Probing collective modes of correlated states of few electrons in semiconductor quantum dots

Low-lying collective excitations above highly correlated ground states of few interacting electrons confined in GaAs semiconductor quantum dots are probed by resonant inelastic light scattering. We highlight that separate studies of the changes in the spin and charge degrees of freedom offer unique access to the fundamental interactions. The case of quantum dots with four electrons is found to be determined by a competition between triplet and singlet ground states that is uncovered in the rich light scattering spectra of spin excitations. These light scattering results are described within a configuration-interaction framework that captures the role of electron correlation with quantitative accuracy. Recent light scattering results that reveal the impact of anisotropic confining potentials in laterally coupled quantum dots are also reviewed. In these studies, inelastic light scattering methods emerge as powerful probes of collective phenomena and spin configurations in quantum dots with few electrons.     

Resonant enhancement of inelastic light scattering in the fractional quantum Hall regime at ν=1/3

Strong resonant enhancements of inelastic light scattering from the long wavelength inter-Landau level magnetoplasmon and the intra-Landau level spin wave excitations are seen for the fractional quantum Hall state at ν=1/3. The energies of the sharp peaks (FWHM 0.2 meV) in the profiles of resonant enhancement of inelastic light scattering intensities coincide with the energies of photoluminescence bands assigned to negatively charged exciton recombination. To interpret the observed enhancement profiles, we propose three-step light scattering mechanisms in which the intermediate resonant transitions are to states with charged excitonic excitations.     

Light scattering observations of spin reversal excitations in the fractional quantum Hall regime

Resonant inelastic light scattering experiments access the low lying excitations of electron liquids in the fractional quantum Hall regime in the range 2/5≥ν≥1/3. Modes associated with changes in the charge and spin degrees of freedom are measured. Spectra of spin reversed excitations at filling factor ν?1/3 and at ν?2/5 identify a structure of lowest spin-split Landau levels of composite fermions (CFs) that is similar to that of electrons. Observations of spin wave excitations enable determinations of energies required to reverse spin. The spin reversal energies obtained from the spectra illustrate the significant residual interactions of composite fermions. At ν=1/3 energies of spin reversal modes are larger but relatively close to spin conserving excitations that are linked to activated transport. Predictions of composite fermion theory are in good quantitative agreement with experimental results.     

Evidence of Landau levels and interactions in low-lying excitations of composite fermions at 1/3<or= nu <or=2/5

Excitation modes in the range 2/5>or=nu>or=1/3 of the fractional quantum Hall regime are observed by resonant inelastic light scattering. Spectra of spin-reversed excitations suggest a structure of lowest spin-split Landau levels of composite fermions that is similar to that of electrons. Spin-flip energies determined from spectra reveal significant composite fermion interactions. The filling factor dependence of mode energies displays an abrupt change in the middle of the range when there is partial population of a composite fermion level.     

A relation between the resonance neutron peak and ARPES data in cuprates

We argue that the resonant peak observed in neutron scattering experiments on superconducting cuprates and the peak/dip/hump features observed in ARPES measurements are byproducts of the same physical phenomenon: both are due to feedback effects on the damping of spin fluctuations in a d-wave superconductor. We argue that in the superconducting phase, the dynamical spin susceptibility possesses the resonance peak at Ω_res∝ξ⁻¹ where ξ is the magnetic correlation length. The scattering of the resonant magnetic excitations by electrons gives rise to a peak/dip/hump behavior of the electronic spectral function, the peak-dip separation is exactly Ω_res.     

Evidence of correlation in spin excitations of few-electron quantum dots

We report inelastic light scattering measurements of spin and charge excitations in nanofabricated AlGaAs/GaAs quantum dots with few electrons. A narrow spin excitation peak is observed and assigned to the intershell triplet-to-singlet monopole mode of dots with four electrons. Configuration-interaction theory provides precise quantitative interpretations that uncover large correlation effects that are comparable to exchange Coulomb interactions.     

