arXiv subjects

Gabriel Bester

Publications and source records attributed to Gabriel Bester.

At least 37 records · Page 2Linked to original sources

arXiv2018

Uniaxial stress flips the natural quantization axis of a quantum dot for integrated quantum photonics

The optical selection rules in epitaxial quantum dots are strongly influenced by the orientation of their natural quantization axis, which is usually parallel to the growth direction. This configuration is well suited for vertically emitting devices, but not for planar photonic circuits because of the poorly controlled orientation of the transition dipoles in the growth plane. Here we show that the quantization axis of gallium arsenide dots can be flipped into the growth plane via moderate in plane uniaxial stress. By using piezoelectric strain actuators featuring strain-amplification we study the evolution of the selection rules and excitonic fine-structure in a regime, in which quantum confinement can be regarded as a perturbation compared to strain in determining the symmetry properties of the system. The experimental and computational results suggest that uniaxial stress, may be the right tool to obtain quantum light sources with ideally oriented transition dipoles and enhanced oscillator strengths for integrated quantum photonics.

quant-ph↗

arXiv2015

Heavy strain conditions in colloidal core-shell quantum dots and their consequences on the vibrational properties from \emph{Ab initio} calculations

We preform large-scale \emph{ab initio} density functional theory calculations to study the lattice strain and the vibrational properties of colloidal semiconductor core-shell nanoclusters with up to one thousand atoms (radii up to 15.6~Å). For all the group IV, III-V and II-VI semiconductors studied, we find that the atom positions of the shell atoms, seem unaffected by the core material. In particular, for group IV core-shell clusters the shell material remains unstrained, while the core adapts to the large lattice mismatch (compressive or tensile strain). For InAs-InP and CdSe-CdS, both the cores and the shells are compressively strained corresponding to pressures up to 20 GPa. We show that this compression, which contributes a large blue-shift of the vibrational frequencies, is counterbalanced, to some degree, by the undercoordination effect of the near-surface shell, which contributes a red-shift to the vibrational modes. These findings lead to a different interpretation of the frequency shifts of recent Raman experiments, while they confirm the speculated interface nature of the low-frequency shoulder of the high frequency Raman peak.

cond-mat.mes-hall↗

arXiv2014

Supercoupling between heavy-hole and light-hole states in self-assembled quantum dots

Spintronics, quantum computing and quantum communication science utilizing cubic semiconductors rely largely on the properties of the hole states, composed of light and heavy hole wavefunction components. The admixture of light-hole (LH) into ground hole state predominately by the heavy hole (HH) would induce unique features of LH in optical transitions, spin relaxation, and spin polarization. We point to an unexpected source of HH-LH mixing in quantum dots, arguing that in contrast with current models the mixing does not reflect the strain between the dot and its matrix and does not scale inversely with the energy splitting between the bulk HH and LH states. Instead, we show via atomistic pseudopotential calculations on a range of strained and unstrained dots of different symmetries that the HH-LH mixing is enabled by the presence in the QD of a dense ladder of intermediate states between the HH and LH states which amplifies and propagates this interaction and leads to "supercoupling" (analogous to super-exchange in magnetism). This explains a number of outstanding puzzles regarding the surprising large coupling seen in unstrained QD (GaAs/AlAs) of ideal shapes and the surprising fact that in strained QD (InAs/GaAs) the coupling is very strong despite the fact that the 12-fold increase in bulk HH-LH splitting overrides the ~4 fold enhancement of the coupling matrix element by strain in comparison with unstrained GaAs QDs.

cond-mat.mes-hall↗

arXiv2013

Semiempirical pseudopotential approach for nitride-based nanostructures and {\it ab initio} based passivation of free surfaces

We present a semiempirical pseudopotential method based on screened atomic pseudopotentials and derived from \textit{ab initio} calculations. This approach is motivated by the demand for pseudopotentials able to address nanostructures, where \textit{ab initio} methods are both too costly and insufficiently accurate at the level of the local-density approximation, while mesoscopic effective-mass approaches are inapplicable due to the small size of the structures along, at least, one dimension. In this work we improve the traditional pseudopotential method by a two-step process: First, we invert a set of self-consistently determined screened {\it ab initio} potentials in wurtzite GaN for a range of unit cell volumes, thus determining spherically-symmetric and structurally averaged atomic potentials. Second, we adjust the potentials to reproduce observed excitation energies. We find that the adjustment represents a reasonably small perturbation over the potential, so that the ensuing potential still reproduces the original wave functions, while the excitation energies are significantly improved. We furthermore deal with the passivation of the dangling bonds of free surfaces which is relevant for the study of nanowires and colloidal nanoparticles. We present a methodology to derive passivant pseudopotentials from {\it ab initio} calculations. We apply our pseudopotential approach to the exploration of the confinement effects on the electronic structure of GaN nanowires.

