93 research outputs found
Exact two-body quantum dynamics of an electron-hole pair in semiconductor coupled quantum wells: a time-dependent approach
We simulate the time-dependent coherent dynamics of a spatially indirect
exciton (an electron-hole pair with the two particles confined in different
layers) in a GaAs coupled quantum well system. We use a unitary wave-packet
propagation method taking into account in full the four degrees of freedom of
the two particles in a two-dimensional system, including both the long-range
Coulomb attraction and arbitrary two-dimensional electrostatic potentials
affecting the electron and/or the hole separately. The method has been
implemented for massively parallel architectures to cope with the huge
numerical problem, showing good scaling properties and allowing evolution for
tens of picoseconds. We have investigated both transient time phenomena and
asymptotic time transmission and reflection coefficients for potential profiles
consisting of i) extended barriers and wells and ii) a single-slit geometry. We
found clear signatures of the internal two-body dynamics, with transient
phenomena in the picosecond time-scale which might be revealed by optical
spectroscopy. Exact results have been compared with mean-field approaches
which, neglecting dynamical correlations by construction, turn out to be
inadequate to describe the electron-hole pair evolution in realistic
experimental conditions.Comment: 12 two-column pages + 3 supplemental material pages, 9 figures, to
appear on Phys.Rev.
Space- and time-dependent quantum dynamics of spatially indirect excitons in semiconductor heterostructures
We study the unitary propagation of a two-particle one-dimensional
Schr\"odinger equation by means of the Split-Step Fourier method, to study the
coherent evolution of a spatially indirect exciton (IX) in semiconductor
heterostructures. The mutual Coulomb interaction of the electron-hole pair and
the electrostatic potentials generated by external gates and acting on the two
particles separately are taken into account exactly in the two-particle
dynamics. As relevant examples, step/downhill and barrier/well potential
profiles are considered. The space- and time-dependent evolution during the
scattering event as well as the asymptotic time behavior are analyzed. For
typical parameters of GaAs-based devices the transmission or reflection of the
pair turns out to be a complex two-particle process, due to comparable and
competing Coulomb, electrostatic and kinetic energy scales. Depending on the
intensity and anisotropy of the scattering potentials, the quantum evolution
may result in excitation of the IX internal degrees of freedom, dissociation of
the pair, or transmission in small periodic IX wavepackets due to dwelling of
one particle in the barrier region. We discuss the occurrence of each process
in the full parameter space of the scattering potentials and the relevance of
our results for current excitronic technologies.Comment: 28 pages, 10 figures, preprint forma
Effect of a Temperature Gradient on the Screening Properties of Ionic Fluids
The electrostatic screening properties of ionic fluids are of paramount
importance in countless physical processes. Yet, the behavior of ionic
conductors out of thermal equilibrium has to date mainly been studied in the
context of thermodiffusion phenomena by virtue of direct extensions of
Debye-H\"uckel theories. We investigate how the static response of a symmetric
ionic fluid is influenced by the presence of a thermal gradient by introducing
a theory of electrostatic screening under a stationary temperature profile. By
borrowing mathematical methods commonly used in the semiclassical approximation
of quantum particles, we find analytical solutions to the asymptotic decay of
the charge density which can be used to describe the non-equilibrium response
of the system to external charge perturbations and for arbitrary ionic
concentrations. Notably, a transition between monotonic and oscillatory
screening regimes is observed as an effect of the temperature variation which
generalizes known results of thermal equilibrium to out of equilibrium
conditions. A final quantitative example on the screening of charged surfaces
in aqueous electrolytes shows that the deviation from thermal equilibrium
predicted by our solutions is generally larger than thermodiffusion effects,
and should therefore be taken into account for a comprehensive description of
the electrical double layer. Our findings pave the way to the rigorous
treatment of non-equilibrium steady states in ionic systems with potential
applications to the study of energy materials, nanostructured systems and
waste-heat-recovery technologies.Comment: 11 pages, 5 figure
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