2,901 research outputs found
Quantum Critical Point in the Spin Glass-Kondo Transition in Heavy Fermion Systems
The Kondo-Spin Glass competition is studied in a theoretical model of a Kondo
lattice with an intra-site Kondo type exchange interaction treated within the
mean field approximation, an inter-site quantum Ising exchange interaction with
random couplings among localized spins and an additional transverse field in
the x direction, which represents a simple quantum mechanism of spin flipping.
We obtain two second order transition lines from the spin-glass state to the
paramagnetic one and then to the Kondo state. For a reasonable set of the
different parameters, the two second order transition lines do not intersect
and end in two distinct QCP.Comment: 20 pages; 1 figure; to appear in Physical Review
Quantum critical point in the spin glass-antiferromagnetism competition in Kondo-lattice systems
A theory is proposed to describe the competition among antiferromagnetism
(AF), spin glass (SG) and Kondo effect. The model describes two Kondo
sublattices with an intrasite Kondo interaction strength and an
interlattice quantum Ising interaction in the presence of a transverse field
. The interlattice coupling is a random Gaussian distributed variable
(with average and variance ) while the field is
introduced as a quantum mechanism to produce spin flipping. The path integral
formalism is used to study this fermionic problem where the spin operators are
represented by bilinear combinations of Grassmann fields. The disorder is
treated within the framework of the replica trick. The free energy and the
order parameters of the problem are obtained by using the static ansatz and by
choosing both and to allow, as previously,
a better comparison with the experimental findings.
The results indicate the presence of a SG solution at low and for
temperature ( is the freezing temperature). When is
increased, a mixed phase AF+SG appears, then an AF solution and finally a Kondo
state is obtained for high values of . Moreover, the behaviors of the
freezing and Neel temperatures are also affected by the relationship between
and the transverse field . The first one presents a slight
decrease while the second one decreases towards a Quantum Critical Point (QCP).
The obtained phase diagram has the same sequence as the experimental one for
, if is assumed to increase with , and
in addition, it also shows a qualitative agreement concerning the behavior of
the freezing and the Neel temperatures.Comment: 11 pages, 3 figures, accepted for publication in J. Phys.
Spin-glass phase transition and behavior of nonlinear susceptibility in the Sherrington-Kirkpatrick model with random fields
The behavior of the nonlinear susceptibility and its relation to the
spin-glass transition temperature , in the presence of random fields, are
investigated. To accomplish this task, the Sherrington-Kirkpatrick model is
studied through the replica formalism, within a one-step
replica-symmetry-breaking procedure. In addition, the dependence of the
Almeida-Thouless eigenvalue (replicon) on the random fields
is analyzed. Particularly, in absence of random fields, the temperature
can be traced by a divergence in the spin-glass susceptibility ,
which presents a term inversely proportional to the replicon . As a result of a relation between and , the
latter also presents a divergence at , which comes as a direct consequence
of at . However, our results show that, in the
presence of random fields, presents a rounded maximum at a temperature
, which does not coincide with the spin-glass transition temperature
(i.e., for a given applied random field). Thus, the maximum
value of at reflects the effects of the random fields in the
paramagnetic phase, instead of the non-trivial ergodicity breaking associated
with the spin-glass phase transition. It is also shown that still
maintains a dependence on the replicon , although in a more
complicated way, as compared with the case without random fields. These results
are discussed in view of recent observations in the LiHoYF
compound.Comment: accepted for publication in PR
Redesign participativo do aplicativo móvel Agritempo: a importância da interação usuário-desenvolvedor.
Este trabalho apresenta o impacto da aplicação do DCU no processo de desenvolvimento de um aplicativo móvel para o produtor agrÃcola por meio de uma validação colaborativa
New spectral data of some flavonoids from Deguelia hatschbachii A.M.G. Azevedo
From the roots of Deguelia hatschbachii, the known flavonoids scandenin (1), methyl robustate (2) and 4',5- dihydroxy-6-(3,3-dimethylalyll)-7-methoxy flavanone (3) were isolated and characterized by comparison of their spectroscopic data with those found in the literature. Now the inclusion of 1D- and 2D-NMR and MS/MS data has allowed the complete assignment of all hydrogen and carbon chemical shifts in their NMR spectra, as well as the elucidation of the fragmentation pathways of 1-3 in the mass spectrometer.Entre os flavonóides isolados de Deguelia hatschbachii¹ encontram-se escandenina (1), robustato de metila (2) e 4',5-diidroxi-6-(3,3-dimetilalila)-7-metoxiflavanona (3) que foram identificados pela comparação dos dados espectroscópicos previamente publicados. A obtenção de novos dados espectroscópicos (RMN, RMN-2D e EM/EM) permitiu a atribuição dos deslocamentos quÃmicos de todos os hidrogênios e carbonos nos espectros de RMN ¹H e 13C destas substâncias, assim como a elucidação dos caminhos de fragmentação de 1-3 no espectrômetro de massas.133137Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de NÃvel Superior (CAPES
As formigas são atraÃdas por folhas herbivoradas de Caryocar brasiliense Camb. (Caryocaraceae)?
Different profiles of secondary compounds are released by plants after herbivore attack. Many of these compounds are used by predators and parasitoids to locate herbivores that are damaging leaves. Such an induced indirect defense was tested with the Caryocar brasiliense-ant system in the dry season, when C. brasiliense has old leaves, and in the rainy season, when C. brasiliense has new leaves. A total of 20 plants were analyzed per season. Two opposite leaves of the same branch were selected for each plant. Approximately 40% of the area of one leaf was removed (treatment leaf) while the other leaf remained intact (control). The number of ants that visited each leaf was counted simultaneously for a period of 15 minutes and the mean difference in ant number was tested by paired t-test. The mean number of ants differed significantly between treatment and control only in the rainy season (t = 3.004, df = 19, p = 0.007). This finding suggests the presence of induced defense in this system only when the leaves are young. The study supports the Optimal Defense Theory since young leaves of C. brasiliense with artificial damage attracted significantly more ants than leaves without damage and represents the first evidence of an induced defense mechanism in the C. brasiliense-ant system.Diferentes compostos quÃmicos são liberados pelas plantas após o ataque dos herbÃvoros. Muitos desses compostos são usados por predadores e parasitoides para localizar os herbÃvoros que estão injuriando as plantas. Esse tipo de defesa induzida indireta foi testada no sistema Caryocar brasiliense e formigas em duas estações: seca (quando C. brasiliense está com folhas maduras) e chuvosa (quando C. brasiliense está com folhas jovens). Nós analisamos 20 plantas por estação do ano. Em cada planta nós selecionamos duas folhas opostas de um mesmo ramo. Nós removemos cerca de 40% da área de uma das folhas, deixando a outra folha intacta. Nós mensuramos simultaneamente o número de formigas que visitaram cada tipo de folha por 15 min e analisamos a diferença entre o número de formigas em cada tipo de folha através de teste t pareado. Nós observamos diferença significativa no número de formigas que patrulham as folhas tratamento (com herbivoria artificial) e controle (sem hervivoria artificial) apenas na estação chuvosa (t = 3,004, df = 19, p = 0,007). Isso sugere que existe defesa induzida nesse sistema somente quando as folhas são jovens. Nosso estudo corrobora a Teoria de Defesa Ótima já que apenas as folhas jovens de C. brasiliense com dano artificial atraÃram significativamente mais formigas do que as folhas sem danos. Esta é a primeira vez que mecanismos de defesa induzida são observados no sistema C. brasiliense-formigas
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