[Show abstract][Hide abstract] ABSTRACT: Single and multi-band (Burt-Foreman) k.p Hamiltonians for GaAs crystal phase
quantum dots are developed and used to assess ongoing experimental activity on
the role of such factors as quantum confinement, spontaneous polarization,
valence band mixing and exciton Coulomb interaction. Spontaneous polarization
is found to be a dominating term. Together with the control of dot thickness
[Vainorious Nano Lett. 15, 2652 (2015)] it enables wide exciton wavelength and
lifetime tunability. Several new phenomena are predicted for small diameter
dots [Loitsch et al. Adv. Mater. 27, 2195 (2015)], including non-heavy hole
ground state, strong hole spin admixture and a type-II to type-I exciton
transition, which can be used to improve the absorption strength and reduce the
radiative lifetime of GaAs polytypes.
[Show abstract][Hide abstract] ABSTRACT: We calculate the spin-orbit induced hole spin relaxation between Zeeman sublevels of vertically stacked InAs quantum dots. The widely used Luttinger-Kohn Hamiltonian, which considers coupling of heavy- and light-holes, reveals that hole spin lifetimes (T 1) of molecular states significantly exceed those of single quantum dot states. However, this effect can be overcome when cubic Dresselhaus spin-orbit interaction is strong. Misalignment of the dots along the stacking direction is also found to be an important source of spin relaxation.
[Show abstract][Hide abstract] ABSTRACT: Strain in colloidal heteronanocrystals with non-centrosymmetric lattices presents a unique opportunity for controlling optoelectronic properties and adds a new degree of freedom to existing wavefunction engineering and doping paradigms. We synthesized wurtzite CdSe nanorods embedded in a thick CdS shell, hereby exploiting the large lattice mismatch between the two domains to generate a compressive strain of the CdSe core and a strong piezoelectric potential along its c-axis. Efficient charge separation results in an indirect ground-state transition with a lifetime of several microseconds, almost one order of magnitude longer than any other CdSe/CdS nanocrystal. Higher excited states recombine radiatively in the nanosecond time range, due to increasingly overlapping excited-state orbitals. k˙p calculations confirm the importance of the anisotropic shape and crystal structure in the buildup of the piezoelectric potential. Strain engineering thus presents an efficient approach to highly tunable single- and multiexciton interactions, driven by a dedicated core/shell nanocrystal design.
[Show abstract][Hide abstract] ABSTRACT: We investigate theoretically the spin purity of single holes confined in
vertically coupled GaAs/AlGaAs quantum dots (QDs) under longitudinal magnetic
fields. A unique behavior is observed for triangular QDs, by which the spin is
largely pure when the hole is in one of the dots, but it becomes strongly mixed
when an electric field is used to drive it into molecular resonance. The spin
admixture is due to the valence band spin-orbit interaction, which is greatly
enhanced in C3h symmetry environments. The strong yet reversible electrical
control of hole spin suggests that molecules with C3-symmetry QDs, like those
obtained with  growth, can outperform the usual C2-symmetry QDs obtained
with  growth for the development of scalable qubit architectures.
Full-text · Article · May 2015 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: We report a systematic study of the spin relaxation anisotropy between single
electron Zeeman sublevels in cuboidal GaAs quantum dots (QDs). The QDs are
subject to an in-plane magnetic field. As the field orientation varies, the
relaxation rate oscillates periodically, showing ``magic'' angles where the
relaxation rate is suppressed by several orders of magnitude. This behavior is
found in QDs with different shapes, heights, crystallographic orientations and
external fields. The origin of these angles can be traced back to the
symmetries of the spin admixing terms of the Hamiltonian. In  grown QDs,
the suppression angles are different for Rashba and Dresselhaus spin-orbit
terms. By contrast, in  grown QDs they are the same, which should
facilitate a thorough suppression of spin-orbit induced relaxation. Our results
evidence that cubic Dresselhaus terms play a critical role in determining the
spin relaxation anisotropy even in quasi-2D QDs.
Full-text · Article · Sep 2014 · New Journal of Physics
[Show abstract][Hide abstract] ABSTRACT: We show that hole states in InAs/GaAs double quantum dots can exhibit spin anticrossings of up to 1 meV, according to simulations with a three dimensional Burt-Foreman Hamiltonian including strain and piezoelectric fields. The spin mixing originates in the valence band spin-orbit interaction plus the spatial symmetry breaking arising from misalignment between the dots and piezoelectric potential. The values we report are in better agreement with experiments than previous theoretical estimates and yield good prospects for efficient hole spin control. (C) 2013 AIP Publishing LLC.
[Show abstract][Hide abstract] ABSTRACT: The main characteristic strain trends in free-standing II–VI wurtzite semiconductor nanorods coated with a few-monolayers shell are reported. Calculations for different aspect ratios and shell thicknesses show that these are key factors for the strength of strain components that can even change their sign. Strain in core-shell nanorods with few monolayers coating is strong and qualitatively different from that of buried dots. Hexagonal symmetry compared to cubic and isotropic approximations reveals that, with the appropriate parameters, isotropic strain mimics very well the strain distributions of wurtzite core-shell nanorods.