Localized electronic excitations in NiO studied with resonant inelastic X-Ray scattering at the Ni M threshold: evidence of spin flip

We studied the neutral electronic excitations of NiO localized at the Ni sites by measuring the resonant inelastic x-ray scattering (RIXS) spectra at the Ni M2,3 edges. The good energy resolution allows an unambiguous identification of several spectral features due to excitations. The dependence of the RIXS spectra on the excitation energy gives evidence of local spin flip and yields a value of 125 +/- 15 meV for the antiferromagnetic exchange interaction. Accurate crystal field parameters are also obtained.     

Inelastic light scattering by electron excitations with large wave vectors in a 2D magnetoplasma

Microscopic mechanisms of inelastic light scattering in an interacting electron plasma in semiconductor heterostructures are considered. In the dipole limit, the cross section consists of two main contributions: the first is related to a disorder-induced mechanism and the second arises from the Coulomb interaction. The spectra of disorder-induced light scattering are described in terms of correlation functions of a random potential. The spectrum induced by the Coulomb interaction arises from two-quasiparticle excitations. The mechanisms which are studied in this paper result in the appearance of large wave vector excitations in the spectra of resonant light scattering. These results can be used to model the experimentally observed appearance of the roton density of states in light scattering spectra in the integer quantum Hall regime of a two-dimensional system. Furthermore, we show that the lineshape of spectra strongly depends on the character of disorder and, in particular, on the spatial positions of impurities with respect to a quantum well. Zh. éksp. Teor. Fiz. 112, 1041–1054 (September 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

An extraction algorithm for core‐level excitations in non‐resonant inelastic X‐ray scattering spectra

Non‐resonant inelastic X‐ray scattering of core electrons is a prominent tool for studying site‐selective, i.e. momentum‐transfer‐dependent, shallow absorption edges of liquids and samples under extreme conditions. A bottleneck of the analysis of such spectra is the appropriate subtraction of the underlying background owing to valence and core electron excitations. This background exhibits a strong momentum‐transfer dependence ranging from plasmon and particle–hole pair excitations to Compton scattering of core and valence electrons. In this work an algorithm to extract the absorption edges of interest from the superimposed background for a wide range of momentum transfers is presented and discussed for two examples, silicon and the compound silicondioxide.     

Spectroscopy of few-electron collective excitations in charge-tunable artificial atoms

We report the investigation of electronic excitations in InGaAs self-assembled quantum dots using resonant inelastic light scattering. The dots can be charged via a gate by N=1, em leader,6 electrons. We observe excitations, which are identified as transitions of electrons, predominantly from the s to the p shell (s-p transitions) of the quasiatoms. We find that the s-p transition energy decreases and the observed band broadens, when the p shell is filled with 1 to 4 electrons. By a theoretical model, which takes into account the full Coulomb interaction in the few-electron artificial atom, we can confirm the experimental results to be an effect of the Coulomb interaction in the quantum dot.     

Spin texture and magnetoroton excitations at nu=1/3

Neutral spin texture (ST) excitations at nu=1/3 are directly observed for the first time by resonant inelastic light scattering. They are determined to involve two simultaneous spin flips. At low magnetic fields, the ST energy is below that of the magnetoroton minimum. With increasing in-plane magnetic field these mode energies cross at a critical ratio of the Zeeman and Coulomb energies of eta(c)=0.020+/-0.001. Surprisingly, the intensity of the ST mode grows with temperature in the range in which the magnetoroton modes collapse. The temperature dependence is interpreted in terms of a competition between coexisting phases supporting different excitations. We consider the role of the ST excitations in activated transport at nu=1/3.     

Estimation of the cross section of neutron scattering by spin waves in thin ferromagnetic layers

Inelastic neutron scattering by magnetic excitations in thin ferromagnetic films has not been observed so far owing to the small cross section of the interaction of neutrons with spin waves. To increase the probability of inelastic magnetic scattering, it has been proposed to implement three-layer structures in which the neutron wave functions exhibit resonant enhancement in a ferromagnetic layer. The cross section of neutron scattering by spin waves in the regime of the resonant enhancement of the neutron wave function has been estimated.     

