Electronic properties of non-circular quantum dots

Quantum dots are nanoscale electronic objects, typically fabricated using two-dimensional semiconductor heterostructures or three-dimensional atomic clusters. In addition to promising technological applications, quantum dots represent excellent sources of interesting many-electron quantum physics. This thesis deals with the theoretical modeling of two-dimensional quantum dots consisting of less than twenty electrons. The aim is to clarify how the geometry of the confining potential affects the ground-state structure of the system with and without the presence of an external magnetic field. The question is of a great importance in understanding and predicting the basic electronic properties of actual quantum-dot devices. The calculations are based on the density-functional theory applied within a real-space multigrid approach, which is shown to be a powerful method for the systems considered in this thesis. The accuracy of the method is, however, highly dependent on the local spin-density approximation used. Hence, different parametrizations for the exchange-correlation energy are compared. Furthermore, the problem of the density-functional theory as a mean-field approach using a single-configuration wave function is analyzed in this thesis. Quantum dots of various shapes are investigated, beginning with polygonal systems to determine the critical densities for the Wigner crystallization. Rectangular dots are shown to be particularly sensitive to the geometry, but a qualitative agreement is obtained with the experimental addition energy spectra. The lack of circular symmetry does not prevent the maximum-density-droplet formation when an external magnetic field is applied, and the high-field limit may be characterized by remarkably regular state oscillations. The symmetry can also be distorted by an external impurity, for which a realistic model is obtained with a comparison to experimental data. Presumably, some of the rich variety of phenomena identified in this thesis for non-circular quantum dots will have a realization in the future nanoelectronics.reviewe

Dynamical Heart Beat Correlations during Running

Fluctuations of the human heart beat constitute a complex system that has been studied mostly under resting conditions using conventional time series analysis methods. During physical exercise, the variability of the fluctuations is reduced, and the time series of beat-to-beat RR intervals (RRIs) become highly non-stationary. Here we develop a dynamical approach to analyze the time evolution of RRI correlations in running across various training and racing events under real-world conditions. In particular, we introduce dynamical detrended fluctuation analysis and dynamical partial autocorrelation functions, which are able to detect real-time changes in the scaling and correlations of the RRIs as functions of the scale and the lag. We relate these changes to the exercise intensity quantified by the heart rate (HR). Beyond subject-specific HR thresholds the RRIs show multiscale anticorrelations with both universal and individual scale-dependent structure that is potentially affected by the stride frequency. These preliminary results are encouraging for future applications of the dynamical statistical analysis in exercise physiology and cardiology, and the presented methodology is also applicable across various disciplines.Comment: 19 pages, 10 figure

Time-series analysis approach to the characteristics and correlations of wastewater variables measured in paper industry

Advanced wastewater treatment technologies and protocols are required to minimize the overall water footprint and environmental cost of paper industries. Time-series analysis provides powerful methods to analyze industrial wastewater data with the aim of understanding and predicting the behavior of relevant wastewater variables such as chemical and biochemical oxygen demand. In this study, we introduce a variety of novel computational methods for complex systems and demonstrate their applicability to data obtained from a wastewater treatment plant, including the preprocessing of the raw data, time-dependent characteristics of individual measured parameters, as well as the mutual correlations between multiple parameters. These methods include the Potts model for preprocessing, empirical mode decomposition for periodic component extraction, detrended fluctuation analysis for noise characterization of the data and finally time-lagged windowed cross-correlation, which is utilized to obtain time-dependent couplings between the measured parameters. The results provide valuable insights into the underlying processes in the wastewater treatment plant. The Potts model effectively processes noisy data, from which periodic variations can be successfully removed with empirical mode decomposition, while preserving the relevant characteristics. Further, detrended fluctuation analysis shows prospects for indicating periods of abnormal behavior in the data. Finally, the correlation analysis reveals the characteristic time delays between influent and effluent chemical oxygen demand, giving an average delay of one day, which has implications in plant control. In conclusion, our methods show promising prospects for further applicability in wastewater analysis.Peer reviewe

Controlled high-fidelity navigation in the charge stability diagram of a double quantum dot

We propose an efficient control protocol for charge transfer in a double quantum dot. We consider numerically a two-dimensional model system, where the quantum dots are subjected to time-dependent electric fields corresponding to experimental gate voltages. Our protocol enables navigation in the charge stability diagram from a state to another through controllable variation of the fields. We show that the well-known adiabatic Landau-Zener transition-when supplemented with a time-dependent field tailored with optimal control theory-can remarkably improve the transition speed. The results also lead to a simple control scheme obtained from the experimental charge stability diagram that requires only a single parameter. Eventually, we can achieve the ultrafast performance of the composite pulse protocol that allows the system to be driven at the quantum speed limit.Fil: Acosta Coden, Diego Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado E Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Cs.exactas Naturales y Agrimensura. Instituto de Modelado E Innovación Tecnologica; ArgentinaFil: Romero, Rodolfo Horacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado E Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Cs.exactas Naturales y Agrimensura. Instituto de Modelado E Innovación Tecnologica; ArgentinaFil: Räsänen, Esa. Tampere University of Technology; Finlandi