Effect of electron-hole spatial correlation on spin relaxation dynamics in InAs submonolayer

Using time-resolved photoluminescence and time-resolved Kerr rotation spectroscopy, we explore the unique electron spin behavior in an InAs submonolayer sandwiched in a GaAs matrix, which shows very different spin characteristics under resonant and non-resonant excitations. While a very long spin relaxation lifetime of a few nanoseconds at low temperature is observed under non-resonant excitation, it decreases dramatically under resonant excitation. These interesting results are attributed to the difference in electron-hole interactions caused by non-geminate or geminate capture of photo-generated electron-hole pairs in the two excitation cases, and provide a direct verification of the electron-hole spatial correlation effect on electron spin relaxation.     

Excitations in charge-transfer atom scattering

The electronic excitations are calculated for a tight-binding model of 25–1000 eV Na atoms scattering off W, assuming a classical trajectory for the Na atoms which interact with a single half-filled band on the substrate. The excitation spectrum consists of substrate electron-hole pairs at low energies, with a jump at the ionization threshold due to electron transfer from the Na to the W. If the Na ionization level crosses the Fermi energy beyond the range of hopping between the Na and the W substrate, the ionization probability is high. As the Na kinetic energy is reduced the ionization probability decreases, but the substrate electron-hole excitations increase in importance, and this is discussed in semi-classical terms.     

Molecular transport junctions: current from electronic excitations in the leads

Using a model comprising a two-level bridge connecting free electron reservoirs we show that coupling of a molecular bridge to electron-hole excitations in the leads can markedly effect the source-drain current through a molecular junction. In some cases, e.g., molecules that exhibit strong charge transfer transitions, the contribution from electron-hole excitations can exceed the Landauer elastic current and dominate the observed conduction.     

Coherent ultrafast dynamics in the quantum Hall effect regime

We review recent investigations of the femtosecond non-linear optical response of the two-dimensional electron gas (2DEG) in the quantum Hall effect regime. We find that the time and frequency profile of the four-wave-mixing non-linear optical spectrum is strongly influenced by Coulomb correlations between the photoexcited electron-hole pairs and the 2DEG collective excitations. We discuss experimental and theoretical results showing non-Markovian memory effects in the polarization dephasing, and an optically induced time-dependent coupling between the two lowest Landau level magnetoexcitons.     

Elementary excitations in nanochannel arrays of quantum wires

We present an analytical model for the Coulomb interaction effects in quantum wires forming a nanochannel array. We study the elementary excitations (plasmons and electron-hole excitations) of electron arrays forming three-dimensional structures. The plasmon spectrum of boson arrays is also calculated. Our model applies to bulk material with one-dimensional conduction channels as realized in organic or polymer crystals and in nanochannel array glasses.     

Resonant inelastic x-ray scattering study of charge excitations in La(2)CuO(4)

We report a resonant inelastic x-ray scattering study of the dispersion relations of charge-transfer excitations in insulating La(2)CuO(4).. These data reveal two peaks, both of which show two-dimensional characteristics. The lowest energy excitation has a gap energy of approximately 2.2 eV at the zone enter, and a dispersion of approximately 1 eV. The spectral weight of this mode becomes dramatically smaller around (pi, pi). The second peak shows a smaller dispersion ( approximately 0.5 eV) with a zone-center energy of approximately 3.9 eV. We argue that these are both highly dispersive exciton modes damped by the presence of the electron-hole continuum.     

Out-of-equilibrium heating of electron liquid: fermionic and bosonic temperatures

We investigate out-of-the equilibrium properties of the electron liquid in a two-dimensional disordered superconductor subject to the electric bias and temperature gradient. We calculate kinetic coefficients and Nyquist noise, and find that they are characterized by distinct effective temperatures: Te, characterizing single-particle excitations, TCp, describing the Cooper pairs, and Teh, corresponding to electron-hole or dipole excitations. Varying the ratio between the electric j and thermal jth currents and boundary conditions one can heat different kinds of excitations tuning their corresponding temperatures. We propose the experiment to determine these effective temperatures.     

