Nuclear-electron spin interaction in low-dimensional semiconductors

The nuclear spins in semiconductors can serve as a quantum mechanical system with minutes long spin relaxation times, enhanced by the decoupling from the surrounding matter in strained nanostructures and the absence of interaction with light. In order to manipulate the nuclear spin system optically, resident electrons confined in the nanostructure are introduced as a mediator. The electron localization leads to an efficient coupling of the electron spin to the nuclear spins via Fermi contact hyperfine interaction. This allows one to polarize the nuclear spins and to detect their dynamics. In this work, the nuclear spin dynamics in a cadmium telluride (CdTe) quantum well is characterized, assisted by nuclear magnetic resonance radio frequency pulses. The independent dynamics of the isotope spins turns out to be harmonized. In the enhanced localization in negatively doped indium gallium arsenide (InGaAs) quantum dots, two nuclear spin polarization protocols are assessed. Firstly, the developed extended pump-probe setup allows to investigate the established nuclei-induced frequency focusing by tracing the electron spin precession with improved spectral resolution. Secondly, a novel technique based on the pulsed excitation with 1 GHz pulse repetition frequency is introduced which allows to establish a substantial nuclear spin polarization in a transverse field while also reducing the fluctuations of the unordered nuclear spins.Die Kernspins in Halbleitern bilden durch die fehlende Wechselwirkung mit Licht ein quantenmechanisches System mit langen Spin-Relaxationszeiten, die durch die Entkopplung vom umgebenden Festkörper in verspannten Nanostrukturen noch verlängert werden. Residente Elektronen in der Nanostruktur erlauben es, die Polarisation des Lichts auf das Kernspinsystem zu übertragen und optisch zugänglich zu machen, weil die Elektronenlokalisierung für eine effiziente Kopplung des Elektronenspins mit den Kernspins über die Fermikontakt-Hyperfeinwechselwirkung sorgt. In dieser Arbeit wird zum einen die isotopenaufgelöste Kernspindynamik in einem Cadmiumtellurid-Quantentopf mit Hilfe von Kernspinresonanzpulsen charakterisiert und eine Angleichung der Dynamik der eigentlich unabhängigen Isotopenspins beobachtet. Für die stärkere Lokalisierung in negativ dotierten Indiumgalliumarsenid-Quantenpunkten werden zum anderen zwei Kernspinpolarisationsprotokolle untersucht. Erstens erlaubt die verbesserte Präzessionsfrequenzauflösung des im Umfeld der Arbeit entwickelten, erweiterten Pump-Probe-Aufbaus die detaillierte Untersuchung der bekannten kerninduzierten Frequenzfokussierung. Zweitens wird die optische Anregung mit einer Pulsfrequenz von 1 GHz beschrieben. Dadurch wird eine signifikante Kernspinpolarisation in einem transversalen Magnetfeld induziert und gleichzeitig die Fluktuation der ungeordneten Kernspins reduziert

Signatures of long-range spin-spin interactions in an (In,Ga)As quantum dot ensemble

We present an investigation of the electron spin dynamics in an ensemble of singly-charged quantum dots subject to an external magnetic field and laser pumping with circularly polarized light. The spectral laser width is tailored such that different groups of quantum dots are coherently pumped. Surprisingly, the dephasing time $T^*$ of the electron spin polarization depends only weakly on the laser spectral width. These findings can be consistently explained by a cluster theory of coupled quantum dots with a long range electronic spin-spin interaction. We present a numerical simulation of the spin dynamics based on the central spin model that includes a quantum mechanical description of the laser pulses as well as a time-independent Heisenberg interaction between each pair of electron spins. We discuss the individual dephasing contributions stemming from the Overhauser field, the distribution of the electron $g$-factors and the electronic spin-spin interaction as well as the spectral width of the laser pulse. This analysis reveals the counterbalancing effect of the total dephasing time when increasing the spectral laser width. On one hand, the deviations of the electron $g$-factors increase. On the other hand, an increasing number of coherently pumped electron spins synchronize due to the spin-spin interaction. We find an excellent agreement between the experimental data and the dephasing time in the simulation using an exponential distribution of Heisenberg couplings with a mean value $\overline{J}\approx 0.26\,\mathrm{\mu eV}$

Tuning the nuclei-induced spin relaxation of localized electrons by the quantum Zeno and anti-Zeno effects

Quantum measurement back action is fundamentally unavoidable when manipulating electron spins. Here we demonstrate that this back action can be efficiently exploited to tune the spin relaxation of localized electrons induced by the hyperfine interaction. In optical pump-probe experiments, powerful probe pulses suppress the spin relaxation of electrons on Si donors in an InGaAs epilayer due to the quantum Zeno effect. By contrast, an increase of the probe power leads to a speed-up of the spin relaxation for electrons in InGaAs quantum dots due to the quantum anti-Zeno effect. The microscopic description shows that the transition between the two regimes occurs when the spin dephasing time is comparable to the probe pulse repetition period

