1,721,013 research outputs found
Electrical control of excitons in a gated two-dimensional semiconductor
Get PDFThe emergence of two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDs), and the ability to build artificial van der Waals heterostructures (vdWHs) by stacking different combinations of materials, has opened a new route for engineering quantum systems. 2D TMDs support bound electron-hole pairs, or excitons, that have particularly large binding energies, such that excitons dominate their optical properties, even at room temperature. The ability to electrically tune their properties via external electric gates is essential for various interesting opto-electronic applications.
A crucial feature of semiconductor nanostructures is the quantum confined Stark effect (QCSE), the change in optical energy on applying an electric field perpendicular to the layers. Using a gated vdWH, we demonstrated that in monolayer MoS optical absorption is strong, but the transition energy is not tunable as the neutral exciton has essentially no permanent out-of-plane electric dipole and is only slightly polarizable. The electrical control of excitons via the QCSE requires larger polarizabilities or a non-zero dipole moment as observed in heterobilayers where the bound electrons and holes reside in different layers. However, the coupling to light in these systems is considerably reduced. To combine best of both worlds, a polarizable yet strong optical dipole, we integrated homobilayer MoS in a dual-gate device structure. In its natural bilayer form, we discovered interlayer excitons which exhibit both a high oscillator strength and highly tunable energies in an applied electric field. Owing to their very large dipole moments, we were able to bring these interlayer excitons energetically close to resonance with the excitons confined to the single layers, and study exciton-exciton interactions in these systems.
Equipping MoS with gates allows electrons to be injected, creating a 2D electron gas. By probing the electronic ground state at various electron densities, we presented experimental evidence for a spontaneous spin-polarization in monolayer MoS. Significantly, the extremely small Bohr radius of an electron in this material suggests that Coulomb effects play an important role at experimental relevant electron densities.
The ability to control the properties of thin semiconductors by electrical means makes these systems a versatile platform for rich exciton physics and unique opto-electronic applications
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Herstellung und Charakterisierung von Mangan dotierten III-V Halbleiterheterostrukturen
No full textGegenstand dieser Arbeit ist das Wachstums von Mangan dotierten III-V Halbleiterheterostrukturen mittels Molekularstrahlepitaxie. Die Charakterisierung erfolgte im Wesentlichen mit Magnetotransportmessungen bei Temperaturen von 300K bis 20mK und Magnetfeldern bis 19T. Es wurden zum einen ferromagnetische GaMnAs Schichten und zum anderen mit Mn magnetisch modulationsdotierte zweidimensionale Ladungsträgersysteme untersucht.
Bei der Herstellung der ferromagnetischen GaMnAs Schichten stand die Verbesserung der elektrischen und magnetischen Eigenschaften im Mittelpunkt. Hauptziel war es, das Wachstum für die Integration der GaMnAs Schichten in weiterführende Heterostrukturen zu optimieren. Die Verbesserung der Kristallqualität und der Einfluss der Pufferschichten, waren von zentralem Interesse. Es konnten GaMnAs Schichten auf (001), (311)A und (311)B orientierten GaAs Substraten mit derzeit aktuellen Curietemperaturen hergestellt werden. Zusätzlich wurde das Wachstum ferromagnetischer GaMnAs Schichten auf nichtpolaren (110) GaAs Substraten etabliert und anschließend auf das Überwachsen von [110] Spaltkanten übertragen. Als erste Anwendung wurden mit diesem Verfahren magnetische bipolare p-n Übergänge hergestellt.
