Bahrs, S., A. Bruchhausen, A. R. Goni, G. Nieva, A. Fainstein, K. Fleischer, W. Richter, and C. Thomsen. "Effect of light on the reflectance anisotropy and chain-oxygen related Raman signal in untwinned, underdoped crystals of YBa2Cu3O7-delta." JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 67, no. 1-3 (2006): 340–343.
Abstract: Recent studies of the optical anisotropy of a detwinned, underdoped YBa2Cu3O7-delta-crystal showed that illumination-induced change and recovery in the surface reflectance can be traced on a time scale of hours. This effect of light is also known from Raman bleaching and electrical transport and shares features of the oxygen reordering processes above room temperature. We report temperature-dependent studies of the optical anisotropy using reflectance anisotropy spectroscopy and present time-dependent data for the optical transitions at 2.2 and 4.4 eV. We compare our results to Raman bleaching and discuss them within the picture of superstructure patterns and oxygen-defect reordering in underdoped YBa2Cu3O7-delta. (c) 2005 Elsevier Ltd. All rights reserved.
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Bahrs, S., A. R. Goni, B. Maiorov, G. Nieva, A. Fainstein, and C. Thomsen. "Raman-study of photoinduced chain-oxygen ordering in RBa2Cu3O7-delta." IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 13, no. 2 (2003): 3192–3195.
Abstract: We investigated Raman-forbidden signals in RBa2Cu3O7-delta with R = Y or Pr, which lose intensity under illumination at low temperatures. These defect-induced,peaks are only visible in oxygen-deficient material and for light polarized parallel to the copper-oxygen chains along the material's b-axis. In a first, two-laser based experiment we established that the bleaching effect of the light is polarization dependent as well. From this and other known properties we conclude that the signal is connected to the copper-oxygen chains, reflecting their state of disorder, and is thus closely related to aging and to the persistent photoconductivity effect in the material. In a second set of experiments we followed the temperature dependence of the Raman spectra and the intensity decrease.
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Bahrs, S., A. R. Goni, C. Thomsen, B. Maiorov, G. Nieva, and A. Fainstein. "Light-induced oxygen-ordering dynamics in (Y,Pr)Ba2Cu3O6.7: A Raman spectroscopy and Monte Carlo study." PHYSICAL REVIEW B 70, no. 1 (2004): 014512.
Abstract: We investigated the time and temperature dependence of photobleaching effects in RBa2Cu3O7-delta single crystals (R=Y,Pr) by Raman spectroscopy and Monte Carlo simulations based on the asymmetric next-nearest-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 chain-fragment 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.
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Bahrs, S., A. R. Goni, C. Thomsen, B. Maiorov, G. Nieva, and A. Fainstein. "Rare-earth dependence of photoinduced chain-oxygen ordering in RBa2Cu3O7-x (x approximate to 0.3) investigated by Raman scattering." PHYSICAL REVIEW B 65, no. 2 (2002): 024522.
Abstract: We investigated a set of oxygen – deficient RBa2Cu3O7-x (xapproximate to0.3) ceramic samples with Raman spectroscopy under resonant conditions for the defect-induced Cu-O chain related modes at 230 and 600 cm(-1). Emphasis was laid on the frequency and intensity of the modes for different rare-earth atoms, and especially on their dependence under illumination. The modes display photoinduced bleaching for all R, including the nonsuperconducting R = Pr. The dynamics of their bleaching following a temperature quench indicates similar complex relaxation processes for all samples. A simple stretched exponential with a common beta = 0.36+/-0.04 and a distribution of time constants ranging from 50 to 1400 s describes our data well. We discuss our results in connection to photoinduced chain-oxygen ordering in oxygen-deficient superconductors.
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Bahrs, S., J. Guimpel, A. R. Goni, B. Maiorov, A. Fainstein, G. Nieva, and C. Thomsen. "Persistent photo-excitation in GdBa2Cu3O6.5 in a simultaneous Raman and electrical-transport experiment." Physical Review B 72, no. 14 (2005): 144501.
Abstract: We investigate the connection between persistent illumination-induced effects in underdoped RBa2Cu3O7-delta 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.
