On the possibility of self-trapping transition of acoustic polarons in two dimensions

A 2D electron-longitudinal-acoustic-phonon interaction Hamiltonian is derived and used to calculate the ground-state energy of the acoustic polarons in two dimensions. The numerical results for the ground-state energy of the acoustic polarons in two and three dimensions are obtained. The 3D results agree with those obtained by using the Feynman path-integral approach. It is found that the critical coupling constant of the transition from the quasifree state to the self-trapped state in the 2D case is much smaller than in the 3D case for a given cutoff wave-vector. The theory has been used to judge the possibility of the self-trapping for several real materials. The results indicate that the self-trappings of the electrons in AlN and the holes in AlN and GaN are expected to be observed in 2D systems.     

Interaction-assisted propagation of Coulomb-correlated electron-hole pairs in disordered semiconductors

A two-band model of a disordered semiconductor is used to analyze dynamical interaction induced weakening of localization in a system that is accessible to experimental verification. The results show a dependence on the sign of the two-particle interaction and on the optical excitation energy of the Coulomb-correlated electron-hole pair. Received 4 September and Received in final form 26 November 1998     

Singlet-triplet splitting of geminate electron-hole pairs in conjugated polymers

The singlet-triplet splitting of geminate polaron pairs in a ladder-type conjugated polymer has been studied by the thermally stimulated luminescence technique. The energy gap separating the singlet and triplet states of the geminate pairs is measured to be in the range of 3-6 meV, depending on the polymer morphology. The results of correlated quantum-chemical calculations on a long ladder-type oligomer are fully consistent with the observed values of the geminate polaron pair singlet-triplet gap. Such low splitting values have important implications for the spin-dependent exciton formation in conjugated polymers.     

Theoretical prediction of intrinsic self-trapping of electrons and holes in monoclinic HfO2

We predict, by means of ab initio calculations, stable electron and hole polaron states in perfect monoclinic HfO2. Hole polarons are localized on oxygen atoms in the two oxygen sublattices. An electron polaron is localized on hafnium atoms. Small barriers for polaron hopping suggest relatively high mobility of trapped charges. The one-electron energy levels in the gap, optical transition energies and ESR g-tensor components are calculated.     

Excitation of electron-hole pairs in low-energy electron scattering from surfaces

We calculate the Distorted Wave Born Aprroximation differential cross section for the inelastic scattering of low-energy electrons reflected from metallic surfaces via the dynamically screened Coulomb potential, and in particular the excitation of low-energy electron-hole pairs. We discuss the angular and energy dependence of the loss spectra, and the application of this method to study the spectrum of the electron-hole excitations at surfaces.     

Detection of defects binding free polarons in colored alkali halide crystals

The assumption has been made that defects binding free polarons in colored alkali halide crystals are F'-center, i.e., defects that slow down the motion of dislocations (photoplastic effect). This assumption has been confirmed by the experiments performed in this study. Thus, the anion vacancy in alkali halide crystals at a low temperature can capture three electrons: two electrons at a deep level (F'-center) and one electron in a bound polaron state. This electron is retained due to the energy gain in the interaction of a local deformation of the polaron and a local deformation surrounding the F'-center, despite the presence of the Coulomb repulsion.     

Cascade avalanche production of electron-hole pairs in type II quantum wells

It is shown that the photon avalanche mechanism can be used for producing nonequilibrium electron-hole pairs by low-intensity IR light with a photon energy smaller than the energy gap of a semiconductor by a factor of 3–5. A type II heterostructure with deep quantum wells is proposed to be employed for this purpose. In the model under investigation, the photon avalanche effect is due to a combination of a cascade of one-and two-photon transitions and Auger-type transitions.     

Geminate recombination of long-lived electron-hole pairs in doped carbazole-containing amorphous molecular semiconductors

This paper reports an investigation into the features of geminate recombination of long-lived electron-hole pairs in films of poly(N-epoxypropylcarbazole) (PEPC) containing intramolecular charge transfer compounds as centers of photogeneration of charge carriers carried out by measuring the attenuation kinetics of isothermal recombination luminescence over a wide range of temperatures (4.2–300 K) for times t>1 s, along with the kinetics of photoinduced EPR signals. A study is made of how the presence of a potential barrier to the reverse transition of carriers to the recombination center and the initial triplet state of the charged pairs affects the geminate recombination. It is established that the kinetics of isothermal recombintation luminescence attenuation follow the law I(t)∝t ^−m for the times under study, where m takes values from 0.65 to 1.05. The first observations of a nonmonotonic change in m with temperature are reported (m passes through a minimum at a certain temperature), and a mechanism is proposed for recombination of charged pairs in PEPC containing intramolecular charge-transfer compounds as an explanation for this phenomenon. Fiz. Tverd. Tela (St. Petersburg) 39, 1183–1187 (July 1997)     

Role of electron-hole pairs in the mechanisms of desorption from semiconductor surfaces

In the presence of a dense electron-hole plasma (due to high power laser irradiation or resulting from a plasma driven phase transition or melting), it is shown that desorption is drastically enhanced. This permits us to propose a test for the validity of the hypothesis of the plasma driven melting in Si. Also the creation of surface excitons is shown to lead to enhanced desorption.     

Multiphoton generation of electron-hole pairs involving free carriers in an indirect-gap crystal

The paper reports on a study of nonlinear absorption of high-intensity laser radiation in the case where the photon energy is less than one half of the indirect-gap width in a crystal. The deficiency in the energy needed for two-photon excitation of the electron-hole pair is made up by the kinetic energy of free electrons, which was acquired in intraband nonlinear absorption of light. The probabilities of Auger-type indirect two-photon interband transitions involving free electrons have been calculated by perturbation theory. It is shown that, for a free-carrier concentration in the conduction band n _c ? 10¹⁵ cm^?3, and the radiation intensity range of interest for the experiment, j ～ 3–10 GW/cm², the calculated probabilities of such transitions exceed by several orders of magnitude those of “conventional” direct and indirect (involving phonons) multiphoton transitions which can take place in the system considered.     

Phase fixation,excitonic and spin superfluidity of electron-hole pairs and antiferromagnetic chromium

The problem of the excitonic and spin superflow in the Bose-condensated electron-hole fluid is considered taking into account the phase fixation due to non-conservation of the electron-hole pair number and the total spin. The ways to observe the superflows in Cr are discussed. There is the analogy between the spin superfluidity in the exitonic state adn in the A-phase of the superfluid He³. The spin superflow is possible not only in the Cr (the itinerant antiferromagnetic) but in other planar antiferromagnetics too.     

Proposal for production and detection of entangled electron-hole pairs in a degenerate electron gas

We demonstrate theoretically that the shot noise produced by a tunnel barrier in a two-channel conductor violates a Bell inequality. The nonlocality is shown to originate from entangled electron-hole pairs created by tunneling events-without requiring electron-electron interactions. The degree of entanglement (concurrence) equals 2(T1T2)(1/2)(T1+T2)(-1), with T1,T2<1 the transmission eigenvalues. A pair of edge channels in the quantum Hall effect is proposed as an experimental realization.