  Effective mass anomalies due to the geometrical effects are investigated in
silicon nanostructures using first-principles calculations for the first time.
In \{111\} and \{110\} biaxially strained Si, it is found that longitudinal
effective mass is extraordinarily enhanced for both thin films and crystals.
This mass enhancement is caused by the change of the band structure with double
minima into that with a single minimum due to strain and confinement. At the
transition point, it is analytically shown that the effective mass diverges.
The dependences of the confinement thickness on the anomalies are qualitatively
explained by an extension of the effective mass approximation.
