arXiv subjects

Juan I. Climente

Publications and source records attributed to Juan I. Climente.

33 records · Page 2Linked to original sources

arXiv2016

Electronic Origin of Linearly Polarized Emission in CdSe/CdS Dot-in-Rod Heterostructures

Seeded CdSe/CdS nanorods exhibit intense polarized emission along the rod main axis. The degree of linear polarization cannot be explained by dielectric effects alone, an additional electronic contribution is present whose nature has not been settled up to date. Using multi-band k.p theory, we analyse the potential influence of several factors affecting excitonic emission and show that shear strain is the main electronic mechanism promoting linear polarization. It favors energetically light hole excitons over heavy hole ones, via deformation potential, and makes their radiative recombination faster via piezoelectricity. Implications of this mechanism are that linear emission can be enhanced by growing long but thin CdS shells around large, prolate CdSe cores, which indeed supports and rationalizes recent experimental findings. Together with the well-known dielectric effects, these results pave the way for controlled degree of linear polarization in dot-in-rods through dedicated structural design.

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arXiv2015

Theory of electrons, holes and excitons in GaAs polytype quantum dots

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.

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arXiv2015

Symmetry induced hole-spin mixing in quantum dot molecules

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 [111] growth, can outperform the usual C2-symmetry QDs obtained with [001] growth for the development of scalable qubit architectures.

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arXiv2014

The Role of Alternance Symmetry in Magnetoconductance

We show that the direction of coherent electron transport across a cyclic system of quantum dots or a cyclic molecule can be modulated by an external magnetic field if the cycle has an odd number of hopping sites, but the transport becomes completely symmetric if the number is even. These contrasting behaviors, which remain in the case of interacting electrons, are a consequence of the absence or presence of alternance symmetry in the system. These findings are relevant for the design of nanocircuits based on coupled quantum dots or molecular junctions.

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arXiv2013

Spin-orbit induced hole spin relaxation in InAs and GaAs quantum dots

We study the effect of valence band spin-orbit interactions on the acoustic phonon assisted spin relaxation of holes confined in quantum dots. Heavy hole-light hole (hh-lh) mixing and all the spin-orbit terms arising from zinc-blende bulk inversion asymmetry (BIA) are considered on equal footing in a fully 3D Hamiltonian. We show that hh-lh mixing and BIA have comparable contributions to the hole spin relaxation in self-assembled QDs, but BIA becomes dominant in gated QDs. The dependence of the hole spin relaxation on the QD geometry and spin splitting energy is drastically different from that of electrons, with a non-monotonic behavior which results from the interplay between SOI terms. Our results reconcile contradictory predictions of previous theoretical works and are consistent with experiments.

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arXiv2013

Magnetic field implementation in multiband k.p Hamiltonians of holes in semiconductor heterostructures

We propose an implementation of external homogeneous magnetic fields in k$\cdot$p Hamiltonians for holes in heterostructures, in which we made use of the minimal coupling prior to introduce the envelope function approximation. Illustrative calculations for holes in InGaAs quantum dot molecules show that the proposed Hamiltonian outperforms standard Luttinger model [Physical Review 102, 1030 (1956)] describing the experimentally observed magnetic response. The present implementation culminates our previous proposal [Phys. Rev. B 82, 155307 (2010)].

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arXiv2012

Scalable qubit architecture based on holes in quantum dot molecules

Spins confined in quantum dots are a leading candidate for solid-state quantum bits that can be coherently controlled by optical pulses. There are, however, many challenges to developing a scalable multibit information processing device based on spins in quantum dots, including the natural inhomogeneous distribution of quantum dot energy levels, the difficulty of creating all-optical spin manipulation protocols compatible with nondestructive readout, and the substantial electron-nuclear hyperfine interaction-induced decoherence. Here, we present a scalable qubit design and device architecture based on the spin states of single holes confined in a quantum dot molecule. The quantum dot molecule qubit enables a new strategy for optical coherent control with dramatically enhanced wavelength tunability. The use of hole spins allows the suppression of decoherence via hyperfine interactions and enables coherent spin rotations using Raman transitions mediated by a hole-spin-mixed optically excited state. Because the spin mixing is present only in the optically excited state, dephasing and decoherence are strongly suppressed in the ground states that define the qubits and nondestructive readout is possible. We present the qubit and device designs and analyze the wavelength tunability and fidelity of gate operations that can be implemented using this strategy. We then present experimental and theoretical progress toward implementing this design.

