68 research outputs found
Realization of the farad from the dc quantum Hall effect with digitally-assisted impedance bridges
A new traceability chain for the derivation of the farad from dc quantum Hall
effect has been implemented at INRIM. Main components of the chain are two new
coaxial transformer bridges: a resistance ratio bridge, and a quadrature
bridge, both operating at 1541 Hz. The bridges are energized and controlled
with a polyphase direct-digital-synthesizer, which permits to achieve both main
and auxiliary equilibria in an automated way; the bridges and do not include
any variable inductive divider or variable impedance box. The relative
uncertainty in the realization of the farad, at the level of 1000 pF, is
estimated to be 64E-9. A first verification of the realization is given by a
comparison with the maintained national capacitance standard, where an
agreement between measurements within their relative combined uncertainty of
420E-9 is obtained.Comment: 15 pages, 11 figures, 3 table
Practical quantum realization of the ampere from the electron charge
One major change of the future revision of the International System of Units
(SI) is a new definition of the ampere based on the elementary charge \emph{e}.
Replacing the former definition based on Amp\`ere's force law will allow one to
fully benefit from quantum physics to realize the ampere. However, a quantum
realization of the ampere from \emph{e}, accurate to within in
relative value and fulfilling traceability needs, is still missing despite many
efforts have been spent for the development of single-electron tunneling
devices. Starting again with Ohm's law, applied here in a quantum circuit
combining the quantum Hall resistance and Josephson voltage standards with a
superconducting cryogenic amplifier, we report on a practical and universal
programmable quantum current generator. We demonstrate that currents generated
in the milliampere range are quantized in terms of
( is the Josephson frequency) with a measurement uncertainty of
. This new quantum current source, able to deliver such accurate
currents down to the microampere range, can greatly improve the current
measurement traceability, as demonstrated with the calibrations of digital
ammeters. Beyond, it opens the way to further developments in metrology and in
fundamental physics, such as a quantum multimeter or new accurate comparisons
to single electron pumps.Comment: 15 pages, 4 figure
Operation of graphene quantum Hall resistance standard in a cryogen-free table-top system
We demonstrate quantum Hall resistance measurements with metrological
accuracy in a small cryogen-free system operating at a temperature of around
3.8K and magnetic fields below 5T. Operating this system requires little
experimental knowledge or laboratory infrastructure, thereby greatly advancing
the proliferation of primary quantum standards for precision electrical
metrology. This significant advance in technology has come about as a result of
the unique properties of epitaxial graphene on SiC.Comment: 15 pages, 9 figure
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New insights into the solid–liquid interface exploiting neutron reflectivity
We outline recent progress exploiting neutron reflectivity for structural and compositional investigations of the solid-liquid interface. There has been extensive activity in this area, with key areas of development: (i) an increased range of accessible substrates (e.g. metals and minerals), (ii) novel liquid phases, and (iii) strong themes in electrochemistry (e.g. batteries), corrosion, polymers and increasing application of extreme conditions
Contributions of precision engineering to the revision of the SI
All measurements performed in science and industry are based on the International System of Units, the SI. It has been proposed to revise the SI following an approach which was implemented for the redefinition of the unit of length, the metre, namely to define the SI units by fixing the numerical values of so-called defining constants, including c, h, e, k and NA. We will discuss the reasoning behind the revision, which will likely be put into force in 2018. Precision engineering was crucial to achieve the required small measurement uncertainties and agreement of measurement results for the defining constants
The ampere and the electrical units in the quantum era
By fixing two fundamental constants from quantum mechanics, the Planck
constant and the elementary charge , 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 and 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
Real-space renormalization group approach to the integer quantum Hall effect: Ortsraum-Renormierungsgruppenansatz für den ganzzahligen Quanten-Hall-Effekt
Gegenstand dieser Dissertation ist die numerische Untersuchung des
ganzzahligen Quanten-Hall-Effekts (QHE). Im Mittelpunkt steht dabei
der Übergang zwischen den charakteristischen Plateaus des
Hall-Leitwertes. Die Beschreibung des Übergangs erfolgt im Rahmen des
Chalker-Coddington-Netzwerkmodells, wobei zusätzlich ein
Ortsraum-Renormierungsgruppenansatz (RG) angewendet wird um hohe
Systemgrößen zu erreichen. Diese Vorgehensweise erlaubt eine einfache,
aber statistisch sehr gute Beschreibung der starken charakteristischen
Fluktuationen am Übergang im Rahmen von Verteilungsfunktionen.
Die RG Resultate werden zunächst mit Ergebnissen anderer Methoden
verglichen. Es werden die kritische Verteilungsfunktion des
Leitwertes am QHE Übergang und deren Momente ermittelt. Aus dem
Verhalten in der Nähe des Übergangs läßt sich der Wert des kritischen
Exponenten der Lokalisierungslänge ableiten. Diese Ergebnisse stimmen
sehr gut mit exakten numerischen Simulationen überein. Die RG Methode
wird daraufhin zur Berechnung der Energieniveaustatistik (ENS)
erweitert. Die kritische ENS der normierten Abstände von benachbarten
Energieniveaus und der kritische Exponent werden bestimmt. Danach
wird der Einfluß von makroskopischen Inhomogenitäten in Form von
langreichweitiger korrelierter Unordnung auf die kritischen
Eigenschaften des QHE Übergangs untersucht. Hierbei zeigt sich ein
Anwachsen des Exponenten mit zunehmender Reichweite und Stärke der
Unordnung. Abschließend wird die RG zur Berechnung des
Hall-Widerstandes eingesetzt. Die kritische Verteilung
des Hall-Widerstandes läßt auf sehr starke Fluktuationen am Übergang
schließen. Abseits des Übergangs in Richtung Isolator wird
divergentes Verhalten des Hall-Widerstandes gefunden. Zusammenfassend
demonstrieren alle Ergebnisse die Robustheit universeller
Eigenschaften am QHE Übergang
Electronic correlation in the quantum Hall regime
Two-dimensional interacting electron systems become strongly correlated if
the electrons are subject to a perpendicular high magnetic field. After
introducing the physics of the quantum Hall regime the incompressible many-
particle ground state and its excitations are studied in detail at fractional
filling factors for spin-polarized electrons. The spin degree of freedom whose
importance was shown in recent experiments is considered by studying the
thermodynamics at filling factor one and near one.Comment: 55 pages, 26 eps-figure
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