On the possibility of superconductivity in electron-hole liquids

It is shown that an electron-hole liquid under suitable conditions can become superconducting. This conclusion is reached by using an effective electron-electron interaction which includes vertex corrections and multiple electron-hole scattering. The mechanism for superconductivity is novel. The intermediate bosons responsible for the superconducting pairing are not acoustic plasmons but correlated pair excitations from the Fermi sea of the holes. Some materials are proposed for an experimental verification of the ideas presented here.     

Laserlike vibrational instability in rectifying molecular conductors

We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor (D-A) type molecular rectifiers. At finite voltage additional nonequilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to the inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser action. We investigate the effect in realistic molecular rectifier structures using first-principles calculations.     

Competition between electron and phonon excitations in the scattering of nitrogen atoms and molecules off tungsten and silver metal surfaces

We investigate the role played by electron-hole pair and phonon excitations in the interaction of reactive gas molecules and atoms with metal surfaces. We present a theoretical framework that allows us to evaluate within a full-dimensional dynamics the combined contribution of both excitation mechanisms while the gas particle-surface interaction is described by an ab initio potential energy surface. The model is applied to study energy dissipation in the scattering of N(2) on W(110) and N on Ag(111). Our results show that phonon excitation is the dominant energy loss channel, whereas electron-hole pair excitations represent a minor contribution. We substantiate that, even when the energy dissipated is quantitatively significant, important aspects of the scattering dynamics are well captured by the adiabatic approximation.     

Dynamics of self-localized excitations in a polyacene chain

Dynamical formation processes of self-localized excitations induced by charge injection or photoexcitations in a polyacene chain are investigated by a nonadiabatic dynamic method. The polyacene chain is treated as two alternatively coupled polyacetylene chains. The initial lattice configuration is taken as the pristine polyacene chain. It contains an interchain-coupled neutral soliton as a consequence of odd-number sites in each of the two chains. The nonadiabatic dynamical processes are carefully investigated in the following physical cases: (1) electron injection; (2) electron transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO); (3) electron transition from HOMO to the localized soliton level, and (4) electron-hole pair excited at the continuum absorbtion edge for light polarized parallel to the chain. It is interestingly found that the centers of the electron and the hole excited by light polarized parallel to the chain are separated. Therefore, the photogenerated charge carriers should be favorable in polyacene, which is remarkably different from those found in a single polyacetylene chain.     

Condensation effects of excitons

The theory of the electronic excitations in a highly excited semiconductor is presented. The relaxation processes, the formation of excitons and excitonic molecules, the interaction among the various forms of electronic excitations, as well as their optical and thermodynamical properties are analyzed. At low temperatures one expects condensations into the quantum statistically degenerate phases of the excitonic molecules and of the electron-hole plasma. The physical properties of these low temperature phases are investigated. Possibilities and previous attempts to observe the Bose-Einstein condensation in excitonic systems are discussed critically. The experimental observations of the electron-hole liquid phase transition are reviewed.     

Supersymmetry and electron-hole excitations in semiconductors at finite temperature

The fermionic and bosonic electron-hole low-lying excitations in a semiconductor are analyzed at finite temperature in a unified way following Nambu's quasi-supersymmetric approach for the BCS model of superconductivity. The effective lagrangian for the fermionic modes and for the bosonic low-lying collective excitations in the semiconductor is no longer supersymmetric in a conventional finite-temperature treatment. However, the bosonic excitations do not couple directly to the heat bath and as a result, quasi-supersymmetry is restored to the effective lagrangian when a redefinition of the coupling constant associated with the collective excitations is performed. Our result shows that although the mass and coupling parameters are now temperature dependent, the fermion and boson excited states pair together and can still be transmuted into one another.     

