Introduction The newly discovered BCS-like superconductor MgB 2 [1, 2] has rapidly attracted the interest of the Raman community. Many contributions [3-6] explored the E 2g phonon mode of MgB 2 with a predicted strong coupling to the electronic conduction s-bands [7]. The published Raman spectra of MgB 2 samples both polycrystalline [3] and small single crystals [4, 5] contain mainly the same structures: broad bands extending from 300 to 1600 cm À1 . The main peak at %600 cm À1 is generally associated with the sought E 2g mode, but its unusually broad width (%270 cm À1 ), as well as the presence of other broad peaks in the spectra make the assignment difficult. On the other hand, TEM, X-ray photoemisson and diffraction studies [8][9][10] indicated the presence of impurity phases such as elemental Mg, MgO, B 2 O 3 , Mg(OH) 2 Á MgCO 3 [6] and MgB 4 [9]. We investigated these impurity phases and present an analysis of the second-order features in the spectra of MgB 2 .

We investigate the connection between persistent illumination-induced effects in underdoped RBa 2 Cu 3 O 7−␦ known as persistent photoconductivity and Raman bleaching. Despite the long-standing assumption that the electrical and optical properties respond to the same light-induced change in the material, they have not been directly compared until now. We present a simultaneous experiment of Raman spectroscopy and electrical transport under visible illumination at low temperatures. The time dependence of the response in the two methods differs by two orders of magnitude, showing that the effects are connected but not identical. We discuss our results within the oxygen-vacancy reordering model of photobleaching and find that different Cu-O chain lengths affect the optical and the electrical response differently. Raman bleaching and persistent photoconductivity thus provide a different perspective on the microscopic oxygen vacancy distribution.

We investigated the time and temperature dependence of photobleaching effects in RBa 2 Cu 3 O 7−␦ single crystals ͑R =Y,Pr͒ by Raman spectroscopy and Monte Carlo simulations based on the asymmetric nextnearest-neighbor Ising model. In a temperature range between 40 and 300 K the bleaching slows down on cooling, displaying a pronounced change in dynamics around 160 K for R = Y. To model this behavior we extended the Ising model by introducing a single energy barrier which impedes oxygen movement in the plane unless the oxygen atoms are excited by light. We obtain a time-and temperature-dependent development of superstructures under illumination with the fastest change at intermediate model temperatures. The chainfragment development in the simulation thus matches the experimental low-temperature dynamics of Raman photobleaching, providing further support for oxygen reordering in the chain plane being at the origin of Raman photobleaching and related effects.

We present a reflectance-anisotropy spectroscopy (RAS) investigation of photoexcitation and annealing effects in oxygen deficient detwinned YBa 2 Cu 3 O 6.7 single crystals. Well-resolved RAS spectral features are either bleached or enhanced on a time scale of hours upon laser illumination with polarization parallel to the Cu͑1͒-O͑1͒ chains. These photoinduced effects recover with room temperature annealing in the dark. Based on previous ellipsometric studies and on cluster models for the oxygen Cu-O͑1͒ chain-fragments we are able to assign the RAS peaks that depend on illumination to optical transitions involving copper atoms located either on short chain-fragments or in isolated Cu-O͑4͒ 2 sites. This provides strong evidence that photoinduced chain-oxygen ordering is indeed at the origin of persistent photoconductivity and Raman vibrational mode bleaching in these materials.

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