Ising t-J model close to half filling: A Monte Carlo study

Within the recently proposed doped-carrier representation of the projected lattice electron operators we derive a full Ising version of the t-J model. This model possesses the global discrete Z_2 symmetry as a maximal spin symmetry of the Hamiltonian at any values of the coupling constants, t and J. In contrast, in the spin anisotropic limit of the t-J model, usually referred to as the t-J_z model, the global SU(2) invariance is fully restored at J_z=0, so that only the spin-spin interaction has in that model the true Ising form. We discuss a relationship between those two models and the standard isotropic t-J model. We show that the low-energy quasiparticles in all three models share the qualitatively similar properties at low doping and small values of J/t. The main advantage of the proposed Ising t-J model over the t-J_z one is that the former allows for the unbiased Monte Carlo calculations on large clusters of up to 10^3 sites. Within this model we discuss in detail the destruction of the antiferromagnetic order by doping as well as the interplay between the AF order and hole mobility. We also discuss the effect of the exchange interaction and that of the next nearest neighbour hoppings on the destruction of the AF order at finite doping. We show that the short-range AF order is observed in a wide range of temperatures and dopings, much beyond the boundaries of the AF phase. We explicitly demonstrate that the local no double occupancy constraint plays the dominant role in destroying the magnetic order at finite doping. Finally, a role of inhomogeneities is discussed.Comment: 24 pages, 10 figure

The structure of chromospheres around late-type giants and supergiants

Observations alpha Tau (K5III) and beta Gru (M2II) made at high resolution are used to confirm line identifications of features blended at low resolution. The high resolution spectra allow selected pairs of lines to be used to find the electron density and the opacity. Methods for determining these factors and the usual emission measure are presented. The electron density and opacity can be used together with the emission measure to place constraints on the structure of the atmosphere. The line formation processes are briefly discussed. Photo-excitation by strong lines appears to be important in these late type atmospheres

Ward Identities and chiral anomalies for coupled fermionic chains

Coupled fermionic chains are usually described by an effective model written in terms of bonding and anti-bonding spinless fields with linear dispersion in the vicinities of the respective Fermi points. We derive for the first time exact Ward Identities (WI) for this model, proving the existence of chiral anomalies which verify the Adler-Bardeen non-renormalization property. Such WI are expected to play a crucial role in the understanding of the thermodynamic properties of the system. Our results are non-perturbative and are obtained analyzing Grassmann functional integrals by means of Constructive Quantum Field Theory methods.Comment: TeX file, 26 pages, 7 figures. Published version, new section added to answer referee remarks and derive the Ward Identites, no modifications in the main resul

Psicologia ambiental e agroecologia: apropriação do espaço por famílias de agricultores.

Trabalhos anteriores (Bassani, Silveira e Ferraz, 2003; Bassani, Ferraz e Silveira, 2004)assinalaram contribuições da Psicologia Ambiental para a Agroecologia, através de metodologias participativas. Este trabalho é um estudo piloto sobre apropriação de espaço e apego ao lugar por famílias de agricultores do município de Araras (SP). O modelo dual de apropriação de espaço (Pol, 2002) envolve ação/transformação do espaço e identificação simbólica e, a partir deste processo, identificam-se características do apego ao lugar. Resultados indicam diferenças entre as famílias nos processos de ação/transformação. Do ponto de vista simbólico, não há diferenças entre as famílias: são mantidos símbolos de identidade da família e da região. Discute-se o papel de políticas públicas na apropriação de espaço dos agricultores e suas famílias

Métodos participativos e agroecologia familiar.

O presente artigo propõe a reflexão sobre a abordagem participativa relacionada ao ambiente complexo da prática agroecológica. Considera-se, que pesquisas participativas assentadas em arranjos metodológicos desta natureza destacam as condições reais de produção familiar e permitem apreender os distintos modos de vida e aspectos particulares presentes na reprodução das famílias rurais. Ademais, seus resultados possibilitam aos agricultores revalorizar suas potencialidades e intensificar o atendimento das suas próprias necessidades

The modulated spin liquid: a new paradigm for URu2_2Si2_2

We argue that near a Kondo breakdown critical point, a spin liquid with spatial modulations can form. Unlike its uniform counterpart, we find that this occurs via a second order phase transition. The amount of entropy quenched when ordering is of the same magnitude as for an antiferromagnet. Moreover, the two states are competitive, and at low temperatures are separated by a first order phase transition. The modulated spin liquid we find breaks $Z_4$ symmetry, as recently seen in the hidden order phase of URu$_2$Si$_2$. Based on this, we suggest that the modulated spin liquid is a viable candidate for this unique phase of matter.Comment: 4 pages, 2 figure

Thymic Squamous Cell Carcinoma? Do you confirm this diagnosis?

Introduction In dogs thymic carcinoma is considered rare and distinguishes itself from thymoma by its cytologically malignant features, extensive local invasion, and a substantial potential for metastasis. Surgical excision is the treatment of choice for most thymic tumours, with the exception of lymphoma. Medical History A 9 years old cross-breed male dog was presented due to cough and respiratory distress with duration of 4 weeks. The animal presented with inspiratory dyspnea and cyanosis. A mass, dorsal to the pharynx, was diagnosed after clinical and radiographical examination, suspected of thyroid neoplasia. Cytology was performed and revealed cells consistent with malignancy, apparently carcinoma with high parameters of aggressiveness. As a form of treatment and to obtain a definitive diagnosis, was decided to surgical removed the mass which measured 7.8x3.7x3.5cm. Material and Methods The sample was fixed in 10 % buffered formalin solution for histological evaluation and sent to the Anatomic Pathology Laboratory of the Agrarian Superior School of Viseu, in Portugal, for histological evaluation. Results Microscopic examination revealed that we were in presence of a lymphoid organ, with peculiar features, namely a small to intermediate-sized lymphoid cells, "starry-sky" pattern and rounded eosinophilic perivascular arrangements resembling Hassall's corpuscles. Concomitantly, typical features of squamous cell carcinoma were evident. Discussion and Conclusion In conclusion, based on its histological features, the authors suggested the diagnosis of Thymic Squamous Cell Carcinoma with an unusual location. Thymic tumours in dogs are predominantly located in the anterior mediastinum but they may extend from the neck to the posterior mediastinum.This work was supported by Portuguese Foundation for Science and Technology, Center for Studies in Education, Technologies and by Health and by strategic project PEst-OE/CED/UI4016/201