CuGeO

CuGeO3, or copper germanate, is an inorganic compound notable for its quasi-one-dimensional magnetic behavior. In its crystal structure, Cu2+ ions form chains along the c axis connected by oxygen bridges to GeO4 tetrahedra. The resulting arrangement yields strong exchange interactions along the chains and much weaker coupling between chains, making CuGeO3 a model system for one-dimensional magnetism.

Magnetically, CuGeO3 is described as a spin-1/2 Heisenberg antiferromagnet along the chains, with only weak interchain interactions. Its magnetic susceptibility shows the characteristic broad maximum of a one-dimensional system at relatively high temperatures, followed by a transition at low temperature where the material changes its magnetic state.

The defining feature of CuGeO3 is its spin-Peierls transition. At about 14 kelvin, the lattice along the chains undergoes dimerization, causing alternating Cu–Cu bond strengths and forming spin singlets on adjacent sites. This dimerization opens a spin gap of a few millielectronvolts, transforming the ground state into a nonmagnetic, gapped singlet state. The transition is accompanied by a lattice distortion, visible in diffraction studies as a superlattice modulation.

CuGeO3 gained prominence as the first inorganic material observed to exhibit a spin-Peierls transition, providing a concrete platform to study spin-lattice coupling in low-dimensional magnets. It has been the subject of extensive neutron scattering, ESR, and optical studies, which detail the nature of the dimerization and the associated spin gap.

Doping and pressure effects further tune the transition. Substituting Cu with nonmagnetic ions or applying pressure can modify or suppress the spin-Peierls state, introducing soliton-like excitations and altering the transition temperature.