“…Since the leading work in [39], the basic features had been investigated theoretically [40,41] and experimentally [42]. These studies have motivated various subsequent works; realistic effects on the resonant tunneling were discussed in [43,44,45,46,47,48,49], and some different theoretical methods which can deal with an arbitrary finite periodic potential were developed in [50,51,52,53,54,55]. Furthermore, the resonant tunneling is still an active area of research for the applications to high-frequency oscillators in recent years [56,57,58].…”