Electron-photon Interaction Research Articles

Inelastic transport through a single molecular dot in the presence of electron-electron interaction

We studied the low-temperature inelastic transport properties of a single quantum dot with the on-site Coulomb repulsive interaction, the electron-phonon interaction, and the electron-photon interaction, using the nonequilibrium Green function (NEGF) approach and the canonical transformation. The time-averaged occupation number, the time-averaged differential conductance and time-averaged current are calculated self-consistently in the quantum dot. The variation in the differential conductance with the frequency of the external field and the incident electron energy is presented. Some interesting electrical transport characteristics come out in the system due to the electron-electron correlation. It is found that the two kinds of phonon-emission peaks may coexist in the time-averaged differential conductance spectrum due to the charge accumulations. One enhances both the time-averaged occupation number and the current even in the low-temperature case, while the other suppresses them. The magnitudes of the phonon peaks are dominated by the time-averaged occupation numbers, so the phonon peaks can be modulated by the external irradiations.

Quantum Phenomena of Carbon Nanotubes

Quantum phenomena of carbon nanotube are overviewed. Optical and transport properties of carbon nanotubes can be understood in terms of electron-photon and electron-phonon interaction. We will discuss the relevance of these interactions to the quantum properties.

Linear polarization of the2p53s→2p6lines following the inner-shell photoionization of sodiumlike ions

The inner-shell photoionization of highly charged, many-electron ions and their subsequent radiative decay are studied theoretically within the multiconfiguration Dirac-Fock approach. Special attention is paid to the linear polarization of the characteristic x-ray radiation. Detailed calculations have been carried out, in particular for the $2{p}^{5}3s\ensuremath{\rightarrow}2{p}^{6}$ radiative transitions following ionization of the $2p$ electron of the sodiumlike iron ${\mathrm{Fe}}^{15+}$ and uranium ${\mathrm{U}}^{81+}$ ions. For these elements, the inner-shell photoionization was found to induce the (relatively) low linear polarization of the subsequently emitted photons which is strongly affected, moreover, by the higher-order (nondipole) effects in the electron-photon interaction.

Nonrelativistic quantum electrodynamic approach to photoemission theory: I. Basic theoretical framework

A new nonrelativistic many-body theory to analyze X-ray photoelectron spectroscopy (XPS) spectra has been developed on the basis of quantum electrodynamic (QED) Keldysh Green’s function approach. To obtain XPS current density we calculate electron Green’s function g < which partly includes electron–photon interactions. We first separate longitudinal and transverse parts of these Green’s functions in the Coulomb gauge. The transverse electron selfenergy describes the electron–photon interaction, whereas the longitudinal electron selfenergy describes the electron–electron interaction. We derive the QED Hedin’s equation from which we obtain systematic skeleton expansion in the power series of the screened Coulomb interaction W and the photon Green’s function D k l . We show the present theory provides a sound theoretical tool to study complicated many-body processes such as the electron propagation damping, intrinsic, extrinsic losses and their interference, and furthermore, resonant photoemission processes. We have also found the importance of the mixed photon Green’s functions D 0 k and D k 0 which have been supposed to be unimportant for the XPS analyses. They, however, directly describe the radiation field screening. In this work, photon field screening effects are discussed in one-step theory, where the electron–photon interaction operator Δ is proved to be replaced by ε − 1 Δ beyond linear approximation. Beyond free photon Green’s function approximation, photon scatterings from the electron density are incorporated within the present QED theory. These photon field effects can directly describe the microscopic photon field spatial variation specific to near the surface region and nanoparticle systems.

Correlated radiative capture of two electrons with formation of heliumlike ion in the 1s2s 1S state

We investigate the correlated double-electron capture by a bare nucleus followed by the emission of a single photon and the formation of heliumlike ion in the excited 2 1 S state. The total cross section of the process is calculated taking into account the leading orders of QED perturbation theory. The present evaluation is focused on the domain of collision energies, which allows for the dipole approximation in treatment of the electron–photon interaction. The universal scaling is found for the ratio of cross sections for capture into the 2 1 S and 1 1 S states. In the low-energy regime, the probability for the population of excited states is shown to exceed significantly that of the ground state.

