Frederic Mila

Li2VO(Si,Ge)O4, a prototype of a two-dimensional frustrated quantum heisenberg antiferromagnet

NMR and magnetization measurements in Li2VOSiO4 and Li2VOGeO4 are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings J1 along the sides of the square and J2 along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J2/J1>0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.

Plaquette valence-bond crystal in the frustrated Heisenberg quantum antiferromagnet on the square lattice

Using both exact diagonalizations and diagonalizations in a subset of short-range valence-bond singlets, we address the nature of the groundstate of the Heisenberg spin-

Orbital Currents in Extended Hubbard Models of High-T c Cuprate Superconductors

1∕2

Phase diagram of the one-dimensional extended Hubbard model with attractive and/or repulsive interactions at quarter filling.

antiferromagnet on the square lattice with competing next-nearest and next-next-nearest neighbor antiferromagnetic couplings (

Shadow Band in the One-Dimensional Infinite-U Hubbard Model.

{J}_{1}\ensuremath{-}{J}_{2}\ensuremath{-}{J}_{3}

Thermodynamics of the One-Dimensional SU(4) Symmetric Spin-Orbital Model

model). A detailed comparison of the two approaches reveals a region along the line

Frustrated three-leg spin tubes : From spin 1/2 with chirality to spin 3/2

({J}_{2}+{J}_{3})∕{J}_{1}=1∕2

Quantum compass model on the square lattice

, where the description in terms of nearest-neighbor singlet coverings is excellent, therefore providing evidence for a magnetically disordered region. Furthermore, a careful analysis of dimer-dimer correlation functions, dimer structure factors and plaquette-plaquette correlation functions provides striking evidence for the presence of a plaquette valence bond crystal order in part of the magnetically disordered region.

Magnetic and thermodynamic properties ofLi2VOSiO4:A two-dimensionalS=1/2frustrated antiferromagnet on a square lattice

Motivated by the recent report of broken time-reversal symmetry and zero momentum magnetic scattering in underdoped cuprates, we investigate under which circumstances orbital currents circulating inside a unit cell might be stabilized in extended Hubbard models that explicitly include oxygen orbitals. Using Gutzwiller projected variational wave functions that treat on an equal footing all instabilities, we show that orbital currents indeed develop on finite clusters and that they are stabilized in the thermodynamic limit if additional interactions, e.g., strong hybridization with apical oxygens, are included in the model.

Poisson vs. GOE Statistics in Integrable and Non-Integrable Quantum Hamiltonians

We study the phase diagram of the one dimensional (1D)