10,910 research outputs found
Experimental demonstration of a mu=-1 metamaterial lens for magnetic resonance imaging
In this work a mu=-1 metamaterial (MM) lens for magnetic resonance imaging
(MRI) is demonstrated. MRI uses surface coils to detect the radiofrequency(RF)
energy absorbed and emitted by the nuclear spins in the imaged object. The
proposed MM lens manipulates the RF field detected by these surface coils, so
that the coil sensitivity and spatial localization is substantially improved.
Beyond this specific application, we feel that the reported results are the
experimental confirmation of a new concept for the manipulation of RF field in
MRI, which paves the way to many other interesting applications.Comment: 9 pages, 3 figure
Percolation for the stable marriage of Poisson and Lebesgue
Let be the set of points (we call the elements of centers) of
Poisson process in , , with unit intensity. Consider the
allocation of to which is stable in the sense of Gale-Shapley
marriage problem and in which each center claims a region of volume . We prove that there is no percolation in the set of claimed sites if
is small enough, and that, for high dimensions, there is percolation
in the set of claimed sites if is large enough.Comment: revised version (only minor correction since v2), 16 pages, 3 figure
The Timing of Nine Globular Cluster Pulsars
We have used the Robert C. Byrd Green Bank Telescope to time nine previously
known pulsars without published timing solutions in the globular clusters M62,
NGC 6544, and NGC 6624. We have full timing solutions that measure the spin,
astrometric, and (where applicable) binary parameters for six of these pulsars.
The remaining three pulsars (reported here for the first time) were not
detected enough to establish solutions. We also report our timing solutions for
five pulsars with previously published solutions, and find good agreement with
past authors, except for PSR J1701-3006B in M62. Gas in this system is probably
responsible for the discrepancy in orbital parameters, and we have been able to
measure a change in the orbital period over the course of our observations.
Among the pulsars with new solutions we find several binary pulsars with very
low mass companions (members of the so-called "black widow" class) and we are
able to place constraints on the mass-to-light ratio in two clusters. We
confirm that one of the pulsars in NGC 6624 is indeed a member of the rare
class of non-recycled pulsars found in globular clusters. We also have measured
the orbital precession and Shapiro delay for a relativistic binary in NGC 6544.
If we assume that the orbital precession can be described entirely by general
relativity, which is likely, we are able to measure the total system mass
(2.57190(73) M_sun) and companion mass (1.2064(20) M_sun), from which we derive
the orbital inclination [sin(i) = 0.9956(14)] and the pulsar mass (1.3655(21)
M_sun), the most precise such measurement ever obtained for a millisecond
pulsar. The companion is the most massive known around a fully recycled pulsar.Comment: Published in ApJ; 33 pages, 5 figures, 7 table
Breakdown of the Fermi-liquid regime in the 2D Hubbard model from a two-loop field-theoretical renormalization group approach
We analyze the particle-hole symmetric two-dimensional Hubbard model on a
square lattice starting from weak-to-moderate couplings by means of the
field-theoretical renormalization group (RG) approach up to two-loop order.
This method is essential in order to evaluate the effect of the
momentum-resolved anomalous dimension which arises in the
normal phase of this model on the corresponding low-energy single-particle
excitations. As a result, we find important indications pointing to the
existence of a non-Fermi liquid (NFL) regime at temperature displaying
a truncated Fermi surface (FS) for a doping range exactly in between the
well-known antiferromagnetic insulating and the -wave singlet
superconducting phases. This NFL evolves as a function of doping into a
correlated metal with a large FS before the -wave pairing
susceptibility finally produces the dominant instability in the low-energy
limit.Comment: 9 pages, 9 figures; published in Phys. Rev.
Evidence for a Massive Neutron Star from a Radial-Velocity Study of the Companion to the Black Widow Pulsar PSR B1957+20
The most massive neutron stars constrain the behavior of ultra-dense matter,
with larger masses possible only for increasingly stiff equations of state.