Three-Coulomb-wave Pluvinage model for Compton double ionization of helium in the region of the cross-section maximum

Double ionization of ground-state helium by the Compton scattering of a photon is calculated using the well-known product of three Coulomb waves (3C) wave function for the final double-continuum state of the atom and the Pluvinage wave function for the initial ground state (also a product of three Coulomb waves). The theoretical model fails to predict both the shape and the overall magnitude of the observed integrated cross section vs incident-photon energy. However, when the monopole part of the electron-photon interaction is removed by an ad hoc procedure, qualitative agreement is obtained, although the absolute size of the cross section is still almost an order of magnitude too large.

Electron-phonon and electron-photon interactions and resonant Raman scattering from the radial-breathing mode of single-walled carbon nanotubes

The resonance Raman profile of the radial-breathing mode is calculated for all 300 single-walled carbon nanotubes in the radius range from $2\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}12\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and for all optical transitions up to $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ using a symmetry-adapted nonorthogonal tight-binding model [V. N. Popov, New J. Phys. 6, 17 (2004)]. The influence of the electron-phonon and electron-photon interactions on the Raman intensity is studied using an approximate expression for the intensity in the vicinity of each optical transition as the product of the electron-phonon coupling matrix element, the momentum matrix element, and the effective mass raised to different powers. The dependence of the latter three quantities and the maximum Raman intensity on the nanotube radius, the chiral angle, and the optical transition energy is discussed in detail. In particular, the points of the corresponding plots exhibit family behavior of three different types. It is shown that the widespread practice to neglect the electron-photon and electron-phonon interactions in the estimation of the intensity can lead to incorrect prediction of the Raman spectra.

Unified theory of consequences of spontaneous emission in aΛsystem

In a $\Lambda$ system with two nearly degenerate ground states and one excited state in an atom or quantum dot, spontaneous radiative decay can lead to a range of phenomena, including electron-photon entanglement, spontaneously generated coherence, and two-pathway decay. We show that a treatment of the radiative decay as a quantum evolution of a single physical system composed of a three-level electron subsystem and photons leads to a range of consequences depending on the electron-photon interaction and the measurement. Different treatments of the emitted photon channel the electron-photon system into a variety of final states. The theory is not restricted to the three-level system.

Relativistic quantum dynamics in strong fields: photon emission from heavy, few-electron ions

Recent progress in the study of the photon emission from highly charged heavy ions is reviewed. These investigations show that high-Z ions provide a unique tool for improving the understanding of the electron–electron and electron–photon interaction in the presence of strong fields. Apart from the bound-state transitions, which are accurately described in the framework of quantum electrodynamics, much information has also been obtained from the radiative capture of (quasi-) free electrons by high-Z ions. Many features in the observed spectra hereby confirm the inherently relativistic behaviour of even the simplest compound quantum systems in nature.

The Band 12 Issue in the Electron Momentum Spectra of Norbornane: A Comparison with Additional Green's Function Calculations and Ultraviolet Photoemission Measurements

In continuation of a recent study of the electronic structure of norbornane [J. Chem. Phys., 2004, 121, 10525] by means of electron momentum spectroscopy (EMS), we present Green's Function calculations of the ionization spectrum of this compound at the ADC(3) level using basis sets of varying quality, along with accurate evaluations at the CCSD(T) level of the vertical (26.5 eV) and adiabatic (22.1 eV) double ionization thresholds under C(2v) symmetry. The obtained results are compared with newly recorded ultraviolet photoemission spectra (UPS), up to binding energies of 40 eV. The theoretical predictions are entirely consistent with experiment and indicate that, in a vertical depiction of ionization, shake-up states at binding energies larger than approximately 26.5 eV tend to decay via emission of a second electron in the continuum. A band of s-type symmetry that has been previously seen at approximately 25 eV in the electron impact ionization spectra of norbornane is entirely missing in the UPS measurements and theoretical ADC(3) spectra. With regard to these results and to the time scales characterizing electron-electron interactions in EMS (10(-17) s) as compared with that (10(-13) s) of photon-electron interactions in UPS, and considering the p-type symmetry of the electron momentum distributions for the nearest 1b(1) and 1b(2) orbitals, this additional band can certainly not be due to adiabatic double ionization processes starting from the ground electronic state of norbornane, or to exceptionally strong vibronic coupling interactions between cationic states derived from ionization of the latter orbitals. It is therefore tentatively ascribed to autoionization processes via electronically excited and possibly dissociating states.

