Phase transitions in a system of two coupled quantum wells

It is predicted that an excitonic liquid is formed in a system of spatially separated electrons (e) and holes (h) in a system of two coupled quantum wells. The ground-state energy and the equilibrium density of the excitonic liquid are calculated as a function of the distance D between the wells. A gas-liquid quantum transition with increasing D is studied. The Berezinskii-Kosterlitz-Thouless transition temperatures at which superfluidity appears in the system are found (for different D). A quantum Mott metal-insulator transition in an anisotropic double-quantum-well structure is investigated. The region of existence of crystalline order in a system of spatially separated e and h is studied. Possible experimental manifestations of the predicted effects are discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 8, 526–531 (25 October 1996)     

Superfluidity of indirect magnetoexcitons in coupled quantum wells

The temperature T _c of the Kosterlitz-Thouless transition to a superfluid state for a system of magnetoexcitons with spatially separated electrons e and holes h in coupled quantum wells is obtained as a function of magnetic field H and interlayer separation D. It is found that T _c decreases as a function of H and D at fixed exciton density n _ex as a result of an increase in the exciton magnetic mass. The highest Kosterlitz-Thouless transition temperature as a function of H increases (at small D) on account of an increase in the maximum magnetoexciton density n _ex versus magnetic field, where n _ex is determined by a competition between the magnetoexciton energy and the sum of the activation energies of incompressible Laughlin fluids of electrons and holes. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 332–337 (10 September 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Metal-insulator transition in a two-layer electron-hole system

This paper discusses a quantum-mechanical metal-insulator transition that occurs in an anisotropic electron-hole system with the electrons and holes separated and confined to a double quantum well. The critical concentration n _c of carriers in the system above which the excitonic (insulating) phase becomes an electron-hole (metallic) phase is investigated, along with its dependence on the distance between wells D. Fiz. Tverd. Tela (St. Petersburg) 39, 1654–1656 (September 1997)     

Excitation spectra of a system of two coupled quantum wells

Quasi-particle spectra, reconstructed by e-h pairing, have been calculated for a system of spatially separated electrons (e) and holes (h) in ground state. The regions of strong pairing interaction and significant correlation effects are reached by abandoning the BCS approximation and using instead interpolation expressions for correlation energies, which depend functionally on the coefficients of the u-v transformation, with subsequent minimization of the total energy of the reconstructed state with respect to the parameters of the u-v transformation. The dependence of the spectra on quantum-well separation and particle concentration in a system of two coupled quantum wells is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 2226–2228 (December 1998)     

Quantum lifetimes and momentum relaxation of electrons and holes in Ga0.7In0.3N0.015As0.985/GaAs quantum wells

Electronic transport in n- and p-type modulation-doped Ga_0.7In_0.3N_0.015As_0.985/GaAs quantum wells are investigated using magneto-transport measurements in the temperature range between T?=?1.8 and 32?K and at magnetic fields up to B?=?11?T. The momentum relaxation and the quantum lifetimes (τ_q ) of electrons and holes are obtained directly from the temperature and magnetic field dependencies of the SdH oscillation amplitudes, respectively. A detailed analysis of quantum and transport life times indicates that the momentum relaxation of holes is forward displaced in k-space, while a large angle-scattering mechanism is prominent for the electrons. This discrepancy is believed to be due to scattering of electrons with nitrogen complexes and to the lack of such a mechanism for holes.     

Electron–hole superlattices in GaAs/Al x Ga1− x As multiple quantum wells

A GaAs/Al _x Ga_{1? x} As semiconductor structure is proposed, which is predicted to superconduct at T _c?≈?2?K. Formation of an alternating sequence of electron- and hole-populated quantum wells (an electron–hole superlattice) in a modulation-doped GaAs/Al _x Ga_{1? x} As superlattice is considered. This superlattice may be analogous to the layered electronic structure of high-T _c superconductors. In the structures of interest, the mean spacing between nearest electron (or hole) wells is the same as the mean distance between the electrons (or holes) in any given well. This geometrical relationship mimics a prominent property of optimally doped high-T _c superconductors. Band bending by built-in electric fields from ionized donors and acceptors induces electron and heavy-hole bound states in alternate GaAs quantum wells. A proposed superlattice structure meeting this criterion for superconductivity is studied by self-consistent numerical simulation.     

