Abstract
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 Δ= 160 K or2Δ/kTc=3.5. The magnetic susceptibility exhibits a strong difference between magnetic-field cooled and zero-field cooled regimes below Tc which also supports a so far unknown superconducting phase resulting from Na doping.
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