[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.
[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. There is current interest in using the spin of carri-ers confined in semiconductor quantum dots (QDs) for single spintronic, optoelectronic and quantum informa-tion research.[1–4] Self-assembled InAs/GaAs QDs have been particularly successful at this regard because they combine high optical activity, which enables precise op-tical preparation and read-out of the spin degrees of freedom,[5, 6] with moderately strong spin-orbit inter-action (SOI), which provides an additional knob for spin control. In general, the spin of electrons and holes in InAs QDs is a fairly good quantum number except in the vicinity of level crossings between states with orthogonal spins, where SOI or hyperfine interaction with the lattice nuclei mix the two states, lifting the degeneracy and forming a spin anticrossing. The importance of spin anticrossings, also referred to as spin hot spots[3, 8, 9], lies in the fact that they lead to fast spin flips. For this reason they have been proposed and used for spin manipulation protocols (see e.g. Refs.10–12). A strong SOI is desirable to ob-tain large spin anticrossings, thereby enabling faster op-erations. In some protocols, large anticrossings are also convenient to enhance the fidelity of the operations. For electrons, spin anticrossings are mainly due to Rashba and Dresselhaus SOI. Takahashi et al. reported gaps of 70−160 µeV between the s and p − orbitals of sin-gle InAs/GaAs QDs. Greilich et al. investigated spin anticrossings between s-shell singlet and triplet states of two electrons in vertically stacked double quantum dots (DQDs), obtaining gaps under 10 µeV . In the same work, it was observed that the corresponding gap for holes was 36 µeV –four times greater–. This is due to the inherent valence band SOI, which is generally stronger than that of the conduction band. Indeed, Doty et al. observed spin anticrossings as large as 200 µeV for holes tunneling in the neutral exciton states of some self-assembled DQD structures. Soon after, the same au-thors reported a gap of 400 µeV on a similar system. This is the largest spin anticrossing observed in the s-shell of InAs QDs so far. * Electronic address: firstname.lastname@example.org
[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.
Journal of Applied Physics 01/2012; 111(1). · 2.21 Impact Factor
[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.
Physical Review B 01/2010; 82(15). · 3.66 Impact Factor
[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
The Journal of Physical Chemistry C 01/2010; 114:8337. · 4.84 Impact Factor
[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.
Physical Review B 01/2009; 80:205312. · 3.66 Impact Factor
[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.
The Journal of Physical Chemistry C 01/2009; 113:11268. · 4.84 Impact Factor
[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.
Journal of Applied Physics 08/2008; · 2.21 Impact Factor
[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
Journal of Applied Physics 01/2008; · 2.21 Impact Factor
[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.
Physical Review B 09/2007; 76(11). · 3.66 Impact Factor
[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.
[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 persistent QR current are considered. These models mimic anisotropic geometry deformations and mass diffusion occuring 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 persistent 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.
[Show abstract][Hide abstract] ABSTRACT: The present paper is an approach to the calculation of strain in quantum dots of arbitrary shape buried in a matrix. We assume the isotropic strain model, which has good performance and is not computationally heavy. We start from the definitions of strain and stress and the fundamental Navier equation. Then, the elasticity formalism is applied to the problem of spherical inclusion. Finally, using the superposition principle, we obtain the strain in an inclusion of arbitrary shape as a sum of effects coming from the inclusion of many small spheres. The resulting strain formula is a surface integral which can be numerically solved and compared to results published in recent scientific literature.
Revista de la Real Sociedad Española de Fisica. 01/2007;
[Show abstract][Hide abstract] ABSTRACT: The conduction band electron states of laterally-coupled semiconductor quantum rings are studied within the frame of the effective mass envelope function theory. We consider the effect of axial and in-plane magnetic fields for several inter-ring distances, and find strong changes in the energy spectrum depending on the coupling regime. Our results indicate that the magnetic response accurately monitors the quantum ring molecule dissociation process. Moreover, the anisotropic response of the electron states to in-plane magnetic fields provides information on the orientation of the quantum ring molecule.
[Show abstract][Hide abstract] ABSTRACT: The electronic states of semiconductor quantum rings (QRs) under tilted magnetic fields are studied in the framework of the effective mass and envelope function approximations. For an axial field, the orbital Zeeman contribution prevails leading to the well-known Aharanov–Bohm spectrum, but it slowly decreases as the magnetic field direction declines. For an in-plane field, only the diamagnetic shift survives and it leads to the formation of double quantum well solutions, this result being relevant for experimental techniques which use in-plane magnetic fields to determine the spin of QR ground states. We also investigate the magnetic response of partially overlapped QRs, which are characteristic of high-density samples of self-assembled rings, and find that the spectrum is quite sensitive to ring coupling.
Physica E Low-dimensional Systems and Nanostructures 01/2006; · 1.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The energy levels and far-infrared absorption spectra of a self-assembled InAs ring with one and two electrons in an external magnetic field are calculated numerically. We use a truly three-dimensional effective mass model which considers finite potential barriers and mass dependence on the energy and position, and includes strain effects. The results obtained indicate that far-infrared spectroscopy of self-assembled rings is very sensitive to electron–electron interactions. The exchange energy leads to aperiodic fractional Aharonov–Bohm oscillations of electronic states and rapid narrowing of the magnetic field windows corresponding to the spin singlet ground state. Our results also suggest that the symmetric form of parabolic confinement potential, which has been widely employed to describe quantum rings, is unsuitable for self-assembled rings as it poorly describes the relevant effects of the inner radius.
[Show abstract][Hide abstract] ABSTRACT: The externally-corrected singles and doubles coupled cluster (CCSD) method, as implemented for high-spin open shell systems by exploiting the unitary group approach and restricted to the first order interacting space (UGA-CCSD(is)) in Part I (Li, X., et al., 1997, J. chem. Phys., 107,90), is applied to several simple radicals in their doublet ground and excited states. The capabilities and limitations of this approach are examined by studying the potential energy surfaces or their suitable cuts involving the dissociation of both single and multiple bonds (OH and CN) or simultaneous dissociation of several single bonds (NH2 and CH3). Using low dimensional CAS-FCI and SOCI wave functions for the internal and external active space excitations, it is shown that corrected CCSD energies are superior to the standard ones in all cases, including those obtained with CI spaces of very modest dimension, and are capable of accounting for the presence of higher than pair clusters even in severe cases of quasi-degeneracy.
[Show abstract][Hide abstract] ABSTRACT: Previously derived expressions for moments of spectral density distribution of an N-electron Hamiltonian defined in a finite-dimensional model space spanned by a set of spin-adapted antisymmetrized products of orthonormal orbitals (full configuration interaction space) are reduced to the low electron density limit, i.e. to the case when the number of electrons is much smaller than the number of orbitals. The limit of a very large number of electrons is also considered.
Journal of Physics A Mathematical and General 12/1997; 30(6):2181-2196. · 1.77 Impact Factor