566 research outputs found
Impacts des barrages sur les caractéristiques des débits moyens annuels en fonction du mode de gestion et de la taille des bassins versants au Québec
Nous avons comparĂ© les impacts des barrages sur les caractĂ©ristiques (volume dâĂ©coulement-frĂ©quence, variabilitĂ© interannuelle et forme de courbe de distribution) des dĂ©bits moyens annuels dans les trois rĂ©gimes rĂ©gularisĂ©s observĂ©s (inversion, homogĂ©nĂ©isation et type naturel) au QuĂ©bec. Nous avons appliquĂ© la mĂ©thode de proportionnalitĂ© qui consiste Ă comparer les caractĂ©ristiques des dĂ©bits des riviĂšres naturelles et celles des riviĂšres rĂ©gularisĂ©es en fonction de la taille des bassins versants. En ce qui concerne le volume dâĂ©coulement-frĂ©quence et sa variabilitĂ© interannuelle, le changement a Ă©tĂ© observĂ© seulement en rĂ©gime dâinversion. Il se traduit par une baisse des dĂ©bits moyens annuels durant les annĂ©es hydrologiques sĂšches et une variabilitĂ© interannuelle relativement forte par rapport aux riviĂšres naturelles. Ces changements sont attribuĂ©s principalement au mode de gestion des rĂ©servoirs car on lĂąche moins dâeau durant ces annĂ©es hydrologiques sĂšches. Enfin, les changements des coefficients dâasymĂ©trie et dâaplatissement ont Ă©tĂ© observĂ©s surtout en rĂ©gime dâhomogĂ©nĂ©isation. Cette Ă©tude dĂ©montre que les barrages peuvent modifier toutes les caractĂ©ristiques des dĂ©bits moyens annuels contrairement Ă lâopinion couramment admise.We compared the impacts of dams on the characteristics (magnitude-frequency, inter-annual variability and distribution curve shape) of the mean annual flows in three regulated flow regimes (inversion, homogenization and natural type) in QuĂ©bec. We applied the âproportionality methodâ, which consists of comparing the flow characteristics of natural rivers to regulated rivers according to watershed size. A change in the flow-frequency volume and its inter-annual variability was observed only in the inversion flow regime. This result translates into a decrease in average annual flows during dry hydrological years and a relatively high inter-annual variability relative to natural rivers. These changes are mainly ascribed to the reservoir management mode because less water is released during dry hydrological years. Finally, the changes of the coefficients of asymmetry and skewness are particularly observed in homogenization flow regime. This study shows that, contrary to the commonly accepted opinion, dams can alter all the characteristics of annual average flows
Redox Modulations, Antioxidants, and Neuropsychiatric Disorders
Although antioxidants, redox modulations, and neuropsychiatric disorders have been widely studied for many years, the field would benefit from an integrative and corroborative review. Our primary objective is to delineate the biological significance of compounds that modulate our redox status (i.e., reactive species and antioxidants) as well as outline their current role in brain health and the impact of redox modulations on the severity of illnesses. Therefore, this review will not enter into the debate regarding the perceived medical legitimacy of antioxidants but rather seek to clarify their abilities and limitations. With this in mind, antioxidants may be interpreted as natural products with significant pharmacological actions in the body. A renewed understanding of these often overlooked compounds will allow us to critically appraise the current literature and provide an informed, novel perspective on an important healthcare issue. In this review, we will introduce the complex topics of redox modulations and their role in the development of select neuropsychiatric disorders
Nernst and Seebeck Coefficients of the Cuprate SuperconductorYBaCuO: A Study of Fermi Surface Reconstruction
The Seebeck and Nernst coefficients and of the cuprate
superconductor YBaCuO (YBCO) were measured in a single crystal with
doping in magnetic fields up to H = 28 T. Down to T=9 K,
becomes independent of field by T, showing that superconducting
fluctuations have become negligible. In this field-induced normal state,
and are both large and negative in the limit, with the
magnitude and sign of consistent with the small electron-like Fermi
surface pocket detected previously by quantum oscillations and the Hall effect.
The change of sign in at K is remarkably similar to that
observed in LaBaCuO, LaNdSrCuO and
LaEuSrCuO, where it is clearly associated with the onset
of stripe order. We propose that a similar density-wave mechanism causes the
Fermi surface reconstruction in YBCO.Comment: Final version accepted for publication in Phys. Rev. Lett. New title,
shorter abstract, minor revision of text and added reference
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
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
Maintenance and broadening of the oceanâs salinity distribution by the water cycle
The global water cycle leaves an imprint on ocean salinity through evaporation and precipitation. It has been proposed that observed changes in salinity can be used to infer changes in the water cycle. Here salinity is characterized by the distribution of water masses in salinity coordinates. Only mixing and sources and sinks of freshwater and salt can modify this distribution. Mixing acts to collapse the distribution, making saline waters fresher and fresh waters more saline. Hence, in steady state, there must be net precipitation over fresh waters and net evaporation over saline waters. A simple model is developed to describe the relationship between the breadth of the distribution, the water cycle, and mixingâthe latter being characterized by an e-folding time scale. In both observations and a state-of-the-art ocean model, the water cycle maintains a salinity distribution in steady state with a mixing time scale of the order of 50 yr. The same simple model predicts the response of the salinity distribution to a change in the water cycle. This study suggests that observations of changes in ocean salinity could be used to infer changes in the hydrological cycle
Lifshitz critical point in the cuprate superconductor YBa2Cu3Oy from high-field Hall effect measurements
The Hall coefficient R_H of the cuprate superconductor YBa2Cu3Oy was measured
in magnetic fields up to 60 T for a hole concentration p from 0.078 to 0.152,
in the underdoped regime. In fields large enough to suppress superconductivity,
R_H(T) is seen to go from positive at high temperature to negative at low
temperature, for p > 0.08. This change of sign is attributed to the emergence
of an electron pocket in the Fermi surface at low temperature. At p < 0.08, the
normal-state R_H(T) remains positive at all temperatures, increasing
monotonically as T \to 0. We attribute the change of behaviour across p = 0.08
to a Lifshitz transition, namely a change in Fermi-surface topology occurring
at a critical concentration p_L = 0.08, where the electron pocket vanishes. The
loss of the high-mobility electron pocket across p_L coincides with a ten-fold
drop in the conductivity at low temperature, revealed in measurements of the
electrical resistivity at high fields, showing that the so-called
metal-insulator crossover of cuprates is in fact driven by a Lifshitz
transition. It also coincides with a jump in the in-plane anisotropy of ,
showing that without its electron pocket the Fermi surface must have strong
two-fold in-plane anisotropy. These findings are consistent with a
Fermi-surface reconstruction caused by a unidirectional spin-density wave or
stripe order.Comment: 16 pages, 13 figures, see associated Viewpoint: M. Vojta, Physics 4,
12 (2011
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
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