Magnetic and Charge Correlations in La2−x−yNdySrxCuO4 : Raman Scattering Study

La 2−x Sr x CuO 4 is one of the most studied Cu-O based layered perovskites [1]. It exhibits some of the most important aspects related to the physics of strongly correlated electrons and, more important, is one of the compounds which belong to the family of high temperature superconducting cuprates. In fact the high T c superconductivity (SC) rush which began in 1986 started with a variant of La 2−x Sr x CuO 4 , a Ba-La-Cu-O based compound [2], where the authors observed a highest onset SC temperature T c in the 30 K range. The phase diagram of La 2−x Sr x CuO 4 is shown in Fig. 1, see also Ref. [3]. Several electronic ground states as well as structural phases evolve with Sr concentration. For x(Sr) ≤ 0.02 the crystals have long range antiferromagnetic (AF) order and one can observe a very rapid suppression of the Néel ordering temperature T N with the amount of Sr. While for x = 0 the AF transition is slightly above room temperature, T N decreases in the 150-200 K range for x = 0.01 and it is completely suppressed above x = 0.02. The phase diagram shows also a SC dome starting at x = 0.05 and ending around x = 0.32. The maximum T c of about 40 K is reached at the optimal doping x = 0.2. The highest SC temperature (T c = 51.5 K) in the La 2−x Sr x CuO 4 family was achieved in thin films under epitaxial strain [4]. There are also two structural phases of the this compound. One is tetragonal and the other one is orthorhombic, see Fig. 2. Sr substitution for La decreases the orthorhombicity and the crystal remains tetragonal at all temperatures at values of x(Sr) which correspond roughly to the region of maximum T c. Other intervening phases shown in Fig. 1, spin glass at low temperatures and low Sr concentration, Fermi or non-Fermi liquid behavior depending on if one is in the far right side of the phase diagram or not, are discussed in literature [1]. The crystal structure of La 2−x Sr x CuO 4 is shown in Fig. 2. The occurrence of several structural phases is typical for perovskites and they generally happen as a result of the lattice strain between the rare-earth and the CuO 2 layers. The strain is often released …