Mixed bulk-filament nature in superconductivity of the charge-density-wave conductor ZrTe3

Abstract

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.

Effects of pressure and magnetic field on superconductivity in ZrTe3: local pair-induced superconductivity

“…This behavior yields the bulk SC. In fact, this picture corresponds to a crossover between the filamentary and bulk SC with varying temperature, which explains the anomalous behavior of the specific heat well, as was discussed in a previous report [15].…”

“…Note that the filamentary SC occurs along the a axis and no visible anomaly in the specific heat [15]. On the other hand, the zero resistance for  I bat 1.4 K corresponds to the bulk SC transition, where a specific heat anomaly has previously been observed [15]. For  I a, the small drops of resistance were observed at 0.50 GPa.…”

“…For 0 GPa, the R a drops to zero at 2.2 K, while the R b decreases to zero at 1.4 K. The highly anisotropic SC transitions are reflected by the filamentary and bulk nature of the SC. Note that the filamentary SC occurs along the a axis and no visible anomaly in the specific heat [15]. On the other hand, the zero resistance for  I bat 1.4 K corresponds to the bulk SC transition, where a specific heat anomaly has previously been observed [15].…”

“…Single crystals of ZrTe 3 were prepared using the iodine (I 2 ) vapor transport method [15]. The crystal structure is shown in figure 1(a).…”

“…In the former, the reduction in R a is attributed to a fluctuation in the bulk SC enhanced by the low dimensionality. In the latter, the transitions of R a and ¢ R b c , correspond to filamentary and bulk SC, which originate from the formation of local pairs, namely bi-polarons, and Cooper pairs induced by the transfer of the local pairs, respectively, owing to a peculiar electronic structure after the CDW transition [15]. At this moment, it remains unclear which interpretation is acceptable.…”