Signatures of Filamentary Superconductivity up to 94 K in Tungsten Oxide WO2.90
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We report results of the search for possible superconducting state in tungsten oxides WO3−x with various oxygen deficiency 0 < x < 1. In samples with one particular composition WO2.9, the signatures of superconductivity with the same transition temperature Tc = 80 K were registered by means of magnetization measurements. By lithium intercalation, the Tc 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 WO2.9 (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.
We thank H. Keller for his support and interest during this work. We also thank R. Khasanov and F. La Mattina for assistance during the ac susceptibility and EPR measurements. Valuable discussions with A. Bishop, A. Bussmann-Holder, J. Purans and Z. Guguchia are gratefully acknowledged.
This work was supported by the Shota Rustavely National Science Foundation of Georgia under grant no. STCU 2017_29.