Wavelength dependence of second-harmonic generation at the copper surface

The wavelength dependence of the second-harmonic (SH) generation efficiency has been measured in copper. A strong enhancement of the SH generation has been observed at the onset of interband transitions. Above the absorption edge the second-order polarization is dominated by the nonlinear response of bound electrons.     

Role of excited states in coherent harmonic bremsstrahlung in a photoionized plasma

It is established for a photoionized plasma formed in the barrier-suppression ionization regime that the preliminary population of the excited states of the ionizing atoms plays an important role. It is established that in this case an anomalously strong (several orders of magnitude) increase occurs in the efficiency of generation of the harmonics of the pump radiation. It is shown that a relative decrease of the harmonics generation efficiency occurs with time as a result of collisions of the electrons produced by ionization.     

Magneto-optics of zero-dimensional electron systems

Photoluminescence spectroscopy has been used to probe the occupied electron states below the Fermi energy of zero-dimensional electron systems (0DESs) in both zero and finite magnetic fields. The arrays of modulation-doped quantum dots investigated were fabricated by both reactive-ion etching and strain-confining GaAs heterojunctions with a -layer of Be present in the GaAs, in order to improve luminescence efficiency. For the etched quantum dots we show that the low magnetic field dispersion T) of the acceptor recombination line is directly related to the magnetic field dependence of the total ground-state energy of interacting electrons in the quantum dots. For the strain-confined 0DESs we have mapped the magneto-dispersion of the quantum confined electron states to reveal 15 electrons per dot.     

Tamm surface states of Hg1-xCdxTe

Using the Kane model, the wave functions and dispersion laws were obtained for Tamm surface states of electrons and holes arising on the ideal surface of Hg_1-xCd_xTe crystal in the parabolic and strong nonparabolic limits. The dependence of surface electrons and holes effective masses, in the parabolic limit, is determined from the parameters of the bulk states. The surface states of heavy holes is shown to be unsensitive to the degree of nonparabolicity of the electron spectrum.     

Effect of screening by two-dimensional charge carriers on the binding energy of excitonic states in GaAs/AlGaAs quantum wells

The spectrum of excitonic excited states in GaAs/AlGaAs quantum wells of different width is studied together with its change due to the screening of electron-hole interaction by two-dimensional electrons. The exciton binding energy decreases sharply with an increase in the concentration of two-dimensional electrons. The temperature dependence of screening parameters is studied for the ground and excited excitonic states down to ultralow temperatures T=50 mK.     

Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage U _cr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (U _cr , pd) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen’s curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam. The mechanism of the formation of the recently created atomospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation. The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.     

On the Stability of the Relativistic Electron-Positron Field

We study the energy of relativistic electrons and positrons interacting through the second quantized Coulomb interaction and a self-generated magnetic field. As states we allow generalized Hartree-Fock states in the Fock space. Our main result is the assertion of positivity of the energy, if the atomic numbers and the fine structure constant are not too big. We also discuss the dependence of the result on the dressing of the electrons (choice of subspaces defining the electrons).     

Attosecond burst and high-harmonic generation in molecular ionization by ultrashort laser pulses

It is shown that the efficiency of attosecond pulse and high-harmonic generation in the ionization of excited molecular structures by a powerful femtosecond optical pulse can appreciably exceed the efficiency of analogous processes in atomic systems. This is due to the presence of a delocalized electron wave-packet component in the nonequilibrium molecular states, resulting in an increase of the number of particles that are effectively involved in the bremsstrahlung generation in the course of recollisions of laser-accelerated electrons with molecular core. Calculations suggest that, by optimizing the nonlinear response of molecular systems in the ionization process, one can develop compact sources of coherent vacuum ultraviolet and X-ray radiation with luminance at a level that is presently achieved only at large-scale accelerator facilities with free-electron lasers.     

Theory of zero bias anomalies due to paramagnetic impurities

The method developed by one of the authors is generalized to include paramagnetic impurities in order to explain zero bias tunneling anomalies. The self-energy of the electrons is supposed to be a local function in space; nonlocal or assisted tunneling effects are beyond the scope of the present theory. The tunneling current is expressed in terms of the local density of states at the barrier which in turn is given in terms of the life-time of the conduction electrons. The change of the local density of states depends on the positions of the impurities relative to the metal-oxide interface. This dependence is thoroughly investigated.     

