Single-crystal growth ; unidirectional solidification of eutectic material or unidirectional demixing of eutectoid material; refining by zone-melting of material ; production of a homogeneous polycrystalline material with defined structure ; single crystals or homogeneous polycrystalline material with defined structure; after-treatment of single crystals or a homogeneous polycrystalline material with defined structure ; apparatus therefor