arXiv subjects

Alex Zunger

Publications and source records attributed to Alex Zunger.

At least 91 records · Page 5Linked to original sources

arXiv2009

Polaronic hole localization and multiple hole binding of acceptors in oxide wide-gap semiconductors

Acceptor-bound holes in oxides often localize asymmetrically at one out of several equivalent oxygen ligands. Whereas Hartree-Fock (HF) theory overly favors such symmetry-broken polaronic hole-localization in oxides, standard local density (LD) calculations suffer from spurious delocalization among several oxygen sites. These opposite biases originate from the opposite curvatures of the energy as a function of the fractional occupation number n, i.e., d2E/dn2 < 0 in HF and d2E/dn2 > 0 in LD. We recover the correct linear behavior, d2E/dn2 = 0, that removes the (de)localization bias by formulating a generalized Koopmans condition. The correct description of oxygen hole-localization reveals that the cation-site nominal single-acceptors in ZnO, In2O3, and SnO2 can bind multiple holes.

cond-mat.mtrl-sci↗

arXiv2008

Highly-reduced Fine-structure splitting in InAs/InP quantum dots offering efficient on-demand 1.55 $μ$m entangled photon emitter

To generate entangled photon pairs via quantum dots (QDs), the exciton fine structure splitting (FSS) must be comparable to the exciton homogeneous line width. Yet in the (In,Ga)As/GaAs QD, the intrinsic FSS is about a few tens $μ$eV. To achieve photon entanglement, it is necessary to Cherry-pick a sample with extremely small FSS from a large number of samples, or to apply strong in-plane magnetic field. Using theoretical modeling of the fundamental causes of FSS in QDs, we predict that the intrinsic FSS of InAs/InP QDs is an order of magnitude smaller than that of InAs/GaAs dots, and better yet, their excitonic gap matches the 1.55 $μ$m fiber optic wavelength, therefore offer efficient on-demand entangled photon emitters for long distance quantum communication.

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↗

arXiv2007

Electronic structures of (In,Ga)As/GaAs quantum dot molecules made of dots with dissimilar sizes

Using single-particle pseudopotential and many-particle configuration interaction methods, we compare various physical quantities of (In,Ga)As/GaAs quantum dot molecules (QDMs) made of dissimilar dots (heteropolar QDMs) with QDMs made of identical dots (homopolar QDMs). The calculations show that the electronic structures of hetero-QDMs and homo-QDMs differ significantly at large inter-dot distance. In particular (i) Unlike those of homo-QDMs, the single-particle molecular orbitals of hetero-QDMs convert to dot localized orbitals at large inter-dot distance. (ii) Consequently, in a hetero-QMD the bonding-antibonding splitting of molecular orbitals at large inter-dot distance is significantly larger than the electron hopping energy whereas for homo-QDM, the bonding-antibonding splitting is very similar to the hopping energy. (iii) The asymmetry of the QDM increases significantly the double occupation for the two-electron ground states, and therefore affects the degree of entanglement of the two electrons.

cond-mat.mtrl-sci↗

arXiv2006

Calculation of conduction-to-conduction and valence-to-valence transitions between bound states in (In,Ga)As/GaAs quantum dots

We have calculated the conduction-to-conduction and valence-to-valence absorption spectrum of bound states in (In,Ga)As/GaAs quantum dots charged with up to three electrons or holes. Several features emerge: (i) In pure (non-alloyed) InAs/GaAs dots, the 1S-1P_1 and 1S-1P_2 conduction intraband transitions are fully in-plane polarized along [1\bar 10] and [110], respectively, while valence transitions are weakly polarized because the hole P states do not show any in-plane preferential orientation. (ii) In alloyed In_{0.6}Ga_{0.4}As/GaAs dots the [110] and [1\bar 10] polarization of the corresponding 1S-1P conduction intraband transitions is weakened since the two 1P states are mixed by alloy fluctuations. The polarization of valence intraband transitions is insensitive to changes in alloy fluctuations. (iii) For light polarized along [001], we find a strong valence-to-valence transition that involves a weakly confined hole state with predominant light-hole character. (iv) When charging the dots with a few electrons, the conduction intraband transitions display spectroscopic shifts of ~1-2 meV. These shifts are a result of correlation effects (captured by configuration-interaction) and not well described within the Hartree-Fock approximation. (v) When charging the dots with holes, valence intraband spectra are more complex than the conduction intraband spectra as hole states are strongly affected by spin-orbit coupling, and configuration mixing is more pronounced. Spectroscopic shifts can no longer be identified unambiguously. These predictions could be tested in single-dot spectroscopy of n-doped and p-doped quantum dots.

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

Nominally forbidden transitions in the interband optical spectrum of quantum dots

We calculate the excitonic optical absorption spectra of (In,Ga)As/GaAs self-assembled quantum dots by adopting an atomistic pseudopotential approach to the single-particle problem followed by a configuration-interaction approach to the many-body problem. We find three types of allowed transitions that would be naively expected to be forbidden. (i) Transitions that are parity forbidden in simple effective mass models with infinite confining wells (e.g. 1S-2S, 1P-2P) but are possible by finite band-offsets and orbital-mixing effects; (ii) light-hole--to--conduction transitions, enabled by the confinement of light-hole states; and (iii) transitions that show and enhanced intensity due to electron-hole configuration mixing with allowed transitions. We compare these predictions with results of 8-band k.p calculations as well as recent spectroscopic data. Transitions in (i) and (ii) explain recently observed satellites of the allowed P-P transitions.

