The total energy of an ionic solid is given by U = -αe24πε0r + Br9, where α is the Madelung constant, and r is the distance between nearest neighbors in the crystal. If r0 is the equilibrium separation, then the constant B is given by:

Among the various materials related to electronics, the energy band gap is maximum in ________.

A cubic sample of edge length L is maintained at a temperature of 4 K. The speed of sound in the material of the sample is 5 × 103 m/s. The minimum value of L required to excite the lowest frequency phonon mode is closest to: