Magnetothermoelectric properties of Bi2Se3

We present a study of entropy transport in Bi2Se3 at low temperatures and high magnetic fields. In the zero-temperature limit, the magnitude of the Seebeck coefficient quantitatively tracks the Fermi temperature of the 3D Fermi surface at \Gamma-point as the carrier concentration changes by two orders of magnitude (10$^{17}$ to 10$^{19}$cm$^{-3}$). In high magnetic fields, the Nernst response displays giant quantum oscillations indicating that this feature is not exclusive to compensated semi-metals. A comprehensive analysis of the Landau Level spectrum firmly establishes a large $g$-factor in this material and a substantial decrease of the Fermi energy with increasing magnetic field across the quantum limit. Thus, the presence of bulk carriers significantly affects the spectrum of the intensively debated surface states in Bi2Se3 and related materials.Comment: 10 pages, 9 figure

Planck pre-launch status : The Planck mission

Peer reviewe

Planck early results II : The thermal performance of Planck

Peer reviewe

Planck early results. II. The thermal performance of Planck

The performance of the Planck instruments in space is enabled by their low operating temperatures, 20 K for LFI and 0.1 K for HFI, achieved through a combination of passive radiative cooling and three active mechanical coolers. The scientific requirement for very broad frequency coverage led to two detector technologies with widely different temperature and cooling needs. Active coolers could satisfy these needs; a helium cryostat, as used by previous cryogenic space missions (IRAS, COBE, ISO, Spitzer, AKARI), could not. Radiative cooling is provided by three V-groove radiators and a large telescope baffle. The active coolers are a hydrogen sorption cooler (<20 K), a 4He Joule-Thomson cooler (4.7 K), and a 3He-4He dilution cooler (1.4 K and 0.1 K). The flight system was at ambient temperature at launch and cooled in space to operating conditions. The HFI bolometer plate reached 93 mK on 3 July 2009, 50 days after launch. The solar panel always faces the Sun, shadowing the rest of Planck, and operates at a mean temperature of 384 K. At the other end of the spacecraft, the telescope baffle operates at 42.3 K and the telescope primary mirror operates at 35.9 K. The temperatures of key parts of the instruments are stabilized by both active and passive methods. Temperature fluctuations are driven by changes in the distance from the Sun, sorption cooler cycling and fluctuations in gas-liquid flow, and fluctuations in cosmic ray flux on the dilution and bolometer plates. These fluctuations do not compromise the science data

H↓ on He : sticking and 2d-superfluidity

The sticking coefficient, which governs the sticking time τs, is discussed for high surface-coverage conditions. We point out that τ s must remain large compared to a characteristic vortex diffusion time, if the system is to display 2d-superfluidity.Le coefficient de collage, qui détermine le temps de séjour en surface τ s, est évalué à taux de couverture élevé. Pour que le système présente les propriétés de superfluidité bidimensionnelle, il faut que τs reste grand devant un temps caractéristique de diffusion de vortex

H - He : effets de l'interaction gaz-surface

A hydrogen atom, adsorbed on liquid helium, slightly polarizes the substrate. Hence a self-energy Σ, associated with an effective mass m* a few percent larger than m. From the gas-surface coupling, we estimate : Σ ≅ - 0.3 K. | Σ | contributes to the adsorption energy ε a and this figure agrees rather well with the measured εa . On the other hand, it leads to a low-temperature « plateau » for the sticking coefficient α which we discuss by reference with available experiments. Again at low temperature, we predict a surface/volume decoupling for the a-atom (↓⇑ - ε↑⇓) population, which opens the possibility of an independent measurement of α. Finally, we give a few remarks on the liquid-3He « substrate ».Un atome d'hydrogène adsorbé à la surface de l'hélium liquide polarise légèrement le substrat. Il en résulte une « self-énergie » Σ, associée à une masse effective m* un peu supérieure à m. A partir du couplage gaz-surface, nous évaluons : Σ ∼ - 0,3 K. Cette valeur rend compte assez bien de l'énergie d'adsorption sur 4He. Par ailleurs, elle conduit à un plateau basse température pour le coefficient de collage α — que nous discutons par référence aux mesures disponibles. Toujours à basse température, nous prédisons un découplage surface/ volume pour la densité des atomes a (état hyperfin : ↓⇑ - ε↑⇓), et nous en tirons le principe d'une mesure indépendante de α. Nous donnons enfin quelques remarques sur le « substrat » 3He liquide

Bruit de courant en conduction non-linéaire

An intermittency-like model for current fluctuations in a nonlinear conductor, is proposed. The lowfrequency noise spectrum, as a function of the driving voltage, agrees with measurements which were performed on the charge-density wave compound TaS3. Phase-slip effects in superconducting weak links provide a physical analogy.Nous présentons un modèle statistique, de type intermittence, pour les fluctuations de courant dans un conducteur non linéaire. La discussion s'appuie sur l'exemple du courant porté par une onde densité de charge, où l'on dispose de mesures de bruit qui s'accordent avec le spectre basse-fréquence prédit par le modèle en fonction de l'écart au seuil de conduction. Une correspondance physique est établie avec les effets de glissement de phase dans les microponts Josephson