On the phase diagram of a two-dimensional electron–hole system

The phase diagram for a system of spatially separated electrons and holes in coupled quantum wells or graphene double layers is studied in the framework of a BCS-like mean-field approach and a Landau expansion in terms of the pairing order parameter. We find a second order transition between an electron–hole plasma and a BCS phase, as well as a first-order transition between the BCS phase and a bosonic Mott phase of tightly bound electron–hole pairs without phase coherence. The electron–hole plasma exists at low and at high densities for weak interaction, the BCS phase at moderate density and the Mott phase at high density and strong interaction.     

Two types of fundamental luminescence of ionization-passive electrons and holes in optical dielectrics—Intraband-electron and interband-hole luminescence (theoretical calculation and comparison with experiment)

A short high-power pulse of ionizing radiation creates a high concentration of nonequilibrium electrons and holes in a dielectric. They quickly lose their energy, generating a multiplicity of secondary quasiparticles: electron—hole pairs, excitons, plasmons, phonons of all types, and others. When the kinetic energy of an electron becomes less that some value E_Δ≈(1.3-2)E_g it loses the ability to perform collisional ionization and electron excitations of the dielectric medium. Such an electron is said to be ionization-passive. It relaxes to the bottom of the lower conduction band by emitting phonons. Similarly a hole becomes ionization-passive when it “floats up” above some level E_H and loses the ability for Auger ionization of the dielectric medium. It continues to float upward to the ceiling of the upper valance band only by emitting phonons. The concentrations of ionization-passive electrons and holes are larger by several orders of magnitude than those of the active electrons and holes and consequently make of a far larger contribution to many kinetic processes such as luminescence. Intraband and interband quantum transitions make the greatest contribution to the fundamental (independent of impurities and intrinsic defects) electromagnetic radiation of ionization-passive electrons and holes. Consequently the brightest types of purely fundamental luminescence of strongly nonequilibrium electrons and holes are intraband and interband luminescence. These forms of luminescence, discovered relatively recently, carry valuable information on the high-energy states of the electrons in the conduction band and of the holes in the valence band of a dielectric. Experimental investigations of these types of luminescence were made, mainly on alkali halide crystals which were excited by nanoseconal pulses of high-current-density electrons and by two-photon absorption of the ultraviolet harmonics of pulsed laser radiation beams of nanosecond and picosecond duration. The present article gives the results of theoretical calculations of the spectra and other characteristics of intraband electron and interband hole luminescence which are compared with the experimental data. Institute of High-Current Electronics, Sibrian Branch of the Russian Academy of Sciences, Polytechnic University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 13–41, November, 1997.     

Relaxation time for a charge carrier due to its scattering from other charge carriers in superlattices

A calculation of relaxation time for (i) electron–electron scattering in a modulation-doped superlattice of type-I and (ii) electron–electron, hole–hole and electron–hole scattering processes in a compositional superlattice of type-II has been performed, using Fermi's golden rule. As compared to a two-dimensional electron gas system, both intralayer and interlayer interactions, between charge carriers in a superlattice, contribute to relaxation time. It is found that scattering processes at all possible value of momentum transfer contribute to relaxation time, for a given value of temperature and carrier density. We further find interlayer interactions in a superlattice make a significant contribution to relaxation time. Relaxation time is found to decrease on increasing temperature, carrier density and single particle energy, in a superlattice. The computed relaxation time for an electron (hole) in a superlattice enhances on increasing the width of layer consisting of electrons (holes). The electron–hole (hole–electron) scattering process in a type-II superlattice yields maximum contribution to the relaxation time when a hole layer lies exactly in between two consecutive electron layers.     

Stabilization energies of charged multiexciton complexes calculated at configuration interaction level

Recombination and stabilization energies of multiexcitons confined in positively and negatively charged semiconductor InGaAs/GaAs quantum dot (QD) samples have been studied by employing large-scale configuration interaction (CI) calculations. The CI calculations show that at most six electrons or two holes can be confined in the QD. Multiply charged multiexciton complexes with up to five excess electrons or two excess holes are also found to be stable, even when a few electron–hole pairs are present in the QD. The chemical potential functions for charged QD samples do not possess the pronounced stepped form as obtained for the corresponding neutral multiexciton complexes. The negatively and the positively charged excitons (negative and positive trions) lie lower in energy as compared to a neutral exciton and a single non-interacting charge carrier in the lowest single-particle state of another quantum dot. The other charged multiexciton complexes studied are not confined with respect to the corresponding neutral multiexciton and a non-interacting charge carrier. To include the contributions from the heavy-hole light-hole (HH–LH) coupling, a perturbative treatment of the band-mixing effects was implemented. The perturbation-theory calculations show that the HH–LH coupling does not shift the energies in the present InGaAs/GaAs QD sample.     

