RVB signatures in the spin dynamics of the square-lattice Heisenberg antiferromagnet

We investigate the spin dynamics of the square-lattice spin-1/2 Heisenberg antiferromagnet by means of an improved mean field Schwinger boson calculation. By identifying both, the long range N\'eel and the RVB-like components of the ground state, we propose an educated guess for the mean field triplet excitation consisting on a linear combination of local and bond spin flips to compute the dynamical structure factor. Our main result is that when this triplet excitation is optimized in such a way that the corresponding sum rule is fulfilled, we recover the low and high energy spectral weight features of the experimental spectrum. In particular, the anomalous spectral weight depletion at $(\pi,0)$ found in recent inelastic neutron scattering experiments can be attributed to the interference of the triplet bond excitations of the RVB component of the ground state. We conclude that the Schwinger boson theory seems to be a good candidate to adequately interpret the dynamic properties of the square-lattice Heisenberg antiferromagnet.Comment: 6 pages with 3 figure

Spin reorientation transition in the incommensurate stripe-ordered phase of La3/2Sr1/2NiO4

The spin ordering of La3/2Sr1/2NiO4 was investigated by magnetization measurements, and by unpolarized- and polarized-neutron diffraction. Spin ordering with an incommensurability epsilon ~ 0.445 is observed below T_so ~ 80 K. On cooling, a spin reorientation is observed at 57 +/- 1 K, with the spin axes rotating from 52 +/- 4 degrees to 78 +/- 3 degrees. This is the first time a spin reorientation has been observed in a La2-xSrxNiO4+delta compound having incommensurate stripe order.Comment: REVTex 4. 4 pages including 4 figures. Minor changes to text. Accepted to be published in Physical Review

Chandra Observation of PSR B1823-13 and its Pulsar Wind Nebula

We report on an observation of the Vela-like pulsar B1823-13 and its synchrotron nebula with Chandra.The pulsar's spectrum fits a power-law model with a photon index Gamma_PSR=2.4 for the plausible hydrogen column density n_H=10^{22} cm^{-2}, corresponding to the luminosity L_PSR=8*10^{31} ergs s^{-1} in the 0.5-8 keV band, at a distance of 4 kpc. The pulsar radiation likely includes magnetospheric and thermal components, but they cannot be reliably separated because of the small number of counts detected and strong interstellar absorption. The pulsar is surrounded by a compact, 25''x 10'', pulsar wind nebula (PWN) elongated in the east-west direction, which includes a brighter inner component, 7''x 3'', elongated in the northeast-southwest direction. The slope of the compact PWN spectrum is Gamma_comp=1.3, and the 0.5-8 keV luminosity is L_comp~3*10^{32} ergs s^{-1}. The compact PWN is surrounded by asymmetric diffuse emission (extended PWN) seen up to at least 2.4' south of the pulsar, with a softer spectrum (Gamma_ext=1.9), and the 0.5-8 keV luminosity L_ext~10^{33}-10^{34} ergs s^{-1}. We also measured the pulsar's proper motion using archival VLA data: \mu_\alpha=23.0+/-2.5 mas yr^{-1}, \mu_\delta=-3.9+/-3.3 mas yr^{-1}, which corresponds to the transverse velocity v_perp=440 km s^{-1}. The direction of the proper motion is approximately parallel to the elongation of the compact PWN, but it is nearly perpendicular to that of the extended PWN and to the direction towards the center of the bright VHE gamma-ray source HESS J1825-137, which is likely powered by PSR B1823-13.Comment: 13 pages, 8 figures and 3 tables; submitted to Ap

Two-loop critical mass for Wilson fermions

We have redone a recent two-loop computation of the critical mass for Wilson fermions in lattice QCD by evaluating Feynman integrals with the coordinate-space method. We present the results for different types of infrared regularization. We confirm both the previous numerical estimates and the power of the coordinate-space method whenever high accuracy is needed.Comment: 13 LaTeX2e pages, 2 ps figures include

Damages caused by cotton rat, Sigmodon hispidus zanjonensis, on sugar cane in San Pedrosula, Honduras

Technical assistance was given to Compañía Azucarera Hondureña, S.A. (Agro-Industrial Co.), Honduras, Central America, to determine if a campaign against noxious rodents to agriculture crops was needed. Several trappings were carried out at different places using snap traps to determine the population structure of rodents associated with the crop, and live traps to determine the index or density of the Sigmodon hispidus rat population, which was identified as being responsible for the damage to sugarcane. Results were 43.24% adult males, 14.86% young males, 31.41% adult females, and 10.47% young females. Of the adult females captured, 54.83% were pregnant with an average of 3 to 4 embryos per rat. A control demonstration combat was carried out at one of the experimental stations with a bait prepared with 2% zinc phosphide in a place where it had been previously determined there was a population of 39 rats per hectare. After such control, the population was reduced to 18 rats per hectare, which represents an efficiency of 53.85%. An evaluation of damages was also measured at different places to determine the degree of loss caused by the rats, which proved to be 22.79% damage. The size of the sample was estimated in 3 samples per hectare, with a level of confidence of 95%

Planetary Formation Scenarios Revistied: Core-Accretion Versus Disk Instability

The core-accretion and disk instability models have so far been used to explain planetary formation. These models have different conditions, such as planet mass, disk mass, and metallicity for formation of gas giants. The core-accretion model has a metallicity condition ([Fe/H] > −1.17 in the case of G-type stars), and the mass of planets formed is less than 6 times that of the Jupiter mass MJ. On the other hand, the disk instability model does not have the metallicity condition, but requires the disk to be 15 times more massive compared to the minimum mass solar nebulae model. The mass of planets formed is more than 2MJ. These results are compared to the 161 detected planets for each spectral type of the central stars. The results show that 90% of the detected planets are consistent with the core-accretion model regardless of the spectral type. The remaining 10% are not in the region explained by the core-accretion model, but are explained by the disk instability model. We derived the metallicity dependence of the formation probability of gas giants for the core-accretion model. Comparing the result with the observed fraction having gas giants, they are found to be consistent. On the other hand, the observation cannot be explained by the disk instability model, because the condition for gas giant formation is independent of the metallicity. Consequently, most of planets detected so far are thought to have been formed by the core-accretion process, and the rest by the disk instability process.Comment: accepted for publication in The Astrophysical Journa