Spectroscopy of soft modes and quantum phase transitions in coupled electron bilayers

Strongly correlated two-dimensional electrons in coupled semiconductor bilayers display remarkable broken symmetry many-body states under accessible and controllable experimental conditions. In the case of continuous quantum phase transitions (QPTs), soft collective modes drive the transformations that link distinct ground states of the electron double layers. In this paper we consider results showing that resonant inelastic light scattering methods detect soft collective modes of the double layers and probe their evolution with temperature and magnetic field. The light scattering experiments offer venues of research of fundamental interactions and continuous QPTs in low-dimensional electron liquids.     

Cyclotron spin-wave in the 2D electron system

The cyclotron spin-wave mode of a two-dimensional electron system have been investigated by inelastic light scattering. It is observed at small electron filling factors, (v～0.1, when the electron system is spin-depolarized. As long as the electron system becomes fully spin-polarized (v>0.2), the cyclotron spin-wave disappears from the inelastic light scattering spectra. It reenters at electron filling factors v>1. Over the range of electron filling factors of 1<v<2, the cyclotron spin-wave energy is insensitive to both the experimentally accessible in-plane momenta and the electron concentration, whereas its inelastic light scattering efficiency is strongly influenced by the spin polarization of the electron system.     

Theory of RIXS in strongly correlated electron systems: Mott gap excitations in cuprates

We theoretically examine the momentum dependence of resonant inelastic X-ray scattering (RIXS) spectrum for one-dimensional and two-dimensional cuprates based on the single-band Hubbard model with realistic parameter values. The spectrum is calculated by using the numerical diagonalization technique for finite-size clusters. We focus on excitations across the Mott gap and clarify spectral features coming from the excitations as well as the physics behind them. Good agreement between the theoretical and existing experimental results clearly demonstrates that the RIXS is a potential tool to study the momentum-dependent charge excitations in strongly correlated electron systems.     

Spectroscopy of Landau transitions of two-dimensional electron gases

We have employed resonant inelastic light scattering spectroscopy to study electronic transitions in multilayer two-dimensional electron gases in magnetic fields of 4–14 T. From polarized spectra, we have evidence for both single particle Landau transitions (Δl = 1 and Δl = 2) as well as their collective counterparts. We show the variation of intensities with B and the resonant behavior, but are unable to identify a scattering mechanism.     

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.     

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.     

A non-local approach to the theory of inelastic dynamic light scattering from solid-state plasmas

The existing theory of inelastic dynamic light scattering from coupled systems of free carriers and lattice excitations in absorbing crystals is extended to include non-local electronic transport effects.     

Comparison of elastic and inelastic scattering of phonons by a three-level spin system

Expressions for the phonon relaxation time due to elastic and inelastic scattering by small concentrations of paramagnetic ions having a simple three-level Zeeman-type spectrum are obtained. Both resonant and nonresonant processes can occur and the frequency dependence of each is examined for a model applicable to Fe²⁺ ions in MgO. Comparison with a Green function theory of coupled spin-phonon modes is made.     

Coordinate space structure in the parton model

We study the relation between the parton model and light cone analyses for highly inelastic leptonic processes. Results are displayed which follow from scaling laws in general, independently of specific parton model predictions. We conclude that the assumptions of the light cone analysis of inelastic electron scattering are supported by the parton model. However, the parton model matrix element for massive muon pair production is not light-cone dominated, nor does it have the same light cone singularity as inelastic electron scattering.     

Soft spin wave near nu=1: evidence for a magnetic instability in Skyrmion systems

The ground state of the two-dimensional electron gas near nu=1 is investigated by inelastic light scattering measurements carried down to very low temperatures. Away from nu=1, the ferromagnetic spin wave collapses and a new low-energy spin wave emerges below the Zeeman gap. The emergent spin wave shows soft behavior as its energy increases with temperature and reaches the Zeeman energy for temperatures above 2 K. The observed softening indicates an instability of the two-dimensional electron gas towards a magnetic order that breaks spin rotational symmetry. We discuss our findings in light of the possible existence of a Skyrme crystal.     

Quasiparticle lifetime in a two-dimensional semimetal

The quasiparticle lifetime in a two-dimensional direct band semimetal limited by inelastic Coulomb scattering has been calculated in the case where two types of charge carriers are present. It has been shown that the electron lifetime at low and high concentrations of holes is mainly determined by the electron-hole and electron-electron scattering, respectively.     

