Inelastic light-scattering spectra of underdoped La2-xSrxCuO4 single crystals are presented which provide direct evidence of the formation of quasi-one-dimensional charged structures in the two-dimensional CuO2 planes. The stripes manifest themselves in a Drude-like peak at low energies and temperatures. The selection rules allow us to determine the orientation to be along the diagonals at x=0.02 and along the principal axes at x=0.10. The electron-lattice interaction determines the correlation length which turns out to be larger in compound classes with lower superconducting transition temperatures. Temperature is the only scale of the response at different doping levels demonstrating the importance of quantum critical behavior.

We present results of inelastic light scattering experiments on singlecrystalline La2−xSrxCuO4 in the doping range 0.00 ≤ x = p ≤ 0.30 and Tl2Ba2CuO 6+δ at p = 0.20 and p = 0.24. The main emphasis is placed on the response of electronic excitations in the antiferromagnetic phase, in the pseudogap range, in the superconducting state, and in the essentially normal metallic state at x ≥ 0.26, where no superconductivity could be observed. In most of the cases we compare B1g and B2g spectra which project out electronic properties close to (π, 0) and (π/2, π/2), respectively. In the channel of electron-hole excitations we find universal behavior in B2g symmetry as long as the material exhibits superconductivity at low temperature. In contrast, there is a strong doping dependence in B1g symmetry: (i) In the doping range 0.20 ≤ p ≤ 0.25 we observe rapid changes of shape and temperature dependence of the spectra. (ii) In La2−xSrxCuO4 new structures appear for x < 0.13 which are superposed on the electron-hole continuum. The temperature dependence as well as model calculations support an interpretation in terms of charge-ordering fluctuations. For x ≤ 0.05 the response from fluctuations disappears at B1g and appears at B2g symmetry in full agreement with the orientation change of stripes found by neutron scattering. While, with a grain of salt, the particle-hole continuum is universal for all cuprates the response from fluctuating charge order in the range 0.05 ≤ p < 0.16 is so far found only in La2−xSrxCuO4. We conclude that La2−xSrxCuO4 is close to static charge order and, for this reason, may have a suppressed Tc.

The two-dimensional rare-earth tritellurides undergo a unidirectional charge-density-wave ͑CDW͒ transition at high temperature and, for the heaviest members of the series, a bidirectional one at low temperature. Raman scattering experiments as a function of temperature on DyTe 3 and on LaTe 3 at 6 GPa provide a clear-cut evidence for the emergence of the respective collective CDW amplitude excitations. In the unidirectional CDW phase, we discover that the amplitude mode develops as a succession of two mean-field BCS-like transitions with different critical temperatures, which we associate with the presence of two adjacent Te planes in the structure.Electronic instabilities are at the origin of phenomena as diverse as the formation of spin-density wave ͑SDW͒ and charge-density wave ͑CDW͒ or superconductivity, the interplay of which is among the most intriguing open questions of modern solid-state physics. Besides determining the ground state of the quantum system in which they occur, electronic instabilities also fundamentally affect its excitation spectrum. In the CDW state, on which we focus here, a gap opens up in the single-particle spectrum, and two new collective modes, associated with the oscillations of the amplitude and of the phase of the CDW, respectively, appear. 1 The paradigm of CDW forming materials are the quasi-one-dimensional compounds, 2 the properties of which are nicely summarized in Ref. 1. But electronically driven CDW states were also found and thoroughly investigated in novel two-dimensional ͑2D͒ layered compounds, 3-7 an effort motivated in part by the fact that high-temperature superconductivity in the copper-oxide systems may indeed emerge from a peculiar charge ordering through the tuning of relevant parameters. 6,8 A family of layered compounds which have attracted a lot of attention recently are the rare-earth ͑R͒ tritellurides RTe 3 , first studied by DiMasi et al. 9 They host a unidirectional incommensurate CDW already well above room temperature for all R elements lighter than Dy, 10,11 while in the heavy rare-earth tritellurides ͑i.e., R = Tm, Er, Ho, Dy͒ the corresponding transition temperature, T CDW1 , lies below ϳ300 K and decreases with increasing R mass. In the latter systems, a further transition to a bidirectional CDW state occurs at T CDW2 , ranging from 180 K for TmTe 3 to 50 K for DyTe 3 . 10,11 The drastic change in transition temperatures with the size of the R ion or externally applied pressure on a given material 12 is accompanied by a similarly large change in the properties of the CDW itself. In particular, the CDW gap of RTe 3 progressively collapses when the lattice constant is reduced, which, in turn, induces a transfer of spectral weight into the metallic component of the excitation spectrum, 13-15 the latter resulting from the fact that the Fermi surface in these materials is only partially gapped by the formation of the CDW. The response of this residual metallic component completely screens all optically active modes ͑in-cluding the collective CDW phase ex...

