Carrier relaxation mechanisms in self-assembled (In,Ga)As/GaAs quantum dots: Efficient P -> S Auger relaxation of electrons

We calculate the P-shell--to-S-shell decay lifetime \tau(P->S) of electrons in lens-shaped self-assembled (In,Ga)As/GaAs dots due to Auger electron-hole scattering within an atomistic pseudopotential-based approach. We find that this relaxation mechanism leads to fast decay of \tau(P->S)~1-7 ps for dots of different sizes. Our calculated Auger-type P-shell--to-S-shell decay lifetimes \tau(P->S) compare well to data in (In,Ga)As/GaAs dots, showing that as long as holes are present there is no need for an alternative polaron mechanism.Comment: Version published in Phys. Rev.

Electron-Phonon Interacation in Quantum Dots: A Solvable Model

The relaxation of electrons in quantum dots via phonon emission is hindered by the discrete nature of the dot levels (phonon bottleneck). In order to clarify the issue theoretically we consider a system of $N$ discrete fermionic states (dot levels) coupled to an unlimited number of bosonic modes with the same energy (dispersionless phonons). In analogy to the Gram-Schmidt orthogonalization procedure, we perform a unitary transformation into new bosonic modes. Since only $N(N+1)/2$ of them couple to the fermions, a numerically exact treatment is possible. The formalism is applied to a GaAs quantum dot with only two electronic levels. If close to resonance with the phonon energy, the electronic transition shows a splitting due to quantum mechanical level repulsion. This is driven mainly by one bosonic mode, whereas the other two provide further polaronic renormalizations. The numerically exact results for the electron spectral function compare favourably with an analytic solution based on degenerate perturbation theory in the basis of shifted oscillator states. In contrast, the widely used selfconsistent first-order Born approximation proves insufficient in describing the rich spectral features.Comment: 8 pages, 4 figure

STM induced hydrogen desorption via a hole resonance

We report STM-induced desorption of H from Si(100)-H(2$\times1$) at negative sample bias. The desorption rate exhibits a power-law dependence on current and a maximum desorption rate at -7 V. The desorption is explained by vibrational heating of H due to inelastic scattering of tunneling holes with the Si-H 5$\sigma$ hole resonance. The dependence of desorption rate on current and bias is analyzed using a novel approach for calculating inelastic scattering, which includes the effect of the electric field between tip and sample. We show that the maximum desorption rate at -7 V is due to a maximum fraction of inelastically scattered electrons at the onset of the field emission regime.Comment: 4 pages, 4 figures. To appear in Phys. Rev. Let

Effect of the Surface on the Electron Quantum Size Levels and Electron g-Factor in Spherical Semiconductor Nanocrystals

The structure of the electron quantum size levels in spherical nanocrystals is studied in the framework of an eight--band effective mass model at zero and weak magnetic fields. The effect of the nanocrystal surface is modeled through the boundary condition imposed on the envelope wave function at the surface. We show that the spin--orbit splitting of the valence band leads to the surface--induced spin--orbit splitting of the excited conduction band states and to the additional surface--induced magnetic moment for electrons in bare nanocrystals. This additional magnetic moment manifests itself in a nonzero surface contribution to the linear Zeeman splitting of all quantum size energy levels including the ground 1S electron state. The fitting of the size dependence of the ground state electron g factor in CdSe nanocrystals has allowed us to determine the appropriate surface parameter of the boundary conditions. The structure of the excited electron states is considered in the limits of weak and strong magnetic fields.Comment: 11 pages, 4 figures, submitted to Phys. Rev.