Scattering of excitons by free carriers in semiconducting quantum well structures

We have theoretically investigated the scattering of excitons by free electrons and holes in a two-dimensional semiconducting quantum well system. The scattering cross-sections have been calculated using the Born approximation for both the elastic and inelastic scattering of the excitons by the free carriers. The threshold for inelastic scattering is increased over the value in a bulk semiconductor because of the enhancement of the exciton binding energy by its confinement. The behavior of the scattering cross-section as a function of the energy of relative motion of the free carriers and the excitons is different than in the bulk and the cross-section is a more sensitive function of the ratio of the electron and hole masses than in the bulk. In fact, the scattering of light hole excitons is suppressed relative to that of heavy hole excitons by several orders of magnitude.     

Electron spectroscopy of iron disilicide

We have reported on the results of a complex investigation of iron disilicide FeSi₂ using characteristic electron energy loss spectroscopy, inelastic electron scattering cross section spectroscopy, and X-ray photoelectron spectroscopy. It has been shown that the main peak in the spectra of inelastic electron scattering for FeSi₂ is a superposition of two unresolved peaks, viz., surface and bulk plasmons. An analysis of the fine structure of the spectra of inelastic electron scattering cross section by their decomposition into Lorentzlike Tougaard peaks has made it possible to quantitatively estimate the contributions of individual energy loss processes to the resulting spectrum and determine their origin and energy.     

Effect of impurities on thermoelectric power due to phonon drag in metals

The effect of inelastic impurity scattering of electrons on the thermoelectric power due to phonon drag in metals has been studied. It is shown that this is the main cause of the thermoelectric power suppression due to doping at low temperatures. The thermoelectric power in a metal with a quadratic electron spectrum has been calculated as a function of temperature and impurity concentration. In addition to the impurity concentration, the correction to the thermoelectric power due to inelastic scattering contains the large factor Θ_D/T. Zh. éksp. Teor. Fiz. 111, 2237–2242 (June 1997)     

Scattering times in AlGaN/GaN two-dimensional electron gas from magnetotransport measurements

The various scattering times of two-dimensional electron gas were investigated in modulation-doped Al_0.22Ga_0.78N/GaN quantum wells by means of magnetotransport measurements. The ratio of transport and quantum scattering times, τ_t/τ_q∼1, shows that the dominant mobility-limiting mechanisms are short-range scattering potentials. The low-field magnetoresistance shows the weak antilocalization and localization phenomenon from which the spin-orbit scattering and inelastic scattering times are obtained. The inelastic scattering time is found to follow the T⁻¹ law, indicating that electron-electron scattering with small energy transfer is the dominant inelastic process.     

Comparative analysis of characteristic electron energy loss spectra and inelastic scattering cross-section spectra of Fe

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.     

Light scattering by electrons and renormalization of the electron spectrum in osmium

The temperature-dependent inelastic light scattering by electrons in osmium is studied at different values and directions of the wave vector. The frequency dependence for the spectra of electronic light scattering is modeled based on the calculated band structure taking properly into account the effects of inelastic scattering. A comparison of the measured and calculated spectra suggests the dominant role of the electron-phonon scattering in the energy range under study. This allows us to estimate the temperature-dependent renormalization of the electron velocity and the decay rate for the electron states.     

Background subtraction in electron spectroscopy by use of reflection electron energy loss spectra

The use of reflected electron energy loss spectra (REELS) in deconvoluting the inelastic background signal from XPS and AES spectra from homogeneous samples is studied. It is demonstrated that under certain assumptions, the cross section for inelastic electron scattering can be extracted from a REELS spectrum. This cross section is applied to deconvolute an experimental XPS spectrum of aluminium. The method, its limitations and its relation to other methods are discussed.     

Synthesis and Growth Mechanism of Ultralong ZnO Nanocombs and Nanobelts on Cu Substrate

In the two-dimensional electron systems with strong coupling in MgZnO/ZnO heterostructures, the thermal behavior of Ising quantum Hall ferromagnets at the filling factor ν = 2 has been studied. The spin polarization of Hall ferromagnets has been detected by measuring the signal related to the inelastic light scattering by intrasubband spin excitons. A stepwise change in the spin polarization at the phase transition at the filling factors ν = 2,3, and 4 in the heterostructures with different electron densities has been observed. The thermal stability of the Hall ferromagnetic phases at ν = 2 has been studied and the Curie temperature has been estimated. It has been shown that the Curie temperature is determined by the formation energy for domain walls in the Ising quantum Hall ferromagnets.