cond-mat.mes-hall↗

arXiv2013

Vibron-vibron coupling from ab initio molecular dynamics simulations of a silicon cluster

We study the temperature dependent dynamical processes of a Si10H16 cluster and obtain a blue shift of the Si-Si vibrational modes with transverse acoustic character and a red shift of the other vibrational modes with increasing temperature. We link this behavior to the bond length expansion and the varying sign of the Grueneisen parameter. We further present a computational approach able to extract the vibron-vibron coupling strength in clusters or molecules. Our approach is based on ab initio Born-Oppenheimer molecular dynamics and a projection formalism able to deliver the individual vibron occupation numbers. From the Fourier transform of the vibron energy autocorrelation function we obtain the coupling strength of each vibron to the most strongly coupled vibronic states. We find vibron-vibron coupling strength up to 2.5 THz with a moderate increase of about 5 % when increasing the temperature from 50 to 150 K.

cond-mat.mtrl-sci↗

arXiv2012

Effects of charged defects on the electronic and optical properties of self-assembled quantum dots

We investigate the effects of point charge defects on the single particle electronic structure, emission energies, fine structure splitting and oscillator strengths of excitonic transitions in strained In$_{0.6}$Ga$_{0.4}$As/GaAs and strain-free GaAs/Al$_{0.3}$Ga$_{0.7}$As quantum dots. We find that the charged defects significantly modify the single particle electronic structure and excitonic spectra in both strained and strain-free structures. However, the excitonic fine structure splitting, polarization anisotropy and polarization direction in strained quantum dots remain nearly unaffected, while significant changes are observed for strain-free quantum dots.

cond-mat.mes-hall↗

arXiv2012

Confinement effects on the vibrational properties of III-V and II-VI nanoclusters

We present a first-principles study of the confinement effects on the vibrational properties of thousand atoms (radii up to 16.2 Å) colloidal III-V and II-VI nanoclusters. We describe how the molecular-type vibrations, such as surface--optical, surface--acoustic and coherent acoustic modes, coexist and interact with bulk-type vibrations, such as longitudinal and transverse acoustic and optical modes. We link vibrational properties to structural changes induced by the surface and highlight the qualitative difference between III-Vs and II-VIs. We describe the size dependence of the vibrations and find good agreement for Raman shifts and for the frequency of coherent acoustic modes with experiments.

cond-mat.mes-hall↗

arXiv2011

Interatomic potentials for the vibrational properties of III-V semiconductor nanostructures

We derive interatomic potentials for zinc blende InAs, InP, GaAs and GaP semiconductors with possible applications in the realm of nanostructures. The potentials include bond stretching interaction between the nearest and next-nearest neighbors, a three body term and a long-range Coulomb interaction. The optimized potential parameters are obtained by (i) fitting to bulk phonon dispersions and elastic properties and (ii) constraining the parameter space to deliver well behaved potentials for the structural relaxation and vibrational properties of nanostructure clusters. The targets are thereby calculated by density functional theory for clusters of up to 633 atoms. We illustrate the new capability by the calculation Kleinman and Grüneisen parameters and of the vibrational properties of nanostructures with 3 to 5.5 nm diameter.

cond-mat.mes-hall↗

arXiv2010

Charged excitons and biexcitons in laterally coupled InGaAs quantum dots

We present results of atomistic empirical pseudopotential calculations and configuration interaction for excitons, positive and negative trions (X\pm), positive and negative quartons (X2\pm) and biexcitons. The structure investigated are laterally aligned InGaAs quantum dot molecules embedded in GaAs under a lateral electric field. The rather simple energetic of excitons becomes more complex in the case of charged quasiparticles but remains tractable. The negative trion spectrum shows four anticrossings in the presently available range of fields while the positive trion shows two. The magnitude of the anticrossings reveals many-body effects in the carrier tunneling process that should be experimentally accessible.

