MBE using hybrid gases such as arsine () and phosphine () have been developed. Generally speaking MBE is a method of using three temperatures in binary systems, e.g., the substrate temperature, the source material temperature of the group III and the group V elements in the case of III-V compounds. The structural quality of the produced superlattices can be verified by means of X-ray diffraction or neutron diffraction spectra which contain characteristic satellite peaks. Other effects associated with the alternating layering are: giant magnetoresistance, tunable reflectivity for X-ray and neutron mirrors, neutron spin polarization, and changes in elastic and acoustic properties. Depending on the nature of its components, a superlattice may be called magnetic, optical or semiconducting. Miniband structure The schematic structure of a periodic superlattice is shown below, where A and B are two semiconductor materials of respective layer thickness a and b (period: ). When a and b are not too small compared with the interatomic spacing, an adequate approximation is obtained by replacing these fast varying potentials by an effective potential derived from the band structure of the original bulk semiconductors. It is straightforward to solve 1D Schrödinger equations in each of the individual layers, whose solutions are linear combinations of real or imaginary exponentials. For a large barrier thickness, tunneling is a weak perturbation with regard to the uncoupled dispersionless states, which are fully confined as well. In this case the dispersion relation , periodic over with over by virtue of the Bloch theorem, is fully sinusoidal: and the effective mass changes sign for : In the case of minibands, this sinusoidal character is no longer preserved. Only high up in the miniband (for wavevectors well beyond ) is the top actually 'sensed' and does the effective mass change sign. The shape of the miniband dispersion influences miniband transport profoundly and accurate dispersion relation calculations are required given wide minibands. The condition for observing single miniband transport is the absence of interminiband transfer by any process. The thermal quantum kBT should be much smaller than the energy difference between the first and second miniband, even in the presence of the applied electric field. Bloch states For an ideal superlattice a complete set of eigenstates states can be constructed by products of plane waves and a z-dependent function which satisfies the eigenvalue equation . As and are periodic functions with the superlattice period d, the eigenstates are