हाल के दशकों में खोजे गए उच्च-तापीय अतिचालक (High-temperature superconductors) मुख्य रूप से किस वर्ग के होते हैं?

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: