We have grown tungsten-containing films by focused-ion-beam FIB -induced chemical vapor deposition. The films lie close to the metal-insulator transition with an electrical conductivity which changes by less than 5% between room temperature and 7 K. The superconducting transition temperature Tc of the films can be controlled between 5.0 and 6.2 K by varying the ion-beam deposition current. The Tc can be correlated with how far the films are from the metal-insulator transition, showing a nonmonotonic dependence, which is well described by the heuristic model of Osofsky et al. (Phys. Rev. Lett. 87, 197004 (2001)). Our results suggest that FIB direct-writing of W composites might be a potential approach to fabricate mask-free superconducting devices as well as to explore the role of reduced dimensionality on superconductivity.Keywords: Direct-writing, nanoscale structures, superconductivity, tungsten, focused-ionbeam deposition IntroductionThe investigation of enhanced superconductivity has been a major activity in solid-state physics and materials science in the last few decades. There are only 27 elements which, in bulk form, are superconductors at ambient pressure. The highest critical temperature, T c , of an elemental bulk material is that of niobium (T c = 9.3 K). Thus the search for higher-T c superconductors was expanded to alloys, compounds and composites. [1][2][3][4][5][6][7][8] In an alloy, the degree of order can be very important in determining the superconducting properties. For high transition-temperature superconductors the stoichiometric composition and the structural order are of great importance in obtaining a maximal T c . By contrast, highly disordered transition metals mostly display a higher T c than their annealed ordered counterparts. Tungsten is the superconductor of choice for fabrication of transition-edge sensors (TESs) for optical and near-infared wavelengths due to the tunability of its superconducting transition temperature in the range 100 mK and its relatively weak electron-phonon coupling at such temperatures.[9] Single-crystal tungsten has a very low transition temperature: T c of alpha (bcc) W is 15 mK and that of beta (A15) W is in the range 1 K to 4 K. Amorphous tungsten films [4][5] and alloys, [6][7][8] when rendered into a disordered or granular state, have a T c which can be up to two orders of magnitude larger than that of single-crystal bcc W.Recently the use of a focused ion-or electron-beam to induce the deposition of composite materials from metal-organic precursors has emerged as an important nanofabrication technique. [10][11][12][13][14][15][16][17][18] Such techniques enable the formation of metallic

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