The Unique Origin of Colors of Armchair Carbon Nanotubes

The colors of suspended metallic colloidal particles are determined by their size-dependent plasma resonance, while those of semiconducting colloidal particles are determined by their size-dependent band gap. Here, we present a novel case for armchair carbon nanotubes, suspended in aqueous medium, for which the color depends on their size-dependent excitonic resonance, even though the individual particles are metallic. We observe distinct colors of a series of armchair-enriched nanotube suspensions, highlighting the unique coloration mechanism of these one-dimensional metals.Comment: 4 pages, 3 figure

First principles theory of inelastic currents in a scanning tunneling microscope

A first principles theory of inelastic tunneling between a model probe tip and an atom adsorbed on a surface is presented, extending the elastic tunneling theory of Tersoff and Hamann. The inelastic current is proportional to the change in the local density of states at the center of the tip due to the addition of the adsorbate. We use the theory to investigate the vibrational heating of an adsorbate below an STM tip. We calculate the desorption rate of H from Si(100)-H(2$\times$1) as function of the sample bias and tunnel current, and find excellent agreement with recent experimental data.Comment: 5 pages, RevTeX, epsf file

Two-vibron bound states in alpha-helix proteins : the interplay between the intramolecular anharmonicity and the strong vibron-phonon coupling

The influence of the intramolecular anharmonicity and the strong vibron-phonon coupling on the two-vibron dynamics in an $\alpha$-helix protein is studied within a modified Davydov model. The intramolecular anharmonicity of each amide-I vibration is considered and the vibron dynamics is described according to the small polaron approach. A unitary transformation is performed to remove the intramolecular anharmonicity and a modified Lang-Firsov transformation is applied to renormalize the vibron-phonon interaction. Then, a mean field procedure is realized to obtain the dressed anharmonic vibron Hamiltonian. It is shown that the anharmonicity modifies the vibron-phonon interaction which results in an enhancement of the dressing effect. In addition, both the anharmonicity and the dressing favor the occurrence of two different bound states which the properties strongly depend on the interplay between the anharmonicity and the dressing. Such a dependence was summarized in a phase diagram which characterizes the number and the nature of the bound states as a function of the relevant parameters of the problem. For a significant anharmonicity, the low frequency bound states describe two vibrons trapped onto the same amide-I vibration whereas the high frequency bound states refer to the trapping of the two vibrons onto nearest neighbor amide-I vibrations.Comment: may 2003 submitted to Phys. Rev.

Multiexcitons confined within a sub-excitonic volume: Spectroscopic and dynamical signatures of neutral and charged biexcitons in ultrasmall semiconductor nanocrystals

The use of ultrafast gating techniques allows us to resolve both spectrally and temporally the emission from short-lived neutral and negatively charged biexcitons in ultrasmall (sub-10 nm) CdSe nanocrystals (nanocrystal quantum dots). Because of forced overlap of electronic wave functions and reduced dielectric screening, these states are characterized by giant interaction energies of tens (neutral biexcitons) to hundreds (charged biexcitons) of meV. Both types of biexcitons show extremely short lifetimes (from sub-100 picoseconds to sub-picosecond time scales) that rapidly shorten with decreasing nanocrystal size. These ultrafast relaxation dynamics are explained in terms of highly efficient nonradiative Auger recombination.Comment: 5 pages, 4 figures, to be published in Phys. Rev.

The effect of Auger heating on intraband carrier relaxation in semiconductor quantumrods

The rate at which excited charge carriers relax to their equilibrium state affects many aspects of the performance of nanoscale devices, including switching speed, carrier mobility and luminescent efficiency. Better understanding of the processes that govern carrier relaxation therefore has important technological implications. A significant increase in carrier-carrier interactions caused by strong spatial confinement of electronic excitations in semiconductor nanostructures leads to a considerable enhancement of Auger effects, which can further result in unusual, Auger-process-controlled recombination and energy-relaxation regimes. Here, we report the first experimental observation of efficient Auger heating in CdSe quantum rods at high pump intensities, leading to a strong reduction of carrier cooling rates. In this regime, the carrier temperature is determined by the balance between energy outflow through phonon emission and energy inflow because of Auger heating. This equilibrium results in peculiar carrier cooling dynamics that closely correlate with recombination dynamics, an effect never before seen in bulk or nanoscale semiconductors.Comment: 7 pages, 4 figure