2,878 research outputs found
Pressure Induced Quantum Phase Transitions
A quantum critical point is approached by applying pressure in a number of
magnetic metals. The observed dependence of Tc on pressure necessarily means
that the magnetic energy is coupled to the lattice. A first order phase
transition occurs if this coupling exceeds a critical value: this is inevitable
if diverges as Tc approaches zero. It is argued that this is the cause of the
first order transition that is observed in many systems. Using Landau theory we
obtain expressions for the boundaries of the region where phase separation
occurs that agree well with experiments done on MnSi and other materials. The
theory can be used to obtain very approximate values for the temperature and
pressure at the tricritical point in terms of quantities measured at ambient
pressure and the measured values of along the second order line. The values of
the tricritical temperature for various materials obtained from Landau theory
are too low but it is shown that the predicted values will rise if the effects
of fluctuations are included.Comment: 12 pages including figure
Constructing electrically charged Riemannian manifolds with minimal boundary, prescribed asymptotics, and controlled mass
In 2015, Mantoulidis and Schoen constructed -dimensional asymptotically Euclidean manifolds with non-negative scalar curvature whose ADM mass can be made arbitrarily close to the optimal value of the Riemannian Penrose Inequality, while the intrinsic geometry of the outermost minimal surface can be "far away" from being round. The resulting manifolds, called \emph{extensions}, are geometrically not "close" to a spatial Schwarzschild manifold. This suggests instability of the Riemannian Penrose Inequality. Their construction was later adapted to dimensions by Cabrera Pacheco and Miao, suggesting instability of the higher dimensional Riemannian Penrose Inequality. In recent papers by Alaee, Cabrera Pacheco, and Cederbaum and by Cabrera Pacheco, Cederbaum, and McCormick, a similar construction was performed for asymptotically Euclidean, electrically charged Riemannian manifolds and for asymptotically hyperbolic Riemannian manifolds, respectively, obtaining -dimensional extensions that suggest instability of the Riemannian Penrose Inequality with electric charge and of the conjectured asymptotically hyperbolic Riemannian Penrose Inequality in dimensions. This paper combines and generalizes all the aforementioned results by constructing suitable asymptotically hyperbolic or asymptotically Euclidean extensions with electric charge in dimensions for . Besides suggesting instability of a naturally conjecturally generalized Riemannian Penrose Inequality, the constructed extensions give insights into an ad hoc generalized notion of Bartnik mass, similar to the Bartnik mass estimate for minimal surfaces proven by Mantoulidis and Schoen via their extensions, and unifying the Bartnik mass estimates in the various scenarios mentioned above
Manejo Ecológico de Capoeiras e Produção de Alimentos por Agricultores Familiares no Bioma Amazônia do Estado do Maranhão.
O objetivo desse trabalho foi o de avaliar o efeito do uso de Crotalária juncea como fonte de adubo verde na cultura do milho sob sistema de plantio direto em capoeiras trituradas no bioma Amazônia. A metodologia consistiu na realização de um ensaio de síntese em área de produtor familiar do município Santa Luzia do Paruá, Estado do Maranhão. Utilizou-se o delineamento inteiramente casualizado, com cinco tratamentos e cinco repetições. Os resultados obtidos mostraram que o uso de Crotalária juncea consorciada com a cultura de milho adicionou em média 64,02 Kg de N. ha-1 e um rendimento médio de grãos de 745 kg.ha-1. Portanto, a técnica de adubação verde pode ser recomendada no manejo ecológico de capoeiras.Edição dos anais do 6º Congresso Brasileiro de Agroecologia; 2º Congresso Latinoamericano de Agroecologia, Curitiba, nov. 2009
Manejo ecológico de capoeiras e produção de alimentos por agricultores familiares no bioma Amazônia do estado do Maranhão.
O objetivo desse trabalho foi o de avaliar o efeito do uso de Crotalária juncea como fonte de adubo verde na cultura do milho sob sistema de plantio direto em capoeiras trituradas no bioma Amazônia. A metodologia consistiu na realização de um ensaio de síntese em área de produtor familiar do município Santa Luzia do Paruá, Estado do Maranhão. Utilizou-se o delineamento inteiramente casualizado, com cinco tratamentos e cinco repetições. Os resultados obtidos mostraram que o uso de Crotalária juncea consorciada com a cultura de milho adicionou em média 64,02 Kg de N. ha-1 e um rendimento médio de grãos de 745 kg.ha-1. Portanto, a técnica de adubação verde pode ser recomendada no manejo ecológico de capoeiras.Publicado também na Revista Brasileira de Agroecologia, v. 4, n. 2, p. 2144-2148, nov. 2009. Disponível em: http://www.aba-agroecologia.org.br/ojs2/index.php/rbagroecologia/article/viewFile/8007/571
Carrier-induced ferromagnetism in n-type ZnMnAlO and ZnCoAlO thin films at room temperature
The realization of semiconductors that are ferromagnetic above room
temperature will potentially lead to a new generation of spintronic devices
with revolutionary electrical and optical properties. Transition temperatures
in doped ZnO are high but, particularly for Mn doping, the reported moments
have been small. We show that by careful control of both oxygen deficiency and
aluminium doping the ferromagnetic moments measured at room temperature in
n-type ZnMnO and ZnCoO are close to the ideal values of 5mB and 3mB
respectively. Furthermore a clear correlation between the magnetisation per
transition metal ion and the ratio of the number of carriers to the number of
transition metal donors was established as is expected for carrier induced
ferromagnetism for both the Mn and Co doped films. The dependence of the
magnetisation on carrier density is similar to that predicted for the
transition temperature for a dilute magnetic semiconductor in which the
exchange between the transition metal ions is through the free carriers.Comment: 14 pages pd
Evidence for polarons in iron pnictides of the Ln-1111 and AE-122 families
Examination of the electrical resistivities of iron pnictides shows that they
can be accounted by conduction by polarons. Their activation energies show a
linear behaviour with the critical temperatures of the spin density waves
(SDW), T*, as both vary with pressure. The slope matches the ratio SDW gap to
T*, while the intercept can be related to the transition temperature of the
lattice distortion, T0. An adapted Landau free energy predicts the observed
order of the transitions, according to which is higher, T* or T0. Simple
arguments favour combined Jahn-Teller antiferromagnetic bipolarons.Comment: 14 pages with 4 Figure
Magneto-optical properties of Co/ZnO multilayer films
Multilayer films of ZnO with Co were deposited on glass substrates then
annealed in a vacuum. The magnetisation of the films increased with annealing
but not the magnitude of the magneto-optical signals. The dielectric functions
for the films were calculated using the MCD spectra. A Maxwell Garnett theory
of a metallic Co/ZnO mixture is presented. The extent to which this explains
the MCD spectra taken on the films is discussed.Comment: This paper was presented at ICM (2009) and is accepted in this form
for the proceeding
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