Far-Infrared Absorption and the Metal-to-Insulator Transition in Hole-Doped Cuprates

摘要

By studying the optical conductivity of Bi_2Sr_(2-x)La_xCuO_6 and Y_(0.97)Ca_(0.03)Ba_2Cu_3O_5, we show that the metal-to-insulator transition in these hole-doped cuprates is driven by the opening of a small gap at low T in the far infrared. Its width is consistent with the observations of angle-resolved photoemission spectroscopy in other cuprates, along the nodal line of the k space. The gap forms as the Drude term turns into a far-infrared absorption, whose peak frequency can be approximately predicted on the basis of a Mott-like transition. Another band in the midinfrared softens with doping but is less sensitive to the metal-to-insulator transition.