20 research outputs found

    Implementation of the Quantum Hall Effect based precision resistance measurement system

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    The integer Quantum Hall Effect (QHE) occurs when a two-dimensional electron gas (2DEG) is subjected to a strong perpendicular magnetic field and when the system is cooled to low temperatures. The QHE harbours a wealth of unique phenomena. Of interest is the existence of the Quantum Hall Resistance (QHR) which had found to be related to two fundamental constants of nature via the von Klitzing constant h e 2 , where e is the charge of the electron and h Planck’s constant. This thesis investigates the properties of the QHE in a low dimensional electron gas system. The von Klitzing constant is determined as well as the electron density n2D and mobility µ of the material measured. The results are compared to the accepted value of the von Klitzing constant as determined by the metrological community. The average von Klitzing constant obtained is 25 783.637 Ω within an accuracy of 1.13 × 10−12. Our results are further interpreted using the Landau quantum mechanical model of electron transport in perpendicular magnetic field. The measurement of standard resistances utilising a standard DC resistance measurement system were also undertaken at the National Metrology Institute of South Africa (NMISA). This ties in with the ongoing project of NMISA to develop an in-house quantum Hall measurement system to provide the full traceability for resistance standard measurements in the Republic of South Africa. The device measured utilised a GaAs/AlGaAs heterostructure structure, grown via Molecular Beam Epitaxy (MBE). A micron sized Hall bar with Ohmic contacts was patterned using standard clean room procedures. Magnetotransport measurements at low temperatures, sub 200 mK were carried out on the device. The transverse and longitudinal resistances were obtained and plotted against the perpendicular magnetic field. Quantum Hall plateaus and Shubnikov de-Haas (SdH) oscillations were observed. Properties of the heterostructure such as the electron density (n2D) and mobility (µ) were determined. The n2D obtained was 2.27 × 1011 cm−2 with µ at 3.5 × 105 cm2V−1 s −1 . All results were compared to current literature values

    The ampere and the electrical units in the quantum era

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    By fixing two fundamental constants from quantum mechanics, the Planck constant hh and the elementary charge ee, the revised Syst\`eme International (SI) of units endorses explicitly quantum mechanics. This evolution also highlights the importance of this theory which underpins the most accurate realization of the units. From 20 May 2019, the new definitions of the kilogram and of the ampere, based on fixed values of hh and ee respectively, will particularly impact the electrical metrology. The Josephson effect (JE) and the quantum Hall effect (QHE), used to maintain voltage and resistance standards with unprecedented reproducibility since 1990, will henceforth provide realizations of the volt and the ohm without the uncertainties inherited from the older electromechanical definitions. More broadly, the revised SI will sustain the exploitation of quantum effects to realize electrical units, to the benefit of end-users. Here, we review the state-of-the-art of these standards and discuss further applications and perspectives.Comment: 78 pages, 35 figure

    Evidence for universality of tunable-barrier electron pumps

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    We review recent precision measurements on semiconductor tunable-barrier electron pumps operating in a ratchet mode. Seven studies on five different designs of pumps have reported measurements of the pump current with relative total uncertainties around 10-6 or less. Combined with theoretical models of electron capture by the pumps, these experimental data exhibits encouraging evidence that the pumps operate according to a universal mechanism, independent of the details of device design. Evidence for robustness of the pump current against changes in the control parameters is at a more preliminary stage, but also encouraging, with two studies reporting robustness of the pump current against three or more parameters in the range of ∼5 × 10-7 to ∼2 × 10-6. This review highlights the need for an agreed protocol for tuning the electron pump for optimal operation, as well as more rigorous evaluations of the robustness in a wide range of pump designs

