Constant Effective Mass Across the Underdoped Region of the High Tc Cuprates

We investigate the hole dynamics in two prototypical high temperature superconducting systems: La$_{2-x}$Sr$_{x}$CuO$_{4}$ and YBa$_{2}$Cu$_{3}$O$_{y}$ using a combination of DC transport and infrared spectroscopy. By exploring the effective spectral weight obtained with optics in conjunction with DC Hall results we find a constant effective mass across the entire underdoped regime. The transition to the Mott insulating state in these systems is of the "vanishing carrier number" type as we observe no mass divergence as one proceeds to undoped phases. We discuss the implications of these results for the understanding of both transport phenomena and pairing mechanism in high-T$_{c}$ systems.