Electronic Raman scattering from high-and low-energy excitations was studied as a function of temperature, extent of hole doping, and energy of the incident photons in Bi 2 Sr 2 CaCu 2 O 8Ϯ␦ superconductors. For underdoped superconductors, short-range antiferromagnetic (AF ) correlations were found to persist with hole doping, and doped single holes were found to be incoherent in the AF environment. Above the superconducting (SC) transition temperature T c , the system exhibited a sharp Raman resonance of B 1g symmetry and energy of 75 millielectron-volts and a pseudogap for electron-hole excitations below 75 millielectron-volts, a manifestation of a partially coherent state forming from doped incoherent quasi particles. The occupancy of the coherent state increases with cooling until phase ordering at T c produces a global SC state.The normal state properties of doped cuprate high-temperature superconductors are markedly different from those of conventional metals and are usually viewed as manifestations of strong electron-electron correlations. These correlations cause the AF state in undoped cuprates. In the SC state, inelastic neutron scattering reveals a novel resonant magnetic excitation (1, 2). Here, we report a Raman active resonance that is present in doped cuprate superconductors above T c and gains strength with cooling.The doping phase diagram of high-T c cuprate superconductors and parent materials can be divided into four doping regions: (i) a spin S ϭ ½ AF insulator with the spin localized on Cu atoms of the CuO 2 planes, a strong superexchange constant J Ϸ 125 meV, and a high Néel temperature T N Ϸ 300 K that rapidly drops with hole doping in the CuO 2 planes until a metal-insulator transition is reached; (ii) an underdoped superconductor, where T c increases with increasing hole doping; (iii) an optimally doped superconductor, where T c reaches its maximum; and (iv) an overdoped superconductor, where the cuprate becomes a better metal as hole doping progresses and T c decreases. Two phase transition lines separate the "normal" high-temperature state from the low-temperature AF and SC phases.In addition to these AF and SC phase transitions, the phase diagram contains two crossover lines that start at high temperatures for low doping, decrease in temperature for the underdoped region, and merge at the SC transition line for the slightly overdoped SC (3). At the upper crossover temperature, short-range AF correlations develop, which, for very low doping and lower temperatures, evolve into the long-range ordered AF state. The second, lower, crossover temperature is experimentally suggested for the underdoped materials. At this temperature, the system develops a suppression of the density of lowenergy excited states, characterized as the result of a "pseudogap" (4). The gap removes only a fraction of the states at the Fermi energy (E F ). The material remains metallic and, moreover, shows an increase in the component of the electrical conductivity parallel to the CuO 2 planes (5). Nuclear magnetic re...

Magnetorotons in the dispersions of collective gap excitation modes of fractional quantum Hall liquids are measured in resonant inelastic light scattering experiments. Two deep magnetoroton minima are observed at nu = 2/5, while a single deep minimum is resolved at nu = 1/3. The observations are the first evidence of multiple roton minima in gap excitations of the quantum liquids. The results support Chern-Simons and composite fermion calculations that predict multiple roton minima for states with nu>1/3.

Excitation modes in the range 2/5>or=nu>or=1/3 of the fractional quantum Hall regime are observed by resonant inelastic light scattering. Spectra of spin-reversed excitations suggest a structure of lowest spin-split Landau levels of composite fermions that is similar to that of electrons. Spin-flip energies determined from spectra reveal significant composite fermion interactions. The filling factor dependence of mode energies displays an abrupt change in the middle of the range when there is partial population of a composite fermion level.