Longitudinal conductivity of Ge/Si heterostructures with quantum dots

The conductance along an island layer of Ge quantum dots buried in silicon was investigated. The sizes of the islands varied in the range D ≈ 12−19 nm. It was found that the charge transport is characterized by two activation energies. The first one is associated with the thermal emission of holes from Ge quantum wells into the valence band of Si. The second one is due to the tunneling of holes between islands under Coulomb blockade conditions and is determined by the electrostatic charging energy of a quantum dot. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 423–426 (25 March 1996)     

Coulomb interaction during coherent transport of electrons in quantum wires

A model is found whereby the Coulomb interaction during electron transport in quantum wires of finite length with current-lead contacts can be taken into account exactly (within the framework of the bosonization method). It is established that the charge density distribution along a wire in the case of the real Coulomb interaction is strongly different from the Luttinger liquid model, where the interaction is assumed to be short-ranged. The charge density near the contacts is found to be much higher than in the model with a short-range interaction, but away from the contacts the two densities are closer to each other. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 184–189 (10 February 1998)     

Formation of zero-dimensional hole states during molecular-beam epitaxy of Ge on Si (100)

The characteristic features of electronic spectra in Ge/Si (100) heterostructures obtained by molecular-beam epitaxy are investigated by capacitance spectroscopy. It is observed that the self-organization of a Ge film into an island film when the effective germanium thickness exceeds six monolayers is accompanied by the appearance of hole bound states, which can be attributed to size quantization and the Coulomb interaction of carriers in the array of Ge quantum dots. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 125–130 (25 July 1998)     

Coulomb interaction controlled room temperature oscillation of tunnel current in porous Si

A novel phenomenon of regular oscillations is observed in I-V characteristics of porous silicon under illumination by visible light. The measurements are performed at room temperature using a scanning tunneling microscope. The heights of the oscillation peaks appear to be a linear function of the oscillation number. The experimental value of the Coulomb energy determined from the oscillation period is much smaller than k _B T. The oscillations are attributed to a Coulomb effect, i.e., to the periodic trapping of a multielectron level in a quantum well within a Si nanocrystal under the combined influence of the voltage variation at the STM tip and the Coulomb interaction among the carriers. Fiz. Tverd. Tela (St. Petersburg) 40, 1147–1150 (June 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Donor states in tunnel-coupled quantum wells

We study the energy spectrum of the impurity states in tunnel-coupled double quantum wells for Coulomb and short-range donor potentials. We calculate the impurity contribution and the density of states and detect the transformation of a localized donor state into a resonant state when the binding energy of the donor in an isolated quantum well is less than the separation of the energy levels of the double quantum wells. In the opposite case, where the binding energy is greater than the level separation, there is tunneling repulsion between adjacent impurity levels, with the degree of degeneracy of the levels changing when there is tunneling mixing of the ground and excited impurity states from different wells. Resonant states emerge in an asymmetric double quantum well, while in a symmetric double quantum well the impurity level at the barrier’s center proves to be localized even against the background of the continuum. The calculations are based on a general expression for the impurity contribution to the density of states in terms of a 2-by-2 matrix Green’s function, i.e., only a pair of tunnel-coupled levels of the double quantum wells is taken into account. For an impurity with a short-range potential, we derive a matrix generalization of the Koster-Slater solution, while the impurity with a Coulomb potential is analyzed by using the approximation of a narrow resonance and close arrangement of the repulsive levels. Zh. éksp. Teor. Fiz. 115, 1337–1352 (April 1999)     

Excitonic state in quantum wells formed from “above-barrier” electronic states

Lines corresponding to localized excitonic states formed from “above-barrier” electron and/or hole states (specifically, excitation lines of excitons formed by an electron localized in a QW and a free heavy hole) have been observed in the photoluminescence excitation spectra of GaAs/Al_0.05Ga_0.95As structures with quantum wells (QWs), each containing one single-particle size-quantization level for charge carriers of each type. A computational method is proposed that permits finding the binding energy and wave functions of excitons in QWs taking the Coulomb potential into account self-consistently. The computed values of the excitonic transition energies agree quite well with the experimental results. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 613–619 (10 November 1999)     

Generation of one-photon states by a quantum dot

A technique for preparation of a one-photon wave packet through action of a classical electromagnetic field on a semiconducting quantum dot is proposed. We demonstrate that the Coulomb repulsion between charge carriers allows one to select the frequency, amplitude, and duration of an electromagnetic pulse so that one electron will transfer from an upper size-quantized level of the valence band to a lower size-quantized level of the conduction band with a probability close to unity. As a result of radiative recombination of the produced electron-hole pair, exactly one photon is emitted (a one-photon wave packet). This source of one-photon states can be used in quantum systems of data transmission and in quantum computers. Zh. éksp. Teor. Fiz. 112, 1257–1272 (October 1997)     