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Bruchhausen, A., S. Bahrs, K. Fleischer, A. R. Goni, A. Fainstein, G. Nieva, A. A. Aligia, W. Richter, and C. Thomsen. "Photoinduced chain-oxygen ordering in detwinned YBa2Cu3O6.7 single crystals studied by reflectance-anisotropy spectroscopy." PHYSICAL REVIEW B 69, no. 22 (2004): 224508.
Abstract: We present a reflectance-anisotropy spectroscopy (RAS) investigation of photoexcitation and annealing effects in oxygen deficient detwinned YBa2Cu3O6.7 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 Cu(1)-O(1) 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-O(1) 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-O(4)(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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Fainstein, A., P. Etchegoin, and J. Guimpel. "Raman study of photoinduced chain-fragment ordering in GdBa2Cu3Ox thin films." PHYSICAL REVIEW B 58, no. 14 (1998): 9433–9439.
Abstract: Raman scattering experiments on oxygen deficient GdBa2Cu3Ox thin films (x=6.53, 6.7, 6.8, and 6.93) as a function of photoexcitation and annealing induced oxygen disorder are presented. Raman lines, unambiguously assigned to copper and oxygen vibrations involving atoms at the end of Cu-O chains, are used as markers for the existence of short chain fragments. The dynamics of chain conjunction and fragmentation was monitored by the peak intensity dependences with photoexcitation time and annealing temperature. These results demonstrate the presence of photoassisted oxygen ordering in these high-T-c superconductors, and give definitive support for its ultimate role in the phenomena of persistent photoconductivity and photoinduced superconductivity. [S0163-1829(98)04037-5].
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Fainstein, A., B. Maiorov, J. Guimpel, G. Nieva, and E. Osquiguil. "Annealing disorder and photoinduced order of oxygen chains in detwinned YBa2Cu3O6.65 single crystals probed by Raman scattering." Physical Review B – Condensed Matter and Materials Physics 61, no. 6 (2000): 4298–4304.
Abstract: Raman scattering in detwinned YBa2Cu3O6.65 single crystals is studied as a function of photoexcitation and annealing. Copper-oxygen chain-related forbidden Raman bands that are known to strongly bleach with illumination at low temperatures, increase their intensity with chain fragmentation induced by annealing at high temperature. This contrasting behavior proves the conjunction of short Cu-O fragments into longer chains on photoexcitation. We interpret the Raman modes as due to vibrations at the end of CuO chain fragments and Cu-O-Cu monomers, and use their evolution with illumination and annealing as anisotropic sensitive markers of oxygen reordering processes. The identification of the “forbidden” Raman bands is discussed in the context of our results and recent literature in the subject. We also present absorption measurements performed on GdBa2Cu3Ox thin films with varying oxygen content. These experiments show that the 2.2-eV absorption and the chain-related Raman peaks have different dependencies with oxygen content and illumination, ruling out an explanation that suggests that the Raman intensity reduction of these modes is due to a photobleaching of intermediate defect states. These results highlight the potentialities of Raman scattering for oxygen dynamics studies and demonstrate the presence of photoinduced oxygen ordering in these high-Tc superconductor compounds.
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Levy, G., B. Maiorov, M. S. Corvalan, A. Fainstein, and G. Nieva. "Oxygen overdoping in superconducting and non-superconducting Y1-xPrxBa2Cu3Oy." PHYSICA B-CONDENSED MATTER 320, no. 1-4 (2002): 333–336.
Abstract: We present the evolution of magnetic and structural properties of Y1-xPrxBa2Cu3Oy (xsimilar to0.5 and x = 1) single crystals and polycrystalline materials when the oxygen concentration y is varied from under- to overdoping. We have found a monotonous evolution of the Pr Neel temperature for x = 1 samples and a maximum of the superconducting critical temperature for the xsimilar to0.5 samples. The structural properties as detected by X-ray diffraction and Raman spectroscopy show no instabilities when crossing the optimal doping region as was found in the x = 0 material. (C) 2002 Elsevier Science B.V. All rights reserved.
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