Tunneling and infrared spectroscopy on high T_c superconductors

We have investigated single crystals of Bi₂Sr₂Ca_1-xY_xCu₂O₈ (x = 0-0.35) by far infrared transmission spectroscopy and by break junction tunneling. The junctions were made at low temperatures in vacuum, and SIS tunneling was measured at various temperatures and magnetic fields. The tunneling results are interpreted in terms of a Josephson current and a single electron tunneling density of states (DOS). Although the DOS is found to be zero at the Fermi level, we do not see a fully developed gap even at temperatures well below the critical temperature. There is qualitative agreement between tunneling and infrared spectroscopy results.