No preview · Article · Jan 2012 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Using four-band k⋅p Hamiltonians, we study how biaxial strain and position-dependent effective masses influence hole tunneling in vertically coupled InAs/GaAs quantum dots. Strain reduces the tunneling and hence the critical interdot distance required for the ground state to change from bonding to antibonding. The reduced spin-orbit interaction in the GaAs matrix, which we account for using position-dependent Luttinger parameters, has the opposite effect. This compensation results in the critical distance being little affected. The possibility to induce the bonding-to-antibonding transition using longitudinal magnetic fields is also investigated. Luttinger-Kohn Hamiltonian predicts a magnetic enhancement of the heavy hole-light hole coupling which, in turn, leads to such transition. No such effect is, however, observed in magnetophotoluminescence experiments. An alternative implementation of the magnetic field in the envelope function Hamiltonian is given which retrieves the experimental behavior.
No preview · Article · Oct 2010 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Using a six-band k · p Hamiltonian for wurtzite lattice, we study the hole ground state symmetry and composition in spherical quantum dots and elongated quantum rods made of CdS and CdSe. The ground state crossovers which occur when changing the nanocrystal size and shape are well understood in terms of hole band mixing. Contrary to previous belief, the quantum rod ground state crossover with increasing length is shown not to occur at a fixed aspect ratio. The geometry and composition that maximize the spin purity and the intensity of linearly polarized light emission are elucidated. The six-band wurtzite Hamiltonian results for CdSe are compared to those obtained with quasi-cubic four-band and one-band Hamiltonians, and the performance of these simplified Hamiltonians is discussed
Full-text · Article · May 2010 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: We study theoretically the effect of thermal population on the emission spectrum of single CdSe nanocrys-
tals. Quantum confinement leads to nonsimple emission band shapes, which have different characteristics for excitons, biexcitons, positive, and negative trions. These effects are particularly pronounced in nanorods. The maximum of the emission band is not necessarily centered at the fundamental transition energy.
Full-text · Article · Nov 2009 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: We calculate the recombination energies and probabilities of neutral excitons (X), singly charged excitons
(X(), and biexcitons (XX) in CdSe quantum dots of variable length. For spherical dots the relative position
of the emission lines is determined by the confinement. As the dot is elongated, however, Coulomb correlation overcomes single-particle effects and the emission line of X becomes more energetic than that of any other excitonic complex. Likewise, the recombination probability (τ-1) of spherical dots is characteristic of the strong confinement regime: τ-1(XX) ∼ 2τ-1(X() ∼ 4τ-1(X). However, these ratios are reduced with increasing length, as correlations enhance the emission of each excitonic complex at a different rate. Our results are compared with available experimental data.
No preview · Article · Jul 2009 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The ground state electronic configuration of semiconductor spherical quantum dots populated with different numbers of excess electrons, for different radii and dielectric constants of the embedding medium is calculated and the corresponding phase diagram drawn. To this end, an extension of the spin density functional theory to study systems with variable effective mass and dielectric constant is employed. Our results show that high/low spin configurations can be switched by appropriate changes in the quantum dot embedding environment and suggest the use of the quantum dot spin as a sensor of the dielectric response of media.
Full-text · Article · Aug 2008 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Delocalized image surface states in free-standing hollow silica nanospheres populated with one or two electrons or an exciton are theoretically predicted for a wide range of internal radii and shell thicknesses. The driving force building up these surface states is the image self-polarization potential originating from the dielectric mismatch between the nanoshell and the surrounding air. The surface states are localized in a spherical crown beyond the nanoshell border. The transition from volume to surface state will then have to overcome the spatial confining potential barrier of the nanoshell. Owing to the different spatial confining barriers of electrons and holes in the silica nanoshell, electron but no hole density can be concentrated in surface distributions. The self-polarization potential looks like a double well potential, each well located just beyond the nanoshell border, with the internal well deeper than the external one, so that an excess carrier is attracted more strongly by the inner interface. This leads the electron density of a surface state to be located mainly in the internal surface of the hollow nanosphere. The shorter the inner nanoshell radius is, the stronger the binding of the excess electron to the surface will be. The volume/surface ground state phase diagrams of the one-electron, two-electron, and exciton systems have been calculated. All three diagrams are quite similar, thus revealing the monoelectronic character of the driving force for the transition from volume to surface states
Full-text · Article · Jan 2008 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: The formation of quantum dot (QD) excitonic surface states induced by dielectric mismatch is theoretically explored in spherical nanocrystals embedded in very high and in very low permittivity media. It is found that the transition from volume to surface exciton states (V-->S) always parallels a sudden drop of exciton brightness if the QD is embedded in low dielectric constant media. This is not the case of a QD buried in high permittivity media. In this case, the V-->S transition is monitored by a reduction in exciton brightness or not depending on the mh*/me* ratio between the effective masses of electron and hole. The presence of a hydrogenic donor impurity at the QD center can drastically reduce the electron-hole density overlap and thus the excitonic binding energy and the drop of brightness that parallels the formation of surface states.
No preview · Article · Sep 2007 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: A comprehensive study of anisotropic quantum rings, QRs, subject to axial and in-plane magnetic field, both aligned and transverse to the anisotropy direction, is carried out. Elliptical QRs for a wide range of eccentricity values and also perfectly circular QRs including one or more barriers disturbing the QR current are considered. These models mimic anisotropic geometry deformations and mass diffusion occurring in the QR fabrication process. Symmetry considerations and simplified analytical models supply physical insight into the obtained numerical results. Our study demonstrates that, except for unusual extremely large eccentricities, QR geometry deformations only appreciably influence a few low-lying states, while the effect of barriers disturbing the QR current is stronger and affects all studied states to a similar extent. We also show that the response of the electron states to in-plane magnetic fields provides accurate information on the structural anisotropy.