We have shown that the superconducting transitions in ZrTe${}_3$ are successive from filamentary to bulk, based on experimental results obtained for a superconducting specific-heat anomaly and the anisotropic superconducting transition curve of resistivity. A small jumplike superconducting specific-heat anomaly with a large and widely extended tail was observed to closely follow the anisotropic zero-resistance transition. We concluded that the jumplike anomaly signifies the onset of the long-range order of Cooper pairs with the opening of a superconducting gap at $T_C$, while the large and widely extended tail indicates behavior induced by Cooper pairs with a very short coherence length. As a limit for a very short coherence length, we propose local pairs with Bose characteristics. As a result, we can understand that the filamentary superconductivity is caused by the local pairs, and the large and widely extended tail is in a crossover region between superconductivity induced by local pairs and that induced by Cooper pairs. Based on a discussion about the origin of the change in the pair coupling from the starting of local pairing to Cooper pairing in ZrTe${}_3$, we conclude that “mixed bulk-filament superconductivity” results from unique electronic structural changes in the quasi-1D+3D (D, dimensional) Fermi surfaces after the CDW transition.

• Received 26 July 2011

DOI:https://doi.org/10.1103/PhysRevB.85.184513