Two-dimensional electron gas in double quantum wells with tilted bands

When a voltage is applied to double quantum wells based on AlGaAs/GaAs heterostructures with contact regions (n-i-n structures), a two-dimensional (2D) electron gas appears in one of the quantum wells. Under illumination which generates electron-hole pairs, the photoexcited holes become localized in a neighboring quantum well and recombine radiatively with the 2D electrons (tunneling recombination through the barrier). The appearance, ground-state energy, and density of the degenerate 2D electron gas are determined from the structure of the Landau levels in the luminescence and luminescence excitation spectra as well as from the oscillations of the radiative recombination intensity in a magnetic field with detection directly at the Fermi level. The electron density is regulated by the voltage between the contact regions and increases with the slope of the bands. For a fixed slope of the bands the 2D-electron density has an upper limit determined by the resonance tunneling of electrons into a neighboring quantum well and subsequent direct recombination with photoexcited holes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 840–845 (10 June 1997)     

Branching of electron current and quantum effects in two-dimensional dislocation barrier of n-type semiconductor

The current flow through the dislocation wall of the bicrystal (or intergrain) boundary in semiconductor is considered. It is known that the broken bonds along the dislocations make up the acceptor levels (being offset by energy ?_d from the bottom of the conductance band) whose population by electrons is defined by the bulk donor concentration n_D, the ?_d-value and the dislocation spacing D_y. At low enough n_D-value (n_D<e²/?_dD_y⁴), electrons captured at the dislocation acceptors produce the periodic 2D-lattice with two comparable periods of the order of n_s^−1/2 where n_s∼(?_dN_D/e²)^1/2 is 2D-density of captured electrons. Charges of those electrons result in the formation of the dislocation potential barrier whose height (with mean value in order of ?_d) is periodically modulated in the wall plane. Naturally, the electron current prefers to flow along the paths with the lowest barrier height, i.e. through the saddle points of the 3D-potential relief positioned in the dislocation wall plane. At low enough temperatures that leads to separation of the electron current into numerous branches. Provided both 2D-lattice periods of the captured charges are smaller as compared with the electron mean free path, all those narrow and short branches make up a filamentary 1D-conductors with the ballistic transport. As this takes place, effects associated with the 1D-conductance quantization define the whole system resistance and could be observed experimentally. In particular, exponentially strong temperature dependence of the whole system resistance vanishes.     

聚苯胺基固态染料敏化太阳电池中电子输运性能的研究

以导电聚苯胺为空穴传输材料，制备了固态染料敏化太阳电池(DSC).利用强度调制光电流谱(IMPS)和强度调制光电压谱(IMVS)研究了TiO₂多孔膜内的电子输运及复合过程.通过TiO₂多孔膜内电子的平均传输时间(τ_d)和电子寿命(τ_n)及对IMPS实验数据的拟合，获得电子在TiO₂膜内的有效扩散系数(D_n)和扩散长度(L_n).这些聚苯胺基电池中的τ_n值为相应的液体型电池的1/10倍左右，表明在该固体电池中存在严重的光生电子的复合过程，这很可能主要是与氧化态染料分子和导电电子间的复合有关.随着TiO₂膜厚的增加，τ_n和τ_d均变小，但D_n和L_n随之增加，只有在合适的膜厚范围内才能获得较高的光伏性能. 关键词： 聚苯胺 染料敏化太阳电池 IMPS IMVS     

Contribution to the theory of “incompressible” regions in an inhomogeneous magnetized 2D electronic system

A modification of the theory of “incompressible” regions in an ideal spinless inhomogeneous magnetized 2D electronic system near points on the electron density profile n(x) with an integer filling factor is proposed. Such regions leads to the appearance of a finite capacitance between the parts of the 2D system that are separated by an incompressible channel, so that capacitive methods can be used to investigate such a system. The Corbino configuration is especially convenient for these purposes. The parameters of the “incompressible” channel in a Corbino disk with a spatially inhomogeneous 2D electronic system in the presence of an individual point, near the channel, on the electron density profile with an integer magnetic filling factor are determined. The magnetocapacitance between the edges of the Corbino disk separated by an incompressible interlayer is found for cases of practical interest. It is shown that this magnetocapacitance contains direct information about the width of the integer strip. Fiz. Tverd. Tela (St. Petersburg) 41, 1103–1109 (June 1999)     

