Spin Dynamics In Quantum Dots Research Articles

Model expressions for the spin-orbit interaction and phonon-mediated spin dynamics in quantum dots

Model expressions for the spin-orbit interaction in a quantum dot are obtained. The resulting form does not neglect cubic terms and allows for a generalized structural inversion asymmetry. We also obtain analytical expressions for the coupling between states for the electron–phonon interaction and use these to derive spin-relaxation rates, which are found to be qualitatively similar to those derived elsewhere in the literature. We find that, due to the inclusion of cubic terms, the Dresselhaus contribution to the ground state spin relaxation disappears for spherical dots. A comparison with previous theory and existing experimental results shows good agreement thereby presenting a clear analytical formalism for future developments. Comparative calculations for potential materials are presented.

Increased coherence time in narrowed bath states in quantum dots

We study the influence of narrowed distributions of the nuclear Overhauser field on the decoherence of a central electron spin in quantum dots. We describe the spin dynamics in quantum dots by the central spin model. We use analytic solutions for uniform couplings and the time dependent density-matrix renormalization group (tDMRG) for nonuniform couplings. With these tools we calculate the dynamics of the central spin for large baths of nuclear spins with or without external magnetic field applied to the central spin. The focus of our study is the influence of initial mixtures with narrowed distributions of the Overhauser field and of applied magnetic fields on the decoherence of the central spin.

Comparison of quantum-mechanical and semiclassical approaches for an analysis of spin dynamics in quantum dots

Two approaches to the description of spin dynamics of electron-nuclear system in quantum dots are compared: the quantum-mechanical one is based on direct diagonalization of the model Hamiltonian and semiclassical one is based on coupled equations for precession of mean electron spin and mean spin of nuclear spin fluctuations. The comparison was done for a model problem describing periodic excitation of electron-nuclear system by optical excitation. The computation results show that scattering of parameters related to fluctuation of the nuclear spin system leads to appearance of an ordered state in the system caused by periodic excitation and to the effect of electron-spin mode locking in an external magnetic field. It is concluded that both models can qualitatively describe the mode-locking effect, however give significantly different quantitative results. This may indicate the limited applicability of the precession model for describing the spin dynamics in quantum dots in the presence of optical pumping.

Preface: Phys. Status Solidi C 5/2012

AbstractThe 11th International Conference on Physics of Light–Matter Coupling in Nanostructures 2011 (PLMCN 11) was held in Berlin, 4–8 April 2011.The idea of the PLMCN conference series is to bring together scientists active in the fields of semiconductor optics and material science dealing with modern technologies of wide‐band‐gap semiconductors and organic materials, both in academia and industry. This conference traditionally combines new generations of opto‐electronic devices like nanolasers, polariton lasers, single photon emitters and basic physical principles like spin dynamics in quantum dots and microcavities. Furthermore, progress in epitaxial growth of Bragg mirrors and quantum structures in arsenides, nitrides and oxides is in the focus of interest.With the 2011 PLMCN conference in Berlin the scientific lifework of Dieter Bimberg was honored who has accomplished tremendous pioneering work in the field of optoelectronic devices like quantum dots and nanolasers, PCB (printed circuit board) lasers, and electrically driven single‐photon emitters.The organizers thank the participants of PLMCN 2011 for continuing the spirit of now 11 annual conferences – Saint Nectaire, France (2000); Rome, Italy (2001); Rithymnon, Greece (2002); Acireale, Italy (2003); Saint‐Petersburg, Russia (2004); Glasgow, Scotland (2005); Magdeburg, Germany (2006); Havana, Cuba (2007); Tokyo, Japan (2008); Lecce, Italy (2009), and Cuernavaca, Mexico (2010) (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of magnetic field on the electron-nuclear spin dynamics in quantum dots

We report on the theoretical study of the joint effect of the hyperfine interaction and an external magnetic field on the electron-nuclear spin dynamics in a quantum dot. The study is based on the quantum-mechanical description of the hyperfine interaction of an electron spin with a finite number of nuclear spins in the frame of the graded box model. This approach clearly demonstrates electron-spin depolarization in the nuclear spin fluctuations and its suppression by the external magnetic field and dynamic nuclear spin polarization.

Hole and trion spin dynamics in quantum dots under excitation by a train of circularly polarized pulses

We have performed pump-probe experiments in $p$-doped InAs/GaAs quantum dots leading to the all-optical initialization and readout of hole spins. In order to describe these experiments, we have modelized the interconnected dynamics of the photoelectron spin and the resident hole spin, triggered through the optical excitation by a train of short pulses. A complete description of this spin dynamics is obtained by including the hyperfine coupling as the common decoherence mechanism for the electron and hole spins. Periodic excitation conditions for arbitrary values of the pump power and the external magnetic field are also included in the model. In particular, a good agreement concerning the temporal behavior of the photoinduced circular dichroism is obtained for zero or low magnetic fields. When the applied magnetic field screens the hole-hyperfine interaction, we show that the agreement between the experimental and calculated time-dependent curves requires an additional relaxation mechanism for holes with a characteristic time in the microsecond range.

