23 research outputs found
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 to 10cm). 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 -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
Field-induced polarisation of Dirac valleys in bismuth
Electrons are offered a valley degree of freedom in presence of particular
lattice structures. Manipulating valley degeneracy is the subject matter of an
emerging field of investigation, mostly focused on charge transport in
graphene. In bulk bismuth, electrons are known to present a threefold valley
degeneracy and a Dirac dispersion in each valley. Here we show that because of
their huge in-plane mass anisotropy, a flow of Dirac electrons along the
trigonal axis is extremely sensitive to the orientation of in-plane magnetic
field. Thus, a rotatable magnetic field can be used as a valley valve to tune
the contribution of each valley to the total conductivity. According to our
measurements, charge conductivity by carriers of a single valley can exceed
four-fifth of the total conductivity in a wide range of temperature and
magnetic field. At high temperature and low magnetic field, the three valleys
are interchangeable and the three-fold symmetry of the underlying lattice is
respected. As the temperature lowers and/or the magnetic field increases, this
symmetry is spontaneously lost. The latter may be an experimental manifestation
of the recently proposed valley-nematic Fermi liquid state.Comment: 14 pages + 5 pages of supplementary information; a slightly modified
version will appear as an article in Nature physic
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