Spiral ground state in the quasi-two-dimensional spin-12system Cu2GeO4

Abstract

We apply density functional theory band structure calculations, the coupled cluster method, and exact diagonalization to investigate the microscopic magnetic model of the spin-$\frac{1}{2}$ compound Cu${}_{2}$GeO${}_{4}$. The model is quasi-two-dimensional, with uniform spin chains along one direction and frustrated spin chains along the other direction. The coupling along the uniform chains is antiferromagnetic, $J\ensuremath{\simeq}130$ K. The couplings along the frustrated chains are ${J}_{1}\ensuremath{\simeq}\ensuremath{-}60$ K and ${J}_{2}\ensuremath{\simeq}80$ K between nearest neighbors and next-nearest neighbors, respectively. The ground state of the quantum model is a spiral, with a reduced sublattice magnetization of 0.62 ${\ensuremath{\mu}}_{B}$ and a pitch angle of ${84}^{\ifmmode^\circ\else\textdegree\fi{}}$, both renormalized by quantum effects. The proposed spiral ground state of Cu${}_{2}$GeO${}_{4}$ opens a way to magnetoelectric effects in this compound.