Coupled electron-nuclear spin dynamics in quantum dots: A graded box model approach

The dynamics of electron and nuclear spin polarization in an ensemble of singly negatively charged quantum dots subject to optical excitation is theoretically studied using a graded box model, in which the electron density is approximated by a sequence of steps. The model is numerically implemented for a limited number of nuclei (up to $N=192$) with spins $I=1/2$ or $I=3/2$. The polarization dynamics is found to depend strongly on the polarized excitation protocol. For excitation by periodic laser pulses, the electron-nuclear spin dynamics evolves coherently and gives rise to a recurrence effect in the electron spin dynamics as well as to a decelerated nuclear polarization. The validity of the model is justified by comparison with experimental data on the electron spin polarization in (In,Ga)As/GaAs quantum dots.

Time-resolved Studies of Carrier and Spin Dynamics in QuantumDots and Wide Band-gap Materials

Time-resolved studies using ultra-short laser pulses unveil interesting aspects of carrier and spin dynamics in compound semiconductors. Here, thermally-activated carrier transfers between CdTe/ZnTe quantum dots (QDs) and ultrafast spin-relaxations in bulk GaN are reported. Carrier transfer among CdTe/ZnTe self-organized QDs was studied using time-resolved photoluminescence measurements. The carriers in the high-energy ground states of small QDs are confirmed to transfer to the lower-energy ground states of larger QDs, even at 10 K. The energy dependence of the PL decay time changes uniquely with increasing temperature. The change in the energy dependence of the PL decay time can be explained by thermally-activated carrier transfer. Excitonic spin-relaxations in hexagonal GaN and cubic GaN are observed. The A-band free exciton in hexagonal GaN shows a sub-picosecond spin-relaxation of 0.47 ps at 150 K. The acceptor-bound exciton in hexagonal GaN shows spin-relaxation times of 1.40 – 1.14 ps at 15 – 50 K. Meanwhile, the spin-relaxation times in cubic GaN at 15 – 75 K are found to be longer than 5 ns. The long nanosecond spin-relaxation time in cubic GaN is consistent with the observation that spin-relaxation time becomes longer for wider-band-gap zincblende semiconductors.

Time to get the nukes out

The ability to electrically control spin dynamics in quantum dots makes them one of the most promising platforms for solid-state quantum-information processing. Minimizing the influence of the nuclear spin environment is an important step towards realizing such promise.

Optical investigations of quantum dot spin dynamics as a function of external electric and magnetic fields

We have performed all-optical measurements of spin relaxation in single self-assembled $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots (QDs) as a function of static external electric and magnetic fields. To study QD spin dynamics, we measure the degree of resonant absorption which results from a competition between optical spin pumping induced by the resonant laser field and spin relaxation induced by reservoirs. Fundamental interactions that determine spin dynamics in QDs are hyperfine coupling to QD nuclear spin ensembles, spin-phonon coupling, and exchange-type interactions with a nearby Fermi sea of electrons. We show that the strength of spin relaxation generated by the three fundamental interactions can be changed by up to 5 orders of magnitude upon varying the applied electric and magnetic fields. We find that the strength of optical spin pumping that we use to study the spin relaxation is determined predominantly by hyperfine-induced mixing of single-electron spin states at low magnetic fields and heavy-light hole mixing at high magnetic fields. Our measurements allow us to determine the rms value of the hyperfine (Overhauser) field to be $\ensuremath{\sim}15\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ with an electron $g$ factor of ${g}_{e}=0.6$ and a hole mixing strength of ${\ensuremath{\mid}{ϵ}_{H}\ensuremath{\mid}}^{2}=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$.

Exactly solvable spin dynamics of an electron coupled to a large number of nuclei; the electron-nuclear spin echo in a quantum dot

The model used to describe the spin dynamics in quantum dots after optical excitation is considered. Problems of the electron-spin polarization decay and the dependence of the steady-state polarization on magnetic field are solved on the basis of exact diagonalization of the model Hamiltonian. An important role of the nuclear state is shown and methods of its calculation for different regimes of optical excitation are proposed. The effect of spin echo generation after application of a π pulse of a magnetic field is predicted for the system under consideration.

Mixing of two-electron spin states in a semiconductor quantum dot

We show that the low lying spin states of two electrons in a semiconductor quantum dot can be strongly mixed by electron-electron asymmetric exchange. This mixing is generated by the coupling of electron spin to its orbital motion and to the relative orbital motion of the two electrons. The asymmetric exchange can be as large as 50% of the isotropic exchange, even for cylindrical quantum dots. The resulting spin mixing contributes to understanding spin dynamics in quantum dots, including light polarization reversal.

Dynamics of localized Mn spins in diluted‐magnetic‐semiconductor nanostructures with quantum dots

AbstractDynamics of spin–lattice relaxation of the magnetic Mn‐ion system is investigated for nanostructures with (Cd,Mn)Te and (Cd,Mn)Se diluted‐magnetic‐semiconductor (DMS) quantum dots. A nonequilibrium phonon technique with optical detection of the induced heating of the magnetic ion system was exploited. A strongly nonexponential decay of the spin dynamics is found to be specific for the DMS quantum dots. It is explained in the frame of a model suggesting that the spin dynamics in quantum dots is strongly affected by a spin diffusion into/from a DMS wetting layer, where the spin–lattice relaxation rate differs from that in the quantum dots. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)