Explanation
Also called tensor, it is a tensor introduced to describe the quasi classical motion of electrons in crystal, representing a proportional coefficient of external force and acceleration when electrons are subjected to external force. If we use the vacuum mass of electrons to describe the quasi classical motion of electrons in crystal, it is very complicated to take into account the combined effect of external forces and periodic potential fields in crystal. After introduction, the motion of electrons can be directly described in the simple form of Newton's second law of motion. Obviously, the effect of periodic potential field of crystal is covered. It's not a constant, it's a function of the wave vector. In general, it is a tensor, which can be simplified as a scalar under certain special conditions. The value of can be positive or negative, always positive near the bottom of the energy band; The effective mass is always negative near the band top.
Classification
Fundamental of Material Science -> Physical and chemical basis of materials
Information materials -> Photosensitive and display materials -> Physical basis