On the contribution of nearly-critical spin and charge collective modes to the Raman spectr.pdf

On the contribution of nearly-critical spin and charge collective modes to the Raman spectra of high-Tc cuprates S. Capraraa*, C. Di Castroa, T. Enssb, and M. Grillia aDipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale Aldo Moro, 2 – 00185 Roma - Italy bPhysik-Department, Technische Universit?t München, James-Franck-Stra?e, 85747 Garching - Germany Elsevier use only: Received date here; revised date here; accepted date here Abstract We discuss how Raman spectra are affected by nearly-critical spin and charge collective modes, which are coupled to charge carriers near a stripe quantum critical point. We show that specific fingerprints of nearly-critical collective modes can indeed be observed in Raman spectra and that the selectivity of Raman spectroscopy in momentum space may also be exploited to distinguish the spin and charge contribution. We apply our results to discuss the spectra of high-Tc superconducting cuprates finding that the collective modes should have masses with substantial temperature dependence in agreement with their nearly critical character. Moreover spin modes should be more diffusive than charge modes indicating that in stripes the charge is nearly ordered, while spin modes are strongly overdamped and fluctuate with high frequency. ? 2008 Elsevier B.V. All rights reserved PACS: 74.72.-h; 78.30.-j; 71.45.Lr; 74.20.Mn Keywords: Raman and optical spectra; Quantum criticality; Stripes * Corresponding author. Tel.: +039-0649914294; fax: +39-064957697. E-mail address: sergio.caprara@roma1.infn.it. 1. Introduction Many experiments [1] and theoretical predictions [2] indicate that the physics of high-Tc cuprates might be controlled by the proximity to a quantum critical point. One of the candidates for the corresponding ordered phase is a finite-wavelength charge and spin density modulation, possibly in the form of stripe- like textures [3]. The question then arises whether a