Nic Shannon

Half-magnetization plateau stabilized by structural distortion in the antiferromagnetic heisenberg model on a pyrochlore lattice

Magnetization plateaus, visible as anomalies in magnetic susceptibility at low temperatures, are one of the hallmarks of frustrated magnetism. We show how an extremely robust half-magnetization plateau can arise from coupling between spin and lattice degrees of freedom in a pyrochlore antiferromagnet and develop a detailed symmetry of analysis of the simplest possible scenario for such a plateau state. The application of this theory to the spinel oxides CdCr2O4 and HgCr2O4, where a robust half-magnetization plateau has been observed, is discussed.

Nematic order in square lattice frustrated ferromagnets.

We present a new scenario for the breakdown of ferromagnetic order in a two-dimensional quantum magnet with competing ferromagnetic and antiferromagnetic interactions. In this, dynamical effects lead to the formation of two-magnon bound states, which undergo Bose-Einstein condensation, giving rise to bond-centered nematic order. This scenario is explored in some detail for an extended Heisenberg model on a square lattice. In particular, we present numerical evidence confirming the existence of a state with d-wave nematic correlations but no long-range magnetic order, lying between the saturated ferromagnetic and collinear antiferromagnetic phases of the ferromagnetic model J1-J2. We argue by continuity of spectra that this phase is also present in a model with 4-spin cyclic exchange.

Finite temperature properties and frustrated ferromagnetism in a square lattice Heisenberg model

Abstract.The spin 1/2 Heisenberg model on a square lattice with antiferromagnetic nearest- and next-nearest neighbour interactions (the J1-J2 model) has long been studied as a paradigm of a two-dimensional frustrated quantum magnet. Only very recently, however, have the first experimental realisations of such systems been synthesized. The newest material, Pb2VO(PO4)2 seems to have mixed ferro- and antiferromagnetic exchange couplings. In the light of this, we extend the semiclassical treatment of the J1-J2 model to include ferromagnetic interactions, and present an analysis of the finite temperature properties of the model based on the exact diagonalization of 8, 16 and 20 site clusters. We propose that diffuse neutron scattering can be used to resolve the ambiguity inherent in determining the ratio and sign of J1 and J2 from thermodynamic properties alone, and use a finite temperature Lanczos algorithm to make predictions for the relevant high temperature spin-spin correlation functions. The possibility of a spin-liquid phase occurring for ferromagnetic J1 is also briefly discussed.

Spin-wave expansion, finite temperature corrections, and order from disorder effects in the double exchange model

The magnetic excitations of a double-exchange (DE) model are usually discussed in terms of an equivalent ferromagnetic Heisenberg model. We argue that this equivalence is valid only at a quasiclassical levelchar22{}both quantum and thermal corrections to the magnetic properties of a DE model differ from any effective Heisenberg model because its spin excitations interact only indirectly, through the exchange of charge fluctuations. To demonstrate this, we perform a large-S expansion for the coupled spin and charge degrees of freedom of the DE model, aimed at projecting out all electrons not locally aligned with core spins. We generalized the Holstein-Primakoff transformation to the case when the length of the spin is an operator by itself, and explicitly construct fermionic and bosonic operators to fourth order in

Magnetocaloric effect in the frustrated square lattice J1-J2 model

1/sqrt{S}.

Fractional charges in pyrochlore lattices

This procedure removes all spin variables from the Hund coupling term, and yields an effective Hamiltonian with an overall scale of electron hopping, for which we evaluate corrections to the magnetic and electronic properties in a

Monte Carlo study of half-magnetization plateau and magnetic phase diagram in pyrochlore antiferromagnetic Heisenberg model

1/S

Mixed valence on a pyrochlore lattice - LiV2O4 as a geometrically frustrated magnet

expansion to order

Quantum spin dynamics of the bilayer ferromagnet La1.2Sr1.8Mn2O7

mathcal{O}{(1/S}^{2}).

Quantum Liquid with Deconfined Fractional Excitations in Three Dimensions

We also consider the effect of a direct superexchange antiferromagnetic interaction between core spins. We find that the competition between ferromagnetic double exchange and an antiferromagnetic superexchange provides on example of an ``order from disorder phenomenonchar22{}when the two interactions are of comparable strength, an intermediate spin configuration (either a canted or a spiral state) is selected by quantum and/or thermal fluctuations.