A numerical method is described for evaluating transverse spin correlations in the random phase approximation. Quantum spin-fluctuation corrections to sublattice magnetization are evaluated for the antiferromagnetic ground state of the half-filled Hubbard model in two and three dimensions in the...

We study the magnetic excitation spectrum of the spin-1 chain with Hamiltonian <Emphasis FontCategory="NonProportional">H=Σ<Subscript> i </Subscript> cos θ <Emphasis Type="Bold">S <Subscript> i </Subscript> · <Emphasis Type="Bold">S <Subscript> i+1</Subscript> + sin θ(<Emphasis Type="Bold">S <Subscript> i </Subscript> · <Emphasis Type="Bold">S <Subscript> i+1</Subscript>)<Superscript>2</Superscript>. We focus on the range 0 ≤ θ ≤ + π/4 where the spin chain is in the gapped Haldane phase. The excitation spectrum and static structure factor is studied...</superscript></subscript></emphasis></subscript></emphasis></subscript></emphasis></subscript></emphasis></subscript></emphasis>

In a magnetic field, spin-ladders undergo two zero-temperature phase transitions at the critical fields H<Subscript>c1</Subscript> and H<Subscript>c2</Subscript>. An experimental review of static and dynamical properties of spin-ladders close to these critical points is presented. The scaling functions, universal to all quantum critical...</subscript></subscript>

We show that non-frustrated and frustrated ladders in a magnetic field can be systematically mapped onto an XXZ Heisenberg model in a longitudinal magnetic field in the limit where the rung coupling is the dominant one. This mapping is valid in the critical region where the magnetization goes...