cond-mat.mes-hall↗

arXiv2010

Heterogeneous confinement in laterally coupled InGaAs/GaAs quantum dot molecules under lateral electric fields

We study the electronic and optical properties of laterally coupled InGaAs/GaAs quantum dot molecules under lateral electric field. We find that electrons perceive the double-dot structure as a compound single object, while the holes discern two well separated dots. Through a combination of predictive atomistic modeling, detailed morphology studies, and single object micro-photoluminescence measurements, we show that this peculiar confinement results in an unusual heterogeneous behavior of electrons and holes with profound consequences on optical properties.

cond-mat.mes-hall↗

arXiv2009

Prediction of large linear-in-k spin splitting for holes in the 2D GaAs/AlAs system

The spin-orbit interaction generally leads to spin splitting (SS) of electron and hole energy states in solids, a splitting that is characterized by a scaling with the wavevector $\bf k$. Whereas for {\it 3D bulk zincblende} solids the electron (heavy hole) SS exhibits a cubic (linear) scaling with $k$, in {\it 2D quantum-wells} the electron (heavy hole) SS is currently believed to have a mostly linear (cubic) scaling. Such expectations are based on using a small 3D envelope function basis set to describe 2D physics. By treating instead the 2D system explicitly in a multi-band many-body approach we discover a large linear scaling of hole states in 2D. This scaling emerges from hole bands coupling that would be unsuspected by the standard model that judges coupling by energy proximity. This discovery of a linear Dresselhaus k-scaling for holes in 2D implies a different understanding of hole-physics in low-dimensions.

cond-mat.mtrl-sci↗

arXiv2007

Calculation of near-field scanning optical images of exciton, charged excition and multiexciton wavefunctions in self-assembled InAs/GaAs quantum dots

The near-field scanning optical microscopy images of excitonic wavefunctions in self-assembled InAs/GaAs quantum dots are calculated using an empirical pseudopotential method, followed by the configuration interaction (CI) treatment of many-particle effects. We show the wavefunctions of neutral exciton $X^0$ of different polarizations, and compare them to those of the biexciton $XX$ and the charged excitons $X^+$ and $X^-$. We further show that the exciton $X(P_h \to S_e)$ transition which is forbidden in the far-field photoluminescence has comparable intensities to that of $X(S_h \to S_e)$ transition in the near-field photoluminescence .

cond-mat.mtrl-sci↗

arXiv2006

Excitonic exchange effects on the radiative decay time of monoexcitons and biexcitons in quantum dots

Electron-hole exchange interactions split the exciton ground state into "dark" and "bright" states. The dynamics of those states depends on the internal relaxation time between bright and dark states (spin-flip time), and on the radiative recombination time of the bright states. On the other hand, the calculated values of these recombination times depend not only on the treatment of correlation effects, but also on the accuracy of the electron and hole wavefunctions. We calculate the radiative decay rates for monoexcitons and biexcitons in (In,As)Ga/GaAs self-assembled and colloidal CdSe quantum dots from atomistic correlated wave functions. We show how the radiative decay time tau_R(X^0) of the monoexciton depends on the spin-flip relaxation time between bright and dark states. In contrast, a biexciton has no bright-dark splitting, so the decay time of the biexciton tau_R(XX^0) is insensitive to this spin-flip time. This results in ratios $τ_R(X^0)/τ_R(XX^0)$ of 4 in the case of fast spin flip, and a ratio of 2 in the case of slow spin flip. For (In,Ga)As/GaAs, we compare our results with the model calculation of Wimmer et al. [M. Wimmer et al., Phys. Rev. B 73, 165305 (2006)]. When the same spin-flip rates are assumed, our predicted tau_R(X^0)/tau_R(XX^0) agrees with that of Wimmer et al., suggesting that our treatment of correlations is adequate to predict the ratio of monoexciton and biexciton radiative lifetimes. Our results agree well with experiment on self-assembled quantum dots when assuming slow spin flip. Conversely, for colloidal dots the agreement with experiment is best for fast spin flip.

cond-mat.mtrl-sci↗

arXiv2006

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 τ(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 τ(P->S)~1-7 ps for dots of different sizes. Our calculated Auger-type P-shell--to-S-shell decay lifetimes τ(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.