Für die Realisierung magnetischer zweidimensionaler Lochsysteme wurden In0.75Al0.25As/In0.75Ga0.25As/InAs Quantenwellstrukturen verwendet. Da für diese Strukturen keine geeigneten Substrate zur Verfügung stehen, müssen auf herkömmliche (001) GaAs Substrate zunächst Pufferschichten zur Verspannungsrelaxierung hergestellt werden, bei denen der In-Gehalt graduell erhöht wird. Zur Kontrolle dieser Schichten wurden zunächst Si-dotierte zweidimensionale Elektronensysteme auf diesen Pufferschichten hergestellt. Es wurde sowohl an den 2DEGs, als auch an den M2DHGs Magnetotransportexperimente durchgeführt. Die Transportmessungen bei tiefen Temperaturen, insbesondere im mK-Bereich lieferten bei den magnetischen zweidimensionalen Lochsystemen einige überraschende Ergebnisse. Während bei allen Si dotierten Elektronen- und den mit Mn nicht invertiert dotierten Lochsysteme um B=0T schwache Lokalisierungseffekte auftreten, zeigen alle invertiert und beidseitig mit Mn dotierten Strukturen bei senkrecht zur Schichtebene angelegtem Magnetfeld einen enormen Widerstandsanstieg um B=0T. Bei höheren Magnetfeldern sind jedoch die für 2D Systeme typischen Shubnikov-de-Haas Oszillationen im Längswiderstand und Hallplateaus im Querwiderstand detektierbar
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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
Optische Eigenschaften von Quasiteilchen in Mono- und Bilagen von TMDCs
No full textIn this thesis, we investigate the optical properties of excitonic quasiparticles in different transition metal dichalcogenides (TMDCs). These novel two-dimensional (2D) semiconductor materials exhibit remarkable mechanical, optical and electronic properties and feature an additional valley degree of freedom. Due to their atomically thin nature, excitons form as quasiparticles with high binding energies in monolayers (ML) and bilayers (BL) of MoSe2 and MoTe2. We combine photoluminescence (PL), pump probe (PP) and 2D coherent spectroscopy (2DCS) to reveal and quantify valley polarization properties and the interaction of phonons with exciton, trion and biexciton.
ML TMDCs are direct semiconductors, whereas BLs in most TMDCs are indirect and less studied. We perform 2DCS measurements of the BL MoSe2 and find a dephasing time for electrons of 20 fs and a scattering of holes of 400 fs, forming an indirect exciton in MoSe2. Moreover, we observe an intra-layer biexciton signature. The BL of MoTe2, which is yet little investigated in literature, shows a bright emission like the ML. We perform a comprehensive spectroscopic study of ML and BL MoTe2 and analyze the lineshape of temperature-dependent PL spectra, comparing ML and BL. The small difference in lineshape can be traced to symmetry and well width, confirming a direct excitonic transition in the BL, unlike in other TMDCs. We further disentangle the homogenous and inhomogeneous parts of the PL linewidth. The inhomogeneous linewidth predicts an inter- and intra-layer trion in the BL separated by 2.2 meV. In general, the trion state in the BL is confirmed by the mass-action law. We deduce a connection between the homogenous linewidth broadening and the phonon-limited mobility, considering acoustical as well as optical phonons. This results in a high acoustical phonon limited mobility of 6000 cm2/Vs (2000 cm2/Vs) and 4300 cm2/Vs for the ML exciton (trion) and BL exciton. The optical phonon limited mobility at room temperature is found to be 300 cm2/Vs and 150 cm2/Vs in ML and BL which are the highest values found so far for TMDCs.
Exciton and trion resonances in TMDCs allow for optical manipulation of the valley degree of freedom. We find that resonant excitation of the exciton in PP results in a high negative polarization degree of −65% which is lost within 1 ps due to electron-hole exchange interactions and bandgap renormalization in MoTe2. The trion is less influenced by these effects resulting in a polarization of 95 % immediately after excitation, depolarizing within 3 ps. Both depolarization timescales are consistent with the fast decay of excitons and trions due to defect-assisted recombination. During the trion formation time of 1.5-2.5 ps, we measure a polarization of 30 % in ML MoTe2. These are the first observations of valley polarization in MoTe2. Offresonant excitation results in a non-zero polarization of the PL when exciting higher excitonic states. Phonon assisted cascade processes conserve spin and valley, where the electron transfer is dominated by spin- (valley-) conserving scattering at low (high) temperatures.In dieser Arbeit studieren wir die optischen Eigenschaften exzitonischer Quasiteilchen in Übergangsmetall-Dichalcogeniden (TMDCs). Dieser zweidimensionale (2D) Halbleiter hat bemerkenswerte mechanische, optische und elektronische Eigenschaften, sowie einen weiteren Valley-Freiheitsgrad. TMDCs sind nur einige Atomlagen dick, und ihre Quasiteilchen weisen hohe Bindungsenergien in Mono- (ML) und Bilagen (BL) auf. Durch die Kombination von Photolumineszenz (PL), Pump-Probe (PP) und 2D kohärenter Spektroskopie (2DCS) untersuchen und quantifizieren wir die Wechselwirkung von Phononen mit Exzitonen, Trionen und Biexzitonen.
