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The Peierls phase transition in the quasi-one-dimensional conductor is investigated by means of elastic and inelastic neutron scattering. The effective critical exponent , extracted from the temperature dependence of the integrated intensity from the CDW satellite reflections, is anomalously low, suggesting that the phase transition may be of first order. The intensity distribution among symmetry-related satellite reflections indicates a domain structure with slowly fluctuating domain populations. Correlation lengths associated with the diverging `central peak' are determined and are found to be nearly isotropic, at variance with results obtained on other quasi-one-dimensional compounds, such as platinum chains (KCP) or blue bronze, . Doping with 1.2% Nb has a severe effect on the modulated state. The low-temperature satellites are replaced by a diffuse scattering distribution elongated along . The absence of a phonon soft mode and the presence of a diverging central peak at the phase transition is interpreted within the framework of strong electron-phonon coupling. Finally, we propose a Ginzburg-Landau phenomenological model, where the interplay between the electronically coupled optical-like order parameter (Ta-atom tetramerization along the chain axis) and the elastic deformations lies at the origin of the phase transition in .

We present a detailed investigation of the low-frequency dielectric response of the charge-density-wave system ͑CDW͒ o-TaS 3 in wide temperature ͑5-300 K͒ and frequency ͑10 mHz to 100 MHz͒ ranges. Although our measurements agree relatively well with data performed in some restricted frequency and temperature ranges and previously published by several groups, we show that they do not correspond to a single lowfrequency process. Instead, three distinctive processes are found to contribute to the dielectric function below the CDW transition temperature. The temperature evolution of the characteristic relaxation time of the three processes bears a close resemblance to the phenomenology of the dielectric response of glasses. The freezing of some of these processes at finite temperatures leads to changes in the CDW properties, as is thoroughly documented in the literature. Based on these results, we propose a consistent model of the temperature evolution of the CDW ground state.

Improved experimental conditions enabled us to increase the signal-to-noise ratio of the photoemission spectra for the superconducting state of Bi2Ca2SrCu20s, taken with high angular and energy resolution. This also enabled us to reveal a pronounced minimum that separates the two basic features of the spectrum, the narrow quasiparticle excitation peak and the still controversial broad band at lower kinetic energies. The minimum is approximately 3A below the Fermi level.PACS numbers: 74.70.Vy, 79.60.Cn High-resolution photoemission spectroscopy has recently emerged as a fundamental probe of the superconducting state for high-temperature superconductivity [1], Early experiments clearly established the presence of a gap in the electronic spectrum [2][3][4][5]. The spectrum in the superconducting state is dominated, of course, by the narrow peak corresponding to quasiparticle excitations [2,6]. This peak, however, is accompanied by a broad excitation band at lower kinetic energies, whose nature has not been clarified and whose relevance to the mechanism of high-temperature superconductivity has been emphasized by several authors [7][8][9].We carefully analyzed the spectrum with angular and energy resolution similar to or better than those of previous high-resolution studies-but with enhanced signalto-noise ratio. This enabled us to clearly reveal an additional feature that appears important for the general interpretation of the spectra and of the corresponding superconducting state: a pronounced minimum that separates the broad band from the narrow peak. The minimum could not be easily seen in previous studies because of the relatively large noise, and therefore its position could not be determined. We found that the position is close to 3A below the Fermi level, where A is the gap parameter (i.e., 2A is the gap). A broad-band threshold at 3A is predicted, for example, by Littlewood and Varma [7,8] based on the "marginal-Fermi-liquid" model, by Miiller, Arnold, and Swithart [10], and by other theorists [11].Our experimental procedure was a standard synchrotron-radiation photoemission approach, enhanced in order to achieve high angular and energy resolution with a high signal-to-noise level; the results are, to the best of our knowledge, state of the art in solid-state photoemission spectroscopy. The main factors for simultaneously reaching high resolution and high signal-to-noise levels were as follows: the use of very high-quality single crystals of Bi2Ca2SrCu2C>8 cleaved in situ under ultrahigh vacuum, whose structural and transport properties were carefully characterized; the use of a 4-m normal-incidence photon monochromator with resolving power up to 4.3 x 10 3 ; the use of a carefully magnetically shielded high-resolution VSW electron analyzer; and the use of synchrotron radiation.In order to test reproducibility, the experiments were performed on twenty different samples from different batches. In each case, many different points of the Brillouin zone were explored by varying the photoelectron collection ge...

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