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The hole spin in a semiconductor quantum dot
Full text linkExtensive research on semiconductor quantum dots (QDs) has been a hot topic in the semiconductor community over the past 20 to 30 years and is still ongoing. In the late 1980s the term "quantum dot" was introduced to describe a semiconductor nano-structure. Some of the motivating prospects driving the research are low-threshold QD lasers, single dots for medical markers, lighting technologies for TVs or single spins for spintronic applications, e.g. quantum information processing. The size and the structure of a QD can vary from a few nanometres in colloidal dots (also known as nanocrystals) to a few hundred nanometres in lithographically defined electrostatic devices. The material components and the fabrication methods can differ a lot between the individual types of QDs. One feature all different kinds of QDs have in common is the restriction of the carrier motion in all three dimensions, which is induced by confinement. That property is the origin of the name zero-dimensional ("0D") structures. A second term often used describes the QD as an "artificial atom". The strong confinement establishes discrete energy states for the localized single carriers inside the QD, which resembles the properties of carriers in atoms.
The QDs investigated in this thesis are self-assembled InAs QDs in a semiconductor heterostructure, laying the focus on the confined positive charged carriers, the holes. The spin properties of the individual quantum states are characterized with advanced optical spectroscopy techniques.
The following thesis is split into four parts. The first part motivates the search for coherent single hole spins and explains how to get from a bulk semiconductor to a single spin. After a short introduction of semiconductor self assembled quantum dots, their optical properties and bandstructure, the requirements to perform single spin physics are described. The advantage to choose the hole spin for a spin qubit instead of the electron spin, regarding their decoherence properties is discussed.
The second section of the introduction covers the experimental techniques and improvements to current systems paving the way to a highly coherent spin qubit via the hole spin and high quality data. The new device structure as well as the sophisticated technique of resonance fluorescence detection are explained here. A description of the laser frequency locking mechanism and a power stabilization concludes the chapter.
In the second part the first experiments of this thesis on coherent hole spins are presented. With the spectroscopic measurement method of coherent population trapping (CPT) long decoherence times are achieved. Charge noise is determined as a hole spin dephasing mechanism. Despite the very promising results the experiment suffers from two disadvantages. First the measurement method via resonant absorption spectroscopy in combination with the unstable measurements conditions offers a very poor signal to noise ratio. Secondly the low frequency charge fluctuations, inherent in the sample, promote dephasing and induce shifts in the CPT resonance position from scan to scan.
The third part covers different approaches to address the noise issue of part two.
The optical linewidth and the noise are closely related in solid state emitters: The linewidth broadening is caused by spin and charge noise in the quantum device.
First, low frequency charge fluctuations are reduced by a feedback scheme, which stabilizes the emission frequency of the quantum dot to a stabilized reference. The feedback loop minimizes the fluctuations in the emission frequency, even over several hours, and eliminates the charge noise in the quantum dot to a large extent. This method realises a frequency stabilized source of single photons in the solid-state.
The next chapter introduces a new sample design in order to reduce spectral fluctuations. The n-i-p device growth sequence is inverted, which prevents the usual contamination of the QDs by the C-doping. The characteristics of the ultra clean p-doped samples are narrow linewidths in combination with high count rates. The "transform-limit" is reached with a fast scanning method. In the sample a voltage dependent blinking behaviour of the positively charged exciton is discovered.
The story of low-noise samples and noise control continues in the next chapter. Transform-limited linewidth of the neutral and the negatively charged exciton are presented. For the neutral exciton this is even true for slow measurements lasting several seconds. For already low-noise structures the residual linewidth broadening is only caused by the nuclear spin noise. A two colour experiment provides control over the nuclear spins, which dominate the exciton dephasing.
In the last part the interaction of the hole spin with its environment is investigated.
The hole spin states interact in an in-plane magnetic field with an external electric field. The interactions result in a tunable hole g-factor, showing a linear dependency over a large electric field range. In contrast the electron g-factor is not influenced by the electric field at all. Theory reproduces the hole g-factor dependence, which arises from a soft hole confining potential, an In concentration gradient and a strong dependence of the material parameters on the In concentration.
The last chapter demonstrates the anisotropic behaviour of the hyperfine interaction between nuclear spins and the hole spin. In the experiment, again with the measurement method of coherent population trapping, a low-noise sample and resonance fluorescence spectroscopy are combined. The resulting high signal to noise ratio and the ultra narrow CPT dip enable the measurement of very precise values for the energy splitting of the hole spin states. This is leading to the main result: a minimal hole hyperfine interaction in an in-plane magnetic field, proofing a decoupling from the hole spin and the nuclear spins
Single particle transport in two-dimensional heterojunction interlayer tunneling field effect transistor
No full textThe single particle tunneling in a vertical stack consisting of monolayers of two-dimensional
semiconductors is studied theoretically, and its application to a novel Two-dimensional
Heterojunction Interlayer Tunneling Field Effect Transistor (Thin-TFET) is proposed and
described. The tunneling current is calculated by using a formalism based on the Bardeen’s transfer
Hamiltonian, and including a semi-classical treatment of scattering and energy broadening effects.
The misalignment between the two 2D materials is also studied and found to influence the
magnitude of the tunneling current but have a modest impact on its gate voltage dependence. Our
simulation results suggest that the Thin-TFETs can achieve very steep subthreshold swing, whose
lower limit is ultimately set by the band tails in the energy gaps of the 2D materials produced by
energy broadening. The Thin-TFET is thus very promising as a low voltage, low energy solid state
electronic switch
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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