A new spinwave expansion for the ordered Kondo lattice

We present a concise introduction to a new spinwave expansion scheme for magnetically ordered Kondo lattice models. This is motivated by consideration of the ferromagnetically ordered phase of the "double exchange" system La(1-x)Ca(x)MnO3. A brief overview is given of the consequences of quantum and thermal fluctuation effects for the magnetic properties of the double exchange ferromagnet.Comment: Introductary lecture given at VI Vietri Training Course, October 2001 10 pages latex, 10 eps figures, uses AIP macro

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

Above 40K, the magnetic susceptibility of the heavy Fermion spinel LiV2O4 has many features in common with those of geometrically frustrated magnetic insulators, while its room temperature resistivity comfortably exceeds the Mott-Regel limit. This suggests that local magnetic moments, and the underlying geometry of the pyrochlore lattice, play an important role in determining its magnetic properties. We extend a recently introduced tetragonal mean field theory for insulating pyrochlore antiferromagnets to the case where individual tetrahedra contain spins of different lengths, and use this as a starting point to discuss three different scenarios for magnetic and electronic transitions in LiV2O4.Comment: 15 pages latex, 12 eps figures, uses EPJ macro

XPS as a Probe of Gap Opening in Many Electron Systems

Core hole photoemission (XPS) provides a powerful indirect probe of the low energy excitations of a many electron system. We argue that XPS can be used to study the way in which a gap opens at a metal-superconductor or metal- insulator transition. We consider the "universal" physics of how the loss of low energy excitations modifies XPS spectra in the context of several simple models, considering in particular the case of a two dimensional d-wave superconductor.Comment: 8 pages, 9 eps figure

Doped Singlet-Pair Crystal in the Hubbard model on the checkerboard lattice

In the limit of large nearest--neighbor and on--site Coulomb repulsions, the Hubbard model on the planar pyrochlore lattice maps, near quarter-filling, onto a doped quantum fully packed loop model. The phase diagram exhibits at quarter filling a novel quantum state of matter, the Resonating Singlet-Pair Crystal, an insulating phase breaking lattice symmetry. Properties of a few doped holes are investigated. In contrast to the doped quantum antiferromagnet, phase separation is restricted to very small hopping leaving an extended``window'' for superconducting pairing. However the later is more fragile for large hopping than in the case of the antiferromagnet.Comment: 4 pages, 5 figure

Nematic, vector-multipole, and plateau-liquid states in the classical O(3) pyrochlore antiferromagnet with biquadratic interactions in applied magnetic field

The classical bilinear-biquadratic nearest-neighbor Heisenberg antiferromagnet on the pyrochlore lattice does not exhibit conventional Neel-type magnetic order at any temperature or magnetic field. Instead spin correlations decay algebraically over length scales r ~ \sqrt{T}, behavior characteristic of a Coulomb phase arising from a strong local constraint. Despite this, its thermodynamic properties remain largely unchanged if Neel order is restored by the addition of a degeneracy-lifting perturbation, e.g., further neighbor interactions. Here we show how these apparent contradictions can be resolved by a proper understanding of way in which long-range Neel order emerges out of well-formed local correlations, and identify nematic and vector-multipole orders hidden in the different Coulomb phases of the model. So far as experiment is concerned, our results suggest that where long range interactions are unimportant, the magnetic properties of Cr spinels which exhibit half-magnetization plateaux may be largely independent of the type of magnetic order present.Comment: 27 pages latex, 25 eps figure