Inelastic-light-scattering measurements show that exchange Coulomb interactions in the twodimensional electron gas of GaAs microstructures are more important than previously anticipated. Small-wave-vector spectra from modulation-doped quantum wells exhibit unexpected single-particle intersubband transitions in addition to collective spin-density and charge-density modes. From the measured spectral energies the direct and exchange intersubband Coulomb interactions are determined and found to be of comparable strengths. PACS numbers: 71.45.Gm, 73.20.Dx, 73.20.Mf, 78.30.Fs Free electrons in semiconductor microstructures reveal new behaviors that arise from fundamental electronelectron interactions and the reduced dimensionality. 1 Strong correlations in the electron gas, as in the fractional quantum Hall effect, are revealed in magnetotransport 2 and also in magneto-optics. 3,4 More generally, Coulomb interactions and reduced dimensionality have strong manifestations in the elementary excitation spectrum of the free electrons. The energies and character of these excitations are directly studied in optical experiments, such as infrared absorption 5 and inelastic light scattering. 6The light-scattering method is especially powerful because both spin-density and charge-density excitations can be measured. 6 ' 7 At small wave vectors the energies of spin-density modes are shifted from single-particle transition energies by the exchange Coulomb interaction. 1 ' 8 Charge-density modes have energy shifts due to direct as well as exchange terms. 16 " 9 However, since exchange interactions were expected to be small in GaAs, 8 spin-density excitations were previously interpreted as the energy spacings of the quantum-well states and referred to as single-particle excitations. 7,10 " 12 Similarly, the shift of charge-density excitations from singleparticle transition energies was considered in terms of direct Coulomb interactions and coupling to polar optical phonons. 6 ' 7,10 This Letter presents new results showing that such widely used interpretations of inelastic-light-scattering experiments require important revisions. In spectra of small-wave-vector intersubband excitations we find unexpected single-particle transitions in addition to the peaks of collective spin-density and charge-density modes. The significant shifts of the spin-density excitations from intersubband transition energies reveal large exchange interactions. With increasing wave vector, the collective spin-density and charge-density modes display enhanced broadening when their energies overlap the continua of single-particle transition (Landau damping). Analysis of the energies of the three excitations yields quantitative determinations of Coulomb interactions in

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To investigate the origin and diversity of domestic guinea-pigs Cavia porcellus (Linnaeus, 1758; Rodentia, Caviidae), we sequenced the mitochondrial cytochrome b gene of 12 domestic and 10 wild specimens from six species, including the two presumed as ancestral to the domestic one: Cavia tschudii and Cavia aperea. All maximum parsimony and maximum likelihood analyses grouped C. porcellus with C. tschudii (mean K2P distance = 3.2 %); best trees had 609 steps (CI = 0.796; Bremer support Index (SI) = 28), and a -Ln = 4419.52, with 100 % and 97 % bootstrap support respectively. This clade, supported by three substitutions and 96 % bootstrap, is also obtained in the cladistic analysis of corresponding amino acids. When the C. aperea node was forced to join C. porcellus, these trees were consistently longer, less likely and robust, and with less defining characters than the optimal one. All C. porcellus sequences also clustered in a node defined by 15 substitutions. The sub-node containing animals from city markets, pet shops and laboratories was characterized by four substitutions (one non-silent, SI = 7, and 91 % bootstrap). Some South American C. porcellus, called "criollos" (creoles) by local breeders, were more diverse. Probably, a particular clade from southern Peru and Chile may represent a pre-Columbian lineage. Mean K2P distance between C. tschudii and C. aperea was rather large, 7.7 %. Cavia appeared as a robust node (100 % bootstrap). These results indicate that C. tschudii is the species most closely related to C. porcellus.Key words: Andes, Caviidae, cytochrome b, domestication, guinea pig, molecular phylogeny. RESUMENPara investigar el origen y la divergencia de los cuyes domésticos Cavia porcellus (Linnaeus, 1758; Rodentia, Caviidae), secuenciamos el gen mitocondrial para citocromo b de 12 especímenes domésticos y 10 silvestres de seis especies, incluyendo las dos que se presumen como ancestro de la doméstica: C. tschudii y C. aperea. Todos los análisis de máxima parsimonia y máxima verosimilitud agruparon a C. porcellus con C. tschudii (promedio de distancias K2P = 3,2 %); los mejores árboles tenían 609 pasos (CI = 0,796; Índice de Apoyo de Bremer (SI) = 28), y un -Ln = 4.419,52; con apoyos de remuestreo de 100 % y 97 % respectivamente. Dicho clado también apareció en el análisis cladístico de los aminoácidos correspondientes, apoyado por tres sustituciones y 96 % remuestreo. Cuando el nodo de C. aperea fue forzado a unirse al de C. porcellus, estos árboles fueron consistentemente más largos, menos probables y robustos, y con menos caracteres definitorios que el óptimo. Todas las secuencias de C. porcellus se agruparon también en un nodo definido por 15 sustituciones. El subnodo con animales de mercados de ciudad, tiendas de mascota y laboratorios se caracterizó por cuatro sustituciones (una no silenciosa, SI = 7 y 91 % remuestreo). Los C. porcellus sudamericanos, llamados "criollos" por los criadores locales, son más diversos. Probablemente, un clado del sur del Perú y Chile representa un lina...

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