Impurity susceptibility and the fate of spin-flop transitions in lightly-doped La(2)CuO(4)

We investigate the occurrence of a two-step spin-flop transition and spin reorientation when a longitudinal magnetic field is applied to lightly hole-doped La(2)CuO(4). We find that for large and strongly frustrating impurities, such as Sr in La(2-x)Sr(x)CuO(4), the huge enhancement of the longitudinal susceptibility suppresses the intermediate flop and the reorientation of spins is smooth and continuous. Contrary, for small and weakly frustrating impurities, such as O in La(2)CuO(4+y), a discontinuous spin reorientation (two-step spin-flop transition) takes place. Furthermore, we show that for La(2-x)Sr(x)CuO(4) the field dependence of the magnon gaps differs qualitatively from the La(2)CuO(4) case, a prediction to be verified with Raman spectroscopy or neutron scattering.Comment: 4 pages, 3 figures, For the connection between spin-flops and magnetoresistance, see cond-mat/061081

Field dependence of the magnetic spectrum in anisotropic and Dzyaloshinskii-Moriya antiferromagnets: I. Theory

We consider theoretically the effects of an applied uniform magnetic field on the magnetic spectrum of anisotropic two-dimensional and Dzyaloshinskii-Moriya layered quantum Heisenberg antiferromagnets. The first case is relevant for systems such as the two-dimensional square lattice antiferromagnet Sr(2)CuO(2)Cl(2), while the later is known to be relevant to the physics of the layered orthorhombic antiferromagnet La(2)CuO(4). We first establish the correspondence betwenn the low-energy spectrum obtained within the anisotropic non-linear sigma model and by means of the spin-wave approximation for a standard easy-axis antiferromagent. Then, we focus on the field-theory approach to calculate the magnetic field dependence of the magnon gaps and spectral intensities for magnetic fields applied along the three possible crystallographic directions. We discuss the various possible ground states and their evolution with temperature for the different field orientations, and the occurrence of spin-flop transitions for fields perpendicular to the layers (transverse fields) as well as for fields along the easy axis (longitudinal fields). Measurements of the one-magnon Raman spectrum in Sr(2)CuO(2)Cl(2) and La(2)CuO(4) and a comparison between the experimental results and the predictions of the present theory will be reported in part II of this research work [L. Benfatto et al., cond-mat/0602664].Comment: 21 pages, 11 figures, final version. Part II of the present work is presented in cond-mat/060266

One-magnon Raman scattering in La(2)CuO(4): the origin of the field-induced mode

We investigate the one-magnon Raman scattering in the layered antiferromagnetic La(2)CuO(4) compound. We find that the Raman signal is composed by two one-magnon peaks: one in the B1g channel, corresponding to the Dzyaloshinskii-Moryia (DM) mode, and another in the B3g channel, corresponding to the XY mode. Furthermore, we show that a peak corresponding to the XY mode can be induced in the planar (RR) geometry when a magnetic field is applied along the easy axis for the sublattice magnetization. The appearance of such field-induced mode (FIM) signals the existence of a new magnetic state above the Neel temperature T_N, where the direction of the weak-ferromagnetic moment (WFM) lies within the CuO(2) planes.Comment: 4 pages, 3 figure

Baryon loading and the Weibel instability in gamma-ray bursts

The dynamics of two counter-streaming electron-positron-ion unmagnetized plasma shells with zero net charge is analyzed in the context of magnetic field generation in GRB internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios.Comment: 6 pages, 4 figures. Accepted for publication in MNRA

Competing impurities and reentrant magnetism in La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) revisited. The role of the Dzyaloshinskii-Moriya and XY anisotropies

We study the order-from-disorder transition and reentrant magnetism in La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) within the framework of a long-wavelength nonlinear sigma model that properly incorporates the Dzyaloshinskii-Moriya and XY anisotropies. Doping with nonmagnetic impurities, such as Zn, is considered according to classical percolation theory, whereas the effect of Sr, which introduces charge carriers into the CuO(2) planes, is described as a dipolar frustration of the antiferromagnetic order. We calculate several magnetic, thermodynamic, and spectral properties of the system, such as the antiferromagnetic order parameter, the Neel temperature, the spin-stiffness, and the anisotropy gaps, as well as their evolution with both Zn and Sr doping. We explain the nonmonotonic and reentrant behavior experimentally observed for T_N by Hucker et al. in Phys. Rev. B 59, R725 (1999), as resulting from the reduction, due to the nonmagnetic impurities, of the dipolar frustration induced by the charge carriers (order-from-disorder). Furthermore, we find a similar nonmonotonic and reentrant behavior for all the other observables studied. Most remarkably, our results show that while for x=2% and z=0 the Dzyaloshinskii-Moriya gap \Delta_{DM}=0, for z=15% it is approximately \Delta_{DM} = 7.5 cm^(-1). The later is larger than the lowest low-frequency cutoff for Raman spectroscopy (~ 5 cm^(-1)), and could thus be observed in one-magnon Raman scattering.Comment: 13 pages, 10 figure

A new data reduction scheme to obtain the mode II fracture properties of Pinus Pinaster wood

In this work a numerical study of the End Notched Flexure (ENF) specimen was performed in order to obtain the mode II critical strain energy released rate (GIIc) of a Pinus pinaster wood in the RL crack propagation system. The analysis included interface finite elements and a progressive damage model based on indirect use of Fracture Mechanics. The difficulties in monitoring the crack length during an experimental ENF test and the inconvenience of performing separate tests in order to obtain the elastic properties are well known. To avoid these problems, a new data reduction scheme based on the equivalent crack concept was proposed and validated. This new data reduction scheme, the Compliance-Based Beam Method (CBBM), does not require crack measurements during ENF tests and additional tests to obtain elastic properties.FCT - POCTI/EME/45573/200