Existence of Néel order in some spin-1/2 Heisenberg antiferromagnets

The methods of Dyson, Lieb, and Simon are extended to prove the existence of Néel order in the ground state of the 3D spin-1/2 Heisenberg antiferromagnet on the cubic lattice. We also consider the spin-1/2 antiferromagnet on the cubic lattice with the coupling in one of the three lattice directions taken to ber times its value in the other two lattice directions. We prove the existence of Néel order for 0.16r1. For the 2D spin-1/2 model we give a series of inequalities which involve the two-point function only at short distances and each of which would by itself imply Néel order.     

Asymmetric barrier model for heavy ion fusion and its relation to channel coupling

A new asymmetric parabolic effective fusion barrier model for heavy ion fusion is developed.     

An STM study of the localized atomic reaction of 1,2- and 1,4-dibromoxylene with Si(1 1 1)-7 × 7

Thermal reactions of 1,2- and 1,4-dibromoxylene (1,2- and 1,4-diBrXy) with Si(1 1 1)-7 × 7 were investigated by STM at room temperature under UHV conditions. Reaction led to the formation of single adsorbed Br-atoms or pairs of Br-atoms, in a ratio approx. 3:1 for both reagents. Experimental results were interpreted in terms of ‘parent-mediated’ (halogen atom accompanied by organic residue), and ‘daughter-mediated’ (no accompanying organic residue) reaction dynamics. Both mechanisms contributed to the bromination of the silicon surface in comparable amounts. For pairs of bromine atoms the Br–Br separation had a most probable value of 7.6 Å for 1,2-diBrXy, and 11.5 Å for 1,4-diBrXy. This separation was in each case greater than that in the diBrXy parent molecule by a few angstroms. For parent-mediated reaction the dynamics were revealed in detail by the STM images which gave the vectorial location of the halogen-atom products (distance and angle of the daughter atoms) relative to the prior location and alignment of the adsorbed parent molecule. Both reagents, 1,2- and 1,4-diBrXy, were found to be less reactive than the corresponding dibromobenzenes studied earlier in this laboratory [S. Dobrin et al., Surf. Sci. 561 (2004) 11], in both parent- and daughter-mediated modes.     

Effect of lunar atmospheric tide on the meson intensity at Kodaikanal

The lunar semidiurnal variation in the meson intensity at Kodaikanal has been determined by the Chapman-Miller method of analysis from bihourly records of the cosmic ray meson intensity registered by three-fold coincidences for two consecutive groups of data, each extending over a period of 18 months. The former period gives a result about two to three times the probable error and can be taken to be significant. While the correlations between the solar diurnal variation of meson intensity and barometric pressure are high and negative, the corresponding correlations between the lunar semidiurnal variations of meson intensity and barometric pressure are positive. The results obtained at Kodaikanal agree with those of Duperier obtained at London. But the latter period does not give any significant result outside the probable error. Hence further investigation has to be continued.     

Hubbard model on decorated lattices

We introduce a family of lattices for which the Hubbard model and its natural extensions can be quasiexactly solved, i.e., solved for the ground and low energy states. In particular, we show rigorously that the ground state of the Hubbard model with off-site Coulomb repulsions on a decorated Kagomè lattice is an ordered array of local currents. The low energy theory describing this chiral state is an S=1 / 2 XY model, where each spin degree of freedom represents the two possible chiralities of each local current.     

Half-monopoles in the Yang-Mills theory

Using a gauge-invariant characterization of monopoles defined via their centres, we investigate the generic topological field pattern for the three-dimensional Yang-Mills theory. This leads to field patterns with one-half winding number. After presenting the main features through the simpler case of half-vortices, we consider half-monopoles in detail.