    Frequenzabhängigkeit eines Primärnormals für Kapazität im Audiofrequenzbereich

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    The Thompson–Lampard theorem is a theorem of electrostatics which describes a so-called cross capacitance independent of its cross sectional dimensions. It is therefore best suited to be used as a capacitance standard for the absolute determination of the unit of capacitance, the farad. As this cross capacitance is defined for DC but must be operated at AC, a frequency correction must be applied the uncertainty of which is one of the largest contributions to the uncertainty budget. The frequency correction can be calculated with a sufficient accuracy using an equivalent circuit model which describes the behaviour of the calculable cross capacitor at AC. Calculable cross capacitors are mostly operated at a frequency of 1592 Hz ( = 104 rad/s) which is convenient for the link between the farad and the ohm. The determination of the frequency behavior of the PTB calculable cross capacitor in the audio frequency range is carried out by applying an equivalent circuit model which considers the influence of distributed admittances and impedances within the calculable cross capacitor. It has been derived taking into account the main currents flowing inside the calculable cross capacitor and the voltage drops caused by these currents. Most of the circuit parameters of the model were directly measured in the actual configuration of the electrode system with a commercial auto-balancing bridge in a frequency range between 1 kHz and 10 kHz. Owing to the uncertainty of the bridge which is 0.1 % for capacitance measurements and 2 % for inductance measurements, the calculated uncertainty of the frequency correction amounts to a few parts in 108. The capacitances on the calculable cross capacitor were found to be frequency independent, but self-inductances and mutual inductances of the electrodes are frequency dependent and linearly proportional to the inverse square root of the frequency in the examined frequency range. The frequency dependence of the calculable cross capacitor has been calculated based on the measured circuit parameters. It has been found that the frequency dependence of the cross capacitance is proportional to the square of the frequency, and that the influence of mutual inductances of opposite electrodes dominate the behavior at higher frequencies. At a frequency of 1592 Hz, the correction of the PTB calculable cross capacitor with a value of 1 pF amounts to 9.2×10-8 pF with an expanded uncertainty (k=2) of 3.1×10-8 pF.Das Thompson-Lampard Theorem ist ein Theorem der Elektrostatik, das eine sogenannte Kreuzkapazität beschreibt, die unabhängig von den Querschnittsabmessungen ist. Sie ist daher bestens als Kapazitätsnormal für die Darstellung der Kapazitätseinheit Farad geeignet. Da die Kreuzkapazität für den statischen Fall definiert ist, jedoch bei Wechselstrom betrieben wird, muss eine Frequenzkorrektur angebracht werden, deren Unsicherheit einen wesentlichen Beitrag zum Unsicherheitsbudget des Kondensators leistet. Die Frequenzkorrektur kann mit ausreichender Genauigkeit unter Verwendung eines Wechselstrom-Ersatzschaltbildes für den Kreuzkondensator berechnet werden. Kreuzkondensatoren werden im Allgemeinen bei einer Frequenz von 1592 Hz, entsprechend einer Kreisfrequenz von 10 rad/s, betrieben, die für die Ableitung der Widerstandseinheit Ohm aus der Kapazitätseinheit Farad am geeignetsten ist. Das Frequenzverhalten des PTB Kreuzkondensators im Tonfrequenzbereich wurde unter Anwendung eines Ersatzschaltbildes bestimmt, das den Einfluss der verteilten Impedanzen und Admittanzen im Kondensator berücksichtigt. Es wurde unter Beachtung der wesentlichen Ströme im Kondensator sowie ihrer Spannungsabfälle an den Impedanzen abgeleitet. Die Mehrzahl der Ersatzschaltbild-Parameter wurde direkt in der originalen Konfiguration des Elektrodensystems mit einer kommerziellen selbst abgleichenden Messbrücke in einem Frequenzbereich von 1 kHz bis 10 kHz bestimmt. Mit der Messunsicherheit der Messbrücke, die 0,1 % für Kapazitätsmessungen und 2% für Induktivitätsmessungen beträgt, ergibt sich für die Frequenzkorrektur eine Unsicherheit von einigen 10-8. Die Kapazitäten des Kreuzkondensators sind im untersuchten Bereich frequenzunabhängig, während die Selbst- und Gegeninduktivitäten sich linear mit der inversen Quadratwurzel der Frequenz ändern. Die Frequenzabhängigkeit des Kreuzkondensators wurde auf der Grundlage der gemessenen Ersatzschaltbild-Parameter berechnet. Sie ist proportional zum Quadrat der Frequenz. Den stärksten Einfluss auf das Frequenzverhalten bei höheren Frequenzen haben die Gegeninduktivitäten gegenüberliegender Elektroden. Bei einer Frequenz von 1592 Hz beträgt die Frequenzkorrektur des PTB Kreuzkondensators bei einer Kapazität von 1 pF 9.2×10-8 pF mit einer erweiterten Unsicherheit (k=2) von 3.1×10-8 pF

    C and X-band communications transponder and transponder test set Final engineering report, 28 Dec. 1965 - 31 Oct. 1967

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    Tradeoff analyses on C and X band frequencies for ground-manned spacecraft communications, and transponder system design dat

    NASA metrology and calibration, 1993

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    Th sixteenth annual workshop of NASA's Metrology and Calibration Working Group was held April 20-22, 1993. The goals of the Working Group are to provide Agencywide standardization of individual metrology programs, where appropriate; to promote cooperation and exchange of information within NASA, with other Government agencies, and with industry; to serve as the primary Agency interface with the National Institute of Standards and Technology; and to encourage formal quality control techniques such as Measurement Assurance Programs. These proceedings contain unedited reports and presentations from the workshop and are provided for information only

    Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

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    We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.Peer ReviewedPostprint (published version
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