Photoexcited transients in disordered semiconductors: Quantum coherence at very short to intermediate times

We study theoretically electron transients in semiconductor alloys excited by light pulses shorter than 100 femtoseconds and tuned above the absorption edge during and shortly after the pulse, when disorder scattering is dominant. We use non-equilibrium Green functions employing the field-dependent self-consistent Born approximation. The propagators and the particle correlation function are obtained by a direct numerical solution of the Dyson equations in differential form. For the purely elastic scattering in our model system the solution procedures for the retarded propagator and for the correlation function can be decoupled.The propagator is used as an input in calculating the correlation function. Numerical results combined with a cumulant expansion permit to separate in a consistent fashion the dark and the induced parts of the self-energy. The dark behavior reduces to propagation of strongly damped quasi-particles; the field induced self-energy leads to an additional time non-local coherence. The particle correlation function is formed by a coherent transient and an incoherent back-scattered component. The particle number is conserved only if the field induced coherence is fully incorporated. The transient polarization and the energy balance are also obtained and interpreted.Comment: Accepted for publication in Phys. Rev. B; 37 pages,17 figure

Generalized gradient expansions in quantum transport equations

Gradient expansions in quantum transport equations of a Kadanoff-Baym form have been reexamined. We have realized that in a consistent approach the expansion should be performed also inside of the self-energy in the scattering integrals of these equations. In the first perturbation order this internal expansion gives new correction terms to the generalized Boltzman equation. These correction terms are found here for several typical systems. Possible corrections to the theory of a linear response to weak electric fields are also discussed.Comment: 20 pages, latex, to appear in Journal of Statistical Physics, March (1997

Relationship between the expansion of drylands and the intensification of Hadley circulation during the late twentieth century

The changes in coverage by arid climate and intensity of the Hadley circulation during the second half of the twentieth century were examined using observations and the multi-model ensemble (MME) mean of Twentieth-Century Coupled Climate Model (20C3M) simulations. It was found that the area of dry climate, which comprises steppe and desert climates following the Köppen climate classification, expanded to an appreciable extent in observation and, to a lesser degree, in MME simulation. The areal extent of steppe climate (the outer boundary of arid climate) tends to encroach on the surrounding climate groups, which, in turn, feeds desert climate (the inner part of arid climate) and causes it to grow. This result indicates the importance of accurate prediction for climate regimes that border steppe climate. Concomitant with the expansion of drylands, the observed intensity of the Hadley cell is persistently enhanced, particularly during boreal winter, suggesting the validity of a self-induction of deserts through a positive biogeophysical feedback (also known as Charney’s cycle). In comparison, the simulated Hadley circulation in the MME mean remains invariant in time. The current climate models, therefore, disagree with the observation in the long-term linkage between desertification and Hadley cell. Finally, the implication of such discrepancy is discussed as a possible guidance to improve models

Quasiparticle Formation and Decay in Pulsed Photoexcitation of Disordered Semiconductors

The quasiparticle states in strongly illuminated semiconductors are light hybridized electron and hole states, Galickii quasiparticles. Their properties, especially if they are photoexcited at small detunings, may be rather complex. A protracted formation period is followed by quantum beats of two decaying resonances corresponding to both sides of the hybridization gap. On an example of elastic scattering on an alloy disorder, these phenomena are demonstrated and analyzed in terms of poles of the retarded Green function and the corresponding residuals

Optically Induced Gaps in Disordered Semiconductors

On the example of an explicitly solvable model of a semiconductor with alloy disorder in the conduction band, it is shown that a slowly varying exciting light pulse can be treated in an adiabatic approximation, that is, the self-energy of an electron can be taken as a continuously evolving series of snapshots of self-energies corresponding to a steady illumination with the instantaneous value of the light strength

Ultrafast Dynamics and Quantum Transport of Electrons in Strongly Disordered Semiconductors

The new experiments on the response of electrons in semiconductors to femtosecond optical pulses call for developing adequate theoretical tools. A promising approach has been found in using the non-equilibrium Green functions approximately factorized on the basis of the so-called generalized Kadanoff-Baym ansatz. The present work investigates the validity of such approach on an example of a semiconductor with an alloy scattering, where the coherent potential approximation allows to construct the non-equilibrium Green functions directly, so that an explicit comparison with the ansatz decoupling is possible. The ansatz for the electron distribution is in this case justified as far as the quasiparticle picture for the individual electrons is appropriate

Generalized master equation for a molecular bridge improved by vertex correction to the Generalized Kadanoff-Baym Ansatz

The equations for the Non-Equilibrium Green's Functions (NEGF) describing the electron transport through molecular bridges are commonly reduced to a Generalized Master Equation (GME) for the one-electron density matrix using the Generalized Kadanoff-Baym Ansatz (GKBA). This works for weak tunneling and flat tunneling functions combined. For a structure, whose tunneling functions have a complex sd spectral structure, and are moderately strong, we derive an improved GME using a novel “corrected GKBA” incorporating a stationary approximation to the vertex part of the exact reconstruction equations whose free term is the standard GKBA. Numerical tests are successful

Coherence of Electron Photoexcitation by Extremely Short Strong Light Pulses

For a semiconductor alloy with a predominant disorder scattering, we show that, under appropriate conditions, electrons photoexcited by a short strong light pulse form a coherent transient at first, while the incoherently backscattered electrons take over only gradually, with a time lag comparable with the pulse duration. The time evolution of the electron distribution is obtained by a direct evaluation of the non-equilibrium Green function

Relation between full NEGF, non-Markovian and Markovian transport equations

This article addresses the problem of an efficient description of the transient electron transport in (primarily small open) quantum systems out of equilibrium. It provides an overview and critical review of the use of causal Ansatzes with the accent on derivation of (quantum) transport equations from the standard Kadanoff–Baym (KB) equations for the non-equilibrium Green’s functions (NEGF). The family of causal Ansatzes originates from the well-known Generalized Kadanoff–Baym Ansatz (GKBA). The Ansatz technique has been fairly successful in practice. Recently, the scope of the method has been extended towards more “difficult” cases and its success can be assessed more precisely. This general picture is demonstrated and analyzed in detail for a variant of the generic molecular island model, an Anderson impurity linked between two bulk metallic leads by tunneling junctions. First, the KB equations are reduced to a non-Markovian generalized master equation (GME) by means of a general causal Ansatz. Further reduction to a Markovian master equation is achieved by partly relaxing the strictly causal character of the theory. For the model narrowed down to ferromagnetic leads, the transient currents are spin polarized and the tunneling functions have a complex spectral structure. This has prompted deriving explicit conditions for the use of an Ansatz. To extend the applicability range of the GME, approximate vertex corrections to the Ansatz were introduced and used with success. Finally, the relation of the GME description to possible non-equilibrium generalizations of the fluctuation–dissipation theorem is shown, extended beyond the present model within the NEGF formalism and physically interpreted in terms of a simplified kinetic theory of non-equilibrium electrons in open quantum systems