We investigate the magnetoresistance of a quasi-two-dimensional electron system subject to a onedimensional superlattice potential created by the field effect in gated AlGaAs/GaAs heterojunctions. At low temperatures this potential gives rise to a new type of magnetoresistance oscillation with a period governed by the ratio of the classical cyclotron diameter 2Rc to the superlattice period a. The oscillations are quantitatively explained in period, phase, and magnitude by the formation of Landau bands in the two-dimensional electron system under the influence of the periodic potential.PACS numbers: 73.40. Kp, 71.25.He Until recently experimental investigations of the effect of a lateral superlattice potential on the electronic transport properties of a two-dimensional electron system (2D ES) have been limited to ''natural" superlattices in metal oxide-semiconductor structures on high-index surfaces of Si^'^ or InSb.^ Modulation-doped AlGaAs/GaAs heterojunctions with a 2D ES of high mobility now make it possible to study lateral superlattice phenomena using artificially created potentials with periods in the submicrometer range. As a first manifestation of such onedimensional (ID) superlattice effects Weiss et al.^ recently observed a new type of oscillation of the lowtemperature magneotresistance at magnetic fields B < 1 T applied perpendicularly to the 2D ES of an AlGaAs/ GaAs heterojunction with a weak periodic electron density modulation. They created the modulation via the persistent photoeffect by exposing the sample to a light grating of submicrometer period a. Similar to Shubnikov-de Haas (SdH) oscillations these new resistance oscillations are periodic in l/B. Weiss et al. found that apart from a phase factor 0, maxima in the magnetoresistance arise whenever the classical cyclotron diameter IRc at the Fermi energy is a multiple of the period a such that IRc = (m-l-0)a, m = 1,2,. . .In their report, however, they did not identify the mechanism causing the oscillations.Here we present experimental studies of these novel magnetoresistance oscillations on AlGaAs/GaAs heterojunctions with a periodically microstructured gate electrode, in which the field effect is used to create a ID periodic potential. We demonstrate that the magnetoresistance oscillations reflect the formation of Landau bands in the 2D ES under the influence of the superlattice potential. Calculating the magnetic band structure in the presence of the periodic potential, ^'^ we show that the bandwidth of the Landau bands at the Fermi energy oscillates periodically with period IRcIa and gives rise to an oscillatory band conductivity. We calculate the magnetoresistance caused by this mechanism and find good agreement with the observed oscillations in period, phase, and magnitude.The samples are prepared on conventional AlGaAs/ GaAs heterojunctions grown by molecular-beam epitaxy. The heterojunctions consist of a l.6-//m-thick GaAs buffer on top of a semi-insulating GaAs substrate, a 20nm nominally undoped Al;cGai-;cAs (x=0.33) spacer, a 50-nm...

Since 1999, when the first high temperature fixed-points based on the metal–carbon eutectic phase transitions were realized, more than 60 papers have been published on this topic. Eutectic based fixed-points are already being considered as secondary reference points for the International Temperature Scale and have been introduced into industrial laboratories. This rapid progress has been possible through the combined effort of scientists around the world, from national metrology institutes, universities and industry. It has been proposed that these fixed-points should be officially adopted as a way to improve the realization and dissemination of temperature scales above the silver point. In radiometry, the availability of stable high temperature fixed-points will give greater flexibility and at some wavelengths the potential for greater accuracy for spectral radiance and irradiance scale realization. This paper summarizes the major progress in eutectic research so far.

The Earth's climate is undoubtedly changing; however, the time scale, consequences and causal attribution remain the subject of significant debate and uncertainty. Detection of subtle indicators from a background of natural variability requires measurements over a time base of decades. This places severe demands on the instrumentation used, requiring measurements of sufficient accuracy and sensitivity that can allow reliable judgements to be made decades apart. The International System of Units (SI) and the network of National Metrology Institutes were developed to address such requirements. However, ensuring and maintaining SI traceability of sufficient accuracy in instruments orbiting the Earth presents a significant new challenge to the metrology community. This paper highlights some key measurands and applications driving the uncertainty demand of the climate community in the solar reflective domain, e.g. solar irradiances and reflectances/radiances of the Earth. It discusses how meeting these uncertainties facilitate significant improvement in the forecasting abilities of climate models. After discussing the current state of the art, it describes a new satellite mission, called TRUTHS, which enables, for the first time, high-accuracy SI traceability to be established in orbit. The direct use of a 'primary standard' and replication of the terrestrial traceability chain extends the SI into space, in effect realizing a 'metrology laboratory in space'.

We report the fourth World Radiometric Reference (WRR)-to-SI comparison. At the National Physical Laboratory we compared three transfer pyrheliometer instruments in power mode with the SI radiometric scale. Compared with the three previous comparisons, we improved the experiment by operating the transfer instruments in vacuum. At the Total solar irradiance Radiometer Facility (TRF) located at the Laboratory for Atmospheric and Space Physics (LASP) in Boulder, we repeated the power comparison of one of the transfer instruments. The TRF also allowed the comparison and characterization of this instrument in irradiance mode. Using the WRR comparisons performed in Davos, we find that the WRR is 0.34% higher than the SI scale. Comparing irradiance mode calibrations with power mode calibrations reveals that previous estimates of stray light of PMO6-type radiometers were very low. The instrument calibrated at TRF was integrated in the space experiment PREMOS on the French satellite PICARD and carries the first vacuum irradiance calibration to space.

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PREMOS is a space experiment scheduled to fly on the French solar mission PICARD. The experiment comprises filter radiometers and absolute radiometers to measure the spectral and total solar irradiance. The aim of PREMOS is to contribute to the long term monitoring of the total solar irradiance, to use irradiance observations for ‘nowcasting’ the state of the terrestrial middle atmosphere and to provide long term sensitivity calibration for the solar imaging instrument SODISM on PICARD. In this paper we describe the calibration of the instruments. The filter radiometer channels in the visible and near IR were characterized at PMOD/WRC and the UV channels were calibrated at PTB Berlin. The absolute radiometers were compared with the World Radiometric Reference at PMOD/WRC and a power calibration relative to a primary cryogenic radiometer standard was performed in vacuum and air at NPL.

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This paper is concerned with bringing together the topics of uncertainty evaluation using a Monte Carlo method, distributed computing for data parallel applications and pseudo-random number generation. A study of a measurement system to estimate the absolute thermodynamic temperatures of two high-temperature blackbodies by measuring the ratios of their spectral radiances is used to illustrate the application of these topics. The uncertainties associated with the estimates of the temperatures are evaluated and used to inform the experimental realization of the system. The difficulties associated with determining model sensitivity coefficients, and demonstrating whether a linearization of the model is adequate, are avoided by using a Monte Carlo method as an approach to uncertainty evaluation. A distributed computing system is used to undertake the Monte Carlo calculation because the computational effort required to evaluate the measurement model can be significant. In order to ensure that the results provided by a Monte Carlo method implemented on a distributed computing system are reliable, consideration is given to the approach to generating pseudo-random numbers, which constitutes a key component of the Monte Carlo procedure.