Breaking the phonon bottleneck in semiconductor nanocrystals via multiphonon emission induced by intrinsic nonadiabatic interactions

We observe ultrafast 1P-to-1S intraband relaxation in PbSe and CdSe nanocrystals (NCs) that have distinct energy spectra. While ultrafast dynamics in CdSe NCs has typically been interpreted in terms of electron-hole energy transfer, this mechanism is not active in PbSe NCs because of sparse densities of states in the conduction and valence bands. Our observations of temperature activation and confinement-enhanced relaxation in PbSe NCs can be explained by efficient multiphonon emission triggered by nonadiabatic electron-phonon interactions and are indicative of large, size-dependent, intraband Huang-Rhys parameters.     

A tight-binding representation of electron-hole exchange interaction in semiconductors

The electron-hole exchange interaction in semiconductors is analyzed in the framework of the empirical tight-binding method. It is demonstrated that intra-atomic and interatomic contributions to the long-range exchange interaction enter in an inequivalent way. In particular, for the Γ₆×Γ₇ exciton in a spherical nanocrystal with a cubic lattice, the dipole-dipole contribution associated only with the intra-atomic (or intra-site) transitions does not lead to singlet-triplet splitting of the exciton level. The interatomic transitions, for example, anion-to-cation transitions between the nearest neighbors in binary semiconductor compounds, determine the so-called monopole-monopole contribution to the exchange splitting of the Γ₆×Γ₇ exciton, and this contribution does not vanish in a spherical nanocrystal.     

Electron-phonon interaction in non-polar quantum dots induced by the amorphous polar environment

We propose a Fröhlich-type electron-phonon interaction mechanism for carriers confined in a non-polar quantum dot surrounded by an amorphous polar environment. Carrier transitions under this mechanism are due to their interaction with the oscillating electric field induced by the local vibrations in the surrounding amorphous medium. We estimate the corresponding energy relaxation rate for electrons in Si nanocrystals embedded in a SiO₂ matrix as an example. When the nanocrystal diameter is larger than 4 nm then the gaps between the electron energy levels of size quantization are narrow enough to allow for transitions accompanied by emission of a single local phonon having the energy about 140 meV. In such Si/SiO₂ nanocrystals the relaxation time is in nanosecond range.     

Implications of time-reversal symmetry for the band structure of single-wall carbon nanotubes

When electron states in carbon nanotubes are characterized by two-dimensional wave vectors with the components K ₁ and K ₂ along the nanotube circumference and cylindrical axis, respectively, then two such vectors symmetric about a M-point in the reciprocal space of graphene are shown to be related by the time-reversal operation. To each carbon nanotube there correspond five relevant M-points with the following coordinates: K ₁⁽¹⁾ = N/2R, K ₂⁽¹⁾= 0; K ₁⁽²⁾ = M/2R, K ₂⁽²⁾= −π/T; K ₁⁽³⁾= (2N −M)/2R, K ₂⁽³⁾= π/T; K ₁⁽⁴⁾= (M + N)/2R, K ₂⁽⁴⁾= -π/T, and K ₁⁽⁵⁾= (N − M)/2R, K ₂⁽⁵⁾= π/T, where M and N are the integers relating the chiral, C _h , symmetry, R, and translational, T, vectors of the nanotube by N R = C _h + M T, T = |T|, and R is the nanotube radius. The states at the edges of the one-dimensional Brillouin zone, which are symmetric about the M-points with K ₂ = ±π/T, are shown to be degenerate due to the time-reversal symmetry.     

Electrodynamical treatment of the electron-hole long-range exchange interaction in semiconductor nanocrystals

We show that the contribution to the fine structure of the ground exciton level in a semiconductor nanocrystal due to the long-range part of the electron-hole exchange interaction can be equivalently described as arising from the mechanical exciton interaction with the exciton-induced macroscopic longitudinal electric field. Particular cases of nanocrystals with cubic and wurtzite crystal lattice in the strong confinement regime are studied taking into account the complex structure of the valence band. A simplified model accounting for the exciton ground-level splitting and exploiting an effective local scalar susceptibility is established.     

Critical conditions for 1D, 2D and 3D magnetic-bipolaron formation

Il Nuovo Cimento D - Interaction of two magnetic polarons formed either by single carriers or by excitons is considered in bulk crystal, quantum well and quantum wire. It is shown that the magnetic...     

Anisotropic exchange splitting of excitonic levels in small quantum systems

A theory of long-range exchange interaction between an electron and a hole in a zero-dimensional exciton has been developed taking into account the retardation effect. It has been shown that, for the anisotropic localizing potential in bulk semiconductors or semiconductor nanostructures, the long-range interaction leads to a splitting of radiative localized-exciton levels. Polarization of split sublevels is determined by the shape of the exciton envelope function. For excitons localized in quantum wells, typical splitting is expected to be tens of μ eV in agreement with recent experiments.     

Exchange interaction and acoustical phonon modes in CdTe nanocrystals

Photoluminescence of CdTe nanocrystals (NC) is excited resonantly in the lowest energy absorption peak. The spectrum shows a luminescence line shifted to a lower energy and acoustical and optical phonon replica. The Stokes shift between the luminescence and excitation lines is attributed to the electron-hole exchange energy in the nanocrystal. By tuning the laser line inside the absorption peak, we are able to measure the Stokes shift as a function of the excitation energy. Calculation of the absorption gap and the Stokes shift is done in a tight-binding theory. It allows us to determine the radius R of NC excited at a given wavelength and to compare the experimental and theoretical values of the exchange energy as a function of R. A very good agreement is obtained. The observed size dependence of the acoustical phonon mode energy provides a further confirmation of our analysis.