363 research outputs found
Thermodynamic transport theory of spin waves in ferromagnetic insulators
We use the Boltzmann transport theory in the relaxation time approximation to
describe the thermal transport of spin waves in a ferromagnet. By treating spin
waves as magnon excitations we are able to compute analytically and numerically
the coefficients of the constitutive thermo-magnetic transport equations. As a
main result, we find that the absolute thermo-magnetic power coefficient
, relating the gradient of the potential of the magnetization
current and the gradient of the temperature, in the limit of low temperature
and low field, is a constant . The theory
correctly describes the low-temperature and magnetic-field dependencies of spin
Seebeck experiments. Furthermore, the theory predicts that in the limit of very
low temperatures the spin Peltier coefficient , relating the heat and
the magnetization currents, tends to a finite value which depends on the
amplitude of the magnetic field. This indicates the possibility to exploit the
spin Peltier effect as an efficient cooling mechanism in cryogenics.Comment: (v1) PDFLaTeX, 10 pages, 5 figures, 1 table, submitted to Phys. Rev.
B; (v2) PDFLaTeX, 12 pages, 5 figures, 1 table; Secs. I, III, IV highly
improved, old-Sec. VI splitted into two new Secs. VI-VII, references added,
typos corrected, revised version re-submitted to Phys. Rev. B; (v3) PDFLaTeX,
12 pages, 5 figures, 1 table; Refs. [3], [27], [36] updated, final version
published in Phys. Rev.
Probing Gravitational Lensing of the CMB with SDSS-IV Quasars
We study the cross-correlation between the Planck CMB lensing convergence map
and the eBOSS quasar overdensity obtained from the Sloan Digital Sky Survey
(SDSS) IV, in the redshift range . We detect the CMB lensing
convergence-quasar cross power spectrum at significance. The cross
power spectrum provides a quasar clustering bias measurement that is expected
to be particularly robust against systematic effects. The redshift distribution
of the quasar sample has a median redshift , and an effective
redshift about . The best fit bias of the quasar sample is , corresponding to a host halo mass of . This is broadly
consistent with the previous literature on quasars with a similar redshift
range and selection. Since our constraint on the bias comes from the
cross-correlation between quasars and CMB lensing, we expect it to be robust to
a wide range of possible systematic effects that may contaminate the auto
correlation of quasars. We checked for a number of systematic effects from both
CMB lensing and quasar overdensity, and found that all systematics are
consistent with null within . The data is not sensitive to a possible
scale dependence of the bias at present, but we expect that as the number of
quasars increases (in future surveys such as DESI), it is likely that strong
constraints on the scale dependence of the bias can be obtained.Comment: 8 pages, 6 figures, 1 table; matches published version on MNRA
The bright optical companion to the eclipsing millisecond pulsar in NGC 6397
We report the possible optical identification of the companion to the
eclipsing millisecond pulsar PSR J1740-5340 in the globular cluster NGC 6397. A
bright variable star with an anomalous red colour and optical variability which
nicely correlates to the orbital period of the pulsar has been found close to
the pulsar position. If confirmed, the optical light curve, reminiscent of
tidal distorsions similar to those observed in detached and contact binaries,
support the idea that this is the first case of a Roche lobe filling companion
to a millisecond pulsar.Comment: 9 pages, 4 embedded figures, submitted to ApJ Letter
Anergy in self-directed B lymphocytes from a statistical mechanics perspective
The ability of the adaptive immune system to discriminate between self and
non-self mainly stems from the ontogenic clonal-deletion of lymphocytes
expressing strong binding affinity with self-peptides. However, some
self-directed lymphocytes may evade selection and still be harmless due to a
mechanism called clonal anergy. As for B lymphocytes, two major explanations
for anergy developed over three decades: according to "Varela theory", it stems
from a proper orchestration of the whole B-repertoire, in such a way that
self-reactive clones, due to intensive interactions and feed-back from other
clones, display more inertia to mount a response. On the other hand, according
to the `two-signal model", which has prevailed nowadays, self-reacting cells
are not stimulated by helper lymphocytes and the absence of such signaling
yields anergy. The first result we present, achieved through disordered
statistical mechanics, shows that helper cells do not prompt the activation and
proliferation of a certain sub-group of B cells, which turn out to be just
those broadly interacting, hence it merges the two approaches as a whole (in
particular, Varela theory is then contained into the two-signal model). As a
second result, we outline a minimal topological architecture for the B-world,
where highly connected clones are self-directed as a natural consequence of an
ontogenetic learning; this provides a mathematical framework to Varela
perspective. As a consequence of these two achievements, clonal deletion and
clonal anergy can be seen as two inter-playing aspects of the same phenomenon
too
Non-equilibrium thermodynamics of the spin Seebeck and spin Peltier effects
We study the problem of magnetization and heat currents and their associated
thermodynamic forces in a magnetic system by focusing on the magnetization
transport in ferromagnetic insulators like YIG. The resulting theory is applied
to the longitudinal spin Seebeck and the spin Peltier effects. By focusing on
the specific geometry with one YIG layer and one Pt layer, we obtain the
optimal conditions for generating large magnetization currents into Pt or large
temperature effects in YIG. The theoretical predictions are compared with
experiments from the literature permitting to derive the values of the
thermomagnetic coefficients of YIG: the magnetization diffusion length m and the absolute thermomagnetic power coefficient TK.Comment: accepted for publication on Physical Review
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