Micro-Raman evidence for charge ordering in superconducting La 2 x Sr x CuO 4 D. Lampakis a, * , D. Palles a , E. Liarokapis a , C. Panagopoulos b a Department of Physics, National Technical University, GR-15780 Athens, Greece b Cavendish Laboratory and IRC in Superconductivity, University of Cambridge, Madingley Road, CB3 0HE, UK Available online 2 April 2007 Abstract A systematic micro-Raman study has been carried out on high quality La 2 x Sr x CuO 4 compounds with x = 0.0–0.45, in the temper- ature range of 10–300 K. It is shown that the behavior of the asymmetry of the apex phonon could be due to doping induced distortions. The variation of the relative intensity of the La/Sr and the apex phonons, of A g symmetry, is found to be correlated with T c even at low temperatures. Symmetry forbidden broad bands appear at 150, 370,and 280 cm 1 with intensities correlated with the doping dependence of the transition temperature. All these results can be attributed to the formation of stripes at temperatures we above the transition temperature. Ó 2007 Elsevier B.V. All rights reserved. PACS: 74.72. h; 78.30.j; 63.20.e Keywords: La 2 x Sr x CuO 4 ; Raman spectroscopy; Local lattice distortions 1. Introduction Two strong modes of A g symmetry, attributed to the vibrations of the La/Sr and the apex oxygen (O z ) atoms along the c-axis, appear in the zz polarization Raman spec- tra of La 2 x Sr x CuO 4 (LSCO) [1].At room temperature (RT) the spectral characteristics (asymmetry and relative intensity) of these two modes show a correlation with the transition temperature dependence on doping [2]. Further- more,two symmetry forbidden IR modes at 150 and 370 cm 1 appear in the xx/yy polarization spectra with a maximum intensity for the optimally doped samples [2]. Finally, in the low temperature spectra a broad band at 280 cm 1 has also been observed [1]. In this work we investigate the temperature dependence of these correla- tions and compare with previously reported RT data. Details of the sample preparation conditions and the experimental setup for the Raman measurements have been published elsewhere [2]. 2. Results and discussion Fig. 1a shows that the correlation between the apex pho non asymmetry and T c exists atthe lowesttemperatures studied.This would imply that another weak peak exists at the high energy side of the apex band. An EXAFS study has indicated two kinds of octahedra in the LSCO struc- ture, normal (undistorted)and distorted with shorter Cu–O z bond lengths and attributed them to the existence of two types of stripes [3]. In such a case, the weak satellit peak would be due to the vibrations of the O z atoms along the c-axis and depends on doping. Fig. 1b verifies that the relative intensity La/Sr to the apex phonon has a maximum value in the optimally doped region for all temperatures studied. The relative intensity is generally known to be affected by the screening caused by the charge carriers. The correlation of the phonon intensity 0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.308 * Corresponding author. Tel.: +30 210 7722950; fax: +30 210 7723312. E-mail address: lampakis@central.ntua.gr (D. Lampakis). www.elsevier.com/locate/physc Physica C 460–462 (2007) 1174–1175