Broad-band infrared ellipsometry measurements of the c-axis response of underdoped YBa2Cu3O7−δ: Spectroscopic distinction between the normal-state pseudogap and the superconducting gap

We present broad-band infrared ellipsometry measurements of the c-axis dielectric response of underdoped YBa2Cu3O7−d single crystals. Our data provide a clear spectroscopic distinction between the normal-state pseudogap and the superconducting gap. In particular, they establish that different energy scales are underlying the respective redistributions of spectral weight. Furthermore, our data are suggestive of a mutual competition between the two gaps and thus of an extrinsic nature of the pseudogap with respect to superconductivity.