Comment on ‘‘Superconducting phases of Bi and Ga induced by deposition on a Ni sublayer’’

Observation of Superconductivity in Granular Bi Nanowires Fabricated by Electrodeposition

“…26,27 The T c is around 4.0 K, and the superconductivity was thought to originate from the stress between Bi and Ni layers which induces a face-centered cubic structure of the Bi film (this phase does not survive in bulk Bi or Bi film without a Ni layer) or a common rhombohedral phase with a (102) preferred orientation. 28 In this paper we report the observation of superconductivity in granular Bi nanowires, fabricated by electrochemical deposition at room temperature and measured at ambient pressure. The superconductivity is subtle, in that the nanowires can be superconducting or non-superconducting, sensitively depending on the details of sample morphology.…”

“…The values, −(d H c /d T) T c , for 7.2 and 8.3 K superconducting phases in our Bi nanowires were estimated to be 2.6 and 8.1 kOe/K, respectively. These values are noticeably smaller than the values of 20−30 kOe/K for amorphous Bi films and much smaller than 167 kOe/K (or 80 kOe/K) for granular Bi composite films at grain size of 4.2 nm (or grain size larger than 5.0 nm) . However, the estimated values in our Bi nanowires lie in the range of 2−15 kOe/K for strong-coupling superconducting films .…”

“…According to Ginzburg-Landau-Abrikosov-Gorkov theory [34][35][36] for a type-II superconductor, the initial slope at T c of the upper critical field H c (T) curve determines the electronic density of states N (E F ) at the Fermi surface, i.e., (dH c /dT 37 and much smaller than 167 kOe/K (or 80 kOe/K) for granular Bi composite films at grain size of 4.2 nm (or grain size larger than 5.0 nm). 28 However, the estimated values in our Bi nanowires lie in the range of 2-15 kOe/K for strongcoupling superconducting films. 37 The differences in the (dH c /dT) Tc values and the difference in T c indicate that the two observed superconducting transitions around 7.2 and 8.3 K in Bi nanowire arrays are not related to those found in amorphous Bi film or composite granular film.…”

“…They speculated that the original rhombohedral structure of Bi grains might become progressively distorted (with higher density of defects) with decreasing grain size until the Bi grains change into an “amorphous” structure at a grain size of about 4.0 nm. (3) Bi film on a thin Ni film substrate (1−4 nm of Ni/40−70 nm of Bi bilayer). , The T c is around 4.0 K, and the superconductivity was thought to originate from the stress between Bi and Ni layers which induces a face-centered cubic structure of the Bi film (this phase does not survive in bulk Bi or Bi film without a Ni layer) or a common rhombohedral phase with a (102) preferred orientation …”

Superconductivity in a Bi/Ni bilayer

“…The XRD data could also be explained by the common rhombohedral phase with the surface oriented along (110) direction instead of the novel fcc structure, as pointed out by J. A. van Hulst et al 20 . Recent experiments 9,14,21 with similar sample growth conditions show that the order of growth does not change its superconducting properties.…”