64,507 research outputs found
Metallic Continuum Quantum Ferromagnets at Finite Temperature
We study via renormalization group (RG) and large N methods the problem of
continuum SU(N) quantum Heisenberg ferromagnets (QHF) coupled to gapless
electrons. We establish the phase diagram of the dissipative problem and
investigate the changes in the Curie temperature, magnetization, and magnetic
correlation length due to dissipation and both thermal and quantum
fluctuations. We show that the interplay between the topological term (Berry's
phase) and dissipation leads to non-trivial effects for the finite temperature
critical behavior.Comment: Corrected typos, new discussion of T=0 results, to appear in
Europhys. Let
Brane Cosmic String Compactification in Brans-Dicke Theory
We investigate an alternative compactification of extra dimensions using
local cosmic string in the Brans-Dicke gravity framework. In the context of
dynamical systems it is possible to show that there exist a stable field
configuration for the Einstein-Brans-Dicke equations. We explore the analogies
between this particular model and the Randall-Sundrum scenario.Comment: RevTex, 5 pages, no figures. To appear in the Physical Review
Towards an hybrid compactification with a scalar-tensor global cosmic string
We derive a solution of the gravitational equations which leads to a
braneworld scenario in six dimensions using a global cosmic string solution in
a low energy effective string theory framework. The final spacetime is composed
by one warped brane with topology and a power
law warp factor, and one noncompact extra dimension transverse to the brane. By
looking at the current experimental bounds, we find a range of parameters in
which, if the on-brane dimension has an acceptable size, it does not solve the
hierarchy problem. In another example this problem is smoothed by the
Brans-Dicke parameter.Comment: RevTex, 7 pages. New version to be published in the JCAP (2008
A time of flight method to measure the speed of sound using a stereo sound card
We present an inexpensive apparatus for measuring the speed of sound, with a
time of flight method, using a computer with a stereo sound board. Students
measure the speed of sound by timing the delay between the arrivals of a pulse
to two microphones placed at different distances from the source. It can serve
as a very effective demonstration, providing a quick measurement of the speed
of sound in air; we have used it with great success in Open Days in our
Department. It can also be used for a full fledged laboratory determination of
the speed of sound in air.Comment: Accepted for publication in The Physics Teache
Growth of seven perennial plant species adapted to the Brazilian Semi-Arid.
Growth of seven perennial plant species adapted to the Brazilian Semi-Arid. Abstract - Measuring plant height (y) and canopy diameter (x) in trees may not be an easy task, but stem diameter (z) can be more easily evaluated. This work's objective was two-fold: evaluate the growth of species adapted to the Brazilian Semi-Arid Region in the first two years of age, and obtain linear equations to estimate y and x from z, in those species. A random block design with four replications was employed. The values for x, z, and y were measured biannually from October/2003 to March/2005. Prosopis juliflora and Mimosa caesalpiniaefolia showed the highest stem diameter and plant height values, respectively, and both showed the highest canopy diameter. In the equations to estimate plant height from the stem diameter the value of the coefficient of determination (R2) ranged from 0.76 (Tamarindus indica and Leucaena leucocephala) to 0.92 (Prosopis juliflora and Azadirachta indica). In the equations that allowed to estimate the crown diameter from the stem diameter the R2 value ranged from 0.70 (Leucaena leucocephala) to 0.92 (Azadirachta indica). Crescimento de sete espĆ©cies vegetais perenes adaptadas ao Semi-Ćrido brasileiro. Resumo - A mensuraĆ§Ć£o da altura da planta (y) e do diĆ¢metro da copa (x) em Ć”rvores pode nĆ£o ser tarefa fĆ”cil, mas o diĆ¢metro do caule (z) pode ser avaliado mais facilmente. Este trabalho teve por objetivos avaliar o crescimento de espĆ©cies adaptadas ao Semi-Ćrido brasileiro, nos primeiros dois anos de idade, e obter equaƧƵes lineares para estimar y e x a partir de z, nessas espĆ©cies. Utilizou-se o delineamento de blocos ao acaso com quatro repetiƧƵes. Os valores de x, z e y foram medidos semestralmente de outubro/2003 a marƧo/2005. Prosopis juliflora e Mimosa caesalpiniaefolia apresentaram os maiores diĆ¢metros do caule e altura da planta, respectivamente, e ambas apresentaram o maior diĆ¢metro da copa. Nas equaƧƵes para estimar a altura da planta a partir do diĆ¢metro do caule, o valor do coeficiente de determinaĆ§Ć£o (R2) variou de 0,76 (Tamarindus indica and Leucaena leucocephala) a 0,92 (Prosopis juliflora and Azadirachta indica). Nas equaƧƵes que permitiram estimar o diĆ¢metro da copa a partir do diĆ¢metro do caule, o valor de R2 variou de 0,70 (Leucaena leucocephala) a 0,92 (Azadirachta indica)
PopulaƧƵes de plantas e adubaĆ§Ć£o orgĆ¢nica em solo degradado na cultura do amendoim
bitstream/CNPA/19705/1/BOLETIM51.pd
- ā¦