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arXiv2011

The excitonic resonance in semiconductor-metal nano-hybrids

We use a configuration interaction approach within the envelope function approximation to study the nature of the excitonic resonance in nano-hybrids, composite nanoparticles (NPs) combining a semiconducting and a metallic segment in contact. With reference to recent experimental reports, we specifically study CdS-based nanorods with metallic NPs deposited at the tips (matchstick) or metallic coatings (core-shell). The excitonic states are computed taking into account both the renormalization of the electron-hole interaction and self-energy effects induced by the the metallic segment on the electron-hole pair, as well as by the dielectric environment, through an induced charge numerical approach. In neutral matchstick structures the metal NP has only a minor influence (~1 meV) on the excitonic states. When the metallic NP is charged the exciton becomes rapidly redshifted and spatially indirect. In contrast, in neutral core-shell structures the exciton energy redshifts by tens of meV

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arXiv2008

Tuning the tunnel coupling of quantum dot molecules with longitudinal magnetic fields

We show that the energy splitting between the bonding and antibonding molecular states of holes in vertically stacked quantum dots can be tuned using longitudinal magnetic fields. With increasing field, the energy splitting first decreases down to zero and then to negative values, which implies a bonding-to-antibonding ground state transition. This effect is a consequence of the enhancement of the valence band spin-orbit interaction induced by the magnetic field, and it provides a flexible mechanism to switch the molecular ground state from bonding to antibonding.

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arXiv2008

Photoluminescence spectroscopy of trions in quantum dots: a theoretical description

We present a full configuration interaction study of the spontaneous recombination of neutral and singly charged excitons (trions) in semiconductor quantum dots, from weak to strong coupling regimes. We find that the enhancement of the recombination rate of neutral excitons with increasing dot size is suppressed for negative trions, and even reversed for positive trions. Our findings agree with recent comprehensive photoluminescence experiments in self-assembled quantum dots [P. Dalgarno et al. Phys. Rev. B {\bf 77}, 245311 (2008)] and confirm the major role played by correlations in the valence band.The effect of the temperature on the photoluminescence spectrum and that of the ratio between the electron and hole wavefunction lengthscales are also described.

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arXiv2007

Signatures of molecular correlations in the few-electron dynamics of coupled quantum dots

We study the effect of Coulomb interaction on the few-electron dynamics in coupled semiconductor quantum dots by exact diagonalization of the few-body Hamiltonian. The oscillation of carriers is strongly affected by the number of confined electrons and by the strength of the interdot correlations. Single-frequency oscillations are found for either uncorrelated or highly correlated states, while multi-frequency oscillations take place in the intermediate regime. Moreover, Coulomb interaction renders few-particle oscillations sensitive to perturbations in spatial directions other than that of the tunneling, contrary to the single-particle case. The inclusion of acoustic phonon scattering does not modify the carrier dynamics substantially at short times, but can damp oscillation modes selectively at long times.

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arXiv2007

Effect of electron-electron interaction on the phonon-mediated spin relaxation in quantum dots

We estimate the spin relaxation rate due to spin-orbit coupling and acoustic phonon scattering in weakly-confined quantum dots with up to five interacting electrons. The Full Configuration Interaction approach is used to account for the inter-electron repulsion, and Rashba and Dresselhaus spin-orbit couplings are exactly diagonalized. We show that electron-electron interaction strongly affects spin-orbit admixture in the sample. Consequently, relaxation rates strongly depend on the number of carriers confined in the dot. We identify the mechanisms which may lead to improved spin stability in few electron (>2) quantum dots as compared to the usual one and two electron devices. Finally, we discuss recent experiments on triplet-singlet transitions in GaAs dots subject to external magnetic fields. Our simulations are in good agreement with the experimental findings, and support the interpretation of the observed spin relaxation as being due to spin-orbit coupling assisted by acoustic phonon emission.

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arXiv2006

Triplet-Singlet Spin Relaxation in Quantum Dots with Spin-Orbit Coupling

We estimate the triplet-singlet relaxation rate due to spin-orbit coupling assisted by phonon emission in weakly-confined quantum dots. Our results for two and four electrons show that the different triplet-singlet relaxation trends observed in recent experiments under magnetic fields can be understood within a unified theoretical description, as the result of the competition between spin-orbit coupling and phonon emission efficiency. Moreover, we show that both effects are greatly affected by the strength of the confinement and the external magnetic field, which may give access to very long-lived triplet states as well as to selective population of the triplet Zeeman sublevels.

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arXiv2006

Effect of the Coulomb interaction on the electron relaxation of weakly-confined quantum dot systems

We study acoustic-phonon-induced relaxation of charge excitations in single and tunnel-coupled quantum dots containing few confined interacting electrons. The Full Configuration Interaction approach is used to account for the electron-electron repulsion. Electron-phonon interaction is accounted for through both deformation potential and piezoelectric field mechanisms. We show that electronic correlations generally reduce intradot and interdot transition rates with respect to corresponding single-electron transitions, but this effect is lessened by external magnetic fields. On the other hand, piezoelectric field scattering is found to become the dominant relaxation mechanism as the number of confined electrons increases. Previous proposals to strongly suppress electron-phonon coupling in properly designed single-electron quantum dots are shown to hold also in multi-electron devices. Our results indicate that few-electron orbital degrees of freedom are more stable than single-electron ones.

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arXiv2004

Far-infrared absorption of self-assembled semiconductor rings

We report a theoretical description of far-infrared spectroscopy experiments on self-assembled quantum rings in a magnetic field [A. Lorke et al., Phys. Rev. Lett. 84, 2223 (2000)] which, for the first time, accounts for the full set of experimental resonances. In our calculations we use a 3D effective-mass model with a realistic finite step-like confinement potential, including strain and Coulomb effects. We assume a bimodal distribution of ring sizes.

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