太赫兹波段石墨烯等离子体的增益特性

基于麦克斯韦方程组和物质本构方程对石墨烯表面等离子体进行了研究.从理论上探索了石墨烯表面等离子体激元在太赫兹波段的增益特性曲线,并且讨论了石墨烯表面等离子体增益与石墨烯中载流子浓度、石墨烯所处温度以及载流子动量弛豫时间的关系.研究结果表明:在太赫兹波段增益峰值随着石墨烯载流子浓度的增加而发生蓝移,并且在所讨论的温度范围内,由于增益峰所对应的频率都大于1 THz,因此温度的变化对增益峰值以及相应频率的影响不大,即在不同的温度下,相同载流子浓度所对应的增益曲线上峰值的位置和强度几乎相同;增益与石墨烯载流子动量弛豫时间相关,随着载流子动量弛豫时间的增加,使得激发态激励的电子增加,从而导致石墨烯表面等离子体增益变得更大,但这种动量弛豫时间的增加却因弛豫时间对受激辐射频率影响较小而并未对增益峰值位置产生影响.     

Multiple bands: A key to high-temperature superconductivity in iron arsenides?

In the framework of four-band model of superconductivity in iron arsenides proposed by Barzykin and Gor’kov we analyze the gap ratios on hole—like and electron—like Fermi—surface cylinders. It is shown that experimentally observed (ARPES) gap ratios can be obtained only within rather strict limits on the values of pairing coupling constants. The difference of T _c values in 1111 and 122 systems is reasonably explained by the relative values of partial densities of states. The multiple bands electronic structure of these systems leads to a significant enhancement of effective pairing coupling constant determining T _c , so that high enough T _c values can be achieved even for the case of rather small intraband and interband pairing interactions. The article is published in the original.     

Spin relaxation of electrons excited in p-doped semiconductor heterostructures

We present a calculation of the spin-relaxation time of photoexcited electrons in p-doped quantum wells of GaAs with the spin-flip mechanism due to the electron–hole exchange interaction. We have observed shorter spin-relaxation times for electrons close to the conduction-band edge when including the spin mixing of the valence-hole states. This spin mixing allows exchange spin-relaxation channels which are energy forbidden in the case of pure-spin holes.     

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.     

Magnetoexcitons in unbalanced double layers

We study theoretically correlations of electrons and holes in unbalanced double-layer electronic systems in strong magnetic fields. Calculations are made using the exact diagonalization and the variational wave function. The ground state of an electron–hole pair in quantized cyclotron orbits possesses an in-plane electric dipole moment, when an electron and a hole are in different Landau orbits with different radii. The resulting attractive interactions between pairs creates the possibility of novel states.     

Electromigration with enhanced green emission in the titanium dioxide nanotube/graphene composite

One of the most studied photoluminescence emission peaks of anatase titanium dioxide (TiO₂) is green, located at about 520 nm, which is assigned to the radiative recombination between a mobile electron in the conduction band and oxygen vacancy defect as a trapped hole in the bandgap. Composite materials of TiO₂ with graphene are normally shown by the gradual quenching of photoluminescence intensity as a result of carrier lifetime extension, which is important to enhance photocatalytic activity. Herein we report an observation of the intensity enhancement of the green PL emission in a composite TiO₂ nanotube (TNT) and graphene produced through facile hydrothermal synthesis. The heterojunction formation of graphene and TNT makes the excited photoelectrons easy to diffuse from TNT to graphene. Hence, the recombination rate of mobile electrons in graphene and trapped holes located on the nanotube surface is enhanced due to the high mobility of electrons in graphene.     

Relaxation of femtosecond photoexcited electrons in a polar indirect band-gap semiconductor nanoparticle

A model calculation is given for the energy relaxation of a non-equilibrium distribution of hot electrons (holes) prepared in the conduction (valence) band of a polar indirect band-gap semiconductor, which has been subjected to homogeneous photoexcitation by a femtosecond laser pulse. The model assumes that the pulsed photoexcitation creates two distinct but spatially interpenetrating electron and hole non-equilibrium subsystems that initially relax non-radiatively through the electron (hole)-phonon processes towards the conduction (valence) band minimum (maximum), and finally radiatively through the phonon-assisted electron-hole recombination across the band-gap, which is a relatively slow process. This leads to an accumulation of electrons (holes) at the conduction (valence) band minimum (maximum). The resulting peaking of the carrier density and the entire evolution of the hot electron (hole) distribution has been calculated. The latter may be time resolved by a pump-probe study. The model is particularly applicable to a divided (nanometric) polar indirect band-gap semiconductor with a low carrier concentration and strong electron-phonon coupling, where the usual two-temperature model [1-4] may not be appropriate.     

Logarithmic corrections to $${\mathcal{N} = 2}$$ black hole entropy: an infrared window into the microstates

Logarithmic corrections to the extremal black hole entropy can be computed purely in terms of the low energy data—the spectrum of massless fields and their interaction. The demand of reproducing these corrections provides a strong constraint on any microscopic theory of quantum gravity that attempts to explain the black hole entropy. Using quantum entropy function formalism we compute logarithmic corrections to the entropy of half BPS black holes in N=2{{\mathcal N}=2} supersymmetric string theories. Our results allow us to test various proposals for the measure in the OSV formula, and we find agreement with the measure proposed by Denef and Moore if we assume their result to be valid at weak topological string coupling. Our analysis also gives the logarithmic corrections to the entropy of extremal Reissner–Nordstrom black holes in ordinary Einstein–Maxwell theory.     

