Spintronics aims to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them 1-3 . This approach is illustrated by the operation of the most basic spintronic device, the spin valve 4-6 , which can be formed if two ferromagnetic electrodes are separated by a thin tunnelling barrier. In most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction 7,8 . The relative difference in resistance, the so-called magnetoresistance, is then positive. However, if the transport of carriers inside the device is spin-or energy-dependent 3 , the opposite can occur and the magnetoresistance is negative 9 . The next step is to construct an analogous device to a field-effect transistor by using this effect to control spin transport and magnetoresistance with a voltage applied to a gate 10,11 . In practice though, implementing such a device has proved difficult. Here, we report on a pronounced gate-field-controlled magnetoresistance response in carbon nanotubes connected by ferromagnetic leads. Both the magnitude and the sign of the magnetoresistance in the resulting devices can be tuned in a predictable manner. This opens an important route to the realization of multifunctional spintronic devices.Early work on spin transport in multiwall carbon nanotubes (MWNTs) with Co contacts showed that spins could propagate coherently over distances as long as 250 nm (ref. 12). The tunnel magnetoresistance (TMR) = (R AP − R P )/R P , defined as the relative difference between the resistances R AP and R P in the antiparallel and parallel magnetization configuration, was found to be positive and amounted to +4% in agreement with Jullière's formula for tunnel junctions 4,13 . A negative TMR of about −30% was reported later for MWNTs contacted with similar Co contacts 14 . In these experiments, the nanotubes did not show quantum dot behaviour. It has been shown, however, that single-wall carbon nanotubes (SWNTs) and MWNTs contacted with nonferromagnetic metals could behave as quantum dots and FabryPérot resonators [15][16][17][18][19] , in which one can tune the position of discrete energy levels with a gate electrode. From this, one can expect to be able to tune the sign and the amplitude of the TMR in nanotubes, in a similar fashion as predicted originally for semiconductor heterostructures 11 .In this letter, we report on TMR measurements of MWNTs and SWNTs that are contacted with ferromagnetic electrodes and capacitively coupled to a back-gate 20 . A typical sample geometry is shown in the inset of Fig. 1. As a result of resonant tunnelling, we observe a striking oscillatory amplitude and sign modulation of the TMR as a function of the gate voltage. We have studied and observed the TMR on nine samples (seven MWNTs and two SWNTs) with various tube lengths L between the ferromagnetic electrodes (see the Methods section). We present here results for one MWNT device and one SWNT device.We first discu...

We describe the results of electronic Raman-scattering experiments in differently doped single crystals of YBa 2 Cu 3 O 6ϩx and Bi 2 Sr 2 (Ca x Y 1Ϫx )Cu 2 O 8 . The data in antiferromagnetic insulating samples suggest that at least the low-energy parts of the spectra of metallic samples originate predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach which has been introduced earlier for the current response. Dynamical scattering rates ⌫()ϭ1/() and mass-enhancement factors 1ϩ()ϭm*()/m of the carriers are obtained. It is found that a strong polarization dependence of the carrier lifetime develops towards low doping. In B 2g (xy) symmetry selecting predominantly electrons with momenta along the diagonals of the CuO 2 planes the Raman data compare well with the results obtained from dc and dynamical transport. In B 1g (x 2 Ϫy 2 ) symmetry projecting out momenta along the Cu-O bonds the dc scattering rates of underdoped materials become temperature independent and considerably larger than in B 2g symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in B 2g symmetry in the range between the superconducting transition at T c and a characteristic temperature T* of the order of room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity-related features in the spectra can be observed only in B 2g symmetry. The peak frequencies scale with T c . We do not find a direct relation between the pseudogap and the superconducting gap.