Phase transitions in a system of spatially separated electrons and holes

The formation of a superfluid exciton liquid in a system of spatially separated electrons and holes in a system of two coupled quantum wells is predicted and its properties are investigated. The ground-state energy and the equilibrium density of the exciton liquid are calculated as functions of distance D between the quantum wells. The properties of a rarefied exciton gas with dipole-dipole repulsions are considered, where this gas is the metastable phase for D<1.9a* and the stable phase for D<1.9a* (a* is the radius of the two-dimensional exciton). The gas-liquid quantum transition is examined for increasing D. The Berezinskii-Kosterlitz-Thouless transition temperatures, at which superfluidity arises in the system, are found for different values of D. Possible experimental manifestations of the predicted effects are discussed. Zh. éksp. Teor. Fiz. 111, 1879–1895 (May 1997)     

Polar optical phonon limited energy and momentum relaxation of hot electrons in a GaAs-quantum well (QW)

The average energy loss rate, the energy- as well as the momentum relaxation time of hot electrons confined in a GaAs-square quantum well are calculated as a function of the external controllable parametersn _s (electron density),T (lattice temperature) andT _e (electron temperature) for the interaction of the charge carriers with bulk- and surface polar optical phonons. Analytical expressions are derived in the limit of vanishing quantum well width at non-degeneracy and degeneracy of the electron system. Both energy-and momentum relaxation time are found to be complicated functions of the ratiosT _D /T _e andT _D /T withT _D being the Debye-temperature of the polar optical phonon involved in the scattering. In a thick (very thin) QW the energy loss rate to bulk PO-phonons is found to be larger (smaller) than the corresponding loss rate to surface modes. The energy- (momentum-) relaxation times are found to be constant (increasing) functions ofn _s at non-degeneracy (degeneracy) of the electron system. Dedicated to Professor Karlheinz Seeger on the occasion of his 60th birthday     

Interwell excitons in a lateral potential well in an inhomogeneous electric field

The luminescence of interwell excitons in double quantum wells based on GaAs/AlGaAs semiconductor heterostructures (n-i-n structures) in a lateral trap prepared with the use of an inhomogeneous electric field was studied at helium temperatures. A rather strong and inhomogeneous electric field occurred in the depth of the heterostructure when a current passed through the contact between the conducting tip of a tunneling microscope and the heterostructure surface to the bulk region containing a built-in gate. Because of the Stark shift of energy bands in the electric field, the photoexcited electrons and holes are spatially separated in neighboring quantum wells by a tunnel-transparent barrier and are bound into interwell quasi-two-dimensional excitons. These excitons have a dipole moment even in the ground state. Therefore, electrostatic forces in the inhomogeneous electric field cause the excitons to move in the plane of quantum wells toward the maximum field region and eventually accumulate in the lateral trap artificially prepared in such a way. The maximum trap depth achieved through the inhomogeneous electric field was 13.5 meV, and its lateral size was about 10 μm. It is shown that, in the traps prepared in this way, photoexcited interwell excitons behave with increasing concentration at sufficiently low temperatures (T=2K) in the same fashion as in the lateral traps caused by large-scale fluctuations of the random potential. At concentrations exceeding the percolation threshold, the interwell excitons condense into the lowest energy state in the trap.     

Concentration and temperature dependence of the refractive index of ethanol-water mixtures: Influence of intermolecular interactions

The temperature and concentration dependence of the refractive index, n _D(x, T) , in ethanol-water mixtures agrees with previous data in the ethanol-rich concentration range. The refractive index versus concentration x determined at 20 ^° C shows the expected maximum at about 41 mol% water (22 mass% water). The temperature derivative of the refractive index, dn _D /dT, shows anomalies at lower water concentrations at about 10 mol% water but no anomaly at 41 mol% water. Both anomalies are related to intermolecular interactions, the one in n_D seems to be due to molecular segregation and cluster formation while the origin of the second one in dn _D /dT is still not clear.     

Acoustically induced dynamic potential dots

Mobile potential dots (dynamic dots, DDs) formed by surface acoustic waves (SAWs) are used to transport photogenerated electrons and holes in GaAs quantum wells (QWs). We investigate the interaction between the transported carriers and microscopic trap centers in the QW plane using spatially and time-resolved photoluminescence (PL) spectroscopy. The carriers recombine at the trap site emitting short (width0.6 ns) light pulses at a repetition rate corresponding to the SAW frequency. The dependence of the PL intensity from the traps on the number of carriers transported per DD n exhibits a well-defined, distinct plateau for n in the range from 5–20, which is attributed to the emission of a well-defined number of photons.     