Phonon-mediated electron pairing in graphene

The possibility of superconducting pairing of electrons in doped graphene due to in-plane and out-of-plane phonons is studied. Quadratic coupling of electrons with out-of-plane phonons is considered in details, taking into account both deformation potential and bond-stretch contributions. The order parameter of electron-electron pairing can have different structures due to four-component spinor character of electrons wave function. We consider s-wave pairing, diagonal on conduction and valence bands, but having arbitrary structure with respect to valley degree of freedom. The sign and magnitude of contribution of each phonon mode to effective electron-electron interaction turns out to depend on both the symmetry of phonon mode and the structure of the order parameter. Unconventional orbital-spin symmetry of the order parameter is found.     

Multi-band pairing of ultrarelativistic electrons and holes in graphene bilayer

Multi-band pairing of effectively ultrarelativistic electrons and holes in asymmetrically biased graphene bilayer in strong coupling regime is considered. In this regime, the pairing affects both conduction and valence bands of the both graphene layers, and the order parameter is a matrix, which indices correspond to the bands. For band-diagonal s-wave pairing, we derive the system of multi-band gap equations for the gaps in the valence and conduction bands and solve it in the approximation of constant gaps and in the approximation of separable pairing potential. For a characteristic width of the pairing region of order of magnitude of the chemical potential, the gap values are not much different from single-band BCS estimations. However, if the pairing region is wider, then the gaps can be much larger and depend exponentially on its energy width. We also predict gapped and soliton-like oscillations of a relative phase of the gaps and unpairing of quarter-vortices at Kosterlitz-Thouless transition.     

Rate of intersubband transitions of charge carriers in semiconductor quantum wells as a result of their Coulomb interaction

The intersubband scattering of charge carriers in semiconductor quantum wells as a result of their Coulomb interaction has been theoretically investigated. Analytical expressions for the rate of intersubband transitions in the process of electron—electron and electron—hole collisions have been derived in the Born approximation. The theoretical and experimental data on the photoluminescence decay time, obtained for the case of a nondegenerate distribution of charge carriers, were in qualitative agreement. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 782–787, November–December, 2005.     

The flavor of quantum gravity

We develop an effective field theory to describe the coupling of non-thermal quantum black holes to particles such as those of the Standard Model. The effective Lagrangian is determined by imposing that the production cross section of a non-thermal quantum black hole be given by the usual geometrical cross section. Having determined the effective Lagrangian, we estimate the contribution of a virtual hole to the anomalous magnetic moment of the muon, μ→eγ transition and to the electric dipole moment of the neutron. We obtain surprisingly weak bounds on the Planck mass due to a chiral suppression factor in the calculated low energy observables. The tightest bounds come from μ→eγ and the limit on the neutron electric dipole moment. These bounds are in the few TeV region.     

Electron–hole two-stream instability in a quantum semiconductor plasma with exchange-correlation effects

The electron–hole two-stream instability in a quantum semiconductor plasma has been studied including electrons and holes quantum recoil effects, exchange-correlation potentials, and degenerate pressures of the plasma species. Typical values of GaAs and GaSb semiconductors are used to estimate the growth rate of the two-stream instability. The effects of electron– and hole–phonon collision, quantum recoil effects, the streaming velocities, and the corresponding threshold on the growth rate are investigated numerically. Considering the phonon susceptibility allows the acoustic mode to exist and the collisional instability arises in combination with drift of the holes.     

Coherent localization of two interacting carriers in one-dimensional quantum dot arrays

The coherent dynamics of two interacting carriers in one-dimensional quantum dot arrays driven by oscillating electric fields is theoretically investigated with the help of numerical calculations. The coherent localization of two electrons and that of an electron–hole pair are studied in this paper. For the two-electron case, the dynamic localization of the electrons is achieved when the Coulomb interaction is large enough. In this coherent localization, the Coulomb repulsion helps the electrons to be localized. For an electron–hole pair, although the dynamic localization of the composite particle does not occur due to charge neutrality, a different type of coherent localization can occur. These phenomena are explained by the quasienergy spectra based on Floquet analysis.     

Exact non-Born-Oppenheimer wave function for the Hooke-Calogero model of the H2 molecule

The Hooke-Calogero model of the H₂ molecule, a four body system consisting of two protons and two electrons with harmonic electron-nuclear, Coulombic electron-electron and inverse quadratic nuclear-nuclear interactions, has been analyzed in–depth. A fully analytical closed–form non-Born-Oppenheimer solution has been found and based on it the properties of both electron–pair and nuclear–pair densities have been studied. Nuclei have been found to be strongly correlated in a way that resembles a sort of Wigner crystallization, for moderate electron–nucleus confinement strengths. Additionally, we have explicitly evaluated the Coulomb holes for the electrons. Analysis of these holes reveals that the similarity between the electron correlation effects of the model as compared to the real Coulombic systems is remarkable.