Interplay of Cs concentration, dimensionality and superconductivity in Cs<f>x</f>WO<f>3</f>

It is known that Cs<f>x</f>WO<f>3</f>, <f>0.3&ges;x&ges;0.19</f>, is a 3D superconductor. Below <f>x=0.19</f> a transition from a metal to an insulator occurs and for <f>x=0.10</f>, for example, no superconductivity is observed. We find, however, that for <f>x&les;0.05</f> superconductivity reappears. For <f>x=0.005</f> nominal concentration we observe a critical transition <f>Tc</f> to the superconducting state at 5.9 K. This is observed in magnetic measurements, where a pronounced 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<f>x</f>WO<f>3</f> doped crystals. The bulk of the crystal is practically pure WO<f>3</f>. Thus during the high temperature crystal growth process, there is a redistribution of the Cs dopant as it migrates to the surface of the WO<f>3</f> crystals. From these observations, we conclude that upon reduction of the Cs nominal concentration below <f>x=0.05</f>, a 2D superconductor is formed, while for <f>x&ges;0.19</f>, the Cs<f>x</f>WO<f>3</f> bronze is a 3D superconductor. [Copyright &y& Elsevier]