Large-UU limit of a Hubbard model in a magnetic field: chiral spin interactions and paramagnetism

We consider the large-$U$ limit of the one-band Hubbard model at half-filling on a non-bipartite two-dimensional lattice. An external magnetic field can induce a three-spin chiral interaction at order $1 / U^2 ~$. We discuss situations in which, at low temperatures, the chiral term may have a larger effect than the Pauli coupling of electron spins to a magnetic field. We present a model which explicitly demonstrates this. The ground state is a singlet with a gap; hence the spin susceptibility is zero while the chiral susceptibility is finite and paramagnetic.Comment: 12 pages, plain TeX, one figure available on request, to appear in Phys. Rev.

An Application of Stochastic Ordering to the Analysis of the Push-Out Mechanism

this paper, we compute the stochastic bounds on the cell loss rates in an ATM switch. The spatial priority in the buffer is controlled by the Push-Out mechanism, while the time priority is managed in the FIFO manner. We consider an i.i.d arrival process of cells and a constant switching time of a cell. Therefore, the system can be modelled by a discret-time Markov chain, however the size of the chain is approximatively