Some aspects of electronic topological transition in 2D system on a square lattice. Excitonic ordered states

We study the ordered "excitonic" states which develop around the quantum critical point (QCP) associated with the electronic topological transition (ETT) in a 2D electron system on a square lattice. We consider the case of hopping beyond nearest neighbors when ETT has an unusual character. We show that the amplitude of the order parameter (OP) and of the gap in the electron spectrum increase with increasing the distance from the QCP, \delta_c - \delta, where \delta = 1-n and "n" is an electron concentration. Such a behavior is different from the ordinary case when OP and the gap decrease when going away from the point which is a motor for instability. The gap opens at "hot spots" and extends untill the saddle points (SP) whatever is the doping concentration. The spectrum gets a characteristic flat shape as a result of hybrydization effect in the vicinity of two different SP's. The shape of the spectrum and the angle dependence of the gap have a striking similarity with the features observed in the normal state of the underdoped high-T$_c$ cuprates. We discuss also details about the phase diagram and the behaviour of the density of states.Comment: 15 pages, 14 EPS figures included, EPJ style included, added references, changed conten

Various ordered states in a 2D interacting electron system close to an electronic topological transition

We consider a 2D electron system on a square lattice with hopping beyond nearest neighbors. The existence of the quantum critical point associated with an electronic topological transition in the noninteracting system results in density wave (DW) and high temperature d-wave superconducting (dSC) instabilities in the presence of an exchange interaction J. We analyse different DW ordering such as isotropic Spin DW (SDW), d-wave SDW, isotropic Charge DW (CDW) and d-wave CDW. The coexistence of dSC and SDW orders leads necessary to the existence of a third order which is a pi triplet superconducting (PTS) order. A new phase diagram with a mixed phase of SDW, dSC and PTS order is found. The theory is applied to high-Tc cuprates.Comment: 2 pages, 1 figure, submitted to LT22 (Physica B

Effective action for the Kondo lattice model. New approach for S=1/2

In the partition function of the Kondo lattice, spin matrices are exactly replaced by bilinear combinations of Fermi operators with the purely imaginary chemical potential lambda=-i.pi.T/2 (Popov representation). This new representation of spin operators allows one to introduce new Green's functions with Matsubara frequencies 2.pi.T(n+1/4) for S=1/2. A simple temperature diagram technique is constructed with the path integral method. This technique is standard and does not contain the complicated combinatoric rules characteristic of most of the known variants of the diagram techniques for spin systems. The effective action for the almost antiferromagnetic Kondo lattice is derived.Comment: 7 pages, Proceedings of SCES98/Paris; one reference adde

The Kondo-Hubbard model at half-filling

We have analyzed the antiferromagnetic (J>0) Kondo-Hubbard lattice with the band at half-filling by means of a perturbative approach in the strong coupling limit, the small parameter is an arbitrary tight-binding band. The results are valid for any band shape and any dimension. We have obtained the energies of elementary charge and spin excitations as well as the magnetic correlations in order to elucidate the magnetic and charge behavior of the Kondo lattice at half-filling. Finally, we have briefly analyzed the ferromagnetic case (J<0), which is shown to be equivalent to an effective antiferromagnetic Heisenberg model.Comment: 4 pages, Proceedings of SCES98/Pari

Spin - glass transition in Kondo lattice with quenched disorder

We use the Popov-Fedotov representation of spin operators to construct an effective action for a Kondo lattice model with quenched disorder at finite temperatures. We study the competition between the Kondo effect and frozen spin order in Ising-like spin glass. We present the derivation of new mean-field equations for the spin-glass order parameter and analyze the effects of screening of localized spins by conduction electrons on the spin-glass phase transition.Comment: 6 pages, jetpl style included, to appear in JETP Letter

Intermediate coupling fixed point study in the overscreened regime of generalized multichannel SU(N) Kondo models

We study a generalized multichannel single-impurity Kondo model, in which the impurity spin is described by a representation of the SU(N) group which combines bosonic and fermionic degrees of freedom. The impurity spin states are described by Abrikosov pseudofermions, and we make use of a method initiated by Popov and Fedotov which allows a proper handling of the fermionic constraint. The partition function is derived within a path integral approach. We use renormalization group techniques to calculate the $\beta$ scaling function perturbatively in powers of the Kondo coupling constant, which is justified in the weak coupling limit. The truncated expansion is valid in the overscreened (Nozieres-Blandin) regime, for an arbitrary SU(N) group and any value of the parameters characterizing the impurity spin representation. The intermediate coupling fixed point is identified. We derive the temperature dependence of various physical quantities at low T, controlled by a unique critical exponent, and show that the physics of the system in the overscreened regime governed by the intermediate coupling fixed point is characterized by a non-Fermi liquid behavior. Our results are in accordance with those obtained by other methods, as Bethe ansatz and boundary conformal field theory, in the case of various impurity spin symmetries. We establish in a unified way that the Kondo models in which the impurity spin is described successively by a fundamental, symmetric, antisymmetric and mixed symmetry representation yield all the same low-energy physics in the overscreened regime. Possible generalizations of the analysis we present to the case of arbitrary impurity spin representations of SU(N) are also discussed.Comment: 21 pages, 7 figures, REVTeX; final version accepted for publicatio

Iron bioavailability in two commercial cultivars of wheat: a comparison between wholegrain and white flour and the effects of nicotianamine and 2'-deoxymugineic acid on iron uptake into Caco-2 cells

Iron bioavailability in unleavened white and wholegrain bread made from two commercial wheat varieties was assessed by measuring ferritin production in Caco-2 cells. The breads were subjected to simulated gastrointestinal digestion and the digests applied to the Caco-2 cells. Although Riband grain contained a lower iron concentration than Rialto, iron bioavailability was higher. No iron was taken up by the cells from white bread made from Rialto flour or from wholegrain bread from either variety, but Riband white bread produced a small ferritin response. The results probably relate to differences in phytate content of the breads, although iron in soluble monoferric phytate was demonstrated to be bioavailable in the cell model. Nicotianamine, an iron chelator in plants involved in iron transport, was a more potent enhancer of iron uptake into Caco-2 cells than ascorbic acid or 2'-deoxymugineic acid, another metal chelator present in plants

Is international agricultural research a global public good? : The case of rice biofortification

The status of international agricultural research as a global public good (GPG) has been widely accepted since the Green Revolution of the 1960s and 1970s. While the term was not used at the time of its creation, the Consultative Group on International Agricultural Research (CGIAR) system that evolved at that time has been described as a 'prime example of the promise, performance and perils of an international approach to providing GPGs'. Contemporary literature on international agricultural research as a GPG tends to support this view and focuses on how to operationalize the concept. This paper adopts a different starting point and questions this conceptualization of the CGIAR and its outputs. It questions the appropriateness of such a 'neutral' concept to a system born of the imperatives of Cold War geopolitics, and shaped by a history of attempts to secure its relevance in a changing world. This paper draws on a multi-sited, ethnographic study of a research effort highlighted by the CGIAR as an exemplar of GPG-oriented research. Behind the ubiquitous language of GPGs, 'partnership' and 'consensus', however, new forms of exclusion and restriction are emerging within everyday practice, reproducing North-South inequalities and undermining the ability of these programmes to respond to the needs of projected beneficiaries

Strategies for biofortification in Brazil.

Micronutrient deficiencies in the developing world. Biofortification as an intervention strategy. Brazil's biofortification programs. Progress in some individual crops. Conclusion