Suppression of nuclear spin fluctuations in an InGaAs quantum dot ensemble by GHz-pulsed optical excitation

The coherent electron spin dynamics of an ensemble of singly charged (In,Ga)As/GaAs quantum dots in a transverse magnetic field is driven by periodic optical excitation at 1 GHz repetition frequency. Despite the strong inhomogeneity of the electron g factor, the spectral spread of optical transitions, and the broad distribution of nuclear spin fluctuations, we are able to push the whole ensemble of excited spins into a single Larmor precession mode that is commensurate with the laser repetition frequency. Furthermore, we demonstrate that an optical detuning of the pump pulses from the probed optical transitions induces a directed dynamic nuclear polarization and leads to a discretization of the total magnetic field acting on the electron ensemble. Finally, we show that the highly periodic optical excitation can be used as universal tool for strongly reducing the nuclear spin fluctuations and preparation of a robust nuclear environment for subsequent manipulation of the electron spins, also at varying operation frequencies

Lead-dominated hyperfine interaction impacting the carrier spin dynamics in halide perovskites

The outstanding optical quality of lead halide perovskites inspires studies of their potential for the optical control of carrier spins as pursued in other materials. Entering largely uncharted territory, time-resolved pump–probe Kerr rotation is used to explore the coherent spin dynamics of electrons and holes in bulk formamidinium caesium lead iodine bromide (FA0.9Cs0.1PbI2.8Br0.2) and to determine key parameters characterizing interactions of their spins, such as the g-factors and relaxation times. The demonstrated long spin dynamics and narrow g-factor distribution prove the perovskites as promising competitors for conventional semiconductors in spintronics. The dynamic nuclear polarization via spin-oriented holes is realized and the identification of the lead (207Pb) isotope in optically detected nuclear magnetic resonance proves that the hole–nuclei interaction is dominated by the lead ions. A detailed theoretical analysis accounting for the specifics of the lead halide perovskite materials allows the evaluation of the underlying hyperfine interaction constants, both for electrons and holes. Recombination and spin dynamics evidence that at low temperatures, photogenerated electrons and holes are localized at different regions of the perovskite crystal, resulting in their long lifetimes up to 44 μs. The findings form the base for the tailored development of spin-optoelectronic applications for the large family of lead halide perovskites and their nanostructures

A new science of mental disorders:Using personalised, transdiagnostic, dynamical systems to understand, model, diagnose and treat psychopathology

The core ideas of a 10-year research program 'New Science of Mental Disorders' are outlined. This research program moves away from the disorder-based 'one-model-fits-all' approach to treating mental disorders, and adopts the network approach to psychopathology as its foundation of research. Its core assumption is that dynamically interacting symptoms constitute the disorder. Our goal is to further develop the network approach by studying (1) dynamic networks of symptoms and other variables (i.e., elements) in a large number of individuals with a wide range of mental disorders from a transdiagnostic perspective (network-based diagnosis; mapping), including both Ecological Momentary Assessment (EMA) and digital phenotyping, (2) the transdiagnostic mechanisms reflecting potential causal relations among elements of the networks by performing experimental (pre-)clinical studies (zooming), and (3) the effectiveness of personalised network-informed interventions (targeting). Challenges to overcome in this research program are discussed, which relate to data collection (e.g., selection of EMA variables) and data analyses (e.g., power considerations), the development and application of network-informed diagnoses and network-informed interventions (e.g., what characteristic(s) of the network to target in interventions), and the implementation in clinical practice (e.g., train therapists in the use of networks in therapy)

Lead-Dominated Hyperfine Interaction Impacting the Carrier Spin Dynamics in Halide Perovskites

The outstanding optical quality of lead halide perovskites inspires studies of their potential for the optical control of carrier spins as pursued in other materials. Entering largely uncharted territory, time-resolved pump-probe Kerr rotation is used to explore the coherent spin dynamics of electrons and holes in bulk formamidinium caesium lead iodine bromide (FA(0.9)Cs(0.1)PbI(2.8)Br(0.2)) and to determine key parameters characterizing interactions of their spins, such as the g-factors and relaxation times. The demonstrated long spin dynamics and narrow g-factor distribution prove the perovskites as promising competitors for conventional semiconductors in spintronics. The dynamic nuclear polarization via spin-oriented holes is realized and the identification of the lead (Pb-207) isotope in optically detected nuclear magnetic resonance proves that the hole-nuclei interaction is dominated by the lead ions. A detailed theoretical analysis accounting for the specifics of the lead halide perovskite materials allows the evaluation of the underlying hyperfine interaction constants, both for electrons and holes. Recombination and spin dynamics evidence that at low temperatures, photogenerated electrons and holes are localized at different regions of the perovskite crystal, resulting in their long lifetimes up to 44 mu s. The findings form the base for the tailored development of spin-optoelectronic applications for the large family of lead halide perovskites and their nanostructures.ISSN:0935-9648ISSN:1521-409