Angular correlations in the two-photon decay of hydrogenlike ions: Relativistic Green’s-function approach

The angular correlations in the two-photon decay of hydrogenlike ions are studied within the framework of second-order perturbation theory, based on Dirac's equation. Particular attention has been paid to the effects which arise from the higher (nondipole) terms in the expansion of the electron-photon interaction. It is shown that the photon-photon angular correlation function, which is found symmetric with respect to the angle $\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}$ in the electric dipole approximation, becomes asymmetric because of the nondipole contributions, and that this effect is enhanced as the nuclear charge $Z$ increases. Detailed computations on the photon-photon angular distribution have been carried out for the $2{s}_{1∕2}\ensuremath{\rightarrow}1{s}_{1∕2}$ and $3{d}_{5∕2}\ensuremath{\rightarrow}1{s}_{1∕2}$ transitions in neutral hydrogen (H) as well as for hydrogenlike xenon $({\mathrm{Xe}}^{53+})$ and uranium $({\mathrm{U}}^{91+})$ ions, and are compared with previous nonrelativistic results by Au [Phys. Rev. A 14, 531 (1976)]).

IR Absorption by Free Charge Carriers with the Participation of Optical Phonons in Structures with Quantum Wells

The absorption of light by free charge carriers with the participation of surface and bulk optical phonons is considered in terms of the Boltzmann statistics and Pekar-Frohlich Hamiltonian for the electron-photon interaction in a polar semiconductor layer within the framework of the model of a rectangular quantum well. It is found that the contributions of the surface and bulk modes to the probability of light absorption depend on the quantum-well width. It is demonstrated that the line of the photon-phonon resonance has a complex structure due to the difference between the frequencies of the bulk and surface optical vibrations. The influence of the adjacent media on the coefficient of light absorption is investigated.

Self-interaction-corrected relativistic theory of magnetic scattering of x rays: Application to praseodymium

A first-principles theory of resonant magnetic scattering of x rays is presented. The scattering amplitudes are calculated using a standard time-dependent perturbation theory to second order in the electron-photon interaction vertex. In order to calculate the cross section reliably an accurate description of the electronic states in the material under investigation is required and this is provided by the density functional theory employing the local spin density approximation combined with the self-interaction corrections. The magnetic x-ray resonant scattering theory has been implemented in the framework of the relativistic spin-polarized linear muffin tin orbital with atomic sphere approximation band structure calculation method. The theory is illustrated with an application to ferromagnetic praseodymium. It is shown that the theory quantitatively reproduces the dependence on the spin and orbital magnetic moments originally predicted qualitatively [Blume, J. Appl. Phys. 57, 3615 (1985)] and yields results that can be compared directly with experiment.

Calculation of the nonlinear free-carrier absorption of terahertz radiation in semiconductor heterostructures

Nonlinear absorption of terahertz waves by electrons in a semiconductor heterostructure is calculated. We solve the quantum transport equation for electrons strongly coupled to terahertz photons. The electrical field of the laser radiation is included exactly, and the electron-impurity interaction is included up to the second order. It is found that Joule heating of the electronic system due to impurity scattering decreases rapidly due to the strong electron-photon interaction. Our result is the dynamic equivalence of electron localization in a strong field. In the limit of weak radiation field, the current is linear in the field strength.

