J Supercond Nov Magn (2008) 21: 145–150 DOI 10.1007/s10948-008-0309-0 Study of the Superconducting Cs-doped WO 3 Crystal Surface by Electron Backscattered Diffraction Z. Barkay · E. Grunbaum · G. Leitus · S. Reich Received: 27 November 2007 / Accepted: 5 December 2007 / Published online: 24 January 2008 © Springer Science+Business Media, LLC 2008 Abstract The crystallographic surface structure of thermal vapor grown Cs X WO 3 0.005 ≤ x ≤ 0.3 crystals was inves- tigated locally by electron backscattered diffraction in the environmental scanning electron microscope. Monoclinic to hexagonal phase transformation was shown to take place upon Cs doping to nominal concentrations of x = 0.005 and x = 0.05, while monoclinic to trigonal phase transition was observed at a concentration of x = 0.3. In particular, the 2D superconducting crystals, of x = 0.005 nominal concentra- tion, were of inhomogeneous crystallographic phase accord- ing to the local Cs doping. The superconducting Cs-doped regions of the hexagonal phase were shown to be epitaxially grown on the WO 3 monoclinic crystal surface, the (0001) of this phase being parallel to the (001) plane of the WO 3 crystal. Our results support previous observations in these 2D superconducting Cs X WO 3 crystals. Keywords Cs-doped WO 3 · Electron backscattered diffraction · Scanning electron microscopy · 2D superconductivity Z. Barkay Wolfson Applied Materials Research Center, Tel-Aviv University, Ramat-Aviv, Tel-Aviv 69978, Israel E. Grunbaum Department of Physical Electronics, Faculty of Engineering, Tel-Aviv University, Ramat-Aviv, Tel-Aviv 69978, Israel G. Leitus · S. Reich ( ) Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel e-mail: shimon.reich@weizmann.ac.il 1 Introduction Conduction electrons in transition metal oxide compounds generally interact strongly with the surrounding lattice. If we dope the surface of such an oxide with Li + , Na + ,K + , Rb + , Cs + or even with hydrogen, the ions may donate one elec- tron to W 6+ , for example, in WO 3 to form W 5+ ions. For low doping, these W 5+ ions form small to medium size po- larons [1]. At low enough temperatures, these polarons may form bipolarons [2] leading to the nucleation of a supercon- ducting phase on the surface of the transition metal oxide crystals. A similar mechanism was suggested for the HTS cuprates [3, 4]. If we succeed to dope the surface, only the polarons will move near the surface for charge compensation reasons, that is, to say in quasi-2D perovskite layers. This is in analogy to the HTS cuprates where the polarons move in CuO 2 perovskite layers [4]. Furthermore, since many of the transition metal oxides are characterized by very high dielectric constants, about 400 in the case of WO 3 [5], the coulomb repulsion between the polarons will be diminished, favoring the formation of bipolarons. It is known that Cs x WO 3 ,0.3 ≥ x ≥ 0.19 is a 3D su- perconductor. Below x = 0.19, a transition from a metal to an insulator occurs and for x = 0.10, for example, no su- perconductivity is observed [6]. We find, however, [7] that for x ≤ 0.05, superconductivity reappears. For x = 0.005 nominal concentration, we observe the transition to the su- perconducting state at the critical temperature T c = 5.9 K. This is observed in magnetic measurements where a pro- nounced Meissner effect is present. X-ray photoelectron spectroscopy and transport measurements indicate that the SC state is confined to mesoscopic islands, some 10 nm thick on the surface of the Cs x WO 3 doped crystals. The bulk of the crystal is practically pure WO 3. Thus, during the high temperature crystal growth process, there is a redistribution