Temperature and field dependence of the intrinsic tunneling structure in overdoped Bi₂Sr₂CaCu₂O₈

Tunneling in stacked ‘intrinsic’ junctions in the high-T_c superconductor doped Bi₂Sr₂CaCu₂O₈ is a bulk probe of the quasiparticle density of states, which can be used to complement surface spectroscopic techniques such as ARPES and scanning tunneling microscopy. However, in order for this technique to give useful data above the superconducting sum-gap voltage, stacks with area of less than 1 (μm)² and containing approximately 10 or fewer intrinsic junctions must be used in order to ensure negligible self-heating.

Here we present intrinsic tunneling data for mesa structures fabricated on three over- and optimally-doped Bi_2.15Sr_1.85CaCu₂O_8+δ crystals, with 0.16–0.19 holes per CuO₂ unit, for a wide range of temperature (T) and applied magnetic field (H). The differential conductance above the gap edge shows a clear dip structure which is highly suggestive of strong coupling to a narrow boson mode. Data below the gap edge gives clear evidence for strong T-dependent pair breaking. These findings could help theorists make a detailed Eliashberg analysis and thereby contribute towards understanding the pairing mechanism.