426 research outputs found
Pseudogap phase of cuprate superconductors confined by Fermi surface topology
The properties of cuprate high-temperature superconductors are largely shaped
by competing phases whose nature is often a mystery. Chiefly among them is the
pseudogap phase, which sets in at a doping that is material-dependent.
What determines is currently an open question. Here we show that the
pseudogap cannot open on an electron-like Fermi surface, and can only exist
below the doping at which the large Fermi surface goes from hole-like
to electron-like, so that . We derive this result from
high-magnetic-field transport measurements in
LaNdSrCuO under pressure, which reveal a large and
unexpected shift of with pressure, driven by a corresponding shift in
. This necessary condition for pseudogap formation, imposed by details
of the Fermi surface, is a strong constraint for theories of the pseudogap
phase. Our finding that can be tuned with a modest pressure opens a new
route for experimental studies of the pseudogap.Comment: 15 pages, 5 figures, 7 supplemental figure
Field-angle dependence of sound velocity in the Weyl semimetal TaAs
The elastic modulus of a single crystal of the Weyl semimetal TaAs
was investigated by measuring relative changes in the sound velocity under
application of a magnetic field up to 10 T. Using an ultrasonic pulsed-echo
technique, we studied the shear response of the crystal when the angle between
the sound wave propagation and the magnetic field is changed. We observe a
broken tetragonal symmetry at fields above 6 T, an anisotropy that is likely
related to a longitudinal negative magnetoresistance and therefore might
provide evidence of the chiral anomaly, one of the main topological signatures
of this class of materials. We also observe quantum oscillations in the sound
velocity whose frequencies vary with magnetic field orientation. A fan diagram
of Landau level indices reveals topological and trivial Berry phases, depending
on the field orientation, indicating a sensitivity to different Fermi surface
pockets that do or do not enclose Weyl nodes respectively
Field-dependent heat transport in the Kondo insulator SmB6 : phonons scattered by magnetic impurities
The thermal conductivity of the Kondo insulator SmB was measured
at low temperature, down to 70 mK, in magnetic fields up to 15 T, on single
crystals grown using both the floating-zone and the flux methods. The residual
linear term at is found to be zero in all samples, for
all magnetic fields, in agreement with previous studies. There is therefore no
clear evidence of fermionic heat carriers. In contrast to some prior data, we
observe a large enhancement of with increasing field. The effect of
field is anisotropic, depending on the relative orientation of field and heat
current (parallel or perpendicular), and with respect to the cubic crystal
structure. We interpret our data in terms of heat transport predominantly by
phonons, which are scattered by magnetic impurities.Comment: publish versio
What historical landfast ice observations tell us about projected ice conditions in Arctic archipelagoes and marginal seas under anthropogenic forcing
Arctic landfast ice extent and duration are examined from observations, ice assimilations,
ocean reanalyses and coupled models. From observations and
assimilations, it is shown that in areas where landfast ice conditions last
more than 5Â months the first-year ice typically grows to more than 2 m and is
rarely less than 1 m. The observed spatial distribution of landfast ice
closely matches assimilation products but less so for ocean reanalyses and
coupled models. Although models generally struggle to represent the landfast
ice necessary to emulate the observed import/export of sea ice in regions
favourable to landfast ice conditions, some do exhibit both a realistic
climatology and a realistic decline of landfast ice extent under an
anthropogenic forcing scenario. In these more realistic simulations,
projections show that an extensive landfast ice cover should remain for at
least 5Â months of the year, well into the end of the 21st century. This is in
stark contrast with the simulations that have an unrealistic emulation of
landfast ice conditions. In these simulations, slow and packed ice conditions
shrink markedly over the same period. In all simulations and in areas with
landfast ice that lasts more than 5Â months, the end-of-winter sea ice thickness
remains between 1 and 2 m, well beyond the second half of the century. It is
concluded that in the current generation of climate models, projections of
winter sea ice conditions in the Canadian Arctic Archipelago and the Laptev
Sea are overly sensitive to the representation of landfast ice conditions and
that ongoing development in landfast ice parameterization will likely better
constrain these projections.</p
Secretion of functional papain precursor from insect cells. Requirement for N-glycosylation of the pro-region.
The synthetic gene coding for the precursor of the cysteine protease papain (EC 3.4.22.2) has been expressed using the baculovirus/insect cell system. The prepropapain gene was cloned into the transfer vector IpDC125 behind the polyhedrin promoter. The recombinant construct was then incorporated by homologous recombination into the Autographa californiaca nuclear polyhedrosis virus genome. The host Spodoptera frugiperda Sf9 cells infected with the recombinant baculovirus secrete an enzymatically inactive N-glycosylated papain precursor. This zymogen could be activated in vitro to yield about 400 nmol of active papain per liter of culture. The recombinant active mature papain was enzymatically indistinguishable from natural papain but the precursor was not processed to the same amino acid residue. The insect cells also accumulated prepropapain and glycosylated propapain intracellularly. This accumulation was an indication that there are rate-limiting steps in the secretion of proteins from insect cells in this expression system. Characterization of mutants of the precursor has shown that entry into the secretory pathway and addition of carbohydrate are prerequisite conditions for the production and secretion of functional propapain
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