4,022 research outputs found
Confinement of two-dimensional excitons in a non-homogeneous magnetic field
The effective Hamiltonian describing the motion of an exciton in an external
non-homogeneous magnetic field is derived. The magnetic field plays the role of
an effective potential for the exciton motion, results into an increment of the
exciton mass and modifies the exciton kinetic energy operator. In contrast to
the homogeneous field case, the exciton in a non-homogeneous magnetic field can
also be trapped in the low field region and the field gradient increases the
exciton confinement. The trapping energy and wave function of the exciton in a
GaAs two-dimensional electron gas for specific circular magnetic field
configurations are calculated. The results show than excitons can be trapped by
non-homogeneous magnetic fields, and that the trapping energy is strongly
correlated with the shape and strength of the non-homogeneous magnetic field
profile.Comment: 9 pages, 12 figure
Exciton trapping in magnetic wire structures
The lateral magnetic confinement of quasi two-dimensional excitons into wire
like structures is studied. Spin effects are take into account and two
different magnetic field profiles are considered, which experimentally can be
created by the deposition of a ferromagnetic stripe on a semiconductor quantum
well with magnetization parallel or perpendicular to the grown direction of the
well. We find that it is possible to confine excitons into one-dimensional (1D)
traps. We show that the dependence of the confinement energy on the exciton
wave vector, which is related to its free direction of motion along the wire
direction, is very small. Through the application of a background magnetic
field it is possible to move the position of the trapping region towards the
edge of the ferromagnetic stripe or even underneath the stripe. The exact
position of this 1D exciton channel depends on the strength of the background
magnetic field and on the magnetic polarisation direction of the ferromagnetic
film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte
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
Production, breeding and potential of cowpea crop in Brazil.
About 50 years ago cowpea was reported as a relatively minor tropical legume. However, in the last years, it has been emerging as one of the most important food legume of the 21st century (SINGH, 2010). Brazil is not out of this panorama. Brazilian agriculture is undergoing major technological changes and, in addition, globalization in agribusiness has caused impacts on the production chain of several crops, particularly those heavily dependent on the use of a large volume of agricultural inputs, mainly fertilizers and pesticides. Such crops have had a higher production cost each year. On the other hand, this situation has brought new opportunities. Business farmers have sought new alternatives for their production arrangements. In this context cowpea constitutes one of the best options.bitstream/item/85620/1/Doc-216-Production.pd
The Specific Heat of a Ferromagnetic Film.
We analyze the specific heat for the vector model on a -dimensional
film geometry of thickness using ``environmentally friendly''
renormalization. We consider periodic, Dirichlet and antiperiodic boundary
conditions, deriving expressions for the specific heat and an effective
specific heat exponent, \alpha\ef. In the case of , for , by
matching to the exact exponent of the two dimensional Ising model we capture
the crossover for \xi_L\ra\infty between power law behaviour in the limit
{L\over\xi_L}\ra\infty and logarithmic behaviour in the limit
{L\over\xi_L}\ra0 for fixed , where is the correlation length in
the transverse dimensions.Comment: 21 pages of Plain TeX. Postscript figures available upon request from
[email protected]
Completeness and consistency of renormalisation group flows
We study different renormalisation group flows for scale dependent effective
actions, including exact and proper-time renormalisation group flows. These
flows have a simple one loop structure. They differ in their dependence on the
full field-dependent propagator, which is linear for exact flows. We
investigate the inherent approximations of flows with a non-linear dependence
on the propagator. We check explicitly that standard perturbation theory is not
reproduced. We explain the origin of the discrepancy by providing links to
exact flows both in closed expressions and in given approximations. We show
that proper-time flows are approximations to Callan-Symanzik flows. Within a
background field formalism, we provide a generalised proper-time flow, which is
exact. Implications of these findings are discussed.Comment: 33 pages, 15 figures, revtex, typos corrected, to be published in
Phys.Rev.