Drag effect of one-dimensional electrons upon photoionization of D<Superscript>(−)</Superscript> centers in a longitudinal magnetic field

A theory is elaborated for the impurity photon drag effect in a semiconductor quantum wire exposed to a longitudinal magnetic field B directed along the axis of the quantum wire. The phonon drag effect is associated with the transfer of the longitudinal photon momentum to localized electrons in optical transitions from D^(?) states to hybrid-quantized states of the quantum wire, which is described by a confinement parabolic potential. An analytical expression for the drag current density is derived within the model of a zero-range potential in the effective mass approximation, and the spectral dependence of the drag current density is examined at different magnitudes of B and parameters of the quantum wire upon electron scattering by a system of impurities with short-range potentials. It is established that the spectral dependence of the drag current density exhibits a Zeeman doublet with a clear beak-shaped peak due to optical transitions of electrons from D^(?) states to states with the magnetic quantum number m=1. The possibility of using the photon drag effect in a longitudinal magnetic field for the development of laser radiation detectors is analyzed.     

Second-harmonic spectroscopy of electronic structure of Si/SiO2 multiple quantum wells

Size effects in the resonant nonlinear optical response of amorphous Si/SiO₂ multiple quantum wells (MQW) are studied by second-harmonic generation (SHG) spectroscopy in a spectral interval of second-harmonic photon energies from 2.5 to 3.4 eV. The sensitivity of SHG spectroscopy to thickness-dependent electronic structure (sub-band energy position and density of states line shape) of MQW is demonstrated. A monotonic red shift of central energies of SHG resonances by 120 m eV upon increase of the well thickness from 2.5 to 10 ? is observed. This is interpreted as a size dependence of the position of singularities in the combined density of states for a 2D gas of electrons moving in an effective potential well. It is shown that, for agreement with experiment, the simplest (rectangular) shape of the well should be modified in order to take into account the lattice-potential distortion at the interfaces. Received: 16 October 2001 / Revised version: 16 April 2002 / Published online: 6 June 2002     

Time domain capacitance spectroscopy of tunneling electrons in the two-dimensional electron gas

We have developed a technique capable of measuring the tunneling current into both localized and conducting states in a 2D electron system (2DES). The method yields I-V characteristics for tunneling with no distortions arising from low 2D in-plane conductivity. We have used the technique to determine the pseudogap energy spectrum for electron tunneling into and out of a 2D system and, further, we have demonstrated that such tunneling measurements reveal spin relaxation times within the 2DEG. Pseudogap: In a 2DEG in perpendicular magnetic field, a pseudogap develops in the tunneling density of states at the Fermi energy. We resolve a linear energy dependence of this pseudogap at low excitations. The slopes of this linear gap are strongly field dependent. No existing theory predicts the observed behavior. Spin relaxation: We explore the characteristics of equilibrium tunneling of electrons from a 3D electrode into a high mobility 2DES. For most 2D Landau level filling factors, we find that electrons tunnel with a single, well-defined tunneling rate. However, for spin-polarized quantum Hall states (ν=1, 3 and 1/3) tunneling occurs at two distinct rates that differ by up to two orders of magnitude. The dependence of the two rates on temperature and tunnel barrier thickness suggests that slow in-plane spin relaxation creates a bottleneck for tunneling of electrons.     

Analysis of electron recombination in dye-sensitized solar cell

A steady-state numerical model of dye-sensitized solar cell is based on continuity and transport equations for electrons, iodide and triiodide ions. The cell model consists of an active layer, where photovoltaic effect including diffusion of electrons in mesoporous TiO₂ and ions in electrolyte takes place, and a bulk electrolyte layer, where only ions diffuse. Exponential distribution of trap states in TiO₂ and Gaussian distributions of energy levels in the electrolyte within active layer are included in modeling of the recombination dynamics, according to Shockley-Read-Hall statistics and Marcus-Gerischer electron transfer theory. Recombinations at the front contact and a voltage drop at the platinum covered back contact are included in the model. Simulation results are compared with the measured current-voltage characteristics at different light intensities. In particular, light intensity dependence of open circuit voltage is studied over 4 decades. Optimization of cell efficiency regarding active layer and electrolyte layer thickness is carried out. Simulation results show that best efficiency is achieved when electrolyte layer thickness is minimized as much as possible and that active layer thickness is traded off with respect to recombination rates and/or diffusion limited current determined with the selection of the electrolyte.     

