Electron spin resonance and magnetic susceptibility suggest superconductivity in Na doped WO3 samples

In WO3 doped with Na (WO3:Na) an Electron Spin Resonance (ESR) signal with unresolved fine and/or hyperfine structure is detected and used as a probe for the state of the carriers. Using the saturation method we determined the spin-lattice relaxation rate 1/T-1 of these paramagnetic centers. Upon cooling below 100 K, 1/T-1 decreases markedly, as known to occur in NMR when a gap opens in the superconducting state. At low temperatures, 1/T-1 quantitatively follows BCS behavior with a gap Delta = 160 K or 2 Delta/kT(c) = 3.5. The magnetic susceptibility exhibits a strong difference between magnetic-field cooled and zero-field cooled regimes below T-c which also supports a so far unknown superconducting phase resulting from Na doping.