Raman spectra of YBa 2 Cu 3 O 72x and Bi 2 Sr 2 ͑Ca 0.62 Y 0.38 ͒Cu 2 O 81d with T c Х 0.65T max c in the underdoped regime of the phase diagram are studied as a function of temperature and polarization. At B 2g ͑xy͒ symmetry a reduction of spectral weight by 10% for frequencies less than 700 cm 21 or 15k B T c is found below approximately 200 K. Below T c , a superconducting gap opens up which closely resembles that observed at higher doping levels. It is compatible with d x 2 2y 2 pairing and its amplitude 2D 0 can be estimated to be 8k B T c . [S0031-9007(97)

We report electrical transport measurements through a semiconducting single-walled carbon nanotube (SWNT) with three additional top-gates. At low temperatures the system acts as a double quantum dot with large inter-dot tunnel coupling allowing for the observation of tunnel-coupled molecular states extending over the whole double-dot system. We precisely extract the tunnel coupling and identify the molecular states by the sequential-tunneling line shape of the resonances in differential conductance.Comment: 5 pages, 4 figure

The Néel IRAM KIDs Array (NIKA) is a fully integrated measurement system based on kinetic inductance detectors (KIDs) currently being developed for millimeter wave astronomy. The instrument includes dual-band optics allowing simultaneous imaging at 150 GHz and 220 GHz. The imaging sensors consist of two spatially separated arrays of KIDs. The first array, mounted on the 150 GHz branch, is composed of 144 lumped-element KIDs. The second array (220 GHz) consists of 256 antenna-coupled KIDs. Each of the arrays is sensitive to a single polarization; the band splitting is achieved by using a grid polarizer. The optics and sensors are mounted in a custom dilution cryostat, with an operating temperature of ∼70 mK. Electronic readout is realized using frequency multiplexing and a transmission line geometry consisting of a coaxial cable connected in series with the sensor array and a lownoise 4 K amplifier. The dual-band NIKA was successfully tested in 2010 October at the Institute for Millimetric Radio Astronomy (IRAM) 30 m telescope at Pico Veleta, Spain, performing in-line with laboratory predictions. An optical NEP was then calculated to be around 2 × 10 −16 W Hz −1/2 (at 1 Hz) while under a background loading of approximately 4 pW pixel −1 . This improvement in comparison with a preliminary run (2009) verifies that NIKA is approaching the target sensitivity for photon-noise limited ground-based detectors. Taking advantage of the larger arrays and increased sensitivity, a number of scientifically relevant faint and extended objects were then imaged including the Galactic Center SgrB2 (FIR1), the radio galaxy Cygnus A, and the NGC1068 Seyfert galaxy. These targets were all observed simultaneously in the 150 GHz and 220 GHz atmospheric windows.

Context. Current generation millimeter wavelength detectors suffer from scaling limits imposed by complex cryogenic readout electronics. These instruments typically employ multiplexing ratios well below a hundred. To achieve multiplexing ratios greater than a thousand, it is imperative to investigate technologies that intrinsically incorporate strong multiplexing. One possible solution is the kinetic inductance detector (KID). To assess the potential of this nascent technology, a prototype instrument optimized for the 2 mm atmospheric window was constructed. Known as the Néel IRAM KID Array (NIKA), it has recently been tested at the Institute for Millimetric Radio Astronomy (IRAM) 30-meter telescope at Pico Veleta, Spain.Aims. There were four principle research objectives: to determine the practicality of developing a giant array instrument based on KIDs, to measure current in-situ pixel sensitivities, to identify limiting noise sources, and to image both calibration and scientificallyrelevant astronomical sources. Methods. The detectors consisted of arrays of high-quality superconducting resonators electromagnetically coupled to a transmission line and operated at ∼100 mK. The impedance of the resonators was modulated by incident radiation; two separate arrays were tested to evaluate the efficiency of two unique optical-coupling strategies. The first array consisted of lumped element kinetic inductance detectors (LEKIDs), which have a fully planar design properly shaped to enable direct absorbtion. The second array consisted of antenna-coupled KIDs with individual sapphire microlenses aligned with planar slot antennas. Both detectors utilized a single transmission line along with suitable room-temperature digital electronics for continuous readout.Results. NIKA was successfully tested in October 2009, performing in line with expectations. The measurement resulted in the imaging of a number of sources, including planets, quasars, and galaxies. The images for Mars, radio star MWC349, quasar 3C345, and galaxy M87 are presented. From these results, the optical NEP was calculated to be around 1 × 10 −15 W/Hz 1/2 . A factor of 10 improvement is expected to be readily feasible by improvements in the detector materials and reduction of performance-degrading spurious radiation.

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