Diamagnetism in quasicrystals

A reliable explanation of diamagnetism in quasicrystals is given. We show that the weak diamagnetism in perfect icosahedral quasicrystals is due to an atomic-like diamagnetic contribution of tightly bound conduction electrons in electron pockets of a multiconnected Fermi surface. The Landau-Peierls diamagnetic term is small due to large effective masses. At temperatures above the Debye temperature the intervalley electron-phonon scattering makes the electrons ‘free’, and the temperature dependence of the Pauli paramagnetism related to a pseudogap in the density of states at the Fermi level becomes important.     

Behavior features of phononless hopping conductivity in the vicinity of the crossover from the linear to quadratic frequency dependence

It was shown that the Coulomb interaction between electrons of “active” (resonance) pairs plays a major role in a wide frequency range. Therefore, the conventional approach to the calculation of the super-linearity of the frequency dependence of ac conductivity, based on the single-particle density of states with a Coulomb gap, is inapplicable to the calculation of high-frequency phononless conductivity. The observed superlinearity of the frequency dependence of the phononless hopping conductivity can manifest itself immediately in the region of the crossover from the linear to quadratic frequency dependence of the conductivity.     

Layer-heterogeneous magnetic states in metallic nanosystems

The formation of layer-heterogeneous periodic magnetic states in metallic systems is explained in terms of the dependence of the energy of itinerant electrons on the magnetic ordering of localized spins. The interaction of the local magnetic moments with conduction electrons with a certain spin projection is described by a set of spin-dependent δ-function potentials. A matrix method is developed permitting one to calculate the energy spectrum and the density of states of itinerant electrons in the presence of a layered magnetic heterogeneity. This method is used to explain oscillations of the interlayer exchange interaction in metallic magnetic superlattices and the stabilization of spin-density wave structures in transition metals and alloys.     

Single attosecond burst generation during ionization of excited atoms by intense ultrashort laser pulses

We develop an analytical approach to describing the generation of a single attosecond burst during barrier-suppression ionization of a hydrogen atom by an intense laser pulse. We derive analytical expressions that describe the evolution of the electron wave packet in the time interval between the detachment from the atom and the collision with the parent ion for an arbitrary initial atomic state by assuming the atom to be fully ionized in one laser-field half-period. For various s-states, we derive expressions for the profile of the attosecond burst generated when the electron packet collides with the ion and analyze the dependence of its generation efficiency on the principal quantum number n of the initial atomic state. The results obtained are compared with the results of three-dimensional numerical calculations. We show that the attosecond pulse generation efficiency can be several orders of magnitude higher than that in the case of ionization from the ground state when pre-excited atomic states are used.     

Efficiency of Si L 2,3 x-ray generation in the SiO2/Si system by electron impact

The paper reports a study of the depth profile of the generation efficiency and escape of the ultrasoft silicon L _2,3 x-ray radiation excited by electrons of various energies. The generation function describing the excitation efficiency is the kernel of an integral equation determining the dependence of x-ray emission intensity on primary-electron energy. To determine the form of this function, a study was made of the dependence of the Si L _2,3 x-ray spectral intensity and of its silicon L _2,3 component bands, from crystalline silicon and amorphous dioxide SiO₂, on primary-electron energy in samples made from dioxide layers of various thicknesses grown on crystalline silicon. These experiments permitted investigation of the generation-function cross sections at the depth of the Si-SiO₂ interface. The theoretical simulation of the generation function made use of the simplest laws governing electron interaction with solids and of the cross section of the inner-level ionization by electron impact in its most general form. A comparison of the experimentally obtained relative contributions of the Si and SiO₂ emissions with the calculations shows them to be in good agreement up to primary-electron energies of 2–3 keV. Fiz. Tverd. Tela (St. Petersburg) 40, 1932–1936 (October 1998)     

Charge carrier dynamics in CdSe nanocrystals: implications for the use of quantum dots in novel photovoltaics

The dynamics of photo-generated electrons and holes in CdSe quantum dots have been studied using the femtosecond fluorescence upconversion technique, permitting an unambiguous examination of the excited state. The band edge emission shows an expected size dependence on the decay rate. We find that the deep trap emission is coupled to the band edge fluorescence, implicating surface states as important factors in the excited state lifetime of the hole. As a factor of the overall efficiency of solar cells, the rate of charge separation and the fate of the exciton are important considerations in the design of nanocrystal-based photovoltaic devices. Received 30 November 2000