We present results of Raman scattering experiments on tetragonal (Y1−yCay)Ba2Cu3O6+x for doping levels p(x, y) between 0 and 0.07 holes/CuO2. Below the onset of superconductivity at psc1 ≈ 0.06, we find evidence of a diagonal superstructure. At psc1, lattice and electron dynamics change discontinuously with the charge and spin properties being renormalized at all energy scales. The results indicate that charge ordering is intimately related to the transition at psc1 and that the maximal transition temperature to superconductivity at optimal doping T max c depends on the type of ordering at p > psc1. 74.20.Mn, In cuprates the maximal transition temperature to superconductivity T max c depends on the compound class. In contrast, the variation of T c with doping p does not, and superconductivity exists between approximately 0.05 and 0.27 holes per CuO 2 formula unit in clean samples [1]. In the presence of disorder this range shrinks [1] leading to a sample-specific onset point of superconductivity at p sc1 ≥ 0.05. In addition to superconductivity, short-range antiferromagnetism with the domains separated by quasi one-dimensional charged stripes can occur [2,3,4,5,6]. In La 2−x Sr x CuO 4 (p = x), this superstructure is oriented along the diagonals of the CuO 2 plane below p sc1 and rotates by 45• at p sc1 [7]. This rotation was also seen in the low-energy electronic Raman spectra where the ordering-related response flips symmetry [8].For p > p sc1 superstructures are observed in all cuprates [3,6,9,10,11,12]. However, the type of ordering and its relationship to superconductivity is rather complicated to pin down [5,13,14]. In a few compounds the lattice stabilizes static spin and charge superstructures and the superconducting transition temperature is reduced or quenched [3,6]. In most of the cases fluctuating order prevails, and it is particularly hard to detect the charge part [5,13]. Raman spectroscopy was found to be a viable method [8].Inelastic (Raman) scattering of light is capable of probing most of the excitations in a solid including lattice vibrations, spins, and electrons, as well as their interactions [15]. Since the polarizations of the incident and the scattered photons can be adjusted independently, many of the excitations can be sorted out via the selection rules. For instance, the transport properties of conduction electrons can be measured independently in different regions of the Brillouin zone [15,16], and the orientation of (fluctuating) charged stripes can be determined [8]. A detailed model calculation [17] demonstrated that in addition to the symmetry selection rules the dependence on energy and temperature of the response related to stripes can be understood quantitatively (Fig. 1) in terms of charge-ordering fluctuations.In this paper, we focus on the "high-T c " compound YBa 2 Cu 3 O 6+x (Y-123) at doping levels 0 ≤ p ≤ 0.07. The purpose is to gain insight into the nature of the onset of superconductivity at p sc1 ≈ 0.06 and into possible discrimination criteria to the "low-T c " comp...

We performed Raman scattering experiments on superconductivity-induced features in Bi2Sr2(Ca1−xYx)Cu2O 8+δ (Bi-2212), YBa2Cu3O6+x (Y-123), and Tl2Ba2CuO 6+δ (Tl-2201) single crystals. The results in combination with earlier ones enable us to systematically analyze the spectral features in the doping range 0.07 ≤ p ≤ 0.24. In B2g (xy) symmetry we find universal spectra and the maximal gap energy ∆0 to scale with the superconducting transition temperature Tc. The B1g (x 2 − y 2 ) spectra in all three compounds show an anomalous increase of the intensity toward overdoping. The energy scale of the corresponding peak is neither related to the pairing energy nor to the pseudogap, but possibly stems from a symmetry breaking transition at the onset point of superconductivity at psc2 ≃ 0.27.