Raman scattering near a d-wave Pomeranchuk instability

Motivated by recent transport and neutron scattering experiments suggesting an orientational symmetry breaking in underdoped cuprates we present a theoretical study of Raman scattering near a d-wave Pomeranchuk instability (PI). The d-wave component of Raman scattering from electrons and phonons allows to study directly order parameter fluctuations associated with the PI. Approaching the PI from the normal state by lowering the temperature a central peak emerges both in electronic and, as an additional low-frequency feature, in phononic scattering. Approaching the PI in the superconducting state at low temperature by decreasing the doping concentration the central peak is replaced by a soft mode with strongly decreasing width and energy and increasing spectral weight. These predicted low-energy features in Raman scattering could confirm in a rather direct way the presence of a PI in high-temperature cuprate superconductors and in Sr3Ru2O7.