Antiferromagnetic, Neutral, and Superconducting Band in La_2CuO_4
  The symmetry of the Bloch functions in the conduction band of tetragonal and orthorhombic La_2CuO_4 is examined for the existence of symmetry-adapted and optimally localizable (usual or spin-dependent) Wannier functions. It turns out that such Wannier functions do not exist in the tetragonal phase. In the orthorhombic phase, on the other hand, the Bloch functions can be unitarily transformed in three different ways into optimally localizable Wannier functions: they can be chosen to be adapted to each of the three phases observed in the pure or doped material, that is, to the antiferromagnetic phase, to the superconducting phase or to the phase evincing neither magnetism nor superconductivity. This group-theoretical result is proposed to be interpreted within a nonadiabatic extension of the Heisenberg model. Within this model, atomiclike states represented by these Wannier functions are responsible for the stability of each of the three phases. However, all the three atomiclike states cannot exist in the tetragonal phase, but are stabilized by the orthorhombic distortion of the crystal. A simple model is proposed which might explain the physical properties of the doped material as a function of the Sr concentration. 
Ekkehard Kr\"uger