Signatures of Filamentary Superconductivity up to 94 K in Tungsten Oxide WO_2.90

Abstract

We report results of the search for possible superconducting state in tungsten oxides WO_3−x with various oxygen deficiency 0 < x < 1. In samples with one particular composition WO_2.9, the signatures of superconductivity with the same transition temperature T_c = 80 K were registered by means of magnetization measurements. By lithium intercalation, the T_c was further increased to 94 K. The observed small superconducting fraction and the absence of clear transition in resistivity measurements indicate that the superconductivity is localized in small regions which do not percolate. Electron Paramagnetic Resonance experiments showed the presence of W⁵⁺ − W⁵⁺ electron bipolarons in reduced tungsten oxide samples. It is proposed that such bipolarons form and cluster within crystallographic shear planes which exist in the Magnéli phase of WO_2.9 (W₂₀O₅₈) and represent charge-carrier rich quasi-1D stripes or puddles. When decreasing temperature, superconducting state can be established locally in such regions similar to cuprates. The obtained results demonstrate that the Magnéli-type tungsten oxides are promising materials to explore high-temperature superconductivity above liquid nitrogen temperature.