Explaining the Unobserved—Why Quantum Mechanics Ain’t Only About Information

A remarkable theorem by Clifton et al [Found Phys. 33(11), 1561–1591 (2003)] (CBH) characterizes quantum theory in terms of information-theoretic principles. According to Bub [Stud. Hist. Phil. Mod. Phys. 35 B, 241–266 (2004); Found. Phys. 35(4), 541–560 (2005)] the philosophical significance of the theorem is that quantum theory should be regarded as a “principle” theory about (quantum) information rather than a “constructive” theory about the dynamics of quantum systems. Here we criticize Bub’s principle approach arguing that if the mathematical formalism of quantum mechanics remains intact then there is no escape route from solving the measurement problem by constructive theories. We further propose a (Wigner-type) thought experiment that we argue demonstrates that quantum mechanics on the information-theoretic approach is incomplete.     

Instability of the two-dimensional metallic phase to a parallel magnetic field

Magnetotransport studies of the unusual two-dimensional metallic phase in high-mobility Si-MOS structures are reported. It is found that a magnetic field applied in the 2D plane suppresses the metallic state, causing the resistivity to increase dramatically (by more than 30 times). The total existence range of the metallic state is found to contain three distinct types of magnetoresistance, related to the corresponding quantum corrections to the conductivity. The data suggest that the unusual metallic state is a consequence of both spin and Coulomb interaction effects. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 12, 887–892 (25 June 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Single-electron computing without dissipation

The possibility of performing single-electron computing without dissipation in an array of tunnel-coupled quantum dots is studied theoretically, taking the spin gate NOT (inverter) as an example. It is shown that the logical operation can be implemented at the stage of unitary evolution of the electron subsystem, although complete switching of the inverter cannot be achieved in a reasonable time at realistic values of model parameters. The optimal input magnetic field is found as a function of the interdot tunneling energy and intradot Coulomb repulsion energy. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 275–279 (25 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Fano-type features in magnetoabsorption in semiconductors

It is shown that bulk semiconductors exhibit strongly asymmetric Fano-type resonance profiles in magnetoabsorption processes involving the formation of hot electron-hole pairs (EHPs) and accompanied by the scattering of the EHPs by defects. This result is valid for transitions to electronic states with large Landau quantum numbers, when the Coulomb interaction plays a small role. The physical reason for such a sharp change in the magnetoabsorption coefficient as compared with the expected result for the ordinary density of states in a quantizing magnetic field is that the electronic excitations are quasi-one-dimensional. The form of the resonance absorption is in good qualitative agreement with the experimental data. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 619–622 (25 April 1996)     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlaAs superlattices, with different widths of the electron and hole minibands, located in a high magnetic field perpendicular to the heterolayers. It is found that the ground state of the indirect excitons formed by electrons and holes and spatially separated between neighboring quantum wells lies between the ls ground state of the direct excitons and the continuum threshold for dissociated exciton states in the minibands. Indirect excitons in superlattices have a significant oscillator strength when the binding energy of the exciton exceeds the order of the width of the resulting miniband. The behavior of the binding energy of direct and indirect heavy hole excitons during changes in the tunneling coupling between the quantum wells is established. It is shown that a strong magnetic field, which intensifies the Coulomb interaction between the electron and hole in an exciton, weakens the bond in a system of symmetrically bound quantum wells. The spatially indirect excitons studied here are analogous to first order Wannier-Stark localized excitons in superlattices with inclined bands (when an electrical bias is applied), but in the present case the localization is of purely Coulomb origin. Zh. éksp. Teor. Fiz. 112, 1106–1118 (September 1997)     

Photon-assisted shot noise due to the charging effect in a quantum dot device

We have investigated the spectral density of shot noise for an ultra-small quantum dot(QD) system in the Coulomb blockade regime when irradiated with microwave fields (MWFs) by employing a nonequilibrium Green’s function technique. The shot noise is sensitive to Coulomb interaction, and the photon-assisted Coulomb blockade behaviour strongly modifies the mesoscopic transport. We have calculated the first and second derivatives of shot noise in the strong and weak coupling regimes to compare the theoretical results with existing experimental results. In the strong coupling regime, the first and second derivatives of shot noise display Fano type peak-valley structures around the charging channel 2E _c due to Coulomb interaction. When the magnitudes of the MWFs are sufficiently large, the system displays channel blockade due to photon irradiation. The photon-assisted and Coulomb blockade steps in the noise — as well as the resonant behaviour in the differential noise — are smeared by increasing temperature. The Coulomb interaction suppresses the shot noise, but the ac fields can either suppress the shot noise(balanced case) or enhance the shot noise(unbalanced case). The suppression of shot noise caused by ac fields in the balanced case is greater than that caused by Coulomb interaction in our system. Super-Poissonian shot noise may be induced due to the compound effects of strong Coulomb interaction and photon absorption-emission processes.     

