Effective mass calculations of SrTiO3-based superlattices for   thermoelectric applications lead to new layer design
  The effective mass is one of the main factors for enlarging the Seebeck coefficient and electronic conductivity of SrTiO3-based thermoelectric materials [1,2]. The goal of this paper is to clarify, how superlattices can change the effective mass and other features of the bandstructure. The natural Ruddlesden-Popper phase (SrTiO3)n(SrO)m with n=2, m=1 the situation changes, because the TiO6-octahedrons are slightly extended, due to diluted density of the SrO layer. Another effect is the deformed electron density, which leads to reduced effective mass perpendicular to the layer, but enlarged parallel to the plane [3]. The average value of the effective mass over this anisotropy of the 2-dimensional electron gas (2DEG) for pure Ruddlesden-Popper phases is smaller, but can increase beyond the value of pure Pervoskite for certain doping elements. In the same way, artificial superlattices (SrTiO3)x/(SrTi1-z(Nb)zO3)y were examined. When fine nanostructures (n=2, m=3 or n=3, m=2) are present the effective mass increases, when the structure becomes coarser (n=4, m=1) smaller values are determined. 
Wilfried Wunderlich, Hiromichi Ohta, Kunichi Koumoto