cond-mat.mtrl-sci↗

arXiv2006

Importance of second-order piezoelectric effects in zincblende semiconductors

We show that the piezoelectric effect that describes the emergence of an electric field in response to a crystal deformation in III-V semiconductors such as GaAs and InAs has strong contributions from second-order effects that have been neglected so far. We calculate the second-order piezoelectric tensors using density functional theory and obtain the piezoelectric field for [111]-oriented In$_x$Ga$_{1-x}$As quantum wells of realistic dimensions and concentration $x$. We find that the linear and the quadratic piezoelectric coefficients have the opposite effect on the field, and for large strains the quadratic terms even dominate. Thus, the piezoelectric field turns out to be a rare example of a physical quantity for which the first- and second-order contributions are of comparable magnitude.

cond-mat.mtrl-sci↗

arXiv2005

Excitons, biexcitons and trions in self-assembled (In,Ga)As/GaAs quantum dots: Recombination energies, polarization and radiative lifetimes versus dot height

We calculate the height dependence of recombination energies, polarization and radiative lifetimes of the optical transitions of various excitonic complexes: neutral excitons (X^0), negatively- (X^-) and positively-charged (X^+) trions, and biexcitons (XX^0) in lens-shaped, self-assembled In_0.6Ga_0.4As/GaAs quantum dots. By using an atomistic pseudopotential method combined with the configuration-interaction method, we predict the following. (i) The recombination energy of the lowest transition of X^- blue-shifts as height increases, whereas that of X^+ red-shifts. Remarkably, the recombination of XX^0 shows a red-shift at small heights, reaches a maximum shift, and then blue-shifts for taller dots. (ii) Changes in dot height lead to a bound-to-unbound crossover for X^-, X^+ and XX^0. (iii) When considering the [110] and [1\bar{1}0] directions, the lowest transitions of X^0 and XX^0 manifest [110] vs [1\bar{1}0] in-plane polarization anisotropy that switches sign as a function of height as well as alloy randomness. $X^-$ and $X^+$ show transitions with negligible polarization anisotropy regardless of height. (iv) The ground state of X^0 is split in a low-energy pair that is forbidden (dark) and a high-energy pair that is allowed; thus, at T=0K the radiative lifetime τ(X^0) is long (~ ms) due to the dark exciton. On the other hand, at T=10K, τ(X^0) decreases moderately as height increases and its magnitude ranges from 2-3ns. The ground state of X^- and X^+, and that of XX^0 is allowed (bright); so, τ(X^-), τ(X^+) and τ(XX^0) are fast (~ ns) even at T=0K. These radiative lifetimes depend weakly on height. In addition, τ(X^-) ~ τ(X^+) ~1.1ns, while τ(XX^0)~0.5ns$. We compare our predictions with available spectroscopic data.

cond-mat.mtrl-sci↗

arXiv2005

Singlet-triplet splitting, correlation and entanglement of two electrons in quantum dot molecules

Starting with an accurate pseudopotential description of the single-particle states, and following by configuration-interaction treatment of correlated electrons in vertically coupled, self-assembled InAs/GaAs quantum dot-molecules, we show how simpler, popularly-practiced approximations, depict the basic physical characteristics including the singlet-triplet splitting, degree of entanglement (DOE) and correlation. The mean-field-like single-configuration approaches such as Hartree-Fock and local spin density, lacking correlation, incorrectly identify the ground state symmetry and give inaccurate values for the singlet-triplet splitting and the DOE. The Hubbard model gives qualitatively correct results for the ground state symmetry and singlet-triplet splitting, but produces significant errors in the DOE because it ignores the fact that the strain is asymmetric even if the dots within a molecule are identical. Finally, the Heisenberg model gives qualitatively correct ground state symmetry and singlet-triplet splitting only for rather large inter-dot separations, but it greatly overestimates the DOE as a consequence of ignoring the electron double occupancy effect.

cond-mat.mtrl-sci↗

arXiv2005

Electronic phase diagrams of carriers in self-assembled InAs/GaAs quantum dots: violation of Hund's rule and the Aufbau principle for holes

We study the orbital and spin configurations of up to six electrons or holes charged into self-assembled InAs/GaAs quantum dots via single-particle pseudopotential and many-particle configuration interaction method. We find that while the charging of {\it electrons} follows both Hund's rule and the Aufbau principle, the charging of {\it holes} follows a non-trivial charging pattern which violates both the Aufbau principle and Hund's rule, and is robust against the details of the quantum dot size. The predicted hole charging sequence offers a new interpretation of recent charging experiments.

cond-mat.mtrl-sci↗