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.     

Coulomb effects in a ballistic one-channel S-S-S device

We develop a theory of Coulomb oscillations in superconducting devices in the limit of small charging energy E _C ≪Δ. We consider a small superconducting grain with finite capacitance connected to two superconducting leads by nearly ballistic single-channel quantum point contacts. The temperature is assumed to be very low, so there are no single-particle excitations on the grain. Then the behavior of the system can be described in terms of the quantum mechanics of the superconducting phase on the island. The Josephson energy as a function of this phase has two minima that become degenerate when the phase difference on the leads equals to π, the tunneling amplitude between them being controlled by the gate voltage on the grain. We find the Josephson current and its low-frequency fluctuations, and predict their periodic dependence with period 2e on the induced charge Q _x =CV _g . Zh. éksp. Teor. Fiz. 114, 640–653 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Bremsstrahlung spectrum for α decay and quantum tunneling

Spectra of the electromagnetic radiation arising during α decay of atomic nuclei as a consequence of the motion of the α particle through a Coulomb potential barrier and in the Coulomb field of the daughter nucleus are calculated via a quantum-mechanical approach. The contributions of the E1 and E2 multipoles are calculated. Model problems of emission during motion of a charged particle through a spherically symmetric, rectangular potential barrier and a “cut-off” Coulomb barrier are treated. Numerical calculations are performed for ^210,214Po and ²²⁶Ra nuclei. Emission spectra are derived for an α particle propagating along classical trajectories in these potentials. Zh. éksp. Teor. Fiz. 116, 390–409 (August 1999)     

Coulomb blockade electrometer with a high-T c island

We report on a successful attempt to fabricate a Coulomb blockade electrometer consisting of an ultrasmall YBa₂Cu₃O_7−δ (YBCO) island coupled to two gold electrodes through a high-resistance native surface tunnel barrier. A third electrode placed near the island was used as an electrostatic gate. Spectra typical for tunneling into the YBCO superconductor were reproducibly measured. At temperatures below 0.5 K the low-bias conductance was suppressed by the Coulomb blockade. The blockade could be periodically varied by the gate potential. An external magnetic field of up to 5 T strongly influenced the transport via the island but without any change in the period of the Coulomb oscillations. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 112–117 (25 January 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Coulomb screening of impurity charge and anomalous tunneling transparency

This paper analyzes the effect of the screened Coulomb interaction between metallic electrons in the sidewalls, on the one hand, and a localized electron in an impurity level, on the other, on the tunneling in doped quantum structures with an intrinsic two-dimensional continuum. We show that Mahan’s non-Fermi-liquid singularity at the Fermi level is unstable against additional scattering due to tunneling. As a result, the current-voltage characteristic changes radically when the Fermi level in the sidewalls is approached by the edge of the two-dimensional band. Specifically, the peak due to the non-Fermi-liquid singularity with a section of negative differential resistance is replaced with a step-like or a two-step feature, which corresponds to a single or split Fermi-liquid resonance near the edge of the 2D band involved in the tunneling process. Zh. éksp. Teor. Fiz. 115, 1843–1859 (May 1999)     

Quantum dot multiexcitons in a magnetic field

Multiexcitons confined in InGaAs/GaAs quantum dots (QDs) with a lateral size slightly exceeding the exciton Bohr radius are investigated by magnetophotoluminescence spectroscopy at 2 K. The Coulomb correlations in the two-exciton complex result in an additional confinement, which increases with decreasing dot size, while a magnetic field reduces this effect. A three-exciton complex is confined only by the geometric confinement potential of the QD. The exciton-exciton repulsion increases with decreasing dot size, while a magnetic field decreases the repulsion strongly when the magnetic length becomes smaller than the lateral size of the QD. A shell model for the QD multiexciton states is proposed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 263–268 (25 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Anomaly-free quantization of a string in two-dimensional space-time

An anomaly-free quantum theory of a relativistic string is constructed in two-dimensional space-time. The states of the string are found to be similar to the states of a massless chiral quantum particle. This result is obtained by generalizing the concept of an “operator” in quantum field theory. Zh. éksp. Teor. Fiz. 113, 1566–1578 (May 1998)