Theory of resonant Raman scattering from low-energy collective excitations of interacting electrons in quantum wires

The Raman spectra of quantum wires in the region of electronic intra-band excitations are investigated using one- and two-band models based on the Luttinger approximation with spin. Structures related to charge and spin density modes are identified, and analyzed with respect to their behavior with photon energy and temperature. It is found that the low-energy peaks in the polarized spectra, close to resonance that are commonly assigned to “single particle excitations”, can be interpreted as the signature of spin density excitations. A broad structure in the resonant depolarized spectrum is predicted above the frequency of the spin density excitations. This is due to simultaneous but independent propagation of spin and charge density modes. The results, when compared with experiment, show, that the electronic collective excitations of quantum wires at low energies are characteristic for a non-Fermi liquid. Received: 25 March 1998 / Accepted: 3 June 1998     

Rapid collapse of spin waves in nonuniform phases of the second Landau level

The spin degree of freedom in quantum phases of the second Landau level is probed by resonant light scattering. The long wavelength spin wave, which monitors the degree of spin polarization, is at the Zeeman energy in the fully spin-polarized state at ν = 3. At lower filling factors, the intensity of the Zeeman mode collapses, indicating loss of polarization. A novel continuum of low-lying excitations emerges that dominates near ν = 8/3 and ν = 5/2. Resonant Rayleigh scattering reveals that quantum fluids for ν < 3 break up into robust domain structures. While the state at ν = 5/2 is considered to be fully polarized, these results reveal unprecedented roles for spin degrees of freedom.     

Observation of intersubband excitations in a multilayer two dimensional electron gas

We report the observation, by resonant inelastic light scattering, of intersubband excitations of the multilayer two dimensional electron gas, in modulation doped GaAsAlGaAs heterojunction superlattices. These are the first measurements of these transitions by any technique, and furnish intersubband energies in good agreement with calculated values. The spectral bands are broad, and nearly Lorentzian in shape: the implied relaxation rates scale linearly with band energy and are significantly faster than transport relaxation rates. Finally, the polarized spectra reveal differences between spin-flip and non spin-flip excitations which are unique to multilayer two dimensional electron gases.     

Observation of collapse of pseudospin order in bilayer quantum Hall ferromagnets

The Hartree-Fock paradigm of bilayer quantum Hall states with finite tunneling at filling factor nu=1 has full pseudospin ferromagnetic order with all the electrons in the lowest symmetric Landau level. Inelastic light scattering measurements of low energy spin excitations reveal major departures from the paradigm at relatively large tunneling gaps. The results indicate the emergence of a novel correlated quantum Hall state at nu=1 characterized by reduced pseudospin order. Marked anomalies occur in spin excitations when pseudospin polarization collapses by application of in-plane magnetic fields.     

Possible observation of cycloidal electromagnons in BiFeO3

We unravel the magnon spectra of BiFeO3 by means of low-energy inelastic light scattering. We show the existence of two species of magnons corresponding to spin wave excitations in and out of the cycloidal plane. These excitations might be interpreted as electromagnon modes. The present observations present an unique opportunity to study the competition between ferroelectric and magnetic orders.     

Spectroscopy of phonons and spin torques in magnetic point contacts

Phonon spectroscopy is used to investigate the mechanism of current-induced spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization excitations observed in the magneto-conductance of the point contacts are pronounced for diffusive and thermal contacts, where the electrons experience significant scattering in the contact region. We find no magnetic excitations in highly ballistic contacts. Our results show that impurity scattering at the N/F interface is the origin of the new single-interface spin torque effect.     

Optical control of spin coherence in singly charged (In,Ga)As/GaAs quantum dots

Electron spin coherence has been generated optically in n-type modulation doped (In,Ga)As/GaAs quantum dots (QDs) which contain on average a single electron per dot. The coherence arises from resonant excitation of the QDs by circularly polarized laser pulses, creating a coherent superposition of an electron and a trion. Time dependent Faraday rotation is used to probe the spin precession of the optically oriented electrons about a transverse magnetic field. The coherence generation can be controlled by pulse intensity, being most efficient for (2n+1)pi pulses.