linear to nonlinear, and to above the chemical potential energies. In order to test this model, we have carried out experiments to measure the momentum-resolved tunneling of electrons (fermions with spin) from/to a wire formed within a GaAs heterostructure. We observe a well-resolved spin-charge separation at low energy with appreciable interaction strength and only a parabolic dispersion of the first-level mode at higher energies. We find a structure resembling the second-level excitations, which dies away rapidly at high momentum in line with the theoretical predictions here.