On the absorption of electromagnetic radiation near a metal surface

We present a discussion on the absorptance of electromagnetic radiation per unit length in the surface region of a free electron like metal. We wish particularly to comment on recent work by Feibelman, Kliewer, Forstmann and Stenscke, who all considered different approximations of the jellium model. We are using a unified treatment from which the results in their models can easily be obtained. Our results confirm the general conclusions by Kliewer about a strong coupling to the electron-hole excitations which lead to a locally negative absorptance. In the model considered by Feibelman there is no such coupling to electron-hole excitations and he obtains the classical results for the absorptance. Forstmann and Stenschke have recently applied a hydrodynamical approach which gives results in disagreement with Kliewer's theory as well as with our own results, and we make some comments about their approach.     

Microscopic theory of optical excitations,photoluminescence, and terahertz response in semiconductors

This article presents a comprehensive many-body theory for optically excited semiconductors. The coupled equations of motion for the correlation functions of the Coulomb-interacting electron-hole system are derived and solved for different excitation conditions. The generation of a coherent excitonic polarization and its conversion into incoherent populations is analyzed. The spontaneous emission properties of the excited system are evaluated using a fully quantized theory. Luminescence from excitonic and electron-hole plasma populations is computed, and significant hole burning in the exciton center of mass distributions is predicted. It is shown how different excitations states of the many-body system can be identified by their characteristic signatures in the absorption spectra of a terahertz probe field.     

Minimum electrical and thermal conductivity of graphene: a quasiclassical approach

We investigate the minimum conductivity of graphene within a quasiclassical approach taking into account electron-hole coherence effects which stem from the chiral nature of low energy excitations. Relying on an analytical solution of the kinetic equation in the electron-hole coherent and incoherent cases, we study both the electrical and the thermal conductivity whose relation satisfies the Wiedemann-Franz law. We find that most of the previous findings based on the Boltzmann equation are restricted to only high mobility samples where electron-hole coherence effects are not sufficient.     

Ground state of a Uranium impurity in a metal: Inclusion of Self-Energy terms

A previous investigation of the system ground state for the mixed-valent Uranium ion fluctuating between two magnetic configurations in a free-electron sea is extended by including the Self-Energy terms (by Self-Energy terms we refer to additional terms in the wave function that contain electron-hole excitations of the Fermi sea) in the ground state variational functions. It is found that the presence of one electron-hole pair excitation in the calculation has risen the singlet to magnetic state energy separation as compared to the zero-order case.     

Excitation of electron-hole pairs in low-energy electron scattering from surfaces

We calculate the Distorted Wave Born Aprroximation differential cross section for the inelastic scattering of low-energy electrons reflected from metallic surfaces via the dynamically screened Coulomb potential, and in particular the excitation of low-energy electron-hole pairs. We discuss the angular and energy dependence of the loss spectra, and the application of this method to study the spectrum of the electron-hole excitations at surfaces.     

Optical properties of highly excited direct gap semiconductors

The electronic excitations in direct gap semiconductors interact strongly with the photon field. We discuss both the experimental and the theoretical aspects of the optical properties of these materials under strong optical excitation. We distinguish between intermediate excitation levels at which the electronic excitations form a dense system of excitons and excitonic molecules and very high excitation levels at which a degenerate electron-hole plasma occurs. The optical spectra of dense excitonic systems, which are mainly observed in copper halides and II–VI compounds, are shown to be determined mainly by the interaction processes between excitonic molecules, polaritons and free carriers. The optical properties of the electron-hole plasma, which has been observed in II–VI and especially in III–V compounds, can be understood only by taking into account many-body effects, such as dynamical screening of the Coulomb interactions, plasmon-assisted transitions and excitonic enhancement.     

The role of electron-hole pair excitations in the optical absorption of metals,particularly metal spheres

It is shown that electron-hole pair excitations can dominate the optical absorption of a small metal sphere over the entire frequency range up to the bulk plasma frequency. The limited size has the effect of giving more surface to volume, thereby giving a larger relative weight to the single particle excitations, predominantly produced in the surface region, as compared to the planar case where they make up for a smaller part of the total absorptance.