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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Bahrs, S., S. Muller, M. Rubhausen, B. Schulz, A. R. Goni, G. Nieva, and C. Thomsen. "Anisotropic ultraviolet raman resonance in underdoped YBa2Cu3O6.7." PHYSICAL REVIEW B 74, no. 2 (2006): 024519.
Abstract: We report an inverse Raman-bleaching anomaly of oxygen-vacancy-activated phonons in underdoped YBa2Cu3O6.7 excited in resonance at 4.1 eV. These modes are closely related to modes that photobleach under visible excitation. The resonance and selection rules match the prediction from the ab in-plane anisotropy of the dielectric function, as does their growth of intensity under illumination. These properties support the oxygen-vacancy-reordering model for photoinduced changes in underdoped YBa2Cu3O7-delta. We suggest that a similar coupling to anisotropic electronic resonances is responsible for the recently discovered ab anisotropies in the phonon dispersion.
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Bahrs, S., S. Reich, A. Zwick, A. R. Goni, W. Bacsa, G. Nieva, and C. Thomsen. "Raman spectroscopy with UV excitation on untwinned single crystals of YBa2Cu3O7-delta." PHYSICA STATUS SOLIDI B-BASIC RESEARCH 241, no. 12 (2004): R63–R66.
Abstract: Recent reflectance anisotropy studies on the material showed a photobleaching effect in the in-plane components of the dielectric function around 2.2 and 4.1 eV [1]. We present Raman spectra of oxygen deficient YBa2Cu3O7-delta detwinned single crystals taken with laser excitation of 3.7 eV, close to the UV feature in the dielectric function, in search for a signal from the defect-induced Raman modes. While two groups of defect-induced mode at 230 cm(-1) and 600 cm(-1) behave very similar at the 2.2 eV resonance, we find clear differences for UV excitation. A peak at 600 cm(-1) is identified as a copper-oxygen-chain related, defect-induced modes by its Raman selection rules. While this mode is enhanced beyond the strength of the Raman-allowed modes, the group at 230 cm(-1) is absent. Low temperature experiments showed no change of the peak height under illumination, though the peaks are known to photobleach in the visible. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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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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