Spontaneous plaquette dimerization in the J1−J2J_1-J_2 Heisenberg model

We investigate the non magnetic phase of the spin-half frustrated Heisenberg antiferromagnet on the square lattice using exact diagonalization (up to 36 sites) and quantum Monte Carlo techniques (up to 144 sites). The spin gap and the susceptibilities for the most important crystal symmetry breaking operators are computed. A genuine and somehow unexpected `plaquette RVB', with spontaneously broken translation symmetry and no broken rotation symmetry, comes out from our numerical simulations as the most plausible ground state for $J_2/J_1 \simeq 0.5$.Comment: 4 pages, 5 postscript figure

Ability Tracking and Social Capital in China's Rural Secondary School System

The goal of this paper is describe and analyze the relationship between ability tracking and student social capital, in the context of poor students in developing countries. Drawing on the results from a longitudinal study among 1,436 poor students across 132 schools in rural China, we find a significant lack of interpersonal trust and confidence in public institutions among poor rural young adults. We also find that there is a strong correlation between ability tracking during junior high school and levels of social capital. The disparities might serve to further widen the gap between the relatively privileged students who are staying in school and the less privileged students who are dropping out of school. This result suggests that making high school accessible to more students would improve social capital in the general population