Energy spectra and quantum crystallization in two-electron quantum dots in a magnetic field

Quantum crystallization of electrons in a quantum dot (QD) subjected to an external magnetic field is considered. Two-electron QDs with two-dimensional (2D) parabolic confining potential in an external transverse magnetic field are calculated. The Hamiltonian is numerically diagonalized in the basis of one-particle functions to find the energy spectra and wave functions for the relative motion of electrons with inclusion of electron-electron interaction for a broad range of the confining-potential steepness (α) and external magnetic fields (B). The region of the external parameters (α, B) within which a gradual transition to quantum crystalline order occurs is numerically determined. In contrast to a 2D unbounded system, a magnetic field acts nonmonotonically on “crystallization” in a quantum dot with several electrons because of a competition between two effects taking place with increasing B, namely, decreasing spread of the electron wave functions and increasing effective steepness of the confining potential, which reduces the average separation between electrons. Fiz. Tverd. Tela (St. Petersburg) 40, 1753–1759 (September 1998)     

Heating and cooling of a two-dimensional electron gas by terahertz radiation

The absorption of terahertz radiation by free charge carriers in n-type semiconductor quantum wells accompanied by the interaction of electrons with acoustic and optical phonons is studied. It is shown that intrasubband optical transitions can cause both heating and cooling of the electron gas. The cooling of charge carriers occurs in a certain temperature and radiation frequency region where light is most efficiently absorbed due to intrasubband transitions with emission of optical phonons. In GaAs quantum wells, the optical cooling of electrons occurs most efficiently at liquid nitrogen temperatures, while cooling is possible even at room temperature in GaN heterostructures.     

Stability of a Model of Relativistic Quantum Electrodynamics

The relativistic “no pair” model of quantum electrodynamics uses the Dirac operator, D(A) for the electron dynamics together with the usual self-energy of the quantized ultraviolet cutoff electromagnetic field A– in the Coulomb gauge. There are no positrons because the electron wave functions are constrained to lie in the positive spectral subspace of some Dirac operator, D, but the model is defined for any number, N, of electrons, and hence describes a true many-body system. In addition to the electrons there are a number, K, of fixed nuclei with charges ≤Z. If the fields are not quantized but are classical, it was shown earlier that such a model is always unstable (the ground state energy E=−∞) if one uses the customary D(0) to define the electron space, but is stable (E > − const.(N+K)) if one uses D(A) itself (provided the fine structure constant α and Z are not too large). This result is extended to quantized fields here, and stability is proved for α= 1/137 and Z≤ 42. This formulation of QED is somewhat unusual because it means that the electron Hilbert space is inextricably linked to the photon Fock space. But such a linkage appears to better describe the real world of photons and electrons. Received: 8 September 2001 / Accepted: 18 March 2002     

Investigation of the Beam Plasma Discharge in Electronegative Gases Using the Second Derivative of Langmuir Probe Characteristics

The distribution of the charge carriers is measured and the influence of the attachment processes on the electron energy distribution function is demonstrated for beam plasma discharges in SF₆, CF₄ and O₂ with the aid of the second derivative of Langmuir probe characteristics. The structuration of the plasma into regions of predominating negative ions and regions of predominating electrons is determined by the the established radial T_e-profile. The dimension of the quasi electronfree plasma changes significantly as well at transition from the turbulent heating mechanism into the electron impact plasma generation as by occuring low-frequency instabilities. With increasing n_?/n_e a deficit of low energetic electrons appears in the electron energy distribution parallel to the formation of the negative ion peak. The saturation currents at n_e/n_?=0 yield the mass ratios between negative and positive ions.     

2D-Electron Heating in Electric and Magnetic Crossed Fields

2D-electron heating in a potential well of a single n-(AlAs) _x (GaAs)_1–x /i-GaAs (x = 0.28) heterojunction is studied for the cases of a classical (weak) magnetic field B and constant and pulsed electric fields at fixed temperatures 77 and 4.2 K. It is shown that the heating of two-dimensional electrons is similar to that of the bulk ones. The magnetic field cools electrons, and this is manifested in the shifts of the characteristic critical electric fields E _{c 1} and E _{c 2} and in the regions of nonlinearity of voltage-current characteristics. The dependence of the effective electron temperature on the electric field T _e(E)_B is determined.