Relativistic QED approach to XPS and XAFS spectra

A many-body relativistic theory to analyze X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) spectra has been developed on the basis of quantum electrodynamic (QED) Keldysh–Green’s function approach. To obtain XPS current density we calculate g <, and to obtain X-ray absorption intensity we calculate the time derivative of the projected photon Green’s function whose selfenergy includes relativistic electron Green’s functions. We first separate longitudinal and transverse parts of these Green’s functions in the Coulomb gauge. The transverse electron selfenergy describes the electron-photon interaction, whereas the longitudinal electron selfenergy describes the electron-electron interaction. For retarded and advanced electron Green’s functions we can apply a useful expansion in terms of correlated quasi-nonrelativistic Green’s functions, which enables us to use well-defined Debye–Waller factors and optical potentials developed within the framework of nonrelativistic theory. We discuss some important many-body effects as resonant effects and loss effects with relativistic corrections. The present theory is easily extended to handle those spectra irradiated by strong field.

Attractive electron correlation in wide band gap semiconductors by electron–photoninteraction

We theoretically demonstrate attractive electron correlation in wide band gapsemiconductors by electron–photon interaction. At low temperature, wavevectorsof electromagnetic waves absorbed in wide band gap semiconductors cannot beneglected for wavevectors of electron waves; that is, electromagnetic waves affectthe movements of electrons. In particular, attractive interaction occurs betweentwo electrons when one electron changes from a valence band to a conductionband and the other electron changes from a conduction band to a valence band.

High-energy photoeffect

Ionization of a many-electron atom by photoabsorption at high incident photon energies ω (but still ω⪡ mc 2) is characterized using examples of a two-electron atom in ground and in excited states. We use an approach (Phys. Rev. A 67 (2003) 022,709) based on asymptotic Fourier transform (AFT) theory in which the behavior of the high-energy photoabsorption matrix elements is understood in terms of behavior of the wave functions and electron–photon interaction in the vicinity of singularities of the many-body Coulomb potential. Within the unified AFT approach, we discuss correlation effects which characterize high-energy behavior of single ionization cross sections from the ground and from excited states and direct double ionization cross sections from the ground state.

Electron angular distributions in the two-photon ionization of hydrogen-like ions: arelativistic description

The angular distribution of the emitted electrons, following the two-photon ionization ofhydrogen-like ions, is studied within the framework of second order perturbation theoryand the Dirac equation. Using a density matrix approach, we have investigated the effectswhich arise from the polarization of the incoming light as well as from the highermultipoles in the expansion of the electron–photon interaction. For medium- andhigh-Z ions, in particular, the non-dipole contributions give rise to a significant changein the angular distribution of the emitted electrons, if compared with theelectric dipole approximation. This includes a strong forward emission while, inthe dipole approximation, the electron emission always occurs symmetricallywith respect to the plane which is perpendicular to the photon beam. Detailedcomputations for the dependence of the photoelectron angular distributions onthe polarization of the incident light are carried out for the ionization of H, andXe53+ and U91+ (hydrogen-like) ions.

High-Q photonic nanocavity in a two-dimensional photonic crystal.

Photonic cavities that strongly confine light are finding applications in many areas of physics and engineering, including coherent electron-photon interactions, ultra-small filters, low-threshold lasers, photonic chips, nonlinear optics and quantum information processing. Critical for these applications is the realization of a cavity with both high quality factor, Q, and small modal volume, V. The ratio Q/V determines the strength of the various cavity interactions, and an ultra-small cavity enables large-scale integration and single-mode operation for a broad range of wavelengths. However, a high-Q cavity of optical wavelength size is difficult to fabricate, as radiation loss increases in inverse proportion to cavity size. With the exception of a few recent theoretical studies, definitive theories and experiments for creating high-Q nanocavities have not been extensively investigated. Here we use a silicon-based two-dimensional photonic-crystal slab to fabricate a nanocavity with Q = 45,000 and V = 7.0 x 10(-14) cm3; the value of Q/V is 10-100 times larger than in previous studies. Underlying this development is the realization that light should be confined gently in order to be confined strongly. Integration with other photonic elements is straightforward, and a large free spectral range of 100 nm has been demonstrated.

QCD corrections to single top quark production in electron-photon interactions

Single top quark production in electron photon interactions provides a clean environment for the measurement of the Cabbibo-Kobayashi-Maskawa matrix element Vtb. Aiming an experimental precision at the percent level the knowledge of radiative corrections is important. In this paper we present results for the radiative corrections in quantum chromodynamics.