Dichotomy in quasiparticles dynamics of underdoped cuprates in the superconducting state

We report electronic Raman scattering (ERS) measurements for a set of underdoped (p = 0.09-0.16) mercury-based HgBa2CuO4+delta (Hg-1201, T-max = 95 K) cuprates. Inelastic light scattering (Raman effect) is a powerful experimental method for studying quasiparticles (QP) dynamics. Changing the polarization of incident and scattered light allows us to probe the QP dynamics in selected areas of the Fermi surface. By exploring the superconducting dome from the optimal doping to the underdoped one, we show unambiguously the existence of two distinct dynamics, respectively for the nodal (B-2g symmetry) and the anti-nodal (B-1g symmetry) QPs. As cuprates are underdoped, both an increasing of the amplitude and a loss of the intensity for the superconducting gap are observed, coherently with other charge spectroscopies such as ARPES or STM. On the contrary, the behavior of nodal QPs is totally different since the peak associated with the superconductivity in this symmetry follows T-c, and thus, shifts to the lower energies as we underdope the system. We finally show that these two features are only seen in the superconducting state. (C) 2007 Elsevier B.V. All rights reserved.