Here, we present evidence that the black widow pulsar, PSR B1957+20, has a high
mass. We took spectra of its strongly irradiated companion and found an
observed radial-velocity amplitude of K_obs=324+/-3 km/s. Correcting this for
the fact that, due to the irradiation, the center of light lies inward relative
to the center of mass, we infer a true radial-velocity amplitude of K_2=353+/-4
km/s and a mass ratio q=M_PSR/M_2=69.2+/-0.8. Combined with the inclination
i=65+/-2 deg inferred from models of the lightcurve, our best-fit pulsar mass
is M_PSR=2.40+/-0.12 M_sun. We discuss possible systematic uncertainties, in
particular in the lightcurve modeling. Taking an upper limit of i<85 deg based
on the absence of radio eclipses at high frequency, combined with a
conservative lower-limit to the motion of the center of mass, K_2>343 km/s
(q>67.3), we infer a lower limit to the pulsar mass of M_PSR>1.66 M_sun.Comment: 7 pages, 3 figures, 1 table, accepted for publication in ApJ;
revision includes more detail on the spectral classification and discussion
of other recent high neutron-star masse
Goals, Strategies and First Discoveries of AO327, the Arecibo All-Sky 327 MHz Drift Pulsar Survey
We report initial results from AO327, a drift survey for pulsars with the
Arecibo telescope at 327 MHz. The first phase of AO327 will cover the sky at
declinations of -1 to 28 degrees, excluding the region within 5 degrees of the
Galactic plane, where high scattering and dispersion make low-frequency surveys
sub-optimal. We record data from a 57 MHz bandwidth with 1024 channels and 125
us sampling time. The 60 s transit time through the AO327 beam means that the
survey is sensitive to very tight relativistic binaries even with no
acceleration searches. To date we have detected 44 known pulsars with periods
ranging from 3 ms to 2.21 s and discovered 24 new pulsars. The new discoveries
include three millisecond pulsars, three objects with periods of a few tens of
milliseconds typical of young as well as mildly recycled pulsars, a nuller, and
a rotating radio transient. Five of the new discoveries are in binary systems.
The second phase of AO327 will cover the sky at declinations of 28 to 38
degrees. We compare the sensitivity and search volume of AO327 to the Green
Bank North Celestial Cap survey and the GBT350 drift survey, both of which
operate at 350 MHz.Comment: Accepted for publication in Ap
Características químicas de quatro solos do Rio Grande do Norte sob irrigação com águas salinas
Os processos de acumulação de sais são decorrentes do material de origem do solo, além das condições ambientais reinantes, com a interferência marcante do clima e relevo, tornando-se bastante pronunciados em zonas de clima árido e semi-árido do mundo, e potencializadas com o uso de águas de má qualidade na irrigação. Por isso, esse trabalho, objetivou avaliar alterações nas características químicas de quatro solos do Rio Grande do Norte irrigados com águas de níveis crescentes de condutividade elétrica (CE) e dois valores de relação de adsorção de sódio (RAS), cultivados com melão. O trabalho foi conduzido em casa de vegetação, utilizando-se quatro solos da região do Agropolo Assu/Mossoró. O delineamento experimental foi o de blocos ao acaso, em esquema fatorial 4 x 8 x 2, constituído pela combinação de quatro solos (Latossolo, Cambissolo, Argissolo e Neossolo Flúvico), oito valores de CE (100, 250, 500, 750, 1.250, 1.750, 2.250 e 3.000 mS/cm) e dois de RAS (4 e 12), com três repetições. Foram avaliados condutividade elétrica (CE), pH, teores de cátions trocáveis e percentagem de sódio trocável (PST) dos solos. O incremento da salinidade da água promoveu o aumento do pH, CE, teores de Ca, Mg, Na e K trocáveis, e a PST dos solos. O uso de águas salinas promoveu aumento na salinidade e sodicidade dos quatro solos estudados, fatores que contribuem para a degradação dos mesmos
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