The quantum Hall effect allows the international standard for resistance to be defined in terms of the electron charge and Planck's constant alone. The effect comprises the quantization of the Hall resistance in two-dimensional electron systems in rational fractions of R(K) = h/e(2) = 25,812.807557(18) Omega, the resistance quantum. Despite 30 years of research into the quantum Hall effect, the level of precision necessary for metrology--a few parts per billion--has been achieved only in silicon and iii-v heterostructure devices. Graphene should, in principle, be an ideal material for a quantum resistance standard, because it is inherently two-dimensional and its discrete electron energy levels in a magnetic field (the Landau levels) are widely spaced. However, the precisions demonstrated so far have been lower than one part per million. Here, we report a quantum Hall resistance quantization accuracy of three parts per billion in monolayer epitaxial graphene at 300 mK, four orders of magnitude better than previously reported. Moreover, by demonstrating the structural integrity and uniformity of graphene over hundreds of micrometres, as well as reproducible mobility and carrier concentrations across a half-centimetre wafer, these results boost the prospects of using epitaxial graphene in applications beyond quantum metrology.
Electron pumps generate a macroscopic electric current by controlled manipulation of single electrons. Despite intensive research towards a quantum current standard over the last 25 years, making a fast and accurate quantized electron pump has proved extremely difficult. Here we demonstrate that the accuracy of a semiconductor quantum dot pump can be dramatically improved by using specially designed gate drive waveforms. our pump can generate a current of up to 150 pA, corresponding to almost a billion electrons per second, with an experimentally demonstrated current accuracy better than 1.2 parts per million (p.p.m.) and strong evidence, based on fitting data to a model, that the true accuracy is approaching 0.01 p.p.m. This type of pump is a promising candidate for further development as a realization of the sI base unit ampere, following a redefinition of the ampere in terms of a fixed value of the elementary charge.
Controlled charge pumping in an AlGaAs/GaAs gated nanowire by single-parameter modulation is studied experimentally and theoretically. Transfer of integral multiples of the elementary charge per modulation cycle is clearly demonstrated. A simple theoretical model shows that such a quantized current can be generated via loading and unloading of a dynamic quasi-bound state. It demonstrates that non-adiabatic blockade of unwanted tunnel events can obliterate the requirement of having at least two phase-shifted periodic signals to realize quantized pumping. The simple configuration without multiple pumping signals might find wide application in metrological experiments and quantum electronics.PACS numbers: 73.23.Hk,73.22.Dj,73.63.Kv An important milestone in the study of single electron transport is the closure of the quantum metrological triangle for frequency, dc current, and dc voltage [1]. Dc voltage is currently realized from the frequency standard through the Josephson effect. Dc current can then be derived using the quantum Hall effect. Direct realization of dc current from frequency is the currently missing side of the triangle. The closure of the quantum metrological triangle provides a test whether the fundamental constants really appear the same in these different systems [2]. The results of this kind of experiment will also impact on a future system of units which might be based on fundamental constants [3].A current source relevant for the above experiments must produce at least nanoampere currents to be measurable with sufficient accuracy. The electron pump based on arrays of Coulomb blockaded quantum dots (see [4] for a review) or quantum interference [2,5,6] is one class of devices being investigated with respect to metrological relevance [7,8,9]. Electron pumps are typically driven by multiple radio frequency (rf) signals with a well maintained phase relationship, producing a quantized current, i.e. limited to certain values according to I = −nef (with n = 1, 2, 3 . . . , e the negative elementary charge and f the driving frequency). Usually, the accuracy in I degrades with increasing f , which has so far prevented the generation of sufficiently accurate nanoampere currents. An alternative, but challenging task would be the parallelization of pumps driven at intermediate frequencies. Here, pumps requiring only a single rf signal would fundamentally reduce the complexity in the parallelization of such devices. However, electron pumps driven by only one gate [10,11,12,13] have so far not experimentally demonstrated the generation of quantized current. In addition, most models of quantized pumping [5,6,14,15,16,17] have assumed at least two parameters modulated out phase, which may be motivated by the fact that in the adiabatic limit a single periodic perturbation cannot determine the direction of the current [18].In this paper we address this issue and report on the first experimental realization of quantized charge pumping in which only one gate is modulated. We demonstrate on a transparent quantu...
We demonstrate the energy- and time-resolved detection of single-electron wave packets from a clock-controlled source transmitted through a high-energy quantum Hall edge channel. A quantum dot source is loaded with single electrons which are then emitted ~150 meV above the Fermi energy. The energy spectroscopy of emitted electrons indicates that at high magnetic field these electrons can be transported over several microns without inelastic electron-electron or electron-phonon scattering. Using a time-resolved spectroscopic technique, we deduce the wave packet size at picosecond resolution. We also show how this technique can be used to switch individual electrons into different electron waveguides (edge channels).
We explore the robust quantization of the Hall resistance in epitaxial graphene grown on Siterminated SiC. Uniquely to this system, the dominance of quantum over classical capacitance in the charge transfer between the substrate and graphene is such that Landau levels (in particular, the one at exactly zero energy) remain completely filled over an extraordinarily broad range of magnetic fields. One important implication of this pinning of the filling factor is that the system can sustain a very high nondissipative current. This makes epitaxial graphene ideally suited for quantum resistance metrology, and we have achieved a precision of 3 parts in 10 10 in the Hall resistance quantization measurements. Graphene is believed to offer an excellent platform for QHE metrology due to the large energy separation between Landau levels (LL) resulting from the Dirac-type "massless" electrons specific for its band structure [12]. The Hall resistance quantization with an accuracy of 3 parts in 10 9 has already been established [7] in Hall-bar devices manufactured from epitaxial graphene grown on Si-terminated face of SiC (SiC/G). However, for graphene to be practically employed as an embodiment of a quantum resistance standard, it needs to satisfy further stringent requirements [11], in particular with respect to robustness over a range of temperature, magnetic field and measurement current. A high measurement current, which a device can sustain at a given temperature without dissipation, is particularly important for precision metrology as it defines the maximum attainable signalto-noise ratio.The extent of the QHE plateaux in conventional 2D electron systems is, usually, set by disorder and temperature. Disorder pins the Fermi energy in the mobility gap of the 2D system, which suppresses dissipative transport at low temperatures over a finite range of magnetic fields around the values corresponding to exactly filled LLs. These values can be calculated from the carrier density n s determined from the low-field Hall resistivity measurements and coincide with the maximum non-dissipative current, the breakdown current. Thus, the breakdown current in conventional two-dimensional semicondutors peaks very close to the field values where the filling factor ν is an even integer [11]. Though less studied experimentally, the behaviour of the breakdown current on the plateaux for the exfoliated graphene, including the ν = 2 plateau corresponding to the topologically protected N = 0 LL, looks quite similar [13].In this Brief Report we explore the robustness of the Hall resistance quantization in SiC/G. Unlike the QHE in conventional 2D systems, where the carrier density is independent of magnetic field, here specifically to SiC/G, we find that the carrier density in graphene varies with magnetic field due to the charge transfer between surface donor states in SiC and graphene. Most importantly, we find magnetic field intervals of several Tesla, where the carrier density in graphene increases linearly with the magnetic field, resulting ...
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