Magnetic quantum phase transitions of the antiferromagnetic J_{1}-J_{2} Heisenberg model

We obtain the complete phase diagram of the antiferromagnetic $J_{1}$-$J_{2}$ model, $0\leq \alpha = J_2/J1 \leq 1$, within the framework of the $O(N)$ nonlinear sigma model. We find two magnetically ordered phases, one with N\' eel order, for $\alpha \leq 0.4$, and another with collinear order, for $\alpha\geq 0.6$, separated by a nonmagnetic region, for $0.4\leq \alpha \leq 0.6$, where a gapped spin liquid is found. The transition at $\alpha=0.4$ is of the second order while the one at $\alpha=0.6$ is of the first order and the spin gaps cross at $\alpha=0.5$. Our results are exact at $N\rightarrow\infty$ and agree with numerical results from different methods.Comment: 4 pages, 5 figure

Distribution and Origin of Iron Oxides in Soils over Limestone

[Resumen] Se pretende conocer la distribuci6n y el origen de los compuestos de hierro en suelos formados sobre calizas en Galicia. Predominan las formas minerales de hierro, encontrando en la fracción arena magnetita, goethita y/o hematites y excepcionalmente ilmenita, que se consideran heredadas del material de partida. En lámina delgada se observaron goethita y hematites como granos aislados, constituyendo nódulos e integrados en fragmentos de caliza. En la arcilla, trazas de goethita están presentes en casi todos los perfiles; s610 en uno de ellos la hematites es el único 6xido de hierro. La goethita puede haberse formado tanto por neoformaci6n como por microdivisi6n a partir de las fracciones gruesas, siendo este último fen6meno el que ha originado la hematites de la fracci6n arcilla. En dos perfiles la goethita va acompañada de magnetita y se discute la posible existencía y formaci6n de maghemita.[Abstract] Iron distribution in soils formed over limestone of Galicia is studied an the origin of the different iron forms is established. Organic iron is very scarce and mineral iron components dominate. Magnetite, goethite and / or hematite, less cornmonly ilmenite, are found in the sand fraction; they are considered as interited from the parent material. In polished sections goethite an hematite appear as single grains, nodules and il1corporated into limestone fragments. In the clay fraction, traces of goethite are present in aH except one of the profiles where only traces of hematite are indentified. Neoformation and microdivision from coarse fractions can act in the genesis of goethite; only microdivision is considered as the origin of hematite. Magnetite accompanies goethite in two profiles; the possible presence of maghemite and its origin is also discusse

A nonextensive insight to the stellar initial mass function

the present paper, we propose that the stellar initial mass distributions as known as IMF are best fitted by $q$-Weibulls that emerge within nonextensive statistical mechanics. As a result, we show that the Salpeter's slope of $\sim$2.35 is replaced when a $q$-Weibull distribution is used. Our results point out that the nonextensive entropic index $q$ represents a new approach for understanding the process of the star-forming and evolution of massive stars.Comment: 5 pages, 2 figures, Accepted to EP

Magnetic susceptibility anisotropies in a two-dimensional quantum Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interactions

The magnetic and thermodynamic properties of the two-dimensional quantum Heisenberg antiferromagnet that incorporates both a Dzyaloshinskii-Moriya and pseudo-dipolar interactions are studied within the framework of a generalized nonlinear sigma model (NLSM). We calculate the static uniform susceptibility and sublattice magnetization as a function of temperature and we show that: i) the magnetic-response is anisotropic and differs qualitatively from the expected behavior of a conventional easy-axis QHAF; ii) the Neel second-order phase transition becomes a crossover, for a magnetic field B perpendicular to the CuO(2) layers. We provide a simple and clear explanation for all the recently reported unusual magnetic anisotropies in the low-field susceptibility of La(2)CuO(4), L. N. Lavrov et al., Phys. Rev. Lett. 87, 017007 (2001), and we demonstrate explicitly why La(2)CuO(4) can not be classified as an ordinary easy-axis antiferromagnet.Comment: 6 pages, 3 figures, Revtex4, accepted for publication in Phys. Rev.

Slavnov-Taylor identities for noncommutative QED4_4

In this work we present an analysis of the one-loop Slavnov-Taylor identities in noncommutative QED$_4$. The vectorial fermion-photon and the triple photon vertex functions were studied, with the conclusion that no anomalies arise.Comment: 24 pages, revtex4, v2: typos correcte

Theory of Magnetodynamics Induced by Spin Torque in Perpendicularly Magnetized Thin Films

A nonlinear model of spin wave excitation using a point contact in a thin ferromagnetic film is introduced. Large-amplitude magnetic solitary waves are computed, which help explain recent spin-torque experiments. Numerical simulations of the fully nonlinear model predict excitation frequencies in excess of 0.2 THz for contact diameters smaller than 6 nm. Simulations also predict a saturation and red shift of the frequency at currents large enough to invert the magnetization under the point contact. The theory is approximated by a cubic complex Ginzburg-Landau type equation. The mode's nonlinear frequency shift is found by use of perturbation techniques, whose results agree with those of direct numerical simulations.Comment: 5 pages, 4 figures, submitted to PR