Time-resolved spectroscopic studies of lanthanides doped inorganic, hybrid and organic systems

PhDIn this thesis, three main chapters are discussed, including an Er3+-doped Y2O3 system, a Yb3+-doped NaYF4 nanoparticle system and a Yb(F-TPIP)3 co-evaporated with a Zn(F-BTZ)2 organic system. The main discussion is about the underlying physics of lanthanides, sensitisation of lanthanide ions by using the time resolved spectroscopy method. The up- and down-conversion processes between Er3+ ion pairs in the Y2O3 system was systematically investigated using the time resolved spectroscopy method regarding different Er3+ doping concentrations. The measured lifetime gives direct evidence for differentiating the excited state absorption (ESA) and energy transfer (ET) mechanisms during up-conversion processes. Also, the looping mechanism is found by measuring the NIR luminescence lifetime. Most importantly, the red to green emission ratio change during up-conversion processes with doping concentration is explained by a combination of up-conversion route change and concentration quenching effect. A novel sensitisation of Yb3+-doped NaYF4 nanoparticles is demonstrated by capping a 1,2,3,4,5,6,7-heptafluoro-8-hydroxyanthracene-9,10-dione (HL) ligand to the nanoparticle surface. The HL ligand acts as an antenna which can absorb visible light (400 nm to 600 nm) and then transfers the excited energy to Yb3+ ions encapsulated in the NaYF4 host. Interestingly, the energy transfers from the HL ligand to Yb3+ ions near the surface and then migrates to the Yb3+ ions inside the core of nanoparticles and is proved by a systematic time resolved spectroscopy study. The integrated organic chromophore based-excitation is almost 300 times higher compared with the Yb3+ intrinsic absorption band based-excitation. The last main discussion is given to the sensitisation of Yb3+ ions in a co-evaporated organic system as the co-evaporated organic system is more optically desirable than either the Y2O3 or the nanoparticle system. Compared with the intrinsic sensitisation, two orders of magnitude times sensitisation of Yb3+ ions is demonstrated by co-evaporating Yb(F-TPIP)3 and the Zn(F-BTZ)2 chromophore, making it a suitable material for a waveguide amplifier. In summary, an Er3+-doped Y2O3 system is investigated as the fundamental study to understand the underlying physics, and then HL ligand and Zn(F-BTZ)2 chromophore are investigated as potential sensitisers for Yb3+-doped nanoparticles and organic systems separately. The study in this thesis should be useful not only for fundamental physics but also as a reference for the applications of lanthanide

Fano Effect through Parallel-coupled Double Coulomb Islands

By means of the non-equilibrium Green function and equation of motion method, the electronic transport is theoretically studied through a parallel-coupled double quantum dots(DQD) in the presence of the on-dot Coulomb correlation, with an emphasis put on the quantum interference. It has been found that in the Coulomb blockage regime, the quantum interference between the bonding and antiboding DQD states or that between their Coulomb blockade counterparts may result in the Fano resonance in the conductance spectra, and the Fano peak doublet may be observed under certain non-equilibrium condition. The possibility of manipulating the Fano lineshape is predicted by tuning the dot-lead coupling and magnetic flux threading the ring connecting the dots and leads. Similar to the case without Coulomb interaction, the direction of the asymmetric tail of Fano lineshape can be flipped by the external field. Most importantly, by tuning the magnetic flux, the function of four relevant states can be interchanged, giving rise to the swap effect, which might play a key role as a qubit in the quantum computation.Comment: 7 pages, 5 figure

Tunable Fano effect in parallel-coupled double quantum dot system

With the help of the Green function technique and the equation of motion approach, the electronic transport through a parallel-coupled double quantum dot(DQD) is theoretically studied. Owing to the inter-dot coupling, the bonding and antibonding states of the artificial quantum-dot-molecule may constitute an appropriate basis set. Based on this picture, the Fano interference in the conductance spectra of the DQD system is readily explained. The possibility of manipulating the Fano lineshape in the tunnelling spectra of the DQD system is explored by tuning the dot-lead coupling, the inter-dot coupling, the magnetic flux threading the ring connecting dots and leads, and the flux difference between two sub-rings. It has been found that by making use of various tuning, the direction of the asymmetric tail of Fano lineshape may be flipped by external fields, and the continuous conductance spectra may be magnetically manipulated with lineshape retained. More importantly, by adjusting the magnetic flux, the function of two molecular states can be exchanged, giving rise to a swap effect, which might play a role as a qubit in the quantum computation.Comment: 9 pages, 10 figure

