Prof. David Sherrington FRS

Abstract:

The excitation spectrum of spin-Peierls antiferromagnets is discussed taking into acount phonon dynamics but treating inter-chain elastic couplings in mean field theory. This gives a ladder of soliton-anti-soliton boundstates, with no soliton continuum, until soliton deconfinement takes place at a transition into a non-dimerized phase.

Abstract:

Nearly one-dimensional magnetic systems can be obtained by growing materials in which the magnetic ions have strong exchange interactions along atomic chains but much weaker interactions between the chains. By suitable choice of the magnetic ions and their environment the interactions can be varied to give realisations of Ising chains, XY chains, and Heisenberg chains, and their excitations can be studied in detail using neutron scattering techniques. In ordered three-dimensional antiferromagnets, the excitations are long lived spin waves. Spin 1/2 chains do not have long range order even at low temperature, and furthermore the quantum fluctuations are important and the excitations are very different from those of three-dimensional antiferromagnets. In these lectures I review the excitations observed for the Ising system, CsCoCl3, and show that the results can be described in terms of domain walls. CuGeO3 is an example of a spin-Peierls system in which the coupling to the lattice causes a alternation of the exchange constants and the ground state becomes a non-magnetic dimer state. The excitations are then long lived but different from spin waves and best described as the triplet excitations of the ground state dimers. The last example is KCuF3 which is close to a Heisenberg spin 1/2 chain for which the excitations are spinons and the neutron scattering can be described as arising from pairs of spinons. The experimental results are described and it is argued that spinons are also best understood in terms of the excitations of dimers.

Abstract:

Rare-earth superlattices have been grown by a number of groups using molecular beam epitaxy, and the chemical and magnetic structures determined with x-ray and neutron-diffraction techniques. Superlattices of Dy/Y, Ho/Y, Ho/Lu, and Ho/Dy show helical structures in which the magnetic moments are aligned in ferromagnetic sheets within each basal plane, but the orientation of the moments changes from one plane to the next. The magnetic order then propagates coherently from one magnetic layer to the next with a coherence in both the turn angle and chirality of the helix. In contrast there is no coherence in the ordering of the Ho moments from one Ho layer to the next for He/Pr superlattices for which Pr has the dhcp structure and Ho the hcp structure. There is also no coherence in the magnetic structures of He/Sc superlattices for which the lattice mis-match is so large that the Ho and Sc layers have different basal-plane lattice constants. The behaviour is more complex for Ho/Er, Ho/Tm, Er/Y and Er/Lu superlattices which show coherent structures at temperatures where the moments order either in the basal plane or along the c-axis bur when both components order the coherence length decreases as the second component of the moment increases. It is argued that these results are consistent with a model in which if the conduction electrons responsible for the magnetism can propagate through the superlattice a coherent structure is obtained but if they are confined to particular layers when the structure is coherent only over single layers.