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

Multiple charging of InAs/GaAs quantum dots by electrons or holes: addition energies and ground-state configurations

Atomistic pseudopotential plus configuration interaction calculations of the energy needed to charge dots by either electrons or holes are described, and contrasted with the widely used, but highly simplified two-dimensional parabolic effective mass approximation (2D-EMA). Substantial discrepancies are found, especially for holes, regarding the stable electronic configuration and filling sequence which defies both Hund's rule and the Aufbau principle.

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↗

arXiv2005

Electric field control and optical signature of entanglement in quantum dot molecules

The degree of entanglement of an electron with a hole in a vertically coupled self-assembled dot molecule is shown to be tunable by an external electric field. Using atomistic pseudopotential calculations followed by a configuration interaction many-body treatment of correlations, we calculate the electronic states, degree of entanglement and optical absorption. We offer a novel way to spectroscopically detect the magnitude of electric field needed to maximize the entanglement.

cond-mat.mtrl-sci↗

arXiv2005

Origin of transition metal clustering tendencies in GaAs based dilute magnetic semiconductors

While isovalent doping of GaAs (e.g. by In) leads to a repulsion between the solute atoms, two Cr, Mn, or Fe atoms in GaAs are found to have lower energy than the well-separated pair, and hence attract each other. The strong bonding interaction between levels with t2 symmetry on the transition metal (TM) atoms results in these atoms exhibiting a strong tendency to cluster. Using first-principles calculations, we show that this attraction is maximal for Cr, Mn and Fe while it is minimal for V. The difference is attributed to the symmetry of the highest occupied levels. While the intention is to find possible choices of spintronic materials that show a reduced tendency to cluster, one finds that the conditions that minimize clustering tendencies also minimize the stabilization of the magnetic state.

cond-mat.mtrl-sci↗

arXiv2005

Pressure effects on neutral and charged excitons in self-assembled InGaAs/GaAs quantum dots

By combining an atomistic pseudopotential method with the configuration interaction approach, we predict the pressure dependence of the binding energies of neutral and charged excitons: $X^0$ (neutral monoexciton), $X^{-}$ and $X^{+}$ (charged trions), and $XX^0$ (biexciton) in lens-shaped, self-assembled In$_{0.6}$Ga$_{0.4}$As/GaAs quantum dots. We predict that (i) with applied pressure the binding energy of $X^0$ and $X^+$ increases and that of $X^-$ decreases, whereas the binding energy of $XX^0$ is nearly pressure independent. (ii) Correlations have a small effect in the binding energy of $X^0$, whereas they largely determine the binding energy of $X^-$, $X^+$ and $XX^0$. (iii) Correlations depend weakly on pressure; thus, the pressure dependence of the binding energies can be understood within the Hartree-Fock approximation and it is controlled by the pressure dependence of the direct Coulomb integrals $J$. Our results in (i) can thus be explained by noting that holes are more localized than electrons, so the Coulomb energies obey $J^{(hh)}>J^{(eh)}>J^{(ee)}$.

cond-mat.mtrl-sci↗

arXiv2005

Asymmetry in self-assembled quantum dot-molecules made of identical InAs/GaAs quantum dots

We show that a diatomic dot molecule made of two identical, vertically stacked, strained InAs/GaAs self-assembled dots exhibits an asymmetry in its single-particle and may-particle wavefunctions. The single-particle wave function is asymmetric due to the inhomogeneous strain, while the asymmetry of the many-particle wavefunctions is caused by the correlation induced localization: the lowest singlet $^1Σ_g$ and triplet $^3Σ$ states show that the two electrons are each localized on different dots within the molecule, for the next singlet states $^1Σ_u$ both electrons are localized on the same (bottom) dot for interdot separation $d>$ 8 nm. The singlet-triplet splitting is found to be $\sim 0.1$ meV at inter-dot separation $d$=9 nm and as large as 100 meV for $d$=4 nm, orders of magnitude larger than the few meV found in the large (50 - 100 nm) electrostatically confined dots.

cond-mat.mtrl-sci↗

arXiv2005

Dependence of the electronic structure of self-assembled InGaAs/GaAs quantum dots on height and composition

While electronic and spectroscopic properties of self-assembled In_{1-x}Ga_{x}As/GaAs dots depend on their shape, height and alloy compositions, these characteristics are often not known accurately from experiment. This creates a difficulty in comparing measured electronic and spectroscopic properties with calculated ones. Since simplified theoretical models (effective mass, k.p, parabolic models) do not fully convey the effects of shape, size and composition on the electronic and spectroscopic properties, we offer to bridge the gap by providing accurately calculated results as a function of the dot height and composition. Prominent results are the following. (i) Regardless of height and composition, the electron levels form shells of nearly degenerate states. In contrast, the hole levels form shells only in flat dots and near the highest hole level (HOMO). (ii) In alloy dots, the electrons' ``s-p'' splitting depends weakly on height, while the ``p-p'' splitting depends non-monotonically. In non-alloyed InAs/GaAs dots, both these splittings depend weakly on height. For holes in alloy dots, the ``s-p'' splitting decreases with increasing height, whereas the ``p-p'' splitting remains nearly unchaged. Shallow, non-alloyed dots have a ``s-p'' splitting of nearly the same magnitude, whereas the ``p-p'' splitting is larger. (iii) As height increases, the ``s'' and ``p'' character of the wavefunction of the HOMO becomes mixed, and so does the heavy- and light-hole character. (iv) In alloy dots, low-lying hole states are localized inside the dot. Remarkably, in non-alloyed InAs/GaAs dots these states become localized at the interface as height increases. This localization is driven by the biaxial strain present in the nanostructure.

cond-mat.mtrl-sci↗