Spectral signatures of the surface anomalous Hall effect in magnetic axion insulators

The topological surface states of magnetic topological systems, such as Weyl semimetals and axion insulators, are associated with unconventional transport properties such as nonzero or half-quantized surface anomalous Hall effect. Here we study the surface anomalous Hall effect and its spectral signatures in different magnetic topological phases using both model Hamiltonian and first-principles calculations. We demonstrate that by tailoring the magnetization and interlayer electron hopping, a rich three-dimensional topological phase diagram can be established, including three types of topologically distinct insulating phases bridged by Weyl semimetals, and can be directly mapped to realistic materials such as MnBi2Te4/(Bi2Te3)n systems. Among them, we find that the surface anomalous Hall conductivity in the axion-insulator phase is a well-localized quantity either saturated at or oscillating around e2/2h, depending on the magnetic homogeneity. We also discuss the resultant chiral hinge modes embedded inside the side surface bands as the potential experimental signatures for transport measurements. Our study is a significant step forward towards the direct realization of long-sought axion insulators in realistic material systems.Comment: 22 pages, 4 figure

A Semi-automatic Approach to Identifying and Unifying Ambiguously Encoded Arabic-Based Characters

In this study, we outline a potential problem in normalising texts that are based on a modified version of the Arabic alphabet. One of the main resources available for processing resource-scarce languages is raw text collected from the Internet. Many less-resourced languages, such as Kurdish, Farsi, Urdu, Pashtu, etc., use a modified version of the Arabic writing system. Many characters in harvested data from the Internet may have exactly the same form but encoded with different Unicode values (ambiguous characters). The existence of ambiguous characters in words leads to word duplication, thus it is important to identify and unify ambiguous characters during the normalisation stage. Here, we demonstrate cases related to ambiguous Kurdish and Farsi characters and propose a semi-automatic approach to identifying and unifying them

Improved Arabic Characters Recognition by Combining Multiple Machine Learning Classifiers

In this paper, we investigate a range of strategies for combining multiple machine learning techniques for recognizing Arabic characters, where we are faced with imperfect and dimensionally variable input characters. Experimental results show that combined confidence-based backoff strategies can produce more accurate results than each technique produces by itself and even the ones exhibited by the majority voting combination

Phonon-assisted Kondo Effect in a Single-Molecule Transistor out of Equilibrium

The joint effect of the electron-phonon interaction and Kondo effect on the nonequilibrium transport through the single molecule transistor is investigated by using the improved canonical transformation scheme and extended equation of motion approach. Two types of Kondo phonon-satellites with different asymmetric shapes are fully confirmed in the spectral function, and are related to the electron spin singlet or hole spin singlet, respectively. Moreover, when a moderate Zeeman splitting is caused by a local magnetic field, the Kondo satellites in the spin resolved spectral function are found disappeared on one side of the main peak, which is opposite for different spin component. All these peculiar signatures that manifest themselves in the nonlinear differential conductance, are explained with a clear physics picture.Comment: 12 pages, 6 figure

I4U Submission to NIST SRE 2018: Leveraging from a Decade of Shared Experiences

The I4U consortium was established to facilitate a joint entry to NIST speaker recognition evaluations (SRE). The latest edition of such joint submission was in SRE 2018, in which the I4U submission was among the best-performing systems. SRE'18 also marks the 10-year anniversary of I4U consortium into NIST SRE series of evaluation. The primary objective of the current paper is to summarize the results and lessons learned based on the twelve sub-systems and their fusion submitted to SRE'18. It is also our intention to present a shared view on the advancements, progresses, and major paradigm shifts that we have witnessed as an SRE participant in the past decade from SRE'08 to SRE'18. In this regard, we have seen, among others, a paradigm shift from supervector representation to deep speaker embedding, and a switch of research challenge from channel compensation to domain adaptation.Comment: 5 page