Pulsar J0453+1559: A Double Neutron Star System with a Large Mass Asymmetry
To understand the nature of supernovae and neutron star (NS) formation, as
well as binary stellar evolution and their interactions, it is important to
probe the distribution of NS masses. Until now, all double NS (DNS) systems
have been measured to have a mass ratio close to unity (q 0.91). Here we
report the measurement of the individual masses of the 4.07-day binary pulsar
J0453+1559 from measurements of the rate of advance of periastron and Shapiro
delay: The mass of the pulsar is 1.559(5) and that of its companion
is 1.174(4) ; q = 0.75. If this companion is also a neutron star
(NS), as indicated by the orbital eccentricity of the system (e=0.11), then its
mass is the smallest precisely measured for any such object. The pulsar has a
spin period of 45.7 ms and a spin derivative of 1.8616(7) x; from these
we derive a characteristic age of ~ 4.1 x years and a magnetic field of
~ 2.9 x G,i.e, this pulsar was mildly recycled by accretion of matter
from the progenitor of the companion star. This suggests that it was formed
with (very approximately) its current mass. Thus NSs form with a wide range of
masses, which is important for understanding their formation in supernovae. It
is also important for the search for gravitational waves released during a
NS-NS merger: it is now evident that we should not assume all DNS systems are
symmetric
Pulsar J1411+2551: A Low Mass New Double Neutron Star System
In this work, we report the discovery and characterization of PSR J1411+2551,
a new binary pulsar discovered in the Arecibo 327 MHz Drift Pulsar Survey. Our
timing observations of the radio pulsar in the system span a period of about
2.5 years. This timing campaign allowed a precise measurement of its spin
period (62.4 ms) and its derivative (9.6 0.7) ; from these, we derive a characteristic age of Gyr and a
surface magnetic field strength of 2.5 G. These numbers
indicate that this pulsar was mildly recycled by accretion of matter from the
progenitor of the companion star. The system has an eccentric ()
2.61 day orbit. This eccentricity allows a highly significant measurement of
the rate of advance of periastron, . Assuming general relativity accurately models the
orbital motion, this implies a total system mass M = . The minimum companion mass is and the maximum
pulsar mass is . The large companion mass and the orbital
eccentricity suggest that PSR J1411+2551 is a double neutron star system; the
lightest known to date including the DNS merger GW 170817. Furthermore, the
relatively low orbital eccentricity and small proper motion limits suggest that
the second supernova had a relatively small associated kick; this and the low
system mass suggest that it was an ultra-stripped supernova.Comment: Accepted for publication in APJ letter
New Discoveries from the Arecibo 327 MHz Drift Pulsar Survey Radio Transient Search
We present Clusterrank, a new algorithm for identifying dispersed
astrophysical pulses. Such pulses are commonly detected from Galactic pulsars
and rotating radio transients (RRATs), which are neutron stars with sporadic
radio emission. More recently, isolated, highly dispersed pulses dubbed fast
radio bursts (FRBs) have been identified as the potential signature of an
extragalactic cataclysmic radio source distinct from pulsars and RRATs.
Clusterrank helped us discover 14 pulsars and 8 RRATs in data from the Arecibo
327 MHz Drift Pulsar Survey (AO327). The new RRATs have DMs in the range pc cm and periods in the range s. The new
pulsars have DMs in the range pc cm and periods in the
range s, and include two nullers and a mode-switching object.
We estimate an upper limit on the all-sky FRB rate of day for
bursts with a width of 10 ms and flux density mJy. The DMs of all
new discoveries are consistent with a Galactic origin. In comparing statistics
of the new RRATs with sources from the RRATalog, we find that both sets are
drawn from the same period distribution. In contrast, we find that the period
distribution of the new pulsars is different from the period distributions of
canonical pulsars in the ATNF catalog or pulsars found in AO327 data by a
periodicity search. This indicates that Clusterrank is a powerful complement to
periodicity searches and uncovers a subset of the pulsar population that has so
far been underrepresented in survey results and therefore in Galactic pulsar
population models.Comment: 41 pages, 16 figures, 4 tables, accepted by ApJ; added minor
corrections to final ApJ proo
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