Effect of light on the reflectance anisotropy and chain-oxygen related Raman signal in untwinned, underdoped crystals of YBa 2 Cu 3 O 7Kd S. Bahrs a, * , A. Bruchhausen b , A.R. Gon ˜i c , G. Nieva b , A. Fainstein b , K. Fleischer a , W. Richter a , C. Thomsen a a Institut fu ¨r Festko ¨rperphysik, Technische Universita ¨t Berlin, Hardenbergstr. 36, 10623 Berlin, Germany b Centro Ato ´mico Bariloche, Comisio ´n Nacional de Energı ´a Ato ´mica, 8400 San Caros de Bariloche, Rı ´o Negro, Argentina c ICREA Research Professor, Institut de Cie `ncia de Materials de Barcelona, Campus de la UAB, 08193 Bellaterra, Spain Abstract Recent studies of the optical anisotropy of a detwinned, underdoped YBa 2 Cu 3 O 7Kd -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 YBa 2 Cu 3 O 7Kd . q 2005 Elsevier Ltd. All rights reserved. PACS: 78.30.Kj; 74.72.Bk; 64.60.Cn; 73.50.Pz Keywords: A. Oxides; A. Superconductors; D. Dielectric properties; D. Electronic structure; D. Optical properties YBa 2 Cu 3 O 7Kd displays a number of effects particularly connected to oxygen defect sites in the underdoped material. Among them, the illumination-induced effects of persistent photoconductivity and Raman defect-peak bleaching have been repeatedly investigated in the past [1–5]. They are attributed to light-induced reordering of oxygen atoms in the chain plane, or partly to carrier trapping in defect sites in the current discussion. Spontaneous reordering into superstructure patterns which affects the conducting properties is known from room-temperature ageing. In a recent study, we documented that the light-induced effects can also be traced in the a–b components of the dielectric function using reflectance anisotropy spectroscopy (RAS), which is very sensitive to differences in the dielectric properties along two alternating directions of polarization [6]. A RAS peak around 2.2 eV, matching the Raman resonance profile of the photobleaching peaks, decreases upon illumination, while a large feature above 4 eV increases. Ab initio and Hubbard-model calculations attribute the features to different structural elements in the copper–oxygen chain plane [7,6]. The effect shares properties with Raman photobleaching regarding the long time scale, the polarization dependence, and the recovery on a similarly slow time scale at room temperature. Here, we report on low- temperature properties of the RAS spectra, showing that the illumination-induced effect also occurs below room tempera- ture. We discuss its properties in comparison to the known temperature dependence of Raman photobleaching, and within the picture of oxygen reordering. We used YBa 2 Cu 3 O 7Kd detwinned single crystals with an oxygen content of 6.7 and 6.65. Their size was approximately 1.0!0.5!0.1 mm 3 (a!b!c). Their preparation followed the method and proportions as described in Refs. [9,10], and detwinning under uniaxial stress was done as described in Ref. [3]. X-ray diffraction and Raman spectroscopy characterization showed excellent detwinning. The RAS spectra were obtained in the spectral range from 1 to 6 eV using a Xe-lamp as white-light source, which was focussed onto the a–b surface of the crystal at near normal incidence. A photoelastic modulator was used to probe the reflectance difference r b Kr a between the b and a crystalline Journal of Physics and Chemistry of Solids 67 (2006) 340–343 www.elsevier.com/locate/jpcs 0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.10.058 * Corresponding author. E-mail address: sabine@physik.tu-berlin.de (S. Bahrs).