ML TMDCs sind direkte Halbleiter, wohingegen BL in den meisten TMDCs indirekt und wenig untersucht sind. Wir führen 2DCS Messungen am MoSe2 BL durch und untersuchen den Transfer von direktem zu indirektem Exziton, wobei das Elektron innerhalb von 20 fs dephasiert und das Loch innerhalb von 400 fs. Außerdem beobachten wir Anzeichen von Biexzitonen. MoTe2 BL wurde bisher wenig untersucht und zeigt eine strake Lumineszenz ähnlich zum ML. Wir führen eine umfassende spektroskopische Studie an ML und BL MoTe2 durch und analysieren temperaturabhängige PL-Linienformen von ML und BL. Wir begründen kleine Veränderungen der BL Linienbreite im Vergleich zum ML durch Veränderungen in der Symmetrie und Schichtdicke. Dies bestätigt eine direkte Bandlücke in MoTe2 BL, welche in anderen TMDCs nicht beobachtet wird. Die PL Linienbreite wird in den homogenen und den inhomogenen Anteil zerlegt. Durch die Analyse des inhomogenen Anteils erwarten wir zwei 2.2 meV energetisch von einander entfernte Trionen: Inter- und Intralagentrionen. Der Trionenzustand an sich wird durch das Massenwirkungsgesetz bestätigt. Homogene Linienbreite und Mobilität hängen zusammen, wobei wir die Einflüsse akustischer und optischer Phononen betrachten. Wir bestimmen hohe, durch akustische Phononen begrenzte, Mobilitäten von 6000 (2000) cm2/Vs und 4300 cm2/Vs für das Exziton (Trion) der ML und BL. Die durch optische Phononen limitierten Mobilitäten bei Raumtemperatur liegen bei 300 cm2/Vs und 150 cm2/Vs für ML und BL. Dies sind die höchsten gemessenen Mobilitäten in TMDCs.
Der Valley-Freiheitsgrad der Quasiteilchen kann optisch manipuliert werden. Eine resonante Anregung der Exzitonen in PP erzeugt eine Polarisation von -65 %, welche in 1 ps durch Elektron-Loch-Wechselwirkung und Bandlückenrenormierung depolarisiert. Die Trionen sind weniger von diesen Mechanismen betroffen und zeigen eine Polarisation von 95 % direkt nach der Anregung, welche in 3 ps depolarisiert. Defektassoziierte Rekombination zeigt vergleichbare Zeitskalen. Während der Trionformation messen wir etwa 30 % Polarisation in MoTe2 ML. Dies sind die ersten Polarisationsmessungen an MoTe2. Wir messen ebenfalls eine Polarisation, wenn wir höhere exzitonische Zustände anregen. Deren Valley- und Spinzustände werden durch phononische Kaskadenprozesse bis in den Grundzustand erhalten, wobei Spin (Valley) bei tiefen (hohen) Temperaturen erhalten wird
Magnetic ordering effects in a Mn-modulation-doped high mobility two-dimensional hole system
Get PDFWe have studied the magnetotransport properties of a manganese (Mn)-modulation-doped high mobility two-dimensional hole system in a strained InAs quantum well (QW) structure. At precisely T=600 mK a phase transition from paramagnetism to ferromagnetism can be observed by a change of the low-field magnetotransport behavior and hysteresis. In the magnetically ordered phase a superposition of positive magnetoresistance and weak antilocalization was detected in the longitudinal resistance R(xx) and in the Hall resistance R(xy) a superposition of normal, anomalous, and planar Hall effects demonstrating spontaneous magnetization in the QW plane was detected. From extensive analysis of the temperature and magnetic field dependence of the Shubnikov-de Haas oscillations we deduce the effective mass, transport, and quantum-scattering times. The latter indicates presence of small-range scattering potential. From corrections to the Drude conductivity we determine the impurity interaction time, which is significantly reduced in the ferromagnetic phase indicating interaction of the two-dimensional free holes' spin with the